Planetary and Space Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Planetary and Space Science
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The exploration of Titan with an orbiter and a lake probe
Giuseppe Mitri a,n, Athena Coustenis b, Gilbert Fanchini c, Alex G. Hayes d, Luciano Iess e,
Krishan Khurana f, Jean-Pierre Lebreton g, Rosaly M. Lopes h, Ralph D. Lorenz i,
Rachele Meriggiola e, Maria Luisa Moriconi j, Roberto Orosei k, Christophe Sotin h,
Ellen Stofan l, Gabriel Tobie a,m, Tetsuya Tokano n, Federico Tosi o
a
Université de Nantes, LPGNantes, UMR 6112, 2 rue de la Houssinière, F-44322 Nantes, France
Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), Observatoire de Paris, CNRS, UPMC University Paris 06,
University Paris-Diderot, Meudon, France
c
Smart Structures Solutions S.r.l., Rome, Italy
d
Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853, United States
e
Dipartimento di Ingegneria Meccanica e Aerospaziale, Università La Sapienza, 00184 Rome, Italy
f
Institute of Geophysics and Planetary Physics, Department of Earth and Space Sciences, Los Angeles, CA, United States
g
LPC2E-CNRS & LESIA-Obs., Paris, France
h
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States
i
Johns Hopkins University, Applied Physics Laboratory, Laurel, MD, United States
j
Istituto di Scienze dell‘Atmosfera e del Clima (ISAC), Consiglio Nazionale delle Ricerche (CNR), Rome, Italy
k
Istituto di Radioastronomia (IRA), Istituto Nazionale di Astrofisica (INAF), Bologna, Italy
l
Proxemy Research, Rectortown, VA, United States
m
CNRS, LPGNantes, UMR 6112, 2 rue de la Houssinière, F-44322 Nantes, France
n
Institut für Geophysik und Meteorologie, Universität zu Köln, Cologne, Germany
o
Istituto di Astrofisica e Planetologia Spaziali (IAPS), Istituto Nazionale di Astrofisica (INAF), via del Fosso del Cavaliere 100, Rome, Italy
b
art ic l e i nf o
a b s t r a c t
Article history:
Received 17 January 2014
Received in revised form
18 July 2014
Accepted 21 July 2014
Fundamental questions involving the origin, evolution, and history of both Titan and the broader
Saturnian system can be answered by exploring this satellite from an orbiter and also in situ. We present
the science case for an exploration of Titan and one of its lakes from a dedicated orbiter and a lake probe.
Observations from an orbit-platform can improve our understanding of Titan's geological processes,
surface composition and atmospheric properties. Further, combined measurements of the gravity field,
rotational dynamics and electromagnetic field can expand our understanding of the interior and
evolution of Titan. An in situ exploration of Titan's lakes provides an unprecedented opportunity to
understand the hydrocarbon cycle, investigate a natural laboratory for prebiotic chemistry and
habitability potential, and study meteorological and marine processes in an exotic environment. We
briefly discuss possible mission scenarios for a future exploration of Titan with an orbiter and a
lake probe.
& 2014 Elsevier Ltd. All rights reserved.
Keyword:
Titan
1. Introduction
Titan, Saturn's largest satellite, is a unique object in the Solar
System, having a substantial nitrogen atmosphere, a surface with a
complex interplay of geological processes (Lopes et al., 2010) and
an outer ice shell overlying a subsurface ocean (Iess et al.,
2012; Mitri et al., 2014). The climate on this icy satellite boasts a
multi-phase alcanological cycle where methane plays a role
n
Corresponding author.
E-mail address: [email protected] (G. Mitri).
similar to water on Earth. The Cassini–Huygens mission to the
Saturn system, which is still ongoing after 10 years of operations,
accomplished a first in-depth exploration of Titan (Lebreton et al.,
2009; Coustenis et al., 2009a). The data revealed hydrocarbon
lakes and seas located mostly in Titan's polar regions (Stofan et al.,
2007; Hayes et al., 2008), which, combined with extensive
equatorial dune deposits (Lorenz et al., 2006), form a major
reservoir of organics in the Solar System. The presence of standing
bodies of liquid, dissected fluvial channels (Lopes et al., 2010),
tectonic features (Radebaugh et al., 2007; Mitri et al., 2010), vast
dune fields (Lorenz et al., 2006) and putative cryovolcanic flows
http://dx.doi.org/10.1016/j.pss.2014.07.009
0032-0633/& 2014 Elsevier Ltd. All rights reserved.
Please cite this article as: Mitri, G., et al., The exploration of Titan with an orbiter and a lake probe. Planetary and Space Science (2014),
http://dx.doi.org/10.1016/j.pss.2014.07.009i
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(Lopes et al., 2007; 2013) shows striking analogies with terrestrial
geological activity. Titan is the only other place in the Solar
System, with the Earth, to have exposed lakes and seas, albeit
with different materials.
The initial discoveries of the Cassini–Huygens mission inspired
an extension of the mission in 2008, first as the Cassini Equinox
Mission and then as the Cassini Solstice Mission to 2017, as well as
multiple proposals to return to the Saturnian system for further
exploration of this enigmatic satellite. Two mission proposals,
Titan and Enceladus Mission TandEM (Coustenis et al., 2009a)
and Titan Explorer (Lorenz, 2009) for ESA and NASA respectively,
merged to form the Titan Saturn System Mission TSSM (Coustenis
et al., 2010; ESA, 2009) which sought further exploration of Titan
with both a dedicated orbiter and in situ probes (balloon and lake
probe) and a large instrument suite. Ultimately ESA and NASA
decided to prioritize the Europa Jupiter System Mission (EJSM),
which would later become ESA's first Large (L) L1 mission of its
Cosmic Vision Program (2015–2025), the JUpiter ICy moon
Explorer (JUICE) with a planned launch date of 2022 (Grasset
et al., 2012). The demise of TSSM inspired mission proposals
smaller in scale and cost including the Titan Mare Explorer TiME
(Stofan et al., 2010, 2013), the Titan Aerial Explorer TAE (Lunine
et al., 2011) and the Journey to Enceladus and Titan JET (Sotin
et al., 2011), using either a probe or an orbiter but not both, and a
small instrument suite. Building on this heritage, the exploration
of Titan with an orbiter and a lake probe was presented as a White
Paper in response to the 2013 European Space Agency (ESA)'s Call
for Science Themes for the Large-class (L) L2 and L3 missions
(Mitri et al., 2013). This paper is based upon the fore-mentioned
White Paper. The Online Supplementary material for this paper
includes the list of the authors and the supporters of the White
Paper from the scientific community.
Future exploration of Titan with a dedicated orbiter and an
in situ lake probe would address central themes regarding the
nature and evolution of this icy satellite and its organic-rich
environment. While the term lake probe is used in this paper
and in previous literature, it should be noted that the ideal target
of any future exploration of the standing liquid bodies on Titan
would mostly likely be one of the large seas found in the north
polar region rather than a lake. Bodies of standing liquid on Titan
with large dimensions are classified as seas (mare) rather than
lakes (lacus). Prior to an orbiter phase around Titan and an in situ
exploration of its lakes, mission scenarios could also involve
investigations of the planet Saturn and its other satellites including the geologically active icy moon Enceladus. A companion
White Paper proposed the exploration of Enceladus with flybys
and Titan with an orbiter and a balloon probe (Tobie et al., 2014,
in this issue). While the exploration of the Saturn system and its
moons, Enceladus and Titan, was ultimately not selected for the L2
and L3 missions, the scientific case for such exploration received
strong support from the scientific community (Mitri et al., 2013;
Tobie et al., 2014, in this issue).
Section 2 summarizes the scientific objectives for Titan's
exploration and how future scientific investigations of Titan
with a dedicated orbiter and lake probe can address some of these
objectives. In Section 3 we discuss how in situ exploration
of Titan's lakes can provide an unprecedented opportunity to
understand the hydrocarbon cycle, investigate a natural laboratory
for prebiotic chemistry and the limits of life, and study meteorological and marine processes in an exotic environment. In Section
4 we present the scientific investigations, measurements and
example approaches. Finally, in Section 5 we review the mission
concept legacy and in Section 6 we present a mission scenario for
the future exploration of Titan with an orbiter and a lake probe.
2. Scientific objectives and investigations for a future
exploration of Titan
In spite of all its advances, the Cassini–Huygens mission, due to
its limited instrumentation and timeline, among other things,
cannot address some fundamental questions regarding Titan such
as (1) What are the complex organic molecules that form the dune
material, and how have they been formed? (2) What is the
composition of the lakes and seas? (3) What are the processes
that form and maintain the atmosphere? and (4) Is Titan endogenically active? In fact, these central themes can be investigated
by a future mission using an orbiter and a lake probe.
To answer these questions among others, the scientific objectives for a future exploration of Titan include (Table 1)
(i) Determine how Titan was formed and evolved, and its internal
structure (Goal 1); (ii) Determine the nature of the geological
activity on Titan (Goal 2); (iii) Characterize the atmosphere of
Titan and determine how the alcanonological cycle works (Goal 3);
and (iv) Determine the marine processes and chemistry of Titan's
lakes and seas (Goal 4).
2.1. Geophysics
The combination of measurements of the gravity field, rotational dynamics and electromagnetic field will contribute to the
achievement of the scientific objective Goal 1 (Table 1). Static and
dynamic gravity field measurements (Iess et al., 2010, 2012), and
shape models (Zebker et al., 2009; Mitri et al., 2014) from the
Cassini–Huygens mission have greatly expanded our knowledge of
the geophysical processes and structure of Titan's interior and as a
consequence its formation and evolution have also become better
understood. Data from the Cassini–Huygens mission suggest that
either Titan has no induced magnetic field or else it is not yet
detectable, while an intrinsic magnetic field is found to be absent
(Wei et al., 2010). While there is still not yet a unique internal
structural model of Titan, constraints provided by gravity, topography and rotation data suggest that Titan's interior has some
degree of differentiation and possesses a water-ice outer shell
separated from its deep interior by a dense subsurface water ocean
(Mitri et al., 2014). Thermal models suggest the presence of a highpressure polymorph ice layer between the deep interior and the
subsurface ocean (Tobie et al., 2005; Mitri and Showman, 2008;
Mitri et al., 2010).
Radio-tracking data registered during multi-flybys of Titan has
yielded gravity field measurements with a spherical harmonic
expansion to degree-3 (Iess et al., 2010, 2012). The quadrupole
(J2 and C22) moments of the gravity field were used to constrain
the normalized axial moment of inertia which value was inferred
Table 1
Scientific objectives for a future exploration of Titan.
Goal
Goal
Goal
Goal
1
2
3
4
Determine how Titan was formed and evolved, and its internal structure
Determine the nature of the geological activity of Titan
Characterize the atmosphere of Titan and determine how the alcanological cycle works
Determine the marine processes and chemistry of Titan's lakes and seas
Please cite this article as: Mitri, G., et al., The exploration of Titan with an orbiter and a lake probe. Planetary and Space Science (2014),
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using the Radau–Darwin approximation as MoI ffi 0:3414 7 0:0005
(Iess et al., 2010). The high moment of inertia suggests that Titan's
deep interior is formed of relatively low-density material
( 2600 kg m 3) that is consistent with a deep interior differentiated and composed of silicate hydrates such as antigorite
(Castillo-Rogez and Lunine, 2010; Fortes et al., 2007) or only
partially differentiated and composed of a mixture of ice and
rocks (Iess et al., 2010). The state of the internal structure of Titan
is still debated, and future investigations are therefore necessary
to determine its internal structure. The interior structure of Titan
can be inferred from a combined measurement of the gravity field,
rotational dynamics and electromagnetic field.
Future gravity measurements from a spacecraft (S/C) in a polar
orbit will provide a significant improvement to the gravity field
determination with respect to Cassini's gravity measurements. We
consider a nominal final orbit of the S/C around Titan as a 1500 km
altitude circular near-polar orbit. Using such an orbit, the static
gravity field of Titan can be resolved to degree-10 and possibly
higher orders. Also the gravity anomalies can be determined with
a spatial resolution lower than 260 km allowing a better characterization of the density contrasts within the ice shell as well as
within the deep interior.
Analysis based on Titan's shape suggests that Titan's outer ice
shell is thicker at the equator and thinner at the poles (Nimmo and
Bills, 2010; Hemingway et al., 2013; Mitri et al., 2014). Estimates
for the thickness of the ice shell vary from thin (50 km) to thick
(200 km) (e.g., Tobie et al., 2005, 2006; Mitri et al., 2008), and
determining which model is correct will require observations
beyond the capability of Cassini mission. The thickness of Titan's
ice shell has major implications for the thermal state of the crust.
In fact, geological activities and processes on Titan could be related
to the onset of thermal convection in the outer ice shell (Tobie
et al., 2005, 2006; Mitri and Showman, 2008). Analysis of Titan's
topography suggests that the ice shell is currently in a conductive
state (Nimmo and Bills, 2010; Hemingway et al., 2013; Mitri et al.,
2014); however, during Titan's evolution it is possible that the ice
shell may have transitioned between a convective to a conductive
state (Mitri et al., 2008, 2010).
The radio science experiment during the Cassini mission
allowed the measurement of the tidal Love number, k2. The tidal
Love number k2 characterizes the tidal deformation of the interior.
The relatively high measured tidal Love number k2 (0.589 70.150)
of Titan indicates that its outer ice shell is decoupled from the
deep interior by a global subsurface ocean (Iess et al., 2012). The
tidal Love number k2 (Iess et al., 2012) in conjunction with
altimetric measurements of the tidal deformation of the outer
ice shell (Love number h2 measurement) could be used to
determine the thickness of the ice shell (see similar experiment
proposed for Europa by Wahr et al. (2006)) during a future
mission. Using a nominal polar circular orbit around Titan with a
1500 km altitude, the measurement of the tidal Love number k2
can be obtained with an expected uncertainty σ k2 5 10 4
improving the accuracy of the determination of k2 achieved during
the Cassini mission by two orders of magnitude. In addition, radar
sounder observations with a penetration depth up to 10 km with
a vertical resolution of 30 m from an orbital platform could
directly determine the relict brittle–ductile transition of the ice
shell revealing its thermal state.
Surface heights derived during the Cassini mission from the
Cassini RADAR altimeter (Elachi et al., 2004) and SAR-Topography
technique (Stiles et al., 2009) are sparsely distributed and cover
only 1% of Titan's surface. Using the available altimetric data,
Zebker et al. (2009) and Mitri et al. (2014) determined the shape of
Titan up to the seventh order of the spherical harmonic expansion.
A quasi-global coverage of altimetric data with a 10 m vertical
resolution for a future mission is necessary to refine the shape
3
model and gravity anomalies of Titan's interior and define the
level of hydrostatic compensation of the shape and the outer ice
shell. A refinement of the shape of Titan at 1 m vertical
resolution will allow the characterization of the tidal deformation
of the outer layer and the determination of the Love number h2.
Radar altimetry is a suitable technique to determine the elevation
of Titan's surface as demonstrated during the Cassini mission.
Interferometric Synthetic Aperture Radar (InSAR) is also a possible
technique that could be used to generate digital elevation models
(DEMs) of the surface and to produce maps of surface deformation
in combination with the imaging of the surface.
The measurement of the imaginary part of the tidal Love
number k2 using gravity field measurements from the radio
science
experiment can be used to estimate the tidal quality factor
Q ¼ k2 =Imðk2 Þ. The estimation of Q would be used to constrain
the present internal tidal dissipation of Titan providing information on the internal heat production as well as on the tidal
evolution of Titan. The estimation of Q would also provide
information on the viscosity of the ice shell and indirectly on the
thermal structure as well as on the thermal state (conductive or
convective) of the ice shell.
The determination of rotational dynamics is crucial to understand the internal structure and geophysical processes on Titan.
Evolution models of the orbital dynamics use the obliquity along
with the eccentricity as a constraint. When the quadrupole gravity
field is known, the obliquity may also provide the moment of
inertia (Bills and Nimmo, 2008), a fundamental quantity to
constrain the internal structure of Titan. The spin rate and the
physical librations in longitude can provide essential information
on the degree of internal differentiation and the ice shell thickness. Currently the only information about the rotational dynamics
of Titan is derived from Cassini mission data (e.g., Stiles et al.,
2008; Meriggiola et al., 2011) and astronomical observations
(Seidelmann et al., 2006).
During Cassini observations the pole location was found to be
not fully compatible with the occupancy of a Cassini state
(Meriggiola et al., 2011). The estimated obliquity (0.31) is not in
agreement with the predicted value (0.151) inferred from the polar,
dimensionless moment of inertia (0.34) derived from gravity field
measurements (Iess et al., 2010). The interpretation of the disagreement between the measured and predicted values is controversial and it seems to point to a differentiated internal
structure characterized by complex rotational dynamics. For this
reason the pole location in terms of obliquity and the occupancy of
a Cassini state should be investigated by a dedicated experiment.
Physical librations in longitude are due to the eccentricity. Titan is
in a 1:1 resonance and it is orbiting around Saturn at the same rate
of its spin motion (Meriggiola et al., 2011). Since Titan's orbit is
eccentric, the Saturn-satellite line and the long axis of the satellite
are not aligned and they differ by a libration angle. The libration
amplitude is inversely proportional to the moment of inertia of the
body, and it is possible, if the quadrupole gravity field is known, to
infer the thickness (h) of the outer icy shell from the libration
amplitude. The determination of the libration amplitude can
provide crucial information on the dimension as well as on the
liquid/solid state of the shell and the deep interior. The rotational
dynamics of Titan can be inferred from tracking surface landmarks
along time using, for example, Synthetic Aperture Radar (SAR)
images with a spatial resolution of 30 to 50 m, and/or from
gravity field measurements (see Cicalò and Milani (2012)).
Observations of Cassini's magnetic field on Titan show that it
currently does not have an intrinsic field generated by a dynamo
(upper limit is 2 nT surface field at the equator). Even though the
Cassini mission has not been successful thus far in inferring an
electromagnetic induction response from a subsurface salty ocean
as measured during the Galileo mission for Europa, Ganymede and
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Callisto, future, orbiting spacecraft can provide better information
on the interior (subsurface ocean) from this technique. The
discovery of periodic changes in the quadrupole gravity field of
Titan at 4% of the static value indicates the presence of an
internal ocean (Iess et al., 2012). Further independent confirmation of Titan's subsurface ocean is necessary.
Detection of electromagnetic induction from the interior of
Titan induced by time-varying external magnetic fields could
provide such a confirmation as has been successfully demonstrated in the discoveries of subsurface oceans of Europa, Callisto
and Ganymede (Khurana et al., 1998; Zimmer et al., 2000; Kivelson
et al., 2002) and the magma ocean of Io (Khurana et al., 2011). The
basic principle of electromagnetic induction relies on the fact that
subsurface conductors generate eddy currents on their surfaces in
response to time varying electric fields imposed by the changing
primary magnetic field. The secondary field generated by the eddy
currents is dipolar in nature if the conductor has a spherical shape
and can be easily characterized by a spacecraft bearing a magnetometer. Modeling of the secondary field provides information on
the thickness and conductivity of the subsurface conductor. For the
Galilean satellites, the time varying driving primary field is
provided by the rotating tilted magnetic field of Jupiter and its
magnetosphere and has an amplitude ranging from 40 nT near
Callisto to 800 nT at Io. However, Saturn's internal magnetic field is
completely axisymmetric and therefore does not impose a timevarying field on its satellites. However a cyclical magnetic field of
unknown origin has recently been detected; this field is supported
by current systems in Saturn's magnetosphere (Andrews et al.,
2010). This magnetic field has an amplitude of 2–4 nT near Titan
and has a period close to 10.8 h which is an ideal sounding
frequency. Assuming that the ice crust has a thickness of 100 km
or less, a highly conducting ocean would then create a secondary
induced field with a magnitude of 0.6 to 1.3 nT just above Titan's
atmosphere (assuming an R 3 falloff for the dipolar response)
which would be detectable by a magnetometer. Another inducing
field with an amplitude exceeding 10 nT is imposed by the
periodic exits of Titan from Saturn's magnetosphere. The several
nT induction response to this signal would not be sinusoidal but
would be easily characterized by an orbiting spacecraft. Unfortunately, the plasma interaction of Titan with the magnetosphere of
Saturn creates magnetic perturbations with an amplitude of 10 nT
or more (Wei et al., 2010), therefore, it has so far not been possible
to confirm the presence of an electromagnetic induction signal
from Titan. This situation can be remedied if continuous observations from an orbiter were available. The spatial forms and
frequency contents of magnetic fields generated by plasma interaction and by electromagnetic induction are distinct and can be
easily separated by harmonic analysis if continuous observations
from an orbiter were available. Such an experiment will be
performed by the JUpiter ICy moons Explorer – JUICE spacecraft
mission at Ganymede where researchers hope to disentangle the
inducing field from its ocean with an amplitude of 60 nT from
the much stronger permanent internal magnetic field with a
magnitude of 1500 nT.
2.2. Surface processes and composition
A combination of measurements and observations from surface
imaging, altimetric data, spectral data and radar sounding will
enable the achievement of the scientific objective Goal 2 (Table 1).
The Cassini mission has revealed for the first time the complexity
of the surface processes and composition of Titan. However, it is
still not understood if Titan is endogenically active and if cryovolcanic activity is present. Other unsolved questions include
determining the role of the lakes, channel networks and dune
fields in the alcanological cycle (e.g. Aharonson et al., 2012).
Sections 2.2.1 and 2.2.2 discuss how observations from the
orbital-platform will improve our understanding of the geological
processes and surface composition, respectively.
2.2.1. Geology
Our current knowledge of Titan's geology comes from Cassini
observations, which show a geologically complex surface strikingly similar to Earth's. Titan has been called the Earth of the outer
Solar System. Titan has a dense atmosphere (1.5 bar) and the
present-day surface–atmospheric interactions make eolian, fluvial,
pluvial and lacustrine processes important on a scale previously
seen only on Earth, despite vast differences in material properties
and ambient conditions. These atmospheric and surface processes
interact to create similar geologic features to Earth's: lakes and
seas filled with liquid, rivers caused by rainfall, and vast fields of
dunes caused by wind (e.g. Lorenz et al., 2006; Stofan et al., 2007;
Hayes et al., 2008; Radebaugh et al., 2008; Barnes et al., 2008; Burr
et al., 2013). The scarcity of impact craters indicates that Titan's
surface is relatively young, with estimates ranging from 200 Ma to
1 Ga (Lorenz et al., 2007; Wood et al., 2010). Mountains indicate
that tectonic activity has taken place (Radebaugh et al., 2007; Mitri
et al., 2010) although it may not necessarily be endogenic in nature
(Moore and Pappalardo, 2011). Several putative cryovolcanic
features have been identified (e.g. Lopes et al., 2007, 2013;
Soderblom et al., 2009) and the existence of some ongoing
volcanic activity has been proposed (Nelson et al., 2009a,
2009b), though the cryovolcanism interpretation has been disputed (Moore and Pappalardo, 2011). The variety of geologic
processes on Titan and their relationship to the methane cycle
make it particularly significant in Solar System studies.
The Cassini RADAR swaths have revealed a wide variety of
geologic features, examples of which are shown in Fig. 1. However,
interpretations of Titan's surface properties and geologic history
have been hindered by observational challenges primarily related
to its dense obfuscating atmosphere. Currently, the highest resolution observations of Titan's surface come from SAR images with a
pixel-scale of 300 m. At this resolution, Cassini has imaged
43% of Titan's surface. Including lower resolution HiSAR data
( few km pixel scale), obtained at further distances from Titan
than standard SAR, Cassini's surface coverage increases to 60%.
A follow-up mission to Titan should image the surface at a pixel
scale of 30 m or better with an almost complete surface coverage
using Synthetic Aperture Radar (SAR) technique. As has been
shown in both the terrestrial and Martian communities, resolution
increases of an order of magnitude or better results in fundamental advances in the interpretability of observed morphologies.
Altimetric data with a vertical resolution of 10 m is necessary for
the geological interpretation of surface features. Both radar altimetry and Interferometric Synthetic Aperture Radar (InSAR) are
suitable techniques to determine the elevation of Titan's surface.
Radar sounder measurements with a penetration depth up to few
km and a vertical resolution of 10–30 m are capable of characterizing the subsurface structures and determining their correlation
with surface features allowing the determination of the internal
processes that form the surface geological features. In addition,
radar sounder measurements will determine the thickness and
structure of surface organic-material deposits, which can be used
to ascertain the global volume of organic material on Titan.
The major geologic unit types and their possible genesis have
been recently reviewed by Lopes et al. (2010). Below, we briefly
describe each of the major geomorphic units that have been
identified and, where appropriate, state how higher resolution data
could provide fundamental advancements in their interpretability.
Hummocky and mountainous terrain: This unit consists of
numerous patches of radar-bright, textured terrains, some of
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Fig. 1. Examples of geological features on Titan. Top row from left: Sinlap crater with its well-defined ejecta blanket. Middle: Crateriform structure (suspected impact crater)
on Xanadu. Right: Ontarius Lacus near Titan's south pole. Middle row, left: Putative cryovolcanic flow Winia Fluctus (lobate, radar bright), note the dunes overlaying part of
flow. The flow overlays radar-dark undifferentiated plains. Right: Dunes abutting against hummocky and mountainous terrain. Bottom left: mountains (radar bright) in the
equatorial region. Right: Elivagar Flumina, interpreted as a large fluvial deposit, showing braided radar-bright channels. Deposits overlay radar-dark undifferentiated plains.
which have been interpreted to be of tectonic origin (Radebaugh
et al., 2007; Mitri et al., 2010). Among these features are long
mountain chains, ridge-like features, elevated blocks that stand
generally isolated, and bright terrains of a hilly or hummocky
appearance. This terrain is mostly found as isolated patches or
long mountain chains, small in areal extent. The exception is
Xanadu, a large area about 4500 km across. The hummocky and
mountainous terrain unit, including Xanadu, is thought to be the
oldest geologic unit exposed on Titan's surface (Lopes et al., 2010).
Deciphering the origin of this feature class awaits high-resolution
imagery that can resolve the complex lineations and brightness
variations that make up the hummocky appearance; altimetric
data can be used to determine the elevation of this terrain.
Impact craters: Few well-preserved impact craters have been
identified on Titan. However, there are numerous structures on
Titan that may have been caused by impacts. Radar-bright complete or incomplete circles are seen all over the surface. Wood
et al. (2010) identified 44 possible impact craters with a wide
variety of morphologies and, since then, others have been recognized and mapped. Neish et al. (2013) found Titan's craters are
shallower than those on Ganymede, probably due to eolian
infilling, with possible contributions from viscous relaxation and
fluvial modification. Radar sounding can be used to determine the
structure of the craters. Detailed morpho-dynamic analysis of
Titan's craters, however, requires high resolution stereo models
that allow quantitative comparison with numerical simulations.
Dunes: Eolian transportation and deposition is a major and
widespread process on Titan. Features resembling linear dunes on
Earth have been identified by SAR, covering regions hundreds,
sometimes thousands, of kilometers in extent (Lorenz et al., 2006).
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Dunes are dark to RADAR and Imaging Science Subsystem (ISS)
and correlate well with dark surface units identified by the Visual
and Infrared Mapping Spectrometer (VIMS). The dunes are
thought to be made of organic particles (Soderblom et al., 2007),
which may have originated as photochemical debris rained out
from the atmosphere, deposited and perhaps later eroded by
fluvial processes into sand-sized particles suitable for dune formation (Atreya et al., 2006; Soderblom et al., 2007). Titan's sand
dunes represent a detailed geomorphic record of both current and
past wind regimes that can reveal global changes in climate and
surface evolution (e.g. Radebaugh et al., 2008). Studies of Titan's
dune morphologies, how the morphologies spatially vary and
how the dunes interact with local topography provide a
basis for interpreting wind direction, sediment supply, and atmosphere conditions such as boundary layer depth. Thus dunes
represent a morphologic unit where even slight increases in image
resolution can reveal patterns that have significant interpretive
implications.
Channels: Many radar-bright and radar-dark channels are
seen in SAR images, widely distributed in latitude and longitude.
It is likely that many more channels exist on Titan, but are too
small for the Cassini spacecraft SAR data to resolve. Channels are
seen connecting some lakes at high latitudes and cutting the
hummocky, mountainous terrain at lower latitudes, possibly
indicating orographic rainfall. Channels, lakes and dunes are
thought to be the youngest geologic units on Titan (Lopes et al.,
2010). The Labyrinthic terrains unit, identified by Malaska et al.
(2010) is a probable end-member of the different types of channel
and valley network morphologies identified on Titan. There are
few areas of this type of terrain exposed, mostly at high latitudes.
Higher resolution images of Titan's channel networks can allow
pattern analysis, such as branching angle distribution, that will
address the strength of the bedrock and origin (e.g., overland flow
vs. subsurface drainage) of the network (e.g., Burr et al., 2013).
Lakes: Hydrocarbon lakes are found preferentially in the polar
regions at the current season on Titan (Stofan et al., 2007). While
Section 3 discusses the benefits of in situ lake exploration, there is
also much to learn from studying the morphology and distribution
of lacustrine features on Titan. Titan's lakes and seas are found
both connected to and independent of regional and local drainage
networks. They have shoreline transitions varying from sharp to
diffuse and include a variety of plan-form shapes ranging from
near-circular to highly-irregular. Exposed surface areas span from
the limits of detection ( 1 km2) to more than 105 km2 (Hayes
et al., 2008). Along with the surrounding landscape topography,
these lacustrine and features express a range of morphologic
characteristics that constrain basin formation and evolution. For
instance, the largest lakes, or seas, in the north polar region,
consisting of Ligeia, Kraken and Punga Mare, have shorelines that
include shallow bays with well-developed drowned river valleys
(Stofan et al., 2007). Such drowned valleys indicate that once welldrained upland landscapes have become swamped by rising fluid
levels where sedimentation is not keeping pace with a rising base
level. These features also imply that liquid levels were previously
lower and stable long enough to allow the currently drowned
networks to be incised into the terrain. In contrast the largest
southern lake, Ontario Lacus, expresses depositional morphologies
along its western shoreline that include lobate structures interpreted as abandoned deltas (Wall et al., 2010). Obtaining higher
resolution images and topography data of Titan's lakes will allow
quantitative morphology analysis that will lead to new hypotheses
for landscape formation and evolution.
Candidate cryovolcanic terrain: The Cassini's SAR swaths also
revealed several lobate, flow-like features and constructs interpreted as resulting from cryovolcanism (Lopes et al., 2007; Wall
et al., 2009) and VIMS data also identified some features that were
thought to be cryovolcanic in origin (Sotin et al., 2005; Barnes
et al., 2006; Nelson et al., 2009a, 2009b). As more data were
acquired, particularly overlapping SAR coverage from which stereo
could be derived, several putative cryovolcanic units were reassessed and thought to be of a different origin (Lopes et al., 2010)
leading Moore and Pappalardo (2011) to argue that there may be
no cryovolcanic units on Titan at all. However, radar stereogrammetry has revealed a tall mountain (Doom Mons) next to a deep
elongated pit (depth 41 km), adjacent to several flow-like features. This is considered the strongest evidence for a cryovolcanic
region on Titan (Lopes et al., 2013). Higher resolution images and
topography are needed in order to further scrutinize potential
cryovolcanic features. Radar sounder measurements can distinguish cryovolcanic features such as calderas from other features
that have similar morphology (e.g. impact craters) imaging the
subsurface structures.
Undifferentiated plains: There are vast expanses of plains,
mostly at mid-latitudes, that appear relatively homogeneous and
radar-dark, and are classified as plains because they are really
extensive, relatively featureless, and generally appear to be of low
relief. The units may be sedimentary in origin, resulting from
fluvial or lacustrine deposition, or they may be cryovolcanic, but
their relatively featureless nature and gradational boundaries
prevent interpretation using SAR data alone. Radar sounder
measurements will determine the layering of the undifferentiated
plains to determine if they are sedimentary or cryovolcanic in
origin.
Mottled plains: The mottled plains are irregularly shaped
terrains having dominantly intermediate backscatter that appear
to have relatively small topographic variations. Mottled terrains
may be erosional and a mix of several other units, for example,
some may be undifferentiated plains covered over in small areas
by isolated dunes.
2.2.2. Surface composition
Observations of Titan's surface are severely hindered by the
presence of an optically thick, scattering and absorbing atmosphere, allowing direct observation of the surface within only a
few spectral windows in the near-infrared. Based on the 1.29/
1.08 μm, 2.03/1.27 μm and 1.59/1.27 μm band ratios measured by
Cassini's VIMS, three main spectral units can be distinguished on
the overall surface of Titan: whitish material mainly distributed in
the topographically high areas; bluish material adjacent to the
bright-to-dark boundaries; and brownish material that correlates
with RADAR-dark dune fields (Barnes et al., 2007; Soderblom et al.,
2007; Jaumann et al., 2008). Even though the spectral units are
distinct, their actual composition remains elusive. Bright materials
may consist of precipitated aerosol dust composed of methanederived organics (Soderblom et al., 2007) superimposed on waterice bedrock. The bluish component might contain some water ice
as its defining feature (Soderblom et al., 2007), even though
organics cannot be ruled out. Finally, the brownish unit reveals
lower relative water-ice content than Titan's average that is
compatible with an enhancement in complex organic material
and/or nitriles clumping together after raining onto the surface
(Barnes et al., 2008).
A number of chemical species, including water ice, have been
proposed to exist on the surface of Titan including carbon dioxide,
ammonia, ammonia hydrate, cyanoacetylene, benzene, hydrogen
cyanide, acetylene, acetonitrile, and liquid alkanes. But only a few
absorptions have been unambiguously detected in Titan's methane
windows from remote sensing observations carried out during
Cassini fly-bys. Evidence from VIMS infrared spectra suggests CO2
frost, which is probably condensed from the atmosphere (McCord
et al., 2008). Liquid ethane has been identified in Ontario Lacus
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(Brown et al., 2008) and, although the exact blend of hydrocarbons
in polar lakes and seas is unknown, liquid methane is almost
certainly present. Finally, the correlation between the 5-μm-bright
materials and RADAR-empty lakes suggests the presence of
sedimentary or organic-rich evaporitic deposits in dry polar
lakebeds (Barnes et al., 2011).
Spectral mapping of Titan's diverse surface at both high spatial
and high spectral resolution could close a major gap in knowledge after the Cassini mission. Cassini's VIMS instrument is
expected to cover only a small percent of Titan's surface at
kilometer-or-better resolution, even after the extended mission.
Thus a prime element in the orbiter payload is a spectral mapper.
In essence this instrument addresses two main goals: highresolution near-IR imaging and spectroscopy of the surface. Other
spectral end-members will likely appear as a dedicated orbiter
mission improves the spatial resolution and coverage by orders of
magnitude. An important augmentation beyond Cassini is that
the spectral mapper should cover the wavelength range beyond
5 μm (ideally up to 6 μm). This spectral region (in a methane
window, where the atmosphere is quite transparent) is highly
diagnostic of several organic compounds expected on Titan's
surface. Cassini VIMS data has already shown some tentative
signatures around 5 μm (possibly associated with aromatic and/
or polycyclic compounds like phenanthrene, indole, etc.), however robust identification is impossible due to inadequate spectral resolution. Cassini VIMS has shown that Titan's surface can be
observed and mapped at 5 μm (Sotin et al., 2012) despite the low
signal-to-noise ratio of the VIMS instrument at this wavelength,
the small amount of solar flux and the small surface albedo. With
the current technologies, detectors in this wavelength domain
could provide high resolution mapping as well as diagnostic
signatures of several organic compounds likely to be present on
Titan's surface either in the dune fields or on the shores of lakes
(Stofan et al., 2007; Barnes et al., 2011). Finally, correlations
between high-resolution infrared hyperspectral images and surface images (e.g. SAR data) will enable the identification of terrain
units at various spatial scales, which are homogeneous both in
terms of surface composition (abundance of non-water-ice components compared to water ice) and surface properties (e.g.,
roughness and porosity) (Tosi et al., 2010).
2.3. Atmosphere
An instrument suite, enabling both remote and in situ measurements and observations of Titan's atmosphere including, for
example, lidar and infrared spectrometers on an orbiter platform
and mass spectrometers, physical property (temperature, pressure,
humidity, wind speed) sensors and an imaging camera onboard a
lake-probe could be used to achieve scientific objective Goal 3
(Table 1). Tobie et al. (2014, in this issue) discuss in detail the
scientific case for the in situ examination of Titan's atmosphere
using a balloon probe.
Unlike the other moons in the Solar System, Titan has a dense
nitrogen-dominated (98.4%) atmosphere, presenting unique atmospheric dynamics, atmospheric chemistry and alcanonological
cycle. Besides nitrogen, the atmosphere consists of 1.5% methane,
as well as trace amounts of hydrocarbons such as ethane, acetylene, and diacetylene, benzene and nitriles, such as hydrogen
cyanide (HCN), cyanogen (C2N2) and cyanoacetylene (HC3N).
Somewhat more complex molecules such as propane, butane,
polyacetylenes, and complex polymeric materials might follow
from these simpler units based on experiments in the laboratory.
In addition, several oxygen compounds, including water vapor, CO
and CO2 exist in the atmosphere, probably of exogenic origin. The
Cassini–Huygens mission detected and measured many of the
constituents in Titan's atmosphere (Coustenis and Hirtzig, 2009).
7
The Cassini mission has revealed the presence of large organic
molecules with high molecular masses, above 100 amu, in the
thermosphere and ionosphere. This is an indication of the photochemical production of Titan's aerosols starting in the upper
atmosphere. While the identities of these molecules are still
unknown, their presence suggests a complex atmosphere that
could hold the precursors for biological molecules such as those
found on Earth.
The thermal structure of Titan's atmosphere is similar to the
Earth's, with a troposphere, a stratosphere, a mesosphere and a
thermosphere; however these regions develop on a larger scale
height due to Titan's lower gravity and lower temperatures.
A detached layer of haze, the result of photochemical reactions
in the upper atmosphere, is visible at altitudes greater than
520 km (Porco et al., 2005). The Voyager spacecraft showed
the vertical structure of the haze to consist of a main layer, located
at an altitude below 200 km, separated from a detached layer of
haze at an altitude of 500 km by a gap of approximately 300 km;
these two layers merge near the north pole (Porco et al., 2005).
The extended haze layer has been found to have reduced its
altitude from 500 km to 380 km between 2007 and 2010 (West
et al., 2011). Cassini's images also confirmed the existence of bright
clouds in Titan's atmosphere, which were already inferred from
ground-based observations (Brown et al., 2002). Titan's clouds are
probably composed of methane, ethane, or other simple organics.
Their presence seems scattered and variable, interspersing the
overall haze.
A striking feature of Titan's atmospheric dynamics is the
strong stratospheric super-rotation in either hemisphere
(Achterberg et al., 2011). The global profile of zonal winds
between 100 and 500 km altitude can be reconstructed from
the temperature data retrieved from Cassini Composite Infrared
Spectrometer (CIRS), except very close to the equator (Achterberg
et al., 2011). On the other hand, the wind profile below 100 km
altitude is known only at the entry site of the Huygens Probe.
Huygens' Doppler Wind Experiment (Bird et al., 2005) found that
the superrotation gradually weakens below 150 km altitude, but
it also detected an unexpected layer of extremely weak winds
near 80 km altitude and a complex wind pattern near the surface.
This demonstrates that in situ measurements can detect wind
patterns especially in the troposphere that cannot be detected
from remote measurements in other parts of the atmosphere. The
wind speed and direction in selected areas of the troposphere can
be constrained by cloud tracking (Turtle et al., 2011a) but only
where clouds exist. The observed orientation and appearance of
dunes have been used to constrain the surface wind, but this led
to ambiguous interpretations on the prevailing surface wind
direction (Lorenz and Radebaugh, 2009; Rubin and Hesp, 2009;
Tokano, 2010). The wind direction near the surface and its
possible seasonal variation are crucial measurements for the
angular momentum budget of the atmosphere as well as
length-of-day variation of the surface (Tokano and Neubauer,
2005). It is unknown from atmospheric observations whether
there is an Earth-like large angular momentum exchange
between the atmosphere and surface, which is equilibrated on
an annual average, or there is almost no angular momentum
exchange. Geodetic observations should ideally be accompanied
with wind observations to establish a link between possible
atmospheric forcing and response of the surface. Near-surface
wind measurements by a lake lander will be meaningful in this
regard since the wind at high latitudes is more indicative of
seasonal wind fluctuations than equatorial winds and there are
no apparently active dunes at high latitudes that could be used to
guess the wind direction.
Another major characteristic of Titan's meteorology is the
methane weather, which most impressively manifests itself in
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the visible clouds (Roe, 2012). During the southern summer most
convective clouds were seen at high southern latitudes but the
main cloud activity moved northward around the recent equinox
(Schaller et al., 2009; Turtle et al., 2011a). More persistent clouds
at higher altitudes were seen near the north pole (Rodriguez et al.,
2009). For many years, the equatorial region was devoid of visible
clouds and thus was expected to be dry. However, the vertical
profile of temperature and pressure (Fulchignoni et al., 2005) and
methane mole fraction (Niemann et al., 2005) measured by
Huygens found that methane condensation and weak precipitation in the form of drizzles occur even near the equator where no
visible cloud was present (Tokano et al., 2006). On the other hand,
occurrence of convective clouds strongly depends on the global
circulation pattern, static stability and methane humidity in the
lower troposphere (Mitchell et al., 2011; Schneider et al., 2012).
While the vertical temperature profile down to the surface can be
sounded by radio occultation (Schinder et al., 2012), the methane
humidity is difficult to measure remotely. In situ measurement of
the near-surface methane humidity in the polar region would
therefore provide valuable insight into the mechanism behind the
seasonality of Titan's methane weather. There is also evidence of
substantial methane rainfall on Titan, but the evidence is indirect
in that it came from temporary surface darkening and temporal
correlation with a cloud system (Turtle et al., 2011b). The precipitation amount is unknown despite the huge role it plays for
Titan's geomorphology. Only direct measurements of rainfall on
the surface would unambiguously show in which seasons it does
or does not rain in the polar region, how much it rains and the
chemical composition of the rains.
The meridional circulation pattern in the troposphere predicted by general circulation models (GCMs) differs from model
to model, particularly at high latitudes. To some extent the global
statistics of clouds constrain the meridional circulation pattern
(Rodriguez et al., 2009). However, clouds do not develop in the
upwelling branch of the circulation if the near-surface air is too
dry and not every condensation causes visible clouds. Therefore, a
more certain quantity to verify the meridional circulation at high
latitudes is the surface pressure, which can be compared to the
in situ surface pressure data at the Huygens landing site
(Fulchignoni et al., 2005) after a sea-level correction. A surface
pressure higher and lower than the Huygens pressure would
indicate downwelling and upwelling, respectively, in the season
of observation.
Theoretical studies predict some significant changes when the
north polar lake area approaches summer during the next few
years. Seasonal change in the global circulation pattern may cause
substantial precipitation in the north polar region (Schneider et al.,
2012), warming of the sea surface may cause a sea breeze/land
breeze depending on the sea composition (Tokano, 2009), freshening of the near-surface wind may cause previously undetected
waves on the seas (Hayes et al., 2013) and evaporation of a large
amount of methane from the seas might even cause tropical
cyclones (Tokano, 2013). These meteorological features cause
substantial surface pressure variations and thus can be detected
in situ by continuous surface pressure measurements and optical
monitoring of the environment. The unknown exact chemical
composition in the sea, as well as in the air over sea, is important
for many questions related to the polar region. Evaporation of
methane from a sea itself is difficult to observe in situ unless
accompanied with fog, but it can be quantified from the methane
vapor pressure and methane partial pressure. The mole fractions
of other species in the sea and air over sea would indicate whether
the lake and atmosphere are in short-term or long-term chemical
equilibrium, if any.
GCMs predict gravitational tides and various waves in the
troposphere, which affect the wind, pressure and temperature
near the surface (Tokano and Neubauer, 2002; Mitchell et al., 2011;
Lebonnois et al., 2012). Some of the waves may become visible as
clouds but most other waves may not. The most reliable data to
verify these waves are in situ time series measurements of surface
wind and atmospheric pressure over several days. The various
waves have different frequencies and amplitudes and may thus be
recognized in the power spectra of the time series.
3. In situ exploration of a Titan sea
The Cassini spacecraft has unveiled a world that is both strange
and familiar to our own, with vast equatorial dune fields (e.g.,
Lorenz et al., 2006), well-organized channel networks that drain
from mountains into large basins (e.g., Burr et al., 2013) and,
perhaps most astonishingly, lakes and seas (e.g., Stofan et al., 2007)
filled with liquid hydrocarbons (Brown et al., 2008). Titan is the
only extraterrestrial body currently known to support standing
bodies of liquid on its surface and is one of only three places in the
Solar System, along with Earth and Mars, that we know to posses
or have possessed an active hydrologic system (e.g., Atreya et al.,
2006). Just as Earth's history is tied to its oceans, Titan's origin and
evolution are chronicled within the nature and evolution of its
lakes and seas and their interactions with the atmosphere and
surface. While Cassini has provided a wealth of information
regarding the distribution of liquid deposits on Titan (e.g., Hayes
et al., 2008; Turtle et al., 2009; Aharonson et al., 2009), it has
provided only the most basic information regarding their composition (e.g., Brown et al., 2008) and role in Titan's volatile cycles
(e.g., Lunine and Atreya, 2008). In order to address these fundamental questions, which link the lakes to Titan's origin and
evolution, we must visit them in situ. In situ exploration of Titan's
lakes and seas presents an unprecedented opportunity to understand its hydrocarbon cycles through the abundance and variability of the liquid's compositional components (methane, ethane,
propane, etc.), glimpse into the history of the moon's evolution as
chronicled by noble gas abundance (argon, krypton, xenon) and
isotopic composition (e.g., 12C vs. 13C), investigate a natural
laboratory for prebiotic chemistry and the limits of life through
an examination of trace organics, and study meteorological and
marine processes in an exotic environment that is drastically
distinct from terrestrial experience (scientific objectives 1, 2, 3,
and 4, Table 1).
Titan's lakes are found mostly poleward of 551 latitude and
encompass 1.2% of the surface that has been observed ( 50%) by
Cassini's instruments (Hayes et al., 2008, 2011). Lacustrine features
are predominantly found in the north, where 11% of the observed
area poleward of 551N ( 60%) is covered by radar-dark lakes.
Poleward of 551S, only 0.5% of the observed area ( 60%) has been
interpreted as potentially liquid-filled. This dichotomy has been
attributed to Saturn's current orbital configuration, in which
Titan's southern summer solstice occurs nearly coincident with
perihelion and results in a 25% higher peak flux than what is
encountered during northern summer (Aharonson et al., 2009).
Over many seasonal cycles, this flux asymmetry can lead to net
transport of volatiles (methane/ethane) from the south to the
north. As the orbital parameters shift, however, the net flux of
northward-bound volatiles is expect to slow and eventually
reverse, predicting a liquid distribution opposite of today's in
35 kyr (i.e., more liquid in the south than the north). If this
hypothesis is correct, the distribution of liquid deposits on Titan is
expected to move between the poles with a period of 50 kyr in a
process analogous to Croll–Milankovich cycles on Earth. In situ
measurement and comparison between the relative abundance of
volatiles that are mobile over these timescales (e.g., methane,
ethane) vs. those that are involatile (e.g., propane, benzene), can be
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Fig. 2. Distribution of lacustrine and fluvial morphologies in Titan's polar regions (see also Hayes et al. (2008) and Aharonson et al. (2012)). Blue polygons depict modern
lakes and seas filled with liquid hydrocarbons. The red polygons show the locations of radar-bright basins interpreted as paleolakes. Cyan polygons depict the distribution of
albedo-dark features that are above the radar noise floor and express lacustrine-like morphologies. These features are believed to represent anything from mud flats to
shallow liquid deposits. Fluvial valleys are represented by blue lines. Dashed lines were drawn on ISS images while the remaining polygons were created using Synthetic
Aperture Radar (SAR) images generated from Cassini RADAR observations. SAR incidence angle and swath coverage are shown. (For interpretation of the references to color
in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Cassini SAR mosaic images of the north polar region showing Kraken, Ligeia
and Punga Maria. Black–yellow color map was applied to the single band data. (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
used to test this hypothesis and understand volatile transport on
thousand year timescales. Volatile transport over shorter timescales (diurnal, tidal, and seasonal) can be investigated via in situ
measurements of the methane evaporation rate and associated
meteorological conditions (e.g., wind speed, temperature, humidity). These measurements can be used to ground-truth methane
transport predictions from global climate models (e.g., Mitchell,
2008; Tokano, 2009; Schneider et al., 2012).
In the north, 87% of the area of observed lacustrine deposits is
contained within the three largest seas, Kraken Mare (496,380 km2),
Ligeia Mare (133,160 km2), and Punga Mare (49,400 km2) (Figs. 2 and
3). Bodies of standing liquid on Titan with large dimensions are
classified as seas (mare) rather than lakes (lacus). The remainder of
north polar lakes approximately follow a lognormal distribution with
a mean diameter of 10 km (Hayes et al., 2008). While Cassini has
provided a wide range of evidence that the largest of Titan's lakes are
indeed liquid-filled basins (e.g., Brown et al., 2008), the returned data
have only been able to provide lower limits on their depth that are
highly dependent on the unknown dielectric properties of the both
the liquid and the lakebed (e.g., Paillou et al., 2008; Hayes et al., 2010,
2011). In the absence of a substantial subsurface reservoir, these
maria represent the dominant liquid hydrocarbon inventory on Titan.
Prior to Cassini, a global ocean of liquid hydrocarbon was predicted
to cover the surface and act as both a source of atmospheric methane
and sink for photolysis products such as ethane and higher order
hydrocarbon/nitriles (Lunine et al., 1983). The absence of this global
reservoir questions the long-term stability of Titan's atmosphere,
which would be depleted in 107 years in the absence of a crustal
source or methane outgassing from the interior (e.g., Tobie et al.,
2006). Determining the bathymetry of one of Titan's Mare, which can
be reliably accomplished in situ, is essential to understanding the
total volume of liquid available for interaction with the atmosphere.
Remote sensing radar techniques should also be used to determine
the lakes' bathymetry. The inventory of methane in Titan's maria,
which requires knowledge of both depth and composition, will
provide a lower limit on the length of time that the lakes can sustain
methane in Titan's atmosphere (Mitri et al., 2007) and help to
quantify the required rate of methane resupply from the interior
and/or crust. Similarly, the absolute abundance of methane photolysis products (e.g., ethane, propane) will determine a lower limit for
the length of time that methane has been abundant enough to drive
photolysis in the upper atmosphere and deposit its products onto the
surface and, ultimately, into the lakes. Furthermore, a comparison
amongst the relative abundance of photolysis products could reveal
fractionation processes such as crustal sequestration, as would
happen if one constituent (e.g., propane) were preferentially introduced into a repository such as crustal clathrate hydrate.
Similar to the Earth's oceans, Titan's seas record a history of
their parent body's origin and evolution. Specifically, the noble gas
and isotopic composition of the sea can provide information
regarding the origin of Titan's atmosphere, reveal the extent of
communication with the interior, potentially constrain the conditions in the Saturn system during formation, and refine estimates
Please cite this article as: Mitri, G., et al., The exploration of Titan with an orbiter and a lake probe. Planetary and Space Science (2014),
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of the methane outgassing history. The Huygens probe detected a
small argon abundance in Titan's atmosphere, but was unable to
detect the corresponding abundance of xenon and krypton
(Niemann et al., 2005). The non-detection of xenon and krypton
supports the idea that Titan's methane was generated by serpentinization of primordial carbon monoxide and carbon dioxide
delivered by volatile depleted planetesimals originating from
within Saturn's subnebula (e.g., Atreya et al., 2006). A competing
model of methane origination, however, suggests that methane is
primordial and was sourced from planetesimals originating from
the outer solar nebula (Mousis et al., 2009). This competing model
was originally suggested as an attempt to explain the measured
ratio of deuterium/hydrogen (D/H) (1:32 10 4 ), which Mousis
et al. (2009) suggest would require a D/H ratio in Titan's water ice
that does match that of Enceladus. In order to support a primordial
methane source, xenon and krypton would both have to be
sequestered from the atmosphere. While xenon is soluble in liquid
hydrocarbon (solubility of 10 3 at 95 K) and could potentially be
sequestered into liquid reservoirs, argon and krypton cannot.
Therefore, the absence of measureable atmospheric krypton
requires either sequestration into non-liquid surface deposits,
such as clathrates (Mousis et al., 2011), or depletion in the noble
gas concentration of the planetesimals that originally delivered
Titan's atmosphere (Owen and Niemann, 2009). The low, albeit
measureable, argon-to-nitrogen ratio has been used to suggest
that Titan's atmospheric nitrogen was sourced from ammonia, as
ammonia is significantly less volatile than both molecular nitrogen
and argon (Niemann et al., 2005), which have similar volatility.
The presence of xenon in Titan's seas would indicate that the
absence of atmospheric krypton is a result of sequestration into
crustal sources (Mousis et al., 2011) as opposed to the result of
carbon delivery via depleted planetesimals (Owen and Niemann,
2009). In addition to noble gas concentration, isotopic ratios can
also be used to decipher the history of Titan's atmosphere. For
example, the 13C/12C ratio of CH4 was used by Mandt et al. (2009)
to conclude that methane last outgassed from the interior 107
years ago. However, this calculation assumes that the exposed
methane reservoir has an isotopic composition that is in equilibrium with the atmosphere. If the carbon isotope ratio of hydrocarbons in Titan's lakes/seas were found to be different than in the
atmosphere, it would imply alteration of the isotopic composition
by evaporation and condensation and indicate a different timescale for the history of methane outgassing.
Titan's lakes and seas collect organic material both directly,
through atmospheric precipitation of photolysis products, and
indirectly, through eolian/fluvial transport of surface materials
(e.g., river systems flowing into the Mare). As a result, the lakes
and seas represent the most complete record of Titan's organic
complexity and present a natural laboratory for studying prebiotic
organic chemistry. Titan's environment is similar to conditions on
Earth four billion year ago and presents an opportunity to study
active systems involving several key compounds of prebiotic
chemistry (Raulin, 2008). For instance, the complex aerosols produced in Titan's atmosphere via photolysis, which are presumably
transported into the lake via pluvial, fluvial, and eolian processes,
are similar to the initial molecules implicated in the origin of life on
Earth. Other organics can act as tracers for specific environmental
conditions. For example, the presence of oxygen-bearing organic
species would suggest that redox reactions have taken place and
that the organic deposits have, at some point, been in at least
transient contact with liquid water. In general, simple abiotic
organic systems exhibit monotonically decreasing abundance with
increasing chain complexity. Distinctive deviations from such a
distribution can mark the signs of prebiotic chemistry and potentially represent the initial markers on a pathway toward selfreplication and a minimally living system.
Finally, lacustrine settings on Titan represent exotic environments that are being sculpted by many of the same meteorological
and oceanographic processes that affect lake and ocean settings on
Earth. In situ exploration of such an environment provides an
opportunity to study these processes under vastly different environmental conditions (scientific objectives 2 and 4, Table 1). While
the Huygens probe provided atmospheric data over equatorial
latitudes around winter solstice, models of Titan's climate predict
that polar weather could be quite different from the equatorial
environment investigated by Huygens. In situ measurements of
such an environment would provide invaluable ground-truth for
global climate models (GCM) of Titan's surface/atmosphere interactions. Ultimately, this could lead to a better understanding of
how to accurately predict the evolution of our own climate system
and even how to model extreme climates on extrasolar planets, as
Titan may represent a common climate system amongst solid
bodies in Earth-like orbits around M-dwarf stars.
4. Scientific investigations, measurements and approaches
All of the previously discussed objectives of the proposed
exploration of Titan and the broader Saturnian system (Table 1)
can be achieved by complementary investigations of a dedicated
orbiter and a lake probe. The in situ exploration of one of Titan's
seas can contribute to the achievement of all of these scientific
objectives. Table 2 summarizes the scientific investigations, measurements and examples of approaches considered for an exploration of Titan from an orbiter and lake probe and ties them to the
scientific objectives presented in Table 1.
5. Mission concept legacy
Since the Cassini spacecraft and Huygens probe arrived at Titan
in 2004, there have been many proposed mission concepts using a
variety of vehicles, including orbiters, landers, balloons and of
course, nautical probes, including floating, propulsion and submersible types, to expand both orbital and in situ exploration of
Titan inspired by the initial discoveries of the Cassini Huygens
mission. In spring of 2013, ESA put out a request for White Papers
to the scientific community to propose Science Themes for its
large (L)-scale L2 and L3 missions. The exploration of Titan with an
orbiter and a lake probe was presented as a White Paper in
response to this request, which is presented in an extended form
in this paper (see Supplementary material). A companion White
Paper was submitted, which presented the scientific case for the
exploration of Titan with an orbiter and a balloon probe (Tobie
et al., 2014, in this issue). The concept for a mission capable of
implementing the science described in previous sections is in part
based on the Titan and Enceladus Mission TandEM (Coustenis
et al., 2009a) and the Titan Explorer (Leary et al., 2007; Lockwood
et al., 2008) mission proposals for ESA and NASA respectively and
which were merged into the joint space agency Titan Saturn
System Mission Study TSSM (Coustenis et al., 2010; ESA, 2009)
for consideration for a Large (L) class or Flagship type mission. The
TSSM mission proposal included an orbiter with two types of
probe vehicles to enable in situ exploration of Titan: a balloon and
a lake lander, with NASA responsible for the orbiter and ESA
responsible for the probe vehicles. The target of the proposed lake
probe was the Kraken Mare. In addition, TSSM included a tour of
the Saturnian system with flybys at Enceladus. The proposed
mission would have had a launch date between 2023 and 2025,
however ultimately the mission was not chosen for further studies
by the involved space agencies. This initiated several proposals for
smaller type missions in scale and cost for both lake, Titan Mare
Please cite this article as: Mitri, G., et al., The exploration of Titan with an orbiter and a lake probe. Planetary and Space Science (2014),
http://dx.doi.org/10.1016/j.pss.2014.07.009i
G. Mitri et al. / Planetary and Space Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎
11
Table 2
Science Investigations, measurements and example of approaches.
Science investigations
Internal structure
Determine the internal structure
(Goal 1)
Measurements
Example of approaches
Gravity field
Obliquity, libration, occupancy of a Cassini state
S/C radio-tracking to measure static gravity field
Imaging (e.g. SAR) (S/C) with spatial resolution
30 to 50 m
S/C radio-tracking to measure static gravity field
S/C radio-tracking to measure static gravity field
Altimetry/stereo imaging/interferometric topography
(S/C) with vertical resolution of 10 m
S/C radio-tracking to measure time dependent
gravity field
S/C radio-tracking to measure static gravity field
Altimetry/stereo imaging/interferometric topography
(S/C) with vertical resolution 10 m
Gravity anomalies in the ice shell and High order gravity field components
deep interior (Goal 1)
Topography
Present tidal quality factor Q (Goal 1) Time variation of the degree-2 gravity field
Subsurface ocean
Hydrostatic compensation (Goal 1)
Gravity field
Topography
Extent of the subsurface ocean
(Goal 1)
Time variation of the degree-2 gravity field (k2)
Detection of electromagnetic induction from a
subsurface ocean
Tidal deformation (h2)
Libration and obliquity
Ice shell and geology Thickness of the ice shell (Goal 1)
and composition
Time variation of the degree-2 gravity field (k2)
Tidal deformation (h2)
Libration
Characterize the surface and
subsurface structures and the
determine their correlation (Goals
1 and 2)
Correlate the surface composition
and their distribution with surface
structures (Goal 2)
Atmosphere
Lakes/seas
Global mapping of the surface
Global topography
Profiles of subsurface structures and thickness of the
surface organic material layer
Global mapping of the surface
Global mapping of the surface composition
Characterize the atmospheric
circulation and the composition of
the atmosphere from orbit and
during probe descent (Goal 3)
Determine the dynamics and thermal state of the
atmosphere including the clouds
Determine the composition
Characterize the lake/atmosphere
interactions in situ (Goal 3)
Determine temperature, pressure, composition,
evaporation rate and physical properties that
characterize lake and atmosphere interactions
Characterize lakes/seas and their
composition/chemistry
(astrobiological potential)
(Goals 4, 1, 2, and 3)
Lake and sea composition, including low/high mass
hydrocarbons, noble gases, and carbon isotopes
Exchange processes at the sea-air interface to help
constrain the methane cycle
Presence and nature of waves and currents
Properties of sea liquids including turbidity and
dielectric constant
Sea depths to constrain basin shape and sea volume
Shoreline characteristics, including evidence for past
changes in sea level
Explorer (TiME) (Stofan et al., 2010, 2013) and balloon, Titan Aerial
Explorer (TAE) (Lunine et al., 2011) probes within NASA and ESA
respectively. To reduce scale and cost, both proposals did not
include an orbiter and both probes would use Direct-to-Earth
(DTE) communication to relay data back to Earth. TiME was a
floating lake probe that would drift across Ligeia Mare for 3–6
months investigating lake composition and processes as well as
local meteorology and atmosphere. TiME was a finalist for NASA's
Discovery mission. TAE was a proposal in response to ESA's call for
a medium (M) M3 class mission for its Cosmic Vision (2015–2025)
program and included a pressurized balloon that would explore
S/C radio-tracking to measure static gravity field
Magnetometer measurements (S/C)
Altimetry and/or interferometric topography (S/C)
with 1 m vertical resolution
Imaging (e.g. SAR) (S/C) with 30–50 m resolution
S/C radio-tracking to measure the static gravity field
S/C radio-tracking to measure time dependent
gravity field
Altimetry and/or interferometric topography (S/C)
with 1 m vertical resolution
SAR imaging (S/C)
S/C tracking to measure the static gravity field
SAR imaging (S/C)
Altimetry/stereo imaging/interferometric topography
(S/C)
Radar sounding (S/C) with 10 km penetration
depth and 30 m vertical resolution
SAR imaging (S/C) with 30 to 50 m resolution
Infrared spectral imaging (S/C) within the methane
band windows
Infrared spectra imaging of the atmosphere (S/C)
Lidar (S/C)
Infrared spectra imaging of the atmosphere (S/C)
Mass spectrometry with direct sampling from orbit
(S/C) and during decent (L/P)
Lidar (S/C)
Mass spectrometry (L/P)
Physical properties package (L/P)
Mass spectrometry/chemical analyzer (L/P)
Atmosphere physical properties package
(temperature sensor, barometer, anemometer)/
meteorological package/thermo-gravimetry (L/P)
Atmosphere physical properties package
(temperature sensor, barometer, anemometer)/
meteorological package (L/P)
Physical properties package (L/P)
Surface imaging ( 250 μrad/pixel)
Lake physical properties package (turbidity and
dielectric constant measurements) (L/P)
Sonar (L/P)
Surface Imaging ( 250 μrad/pixel)
Descent camera imaging/stereogrammetry (L/P)
the lower atmosphere of Titan's equatorial regions for 3–6 months.
While both TiME and TAE discarded the orbiter, another NASA
Discovery candidate, Journey to Enceladus and Titan (JET), jettisoned the probes and proposed examination of Enceladus' plume
and Titan's atmosphere and surface using only an orbiter with an
instrument suite consisting of two instruments and a radio science
experiment (Sotin et al., 2011). JPL prepared a study on Titan lake
probe missions for the NASA Planetary Science Decadal Survey and
investigated several potential mission architectures and nautical
probe types for the exploration of Titan's lakes and seas including
a relay orbiter with a nautical probe with floating and submersible
Please cite this article as: Mitri, G., et al., The exploration of Titan with an orbiter and a lake probe. Planetary and Space Science (2014),
http://dx.doi.org/10.1016/j.pss.2014.07.009i
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12
components, a floating probe with DTE communication capabilities and submersible and floating probes both with relay communication capabilities (JPL, 2010).
Several other possible missions for in situ examination of Titan
were proposed in the literature including an airplane (AVIATR)
probe that called for aerial reconnaissance of Titan's atmosphere
(Barnes et al., 2010) and a propelled lake probe known as Titan
Lake In-situ Sampling Propelled Explorer TALISE (Urdampilleta
et al., 2012).
6. Proposed mission concept
The proposed mission concept consists of two elements, the
spacecraft (S/C), and a lake probe (lander). The S/C will carry a
scientific payload consisting of remote sensing experiments and an
Entry, Descent and Landing (EDL) module containing a lake lander
equipped with an instrument suite capable of carrying out in situ
measurements of Titan's polar lakes. During the descent the probe
will make in situ measurements of the atmosphere. Once a
successful splashdown has been achieved, the lake lander will be
take measurements, including the sampling of both the liquid of
the sea and the low-atmosphere. A valuable augmentation may be
the capability to sample solid material on the shoreline or on the
shallow sea-bed. The possible targets of the lake probe are Ligeia
Mare (781N, 2501W) and Kraken Mare (681N, 3101W). Fig. 4 shows
the design of the lake probe and the internal view with the
location of the scientific instruments by the MasterSAT10 Team at
University La Sapienza (Italy).
The duration of the cruise from Earth to Saturn is estimated to
be on the order of 10 years or less, depending on the mass of the
spacecraft and the gravitational pull options. Following orbit
insertion at Saturn, the S/C will conduct a tour of the satellite
system, with the possibility of several flybys at Enceladus, while
flybys at Titan would be used both as a scientific opportunity and
as a way to modify the trajectory in order to finally enter orbit
around Titan. A different mission scenario proposes that the orbit
around Titan could be achieved through an aero-capture maneuver, followed by other maneuvers to refine the spacecraft's final
1500 km circular high-inclination orbit. Release and entry of
the lake probe, and relay of its data to Earth will take place either
during (1) Titan flybys phase, or (2) Titan orbital phase. The
architecture of the mission will be defined by the scientists and
the agencies in dedicated studies. We considered as a possible
nominal final orbit of the S/C (orbiter) around Titan a 1500 km
altitude circular near-polar orbit. A polar orbit optimizes the
gravity field measurements and maximizes the imaging coverage
of the surface. A 1500 km altitude orbit has negligible drag effect
on the S/C; however lower altitude orbits are also possible
increasing the orbit maintenance. After orbit insertion around
Titan, mapping of Titan with remote sensing experiments will
begin, lasting from several months to a few years. The lake probe
will relay data to the orbiter, which will serve as the communications link between the probe and Earth. Direct-to-Earth (DTE)
communication from the lake lander (feasible with Earth view
from northern polar seas starting from 2036 to 2050 s and later)
allows for augmented communication and tracking (Coustenis
et al., 2009b). Scenarios for the lake probe are (1) the lake probe
will have propulsion capabilities and mobility, (2) the lake probe
will not have propulsion capabilities, rather it will float around the
lake driven by winds or waves (although they are expected to be
rather small) and possible tides. Fig. 5 shows simulations of five
splashdowns in Ligeia Mare and subsequent drift tracks using
winds predicted by the GCM (Lorenz et al., 2012). The wind
direction varies over the course of a Titan day, and so the
trajectories have loops and curves. For predicted winds in the
Fig. 5. Simulations of five splashdowns in Ligeia Mare and subsequent drift tracks
using winds predicted by the GCM (see Lorenz et al. (2012)). The wind direction
varies over the course of a Titan day, and so the trajectories have loops and curves.
For predicted winds in the season shown (2023) the vehicle makes landfall on the
western shore of Ligeia after several weeks, except in one instance where it reaches
an island.
Fig. 4. Design of the lake probe and the internal view of the scientific instruments (Courtesy of the MasterSAT10 Team at University La Sapienza, Italy).
Please cite this article as: Mitri, G., et al., The exploration of Titan with an orbiter and a lake probe. Planetary and Space Science (2014),
http://dx.doi.org/10.1016/j.pss.2014.07.009i
G. Mitri et al. / Planetary and Space Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎
season shown (2023) the vehicle would make landfall on the
western shore of Ligeia after several weeks, except in one instance
where it reaches an island.
The power source both for the S/C and the lake probe is critical
for this mission on Titan and the Saturn system. Preliminary
analysis indicates that, to meet power requirements through solar
power, the orbiter would need large solar arrays (total surface
130–200 m2). Given the opacity of Titan's atmosphere and the low
solar elevation from the polar seas, the use of a solar powered
generator for the lake-probe is not feasible. The use of batteries for
the lake probe will only provide power to the lander on the order
of hours. The development of radioisotope space power systems
which minimize or replace 238Pu fuel within the European Space
Agency (ESA) funded programs includes the development of
241
Am fueled Radioisotope Thermoelectric Generator (RTG) (e.g.
Ambrosi et al., 2012) that should be considered as a possible
power source for a ESA-led Titan mission.
7. Conclusions
The exploration of Titan with a dedicated orbiter and in situ
lake probe will address fundamental questions involving the
origin, evolution, and history of both Titan and the broader
Saturnian system. Titan's ongoing organic chemistry has direct
applicability to the early prebiotic chemistry on Earth, allowing for
the investigation of reactions and timescales inaccessible to
terrestrial labs. Titan also represents a potentially common climate
system amongst extrasolar planets and a detailed understanding
of its nature and evolution will help to interpret data from these
distant objects. Finally, it is important to emphasize the educational potential of this mission concept. The novel idea of sending
a “boat” to an extraterrestrial sea, which is both innovative and
awe-inspiring, can be used as an inspirational tool at all levels of
education and public outreach.
Acknowledgments
The authors thank the Reviewer Jani Radebaugh for her constructive comments. Some of this work was carried out at the Jet
Propulsion Laboratory, California Institute of Technology, under
contract with NASA.
Appendix A. Supplementary material
Supplementary data associated with this article can be found in
the online version at http://dx.doi.org/10.1016/j.pss.2014.07.009.
References
Achterberg, R.K., Gierasch, P.J., Conrath, B.J., Flasar, F.M., Nixon, C.A., 2011. Temporal
variations of Titan's middle-atmospheric temperatures from 2004 to 2009
observed by Cassini/CIRS. Icarus 211, 686–698.
Aharonson, O., Hayes, A.G., Lunine, J.I., Lorenz, R.D., Allison, M.D., Elachi, C., 2009.
An asymmetric distribution of lakes on Titan as a possible consequence of
orbital forcing. Nat. Geosci. 2, 851–854.
Aharonson, O., Hayes, A.G., Lopes, R.M.C., Lucas, A., Hayne, P., Perron, T., Soderblom,
L.A., 2012. Titan's Surface Geology. In: Mueller-Wodarg, I. (Ed.), Titan: Surface,
Atmosphere, and Magnetosphere. Cambridge Planetary Science Series. Cambridge University Press, Cambridge (646 pp.).
Ambrosi, R.M., Williams, H.R., Samara-Ratna, P., Bannister, N.P., Vernon, D.,
Crawford, T., Bicknell, C., Jorden, A., Slade, R., Deacon, T., Konig, J., Jaegle, M.,
Stephenson, K., 2012. Development and testing of Americium-241 radioisotope
thermoelectric generator: concept designs and breadboard system. Nucl.
Emerg. Technol. Space 3042.
Andrews, D.J., Cowley, S.W.H., Dougherty, M.K., Provan, G., 2010. Magnetic field
oscillations near the planetary period in Saturn's equatorial magnetosphere:
variation of amplitude and phase with radial distance and local time.
J. Geophys. Res. 115, A04212.
13
Atreya, S.K., Adams, E.Y., Niemann, H.B., Demick-Montelara, J.E., Owen, T.C.,
Fulchignoni, M., Ferri, F., Wilson, E.H., 2006. Titan's methane cycle. Planet.
Space Sci. 54, 1177–1187.
Barnes, J.W., Brown, R.H., Radebaugh, J., Buratti, B.J., Sotin, C., Le Mouelic, S.,
Rodriguez, S., Turtle, E.P., Perry, J., Clark, R., Baines, K.H., Nicholson, P.D., 2006.
Cassini observations of flow-like features in western Tui Regio, Titan. Geophys.
Res. Lett. 33, L16204. http://dx.doi.org/10.1029/2006GL026843.
Barnes, J.W., Brown, R.H., Soderblom, L., Buratti, B.J., Sotin, C., Rodriguez, S.,
Le Mouèlic, S., Baines, K.H., Clark, R., Nicholson, P., 2007. Global-scale
surface spectral variations on Titan seen from Cassini/VIMS. Icarus 186,
242–258.
Barnes, J.W., Brown, R.H., Soderblom, L., Sotin, C., Le Mouèlic, S., Rodriguez, S.,
Jaumann, R., Beyer, R.A., Buratti, B.J., Pitman, K., Baines, K.H., Clark, R.,
Nicholson, P., 2008. Spectroscopy, morphometry, and photoclinometry of
Titan's dunefields from Cassini/VIMS. Icarus 195, 400–414.
Barnes, J.W., Bow, J., Schwartz, J., Brown, R.H., Soderblom, J.M., Hayes, A.G.,
Vixie, G., Le Mouélic, S., Rodriguez, S., Sotin, C., Jaumann, R., Stephan, K.,
Soderblom, L.A., Clark, R.N., Buratti, B.J., Baines, K.H., Nicholson, P.D., 2011.
Organic sedimentary deposits in Titan's dry lakebeds: probable evaporite.
Icarus 216, 136–140.
Barnes, J.W., McKay, C., Lemke, L., Beyer, R.A., Radebaugh, J., Atkinson, D., 2010.
AVIATR: Aerial Vehicle for In-Situ and Airborne Titan Reconnaissance. In:
Proceedings of the Lunar and Planetary Science Conference vol. 41, p. 2551.
Bird, M.K., Allison, M., Asmar, S.W., Atkinson, D.H., Avruch, I.M., Dutta-Roy, R.,
Dzierma, Y., Edenhofer, P., Folkner, W.M., Gurvits, L.I., Johnston, D.V., Plettemeier, D., Pogrebenko, S.V., Preston, R.A., Tyler, G.L., 2005. The vertical profile of
winds on Titan. Nature 438, 800–802.
Bills, B.G., Nimmo, F., 2008. Forced obliquity and moments of inertia of Titan. Icarus
196, 293–296.
Brown, M.E., Bouchez, A.H., Griffith, C.A., 2002. Direct detection of variable tropospheric clouds near Titan's south pole. Nature 420, 795–797.
Brown, R.H., Soderblom, L.A., Soderblom, J.M., Clark, R.N., Jaumann, R., Barnes, J.W.,
Sotin, C., Buratti, B., Baines, K.H., Nicholson, P.D., 2008. The identification of
liquid ethane in Titan's Ontario Lacus. Nature 454 (7204), 607–610.
Burr, D.M., Perron, J.T., Lamb, M.P., Irwin III, R.P., Collins, G.C., Howard, A.D.,
Sklar, L.S., Moore, J.M., Á dámkovics, M., Baker, V.R., Drummond, S.A.,
Black, B.A., 2013. Fluvial features on Titan: insights from morphology and
modeling. Geol. Soc. Am. Bull. 125, 299–321.
Castillo-Rogez, J., Lunine, J.I., 2010. Evolution of Titan's rocky core constrained by
Cassini observations. Geophys. Res. Lett. 37, L20205.
Cicalò, S., Milani, A., 2012. Determination of the rotation of Mercury from satellite
gravimetry. Mon. Not. R. Astron. Soc. 427 (1), 468–482.
Coustenis, A., Hirtzig, M., 2009. Cassini–Huygens results on Titan's surface. Res.
Astron. Astrophys. 9, 249–268.
Coustenis, A., Atreya, S.K., Balint, T., Brown, R.H., Dougherty, M.K., Ferri, F.,
Fulchignoni, M., Gautier, D., Gowen, R.A., Griffith, C.A., Gurvits, L.I.,
Jaumann, R., Langevin, Y., Leese, M.R., Lunine, J.I., McKay, C.P., Moussas, X.,
Mueller-Wodarg, I., Neubauer, F., Owen, T.C., Raulin, F., Sittler, E.C., Sohl, F.,
Sotin, C., Tobie, G., Tokano, T., Turtle, E.P., Wahlund, J., Waite, J.H., Baines, K.H.,
Blamont, J., Coates, A.J., Dandouras, I., Krimigis, T., Lellouch, E., Lorenz, R.D.,
Morse, A., Porco, C.C., Hirtzig, M., Saur, J., Spilker, T., Zarnecki, J.C., Choi, E.,
Achilleos, N., Amils, R., Annan, P., Atkinson, D.H., Bénilan, Y., Bertucci, C., Bézard,
B., Bjoraker, G.L., Blanc, M., Boireau, L., Bouman, J., Cabane, M., Capria, M.T.,
Chassefière, E., Coll, P., Combes, M., Cooper, J.F., Coradini, A., Crary, F., Cravens,
T., Daglis, I.A., de Angelis, E., de Bergh, C., de Pater, I., Dunford, C., Durry, G.,
Dutuit, O., Fairbrother, D., Flasar, F.M., Fortes, A.D., Frampton, R., Fujimoto, M.,
Galand, M., Grasset, O., Grott, M., Haltigin, T., Herique, A., Hersant, F.,
Hussmann, H., Ip, W., Johnson, R., Kallio, E., Kempf, S., Knapmeyer, M.,
Kofman, W., Koop, R., Kostiuk, T., Krupp, N., Küppers, M., Lammer, H.,
Lara, L.M., Lavvas, P., Le Mouélic, S., Lebonnois, S., Ledvina, S., Li, J., Livengood,
T.A., Lopes, R.M., Lopez-Moreno, J.J., Luz, D., Mahaffy, P.R., Mall, U., MartinezFrias, J., Marty, B., McCord, T., Menor Salvan, C., Milillo, A., Mitchell, D.G.,
Modolo, R., Mousis, O., Nakamura, M., Neish, C.D., Nixon, C.A., Nna Mvondo, D.,
Orton, G., Paetzold, M., Pitman, J., Pogrebenko, S., Pollard, W., Prieto-Ballesteros,
O., Rannou, P., Reh, K., Richter, L., Robb, F.T., Rodrigo, R., Rodriguez, R.,
Romani, P., Ruiz Bermejo, M., Sarris, E.T., Schenk, P., Schmitt, B., Schmitz, N.,
Schulze-Makuch, D., Schwingenschuh, K., Selig, A., Sicardy, B., Soderblom, L.,
Spilker, L.J., D. Stam, D., Steele, A., Stephan, K., Strobel, D.F., Szego, K., Szopa, C.,
Thissen, R., Tomasko, MG., Toublanc, D., Vali, H., Vardavas, I., Vuitton, V.,
West, R.A., Yelle, R., Young, E.F., 2009a. TandEM: Titan and Enceladus mission.
Exp. Astron. 23, 893–946.
Coustenis, A., Lunine, J., Lebreton, J.-P., Matson, D., Erd, C.h., Reh, K., Beauchamp, P.,
Lorenz, R., Waite, H., Sotin, Ch., Gurvits, L., Hirtizg, M., 2009b. Earth-based
support for the Titan–Saturn Mission. Earth Moon Planets 105, 135–142.
Coustenis, A., Lunine, J., Matson, D., Hansen, C., Reh, K., Beauchamp, P., Lebreton, J.-P.,
Erd, C., 2010. The joint NASA–ESA Titan–Saturn System Mission (TSSM) study.
In: Proceedings of the 40th Lunar and Planetary Science Conference, pp. 1060.
Elachi, C., Allison, M.D., Borgarelli, L., Encrenaz, P., Im, E., Janssen, M.A., Johnson, W.
T.K., Kirk, R.L., Lorenz, R.D., Lunine, J.I., Muhleman, D.O., Ostro, S.J., Picardi, G.,
Posa, F., Rapley, C.G., Roth, L.E., Seu, R., Soderblom, L.A., Vetrella, S., Wall, S.D.,
Wood, C.A., Zebker, H.A., 2004. Radar: the Cassini Titan radar mapper. Space
Sci. Rev. 115 (1), 71–110.
ESA, 2009. TSSM in-situ elements. Assessment Study Report, ESA-SRE(2008)4,
European Space Agency, 12 February 2009.
Fortes, A.D., Grindrod, P.M., Trickett, S.K., Vočadlo, L., 2007. Ammonium sulfate on
Titan: possible origin and role in cryovolcanism. Icarus 188, 139–153.
Please cite this article as: Mitri, G., et al., The exploration of Titan with an orbiter and a lake probe. Planetary and Space Science (2014),
http://dx.doi.org/10.1016/j.pss.2014.07.009i
14
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Fulchignoni, M., Ferri, F., Angrilli, F., Ball, A.J., Bar-Nun, A., Barucci, M.A., Bettanini, C.,
Bianchini, G., Borucki, W., Colombatti, G., Coradini, M., Coustenis, A., Debei, S.,
Falkner, P., Fanti, G., Flamini, E., Gaborit, V., Grard, R., Hamelin, M., Harri, A.M.,
Hathi, B., Jernej, I., Leese, M.R., Lehto, A., Lion Stoppato, P.F., López-Moreno, J.J.,
Mäkinen, T., McDonnell, J.A.M., McKay, C.P., Molina-Cuberos, G., Neubauer, F.M.,
Pirronello, V., Rodrigo, R., Saggin, B., Schwingenschuh, K., Seiff, A., Simões, F.,
Svedhem, H., Tokano, T., Towner, M.C., Trautner, R., Withers, P., Zarnecki, J.C.,
2005. In situ measurements of the physical characteristics of Titan's environment. Nature 438, 785–791.
Grasset, O., Dougherty, M.K., Coustenis, A., Bunce, E.J., Erd, C., Titov, D., Blanc, M.,
Coates, A., Drossart, P., Fletcher, L.N., Hussmann, H., Jaumann, R., Krupp, N.,
Lebreton, J.-P., Prieto-Ballesteros, O., Tortora, P., Tosi, F., Van Hoolst, T., 2012.
JUpiter ICy moons Explorer (JUICE): an ESA mission to orbit Ganymede and to
characterise the Jupiter system. Planet. Space Sci. 78, 1–21. http://dx.doi.org/
10.1016/j.pss.2012.12.002.
Hayes, A., Aharonson, O., Callahan, P., Elachi, C., Gim, Y., Kirk, R., Lewis, K., Lopes, R.,
Lorenz, R., Lunine, J., Mitchell, K., Mitri, G., Stofan, E., Wall, S., 2008. Hydrocarbon lakes on Titan: distribution and interaction with a porous regolith.
Geopyhs. Res. Lett. 35, L09204.
Hayes, A.G., Wolf, A.S., Aharonson, O., Zebker, H., Lorenz, R., Kirk, R.L.,
Paillou, P., Lunine, J., Wye, L., Callahan, P., Wall, S., Elachi, C., 2010. Bathymetry
and absorptivity of Titan's Ontario Lacus. J. Geophys. Res.: Planets 115,
9009.
Hayes, A.G., Aharonson, O., Lunine, J.I., Kirk, R.L., Zebker, H.A., Wye, L.C., Lorenz, R.D.,
Turtle, E.P., Paillou, P., Mitri, G., Wall, S.D., Stofan, E.R., Mitchell, K.L., Elachi, C.,
Cassini Radar Team, 2011. Transient surface liquid in Titan's polar regions from
Cassini. Icarus 211, 655–671.
Hayes, A.G., Lorenz, R.D., Donelan, M.A., Manga, M., Lunine, J.I., Schneider, T., Lamb,
M.P., Mitchell, J.M., Fischer, W.W., Graves, S.D., Tolman, H.L., Aharonson, O.,
Encrenaz, P.J., Ventura, B., Casarano, D., Notarnicola, C., 2013. Wind driven
capillary-gravity waves on Titan's lakes: hard to detect or non-existent? Icarus
225, 403–412.
Hemingway, D., Nimmo, F., Zebker, H., Iess., L., 2013. A rigid and weathered ice shell
on Titan. Nature 500, 550–552.
Iess, L., Jacobson, R.A., Ducci, M., Stevenson, D.J., Lunine, J.I., Armstrong, J.W., Asmar,
S.W., Racioppa, P., Rappaport, N.J., Tortora, P., 2012. The Tides of Titan. Science
337, 457.
Iess, L., Rappaport, N.J., Jacobson, R.A., Racioppa, P., Stevenson, D.J., Tortora, P.,
Armstrong, J.W., Asmar, S.W., 2010. Gravity field, shape, and moment of inertia
of Titan. Science 327, 1367.
Jaumann, R., Brown, R.H., Stephan, K., Barnes, J.W., Soderblom, L.A., Sotin, C.,
Le Mouélic, S., Clark, R.N., Soderblom, J., Buratti, B.J., Wagner, R., McCord, T.B.,
Rodriguez, S., Baines, K.H., Cruikshank, D.P., Nicholson, P.D., Griffith, C.A.,
Langhans, M., Lorenz, R.D., 2008. Fluvial erosion and post-erosional processes
on Titan. Icarus 197, 526–538.
JPL, 2010. Planetary Science Decadal Survey JPL Team X Titan Lake Probe Study,
Final report, Jet Propulsion Laboratory, April.
Khurana, K.K., Kivelson, M.G., Stevenson, D.J., Schubert, G., Russell, C.T., Walker, R.J.,
Polanskey, C., 1998. Induced magnetic fields as evidence for subsurface oceans
in Europa and Callisto. Nature 395, 749.
Khurana, Krishan K., Jia, X., Kivelson, M.G., Nimmo, F., Schubert, G., Russell, C.T.,
2011. Evidence of a global magma ocean in Io's interior. Science 332,
1186–1189.
Kivelson, M.G., Khurana, K.K., Volwerk, M., 2002. The permanent and inductive
magnetic moments of Ganymede. Icarus 157 (2), 507.
Leary, J., Jones, C., Lorenz, R., Strain, R.D., Waite, J.H., 2007. Titan Explorer NASA
Flagship Mission Study. JHU Applied Physics Laboratory, August.
Lebonnois, S., Burgalat, J., Rannou, P., Charnay, B., 2012. Titan global climate model:
a new 3-dimensional version of the IPSL Titan GCM. Icarus 218, 707–722.
Lebreton, J.-P., Coustenis, A., Lunine, J., Raulin, F., Owen, T., Strobel, D., 2009. Results
from the Huygens probe on Titan. Astron. Astrophys. Rev. 17, 149–179.
Lockwood, M.K., Leary, J.C., Lorenz, R., Waite, H., Reh, K., Prince, J., Powell, R., 2008.
Titan explorer. In: Proceedings of the AIAA-2008-7071, AIAA/AAS Astrodynamics Specialist Conference. Honolulu, Hawaii, August.
Lopes, R.M.C., Mitchell, K.L., Stofan, E.R., Lunine, J.I., Lorenz, R., Paganelli, F.,
Kirk, R.L., Wood, C.A., Wall, S.D., Robshaw, L.E., Fortes, A.D., Neish, C.D.,
Radebaugh, J., Reffet, E., Ostro, S.J., Elachi, C., Allison, M.D., Anderson, Y.,
Boehmer, R., Boubin, G., Callahan, P., Encrenaz, P., Flamini, E., Francescetti, G.,
Gim, Y., Hamilton, G., Hensley, S., Janssen, M.A., Johnson, W.T.K., Kelleher, K.,
Muhleman, D.O., Ori, G., Orosei, R., Picardi, G., Posa, F., Roth, L.E., Seu, R.,
Shaffer, S., Soderblom, L.A., Stiles, B., Vetrella, S., West, R.D., Wye, L., Zebker, H.A.,
2007. Cryovolcanic features on Titan's surface as revealed by the Cassini Titan
radar mapper. Icarus 186, 395–412.
Lopes, R.M.C., Stofan, E.R., Peckyno, R., Radebaugh, J., Mitchell, K.L., Mitri, G.,
Wood, C.A., Kirk, R.L., Wall, S.D., Lunine, J.I., Hayes, A., Lorenz, R., Farr, T.,
Wye, L., Craig, J., Ollerenshaw, R.J., Janssen, M., Legall, A., Paganelli, F., West, R.,
Stiles, B., Callahan, P., Anderson, Y., Valora, P., Soderblom, L., The Cassini Radar
Team, 2010. Distribution and interplay of geologic processes on Titan from
Cassini RADAR data. Icarus 205, 540–588.
Lopes, R.M.C., Kirk, R.L., Mitchell, K.L., Legall, A., Barnes, J.W., Hayes, A., Kargel, J.,
Wye, L., Radebaugh, J., Stofan, E.R., Janssen, M.A., Neish, C.D., Wall, S.D., Wood,
C.A., Lunine, J.I., Malaska, M.J., 2013. Cryovolcanism on Titan: new results from
Cassini RADAR and VIMS. J. Geophys. Res.: Planets 118, 1–20.
Lorenz, R.D., Wall, S., Radebaugh, J., Boubin, G., Reffet, E., Janssen, M., Stofan, E.,
Lopes, R., Kirk, R., Elachi, C., Lunine, J., Mitchell, K., Paganelli, F., Soderblom, L.,
Wood, C., Wye, L., Zebker, H., Anderson, Y., Ostro, S., Allison, M., Boehmer, R.,
Callahan, S., Encrenaz, P., Ori, G.G., Francescetti, G., Gim, Y., Hamilton, G.,
Hensley, S., Johnson, W., Kelleher, K., Muhleman, D., Picardi, G., Posa, F., Roth, L.,
Seu, R., Shaffer, S., Stiles, B., Vetrella, S., Flamini, E., West, R., 2006. The sand seas
of Titan: Cassini RADAR observations of equatorial fields of longitudinal dunes.
Science 312, 724–727.
Lorenz, R.D., Wood, C.A., Lunine, J.I., Wall, S.D., Lopes, R.M., Mitchell, K.L., Paganelli,
F., Anderson, Y.Z., Wye, L., Tsai, C., Zebker, H., Stofan, E.R., 2007. Titan's young
surface: initial impact crater survey by Cassini RADAR and model comparison.
Geophys. Res. Lett. 34, L07204.
Lorenz, R.D., Radebaugh, J., 2009. Global pattern of Titan's dunes: Radar survey
from the Cassini prime mission. Geophys. Res. Lett. 36, L03202.
Lorenz, R.D., 2009. Titan mission studies – a historical review. J. Br. Interplanet. Soc.
62, 162–174.
Lorenz, R.D., Newman, C.E., Tokano, T., Mitchell, J.L., Charnay, B., Lebonnois, S.,
Achterberg, R.K., 2012. Formulation of a wind specification for Titan late polar
summer exploration. Planet. Space Sci. 70, 73–83.
Lunine, J.I., Stevenson, D.J., Yung, Y.L., 1983. Ethane ocean on Titan. Science 222,
1229.
Lunine, J., Atreya, S., 2008. The methane cycle on Titan. Nat. Geosci. 1, 335.
Lunine, J.I., Reh, K., Sotin, C., Couzin, P., Vargas, A., 2011. Titan aerial explorer: a
mission to circumnavigate Titan. In: Proceedings of the Lunar and Planetary
Science Conference vol. 42, 1230.
Malaska, M., Radebaugh, J., Lorenz, R., Mitchell, K., Farr, T., Stofan, E., 2010.
Identification of Karst-like terrain on Titan from valley analysis. In: Lunar and
Planetary Institute Science Conference Abstracts, vol. 41, pp. 1544.
Mandt, K.E., Waite, J.H., Lewis, W., Magee, B., Bell, J., Lunine, J., Mousis, O., Cordier, D.,
2009. Isotopic evolution of the major constituents of Titan's atmosphere based on
Cassini data. Planet. Space Sci. 57, 1917–1930.
McCord, T.B., Hayne, P., Combe, J.-P., Hansen, G.B., Barnes, J.W., Rodriguez, S.,
Le Mouélic, S., Baines, E.K.H., Buratti, B.J., Sotin, C., Nicholson, P., Jaumann, R.,
Nelson, R., the Cassini VIMS Team, 2008. Titan's surface: search for spectral
diversity and composition using the Cassini VIMS investigation. Icarus 194,
212–242.
Meriggiola, R., Iess, L., Stiles, B. W. 2011. The rotation of Titan by latest Cassini data.
AGU Fall Meeting Abstracts.
Mitchell, J.L., 2008. The drying of Titan's dunes: Titan's methane hydrology and its
impact on atmospheric circulation. J. Geophys. Res.: Planets 113, 8015.
Mitchell, J.L., Ádámkovics, M., Caballero, R., Turtle, E.P., 2011. Locally enhanced
precipitation organized by planetary-scale waves on Titan. Nat. Geosci. 4,
589–592.
Mitri, G., Meriggiola, R., Hayes, A., Lefevre, A., Tobie, G., Genova, A., Lunine, J.I.,
Zebker, H., 2014. Shape, topography, gravity anomalies and tidal deformation of
Titan. Icarus 236, 169–177. http://dx.doi.org/10.1016/j.icarus.2014.03.018.
Mitri, G., Bland, M.T., Showman, A.P., Radebaugh, J., Stiles, B., Lopes, R.M.C.,
Lunine, J.I., Pappalardo, R.T., 2010. Mountains on Titan: modeling and
observations. J. Geophys. Res. 115, 10002.
Mitri, G., Showman, A.P., Lunine, J.I., Lopes, R.M.C., 2008. Resurfacing of Titan by
ammonia–water cryomagma. Icarus 196, 216–224.
Mitri, G., Showman, A.P., 2008. Thermal convection in ice-I shells of Titan and
Enceladus. Icarus 193, 387–396.
Mitri, G., Showman, A.P., Lunine, J.I., Lorenz, R.D., 2007. Hydrocarbon lakes on Titan.
Icarus 186, 385–394.
Mitri, G., Coustenis, A., Fanchini, G., Hayes, A.G., Khurana, K., Lebreton, J.-P., Lopes, R.
M., Lorenz, R.D., Iess, L., Meriggiola, R., Moriconi, M. L., Orosei, R., Sotin, C.,
Stofan, E., Tobie, G., Tokano, T., Tosi, F., 2013. The Exploration of Titan with an
Orbiter and a Lake-Probe. White Paper written in response to the 2013
European Space Agency (ESA)’s Call for Science Themes for the Large-class
(L) L2 and L3 missions.
Moore, J.M., Pappalardo, R.T., 2011. Titan: an exogenic world? Icarus 212, 790–806.
Mousis, O., Lunine, J.I., Pasek, M., Cordier, D., Hunter Waite, J., Mandt, K.E.,
Lewis, W.S., Nguyen, M.J., 2009. A primordial origin for the atmospheric
methane of Saturn's moon Titan. Icarus 204, 749–751.
Mousis, O., Lunine, J.I., Picaud, S., Cordier, D., Waite Jr., J.H., Mandt, K.E., 2011.
Removal of Titan's atmospheric noble gases by their sequestration in surface
clathrates. Astrophys. J. 740, L9.
Neish, C.D., Kirk, R.L., Lorenz, R.D., Bray, V.J., Schenk, P., Stiles, B.W., Turtle, E.,
Mitchell, K., Hayes, A., Cassini Radar Team, 2013. Crater topography on Titan:
implications for landscape evolution. Icarus 223, 82–90.
Nelson, R.M., Kamp, L.W., Matson, D.L., Irwin, P.G.J., Baines, K.H., Boryta, M.D.,
Leader, F.E., Jaumann, R., Smythe, W.D., Sotin, C., Clark, R.N., Cruikshank, D.P.,
Drossart, P., Pearl, J.C., Hapke, B.W., Lunine, J., Combes, M., Bellucci, G., Bibring,
J.-P., Capaccioni, F., Cerroni, P., Coradini, A., Formisano, V., Filacchione, G.,
Langevin, R.Y., McCord, T.B., Mennella, V., Nicholson, P.D., Sicardy, B., 2009a.
Saturn's Titan: surface change, ammonia, and implications for atmospheric and
tectonic activity. Icarus 199, 429–441.
Nelson, R.M., Kamp, L.W., Lopes, R.M.C., Matson, D.L., Kirk, R.L., Hapke, B.W.,
Wall, S.D., Boryta, M.D., Leader, F.E., Smythe, W.D., Mitchell, K.L., Baines, K.H.,
Jaumann, R., Sotin, C., Clark, R.N., Cruikshank, D.P., Drossart, P., Lunine, J.I.,
Combes, M., Bellucci, G., Bibring, J.-P., Capaccioni, F., Cerroni, P., Coradini, A.,
Formisano, V., Filacchione, G., Langevin, Y., McCord, T.B., Mennella, V., Nicholson, P.D., Sicardy, B., Irwin, P.G.J., Pearl, J.C., 2009b. Photometric changes on
Saturn's Moon Titan: evidence for cryovolcanism. Geophys. Res. Lett. 36,
L04202.
Niemann, H.B., Atreya, S.K., Bauer, S.J., Carignan, G.R., Demick, J.E., Frost, R.L.,
Gautier, D., Haberman, J.A., Harpold, D.N., Hunten, D.M., Israel, G., Lunine, J.I.,
Kasprzak, W.T., Owen, T.C., Paulkovich, M., Raulin, F., Raaen, E., Way, S.H., 2005.
Please cite this article as: Mitri, G., et al., The exploration of Titan with an orbiter and a lake probe. Planetary and Space Science (2014),
http://dx.doi.org/10.1016/j.pss.2014.07.009i
G. Mitri et al. / Planetary and Space Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎
The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe. Nature 438, 779–784.
Nimmo, F., Bills, B.G., 2010. Shell thickness variations and the long wavelength
topography of Titan. Icarus 208, 896–904.
Owen, T., Niemann, H.B., 2009. The origin of Titan's atmosphere: some recent
advances. R. Soc. Lond. Philos. Trans. Ser. A 367, 607–615.
Paillou, P., Lunine, J., Ruffié, G., Encrenaz, P., Wall, S., Lorenz, R., Janssen, M., 2008.
Microwave dielectric constant of Titan-relevant materials. Geophys. Res. Lett.
35, 18202.
Porco, C.C., Baker, E., Barbara, J., Beurle, K., Brahic, A., Burns, J.A., Charnoz, S., Cooper, N.,
Dawson, D.D., Del Genio, A.D., Denk, T., Dones, L., Dyudina, U., Evans, M.W.,
Fussner, S., Giese, B., Grazier, K., Helfenstein, P., Ingersoll, A.P., Jacobson, R.A.,
Johnson, T.V., McEwen, A., Murray, C.D., Neukum, G., Owen, W.M., Perry, J.,
Roatsch, T., Spitale, J., Squyres, S., Thomas, P., Tiscareno, M., Turtle, E.P., Vasavada,
A.R., Veverka, J., Wagner, R., West, R., 2005. Imaging of Titan from the Cassini
spacecraft. Nature, 434; , pp. 159–168.
Radebaugh, J., Lorenz, R.D., Kirk, R.L., Lunine, J.I., Stofan, E.R., Lopes, R.M.C.,
Wall, S.D., the Cassini Radar Team, 2007. Mountains on Titan observed by
Cassini RADAR. Icarus 192, 77–91.
Radebaugh, J., Lorenz, R.D., Lunine, J.I., Wall, S.D., Boubin, G., Reffet, E., Kirk, R.L.,
Lopes, R.M., Stofan, E.R., Soderblom, L., Allison, M., Janssen, M., Paillou, P.,
Callahan, P., Spencer, C., the Cassini Radar Team, 2008. Dunes on Titan observed
by Cassini RADAR. Icarus 194, 690–703.
Raulin, F., 2008. Planetary science: organic lakes on Titan. Nature 454, 587–589.
Rodriguez, S., Le Mouélic, S., Rannou, P., Tobie, G., Baines, K.H., Barnes, J.W., Griffith,
C.A., Hirtzig, M., Pitman, K.M., Sotin, C., Brown, R.H., Buratti, B.J., Clark, R.N.,
Nicholson, P.D., 2009. Global circulation as the main source of cloud activity on
Titan. Nature 459, 678–682.
Roe, H.G., 2012. Titan's methane weather. Annu. Rev. Earth Planet. Sci. 40, 355–382.
Rubin, D.M., Hesp, P.A., 2009. Multiple origins of linear dunes on Earth and Titan.
Nat. Geosci. 2, 653–658.
Schaller, E.L., Roe, H.G., Schneider, T., Brown, M.E., 2009. Storms in the tropics of
Titan. Nature 460, 873–875.
Schinder, P.J., Flasar, F.M., Marouf, E.A., French, R.G., McGhee, C.A., Kliore, A.J.,
Rappaport, N.J., Barbinis, E., Fleischman, D., Anabtawi, A., 2012. The structure of
Titan's atmosphere from Cassini radio occultations: occultations from the
Prime and Equinox missions. Icarus 221, 1020–1031.
Schneider, T., Graves, S.D.B., Schaller, E.L., Brown, M.E., 2012. Polar methane
accumulation and rainstorms on Titan from simulations of the methane cycle.
Nature 481, 58–61.
Seidelmann, P.K., Archinal, B.A., A’Hearn, M.F., Conrad, A., Consolmagno, G.J.,
Hestroffer, D., Hilton, J.L., Krasinsky, G.A., Neumann, G., Oberst, J., Stooke, P.,
Tedesco, E.F., Tholen, D.J., Thomas, P.C., Williams, I.P., 2006. Report of the IAU/
IAG Working Group on cartographic coordinates and rotational elements: 2006.
Celest. Mech. Dyn. Astron. 98, 155–180.
Soderblom, L.A., Kirk, R.L., Lunine, J.I., Anderson, J.A., Baines, K.H., Barnes, J.W.,
Barrett, J.M., Brown, R.H., Buratti, B.J., Clark, R.N., Cruikshank, D.P., Elachi, C.,
Janssen, M.A., Jaumann, R., Karkoschka, E., Mouélic, S., Le Lopes, R.M.,
Lorenz, R.D., McCord, T.B., Nicholson, P.D., Radebaugh, J., Rizk, B., Sotin, C.,
Stofan, E.R., Sucharski, T.L., Tomasko, M.G., Wall, S.D., 2007. Correlations
between Cassini VIMS spectra and RADAR SAR images: implications for Titan's
surface composition and the character of the Huygens probe landing site.
Planet. Space Sci. 55, 2025–2036.
Soderblom, L.A., Brown, R.H., Soderblom, J.M., Barnes, J.W., Kirk, R.L., Sotin, C.,
Jaumann, R., Mackinnon, D.J., Mackowski, D.W., Baines, K.H., Buratti, B.J., Clark,
R.N., Nicholson, P.D., 2009. The geology of Hotei Regio, Titan: correlation of
Cassini VIMS and RADAR. Icarus 204, 610–618.
Sotin, C., Jaumann, R., Buratti, B.J., Brown, R.H., Clark, R.N., Soderblom, L.A., Baines, K.H.,
Bellucci, G., Bibring, J.-P., Capaccioni, F., Cerroni, P., Combes, M., Coradini, A.,
Cruikshank, D.P., Drossart, P., Formisano, V., Langevin, Y., Matson, D.L., McCord, T.B.,
Nelson, R.M., Nicholson, P.D., Sicardy, B., Le Mouelic, S., Rodriguez, S., Stephan, K.,
Scholz, C.K., 2005. Release of volatiles from a possible cryovolcano from nearinfrared imaging of Titan. Nature 435, 786–789.
Sotin, C., Lawrence, K.J., Reinhardt, B., Barnes, J.W., Brown, R.H., Hayes, A.G.,
Le Mouélic, S., Rodriguez, S., Soderblom, J.M., Soderblom, L.A., Baines, K.H.,
Buratti, B.J., Clark, R.N., Jaumann, R., Nicholson, P.D., Stephan, K., 2012.
Observations of Titan's northern lakes at 5 μm: implications for the organic
cycle and geology. Icarus 221, 768–786.
Sotin, C., Altwegg, K., Brown, R.H., Hand, K., Lunine, J.I., Soderblom, J., Spencer, J.,
Tortora, P., JET, Team, 2011. JET: Journey to Enceladus and Titan. In: Proceedings
of the Lunar and Planetary Science Conference vol. 42, p. 1326.
Stiles, B.W., Kirk, R.L., Lorenz, R.D., Hensley, S., Lee, E., Ostro, S.J., Allison, M.D.,
Callahan, P.S., Gim, Y., Iess, L., Perci del Marmo, P., Hamilton, G., Johnson, W.T.K.,
West, R.D., Cassini RADAR Team, 2008. Determining Titan's spin state from
Cassini RADAR images. Astron. J. 135, 1669–1680.
Stiles, Bryan W., Hensley, Scott, Gim, Yonggyu, Bates, David M., Kirk, Randolph L.,
Hayes, Alex, Radebaugh, Jani, Lorenz, Ralph D., Mitchell, Karl L., Callahan, Philip
S., Zebker, Howard, Johnson, William T.K., Wall, Stephen D., Lunine, Jonathan I.,
Wood, Charles A., Janssen, Michael, Pelletier, Frederic, West, Richard D.,
Veeramacheneni, Chandini, The Cassini Radar Team, 2009. Determining Titan
surface topography from Cassini SAR data. Icarus 202, 584–598.
Stofan, E.R., Elachi, C., Lunine, J.I., Lorenz, R.D., Stiles, B., Mitchell, K.L., Ostro, S.,
Soderblom, L., Wood, C., Zebker, H., Wall, S., Janssen, M., Kirk, R., Lopes, R.,
Paganelli, F., Radebaugh, J., Wye, L., Anderson, Y., Allison, M., Boehmer, R.,
Callahan, P., Encrenaz, P., Flamini, E., Francescetti, G., Gim, Y., Hamilton, G.,
Hensley, S., Johnson, W.T.K., Kelleher, K., Muhleman, D., Paillou, P., Picardi, G.,
15
Posa, R., Roth, L., Seu, R., Shaffer, S., Vetrella, S., West, R., 2007. Nature 445,
61–64.
Stofan, E.R., Lorenz, R.D., Lunine, J.I., Aharonson, O., Bierhaus, E., Clark, B., Griffith, C.,
Harri, A.M., Karkoschka, E., Kirk, R., Kantsiper, B., Mahaffy, P., Newman, C.,
Ravine, M., Trainer, M., Waite, H., Zarnecki, J., 2010. Titan Mare Explorer (TiME):
first in situ exploration of an extraterrestrial sea. LPI Contrib. 1538, 5270.
Stofan, E., Lorenz, R., Lunine, J., Bierhaus, E., Clark, B., Mahaffy, P., Ravine, M., 2013.
TiME – The Titan Mare Explorer. In: Proceedings of the IEEE Aerospace
Conference. Big Sky, MT, March, paper #2434.
Tobie, G., Teanby, N.A., Coustenis, A., Jaumann, R., Raulin, F., Schmidt, J., Carrasco, N.,
Coates, A.J., Cordieri, D., De Kok, R., Geppert, W.D., Lebreton, J.-P., Lefevre, A.,
Livengood, T.A., Mandt, K.E., Mitri, G., Nimmo, F., Nixon, C.A., Norman, L.,
Pappalardo, R.T., Postberg, F., Rodriguez, S., Schulze-Makuch, D., Soderblom, J.
M., Solomonidou, A., Stephan, K., Stofan, E.R., Turtle, E.P., Wagner, R.J., West, R.
A., Westlake, J.H., 2014. The science goals and mission concept for a future
exploration of Titan and Enceladus. Space Planet. Sci. (submitted for
publication).
Tobie, G., Lunine, J.I., Sotin, C., 2006. Episodic outgassing as the origin of atmospheric methane on Titan. Nature 440, 61–64.
Tobie, G., Grasset, O., Lunine, J.I., Mocquet, A., Sotin, C., 2005. Titan's internal
structure inferred from a coupled thermal-orbital model. Icarus 175, 496–502.
Tobie, G., Teanby, N., Coustenis, A., Jaumann, R., Raulin, F., Schmidt, J., Carrasco, N.,
Coates, A.J., Cordier, D., De Kok, R., Geppert, W.D., Lebreton, J.-P., Lefevre, A.,
Livengood, T.A., Mandt, K.E., Mitri, G., Nimmo, F., Nixon, C.A., Norman, L.,
Pappalardo, R.T., Postberg, F., Rodriguez, S., Schulze-Makuch, D., Soderblom, J.
M., Solomonidou, A., Stephan, K., Stopan, E.R., Turtle, E.P., Wagner, R.J., West, R.
A., Westlake, J.H., 2014. The science goals and mission concept for a future
exploration of Titan and Enceladus. Planet. Space Sci., in this issue.
Tokano, T., 2009. Impact of seas/lakes on polar meteorology of Titan: simulation by
a coupled GCM-sea model. Icarus 204, 619–636.
Tokano, T., 2010. Relevance of fast westerlies at equinox for the eastward elongation
of Titan's dunes. Aeolian Res. 2, 113–127.
Tokano, T., 2013. Are tropical cyclones possible over Titan's polar seas? Icarus 223,
766–774.
Tokano, T., Neubauer, F.M., 2002. Tidal wind on Titan caused by Saturn. Icarus 158,
499–515.
Tokano, T., Neubauer, F.M., 2005. Wind-induced seasonal angular momentum
exchange at Titan's surface and its influence on Titan's length-of-day. Geophys.
Res. Lett. 32, L24203.
Tokano, T., McKay, C.P., Neubauer, F.M., Atreya, S.K., Ferri, F., Fulchignoni, M.,
Niemann, H.B., 2006. Methane drizzle on Titan. Nature 442, 432–435.
Tosi, F., Orosei, R., Seu, R., Coradini, A., Lunine, J.I., Filacchione, G., Gavrishin, A.I.,
Capaccioni, F., Cerroni, P., Adriani, A., Moriconi, M.L., Negrão, A., Flamini, E.,
Brown, R.H., Wye, L.C., Janssen, M., West, R.D., Barnes, J.W., Wall, S.D.,
Clark, R.N., Cruikshank, D.P., McCord, T.B., Nicholson, P.D., Soderblom, J.M.,
Cassini VIMS and RADAR Teams, 2010. Correlations between VIMS and RADAR
data over the surface of Titan: implications for Titan's surface properties. Icarus
208, 366–384.
Turtle, E.P., Perry, J.E., McEwen, A.S., Del Genio, A.D., Barbara, J., West, R.A., Dawson,
D.D., Porco, C.C., 2009. Cassini imaging of Titan's high-latitude lakes, clouds,
and south-polar surface changes. Geophys. Res. Lett. 36, 2204.
Turtle, E.P., Del Genio, A.D., Barbara, J.M., Perry, J.E., Schaller, E.L., McEwen, A.S.,
West, R.A., Ray, T.L., 2011a. Seasonal changes in Titan's meteorology. Geophys.
Res. Lett. 38, L03203.
Turtle, E.P., Perry, J.E., Hayes, A.G., Lorenz, R.D., Barnes, J.W., McEwen, A.S.,
West, R.A., Del Genio, A.D., Barbara, J.M., Lunine, J.I., Schaller, E.L., Ray, T.L.,
Lopes, R.M.C., Stofan, E.R., 2011b. Rapid and extensive surface changes near
Titan's equator: evidence of April showers. Science 331, 1414–1417.
Urdampilleta, I., Prieto-Ballesteros, O., Rebolo, R., Sancho, J., 2012. TALISE: Titan
Lake In-situ Sampling Propelled Explorer. In: Proceedings of the European
Planetary Science Congress, p. 64.
Wall, S.D., Lopes, R.M., Stofan, E.R., Wood, C.A., Radebaugh, J.L., Hörst, S.M., Stiles, B.
W., Nelson, R.M., Kamp, L.W., Janssen, M.A., Lorenz, R.D., Lunine, J.I., Farr, T.G.,
Mitri, G., Paillou, P., Paganelli, F., Mitchell, K.L., 2009. Cassini RADAR images at
Hotei Arcus and Western Xanadu, Titan: evidence for recent cryovolcanic
activity. Geophys. Res. Lett. 36, L04203.
Wall, S., Hayes, A., Bristow, C., Lorenz, R., Stofan, E., Lunine, J., Le Gall, A., Janssen, M.,
Lopes, R., Wye, L., Soderblom, L., Paillou, P., Aharonson, O., Zebker, H., Farr, T.,
Mitri, G., Kirk, R., Mitchell, K., Notarnicola, C., Casarano, D., Ventura, B., 2010.
Active shoreline of Ontario Lacus, Titan: a morphological study of the lake and
its surroundings. Geophys. Res. Lett. 37, 5202.
Wahr, J.M., Zuber, M.T., Smith, D.E., Lunine, J.I., 2006. Tides on Europa and the
thickness of Europa's icy shell. J. Geophys. Res. 111, E12005.
Wei, H.Y., Russell, C.T., Dougherty, M.K., Neubauer, F.M., Ma, Y.J., 2010. Upper limits
on Titan's magnetic moment and implications for its interior. J. Geophys. Res.
115, E10007.
West, R.A., Balloch, J., Dumont, P., Lavvas, P., Lorenz, R., Rannou, P., Ray, T.,
Turtle, E.P., 2011. The evolution of Titan's detached haze layer near equinox
in 2009. Geophys. Res. Lett. 38, 6204.
Wood, C.A., Lorenz, R., Kirk, R., Lopes, R., Mitchell, K., Stofan, E., Cassini Radar Team,
2010. Impact craters on Titan. Icarus, 334–344.
Zebker, H.A., Stiles, B., Hensley, S., Lorenz, R., Kirk, R.L., Lunine, J., 2009. Size and
shape of Saturn's Moon Titan. Science 324, 921–923.
Zimmer, C., Khurana, K.K., Kivelson, M.G., 2000. Subsurface oceans on Europa and
Callisto: constraints from Galileo magnetometer observations. Icarus 147, 329.
Please cite this article as: Mitri, G., et al., The exploration of Titan with an orbiter and a lake probe. Planetary and Space Science (2014),
http://dx.doi.org/10.1016/j.pss.2014.07.009i