TEANBY: PLANETARY ATMOSPHERES
Cassini at Titan
Cassini’s first year examining
Saturn and its moons has
uncovered many surprises, not
least on the enigmatic moon
Titan. Nick Teanby reviews the
progress made and discusses
prospects for the future.
T
he launch of the Cassini probe on 15
October 1997 was the start of an epic
seven-year journey that included flybys of
Earth, Venus and Jupiter and eventually arrived
at Saturn in July 2004. The Cassini/Huygens
probe and mission are summarized in “The
Cassini/Huygens Mission” p5.22. Titan was last
visited by Voyager I in 1981 during its grand tour
of the solar system. Unlike Voyager, which only
encountered Titan during a brief flyby, Cassini
will remain in the Saturnian system for many
years, providing the opportunity for detailed
mapping of Titan’s surface and atmosphere and
studies of seasonal variations. This article
focuses on recent results about the atmosphere
and surface of Saturn’s largest moon, Titan.
Titan is one of the main science objectives of
the Cassini/Huygens mission. It is the second
largest moon in the solar system, slightly smaller
than Jupiter’s moon Ganymede and bigger than
the planet Mercury. Titan is of especial interest
to planetary scientists because it is the only
moon to have a substantial atmosphere – with a
pressure of 1.5 bar at the surface. Knowledge of
Titan prior to Cassini’s arrival is reviewed by
Taylor and Coustenis (1998), Coustenis and
Taylor (1999), and Lorenz and Mitton (2002).
What makes Titan so fascinating is that many
processes that occur on the Earth are thought to
have analogues on Titan, at much colder temperatures because Titan is so much farther from
the Sun. Such processes may include: tectonics;
weather, including rain; erosion by winds and
liquids; formation of complex organic compounds; a greenhouse effect; and volcanism.
Titan’s atmosphere is up to 98% nitrogen and
2–6% methane, which makes it the only object in
the solar system, apart from Earth, to have nitrogen as its main atmospheric component. Table 1
shows a comparison of Earth and Titan, and illustrates the many similarities and differences. The
nitrogen–methane atmosphere could give rise to
many interesting organic compounds – created
when solar UV photons and energetic electrons
from Saturn’s radiation belt cause dissociation of
5.20
ABSTRACT
Saturn’s giant moon Titan is the second largest moon in the solar system and is the only
planetary body, other than our Earth, to have a substantial nitrogen-based atmosphere.
Many exotic chemical reactions, driven by solar radiation, result in an atmosphere awash
with primitive organic compounds, which eventually rain down onto the surface. It is now just
over one year into the Cassini/Huygens mission to explore the Saturnian system, and already
Titan is proving to be a very curious moon indeed. The atmosphere contains clouds made of
hydrocarbons, which race round the planet, blown by winds rotating faster than the planet
itself. There is evidence of a varied surface laced with drainage channels, tectonic features,
dunes and even volcanoes and hydrocarbon lakes. Strange hazes adorn the sky, which made
studying Titan’s surface difficult prior to Cassini’s arrival. This article summarizes recent
discoveries about the atmosphere and surface of Titan from the mission so far.
Table 1: Comparison of properties of Earth and Titan
Atmospheric composition
Surface pressure
Surface temperature
Atmosphere thickness
(at top of stratosphere)
Clouds/rain
Radius
Axial tilt
Distance from Sun
Solar energy
Length of year
Length of day
Earth
78% N2, 21% O2, <1% H2O
1bar
296K
Titan
98% N2, 2% CH4
1.5bar
95K
50km
H2O
6371km
23.5°
1AU
1368Wm–2
1
1
300km
CH4
2575km
26.7°
9.5AU
15.0Wm–2
29.5
15.9
nitrogen and methane in the upper atmosphere.
The resulting radicals combine to form complex
hydrocarbons and nitriles such as ethane, acetylene, hydrogen cyanide and benzene (Wilson
and Atreya 2004). The photochemical processes
in Titan’s atmosphere are thought to be similar to
those that occurred on the early Earth. But
because of its low temperature and predicted
replenishment of atmospheric methane from the
surface or interior, Titan has preserved much of
this primordial atmospheric composition. It is
hoped that studying Titan will eventually provide
clues to the formation of the compounds that preceeded the development of life on Earth.
Eventually the photochemically produced
organic compounds filter down from the upper
atmosphere towards the surface and lower
atmosphere. Temperatures are colder here so the
organics condense and rain out onto the surface.
The heavier hydrocarbons and nitriles combine
to form fine particles of orange-coloured
organic haze, which gives Titan its characteristic
orange hue at visible wavelengths (figure 1).
This haze shrouds the surface from view at
many wavelengths, a fact that has allowed
astronomers to indulge in much speculation
about processes on the surface – processes that
Cassini is now starting to unravel.
Titan is thought to have a rocky core made of
silicates overlain by a crust mainly composed of
water ice. The surface temperature on Titan is
around 95 K, far too low for the existence of liquid water. In fact, water ice behaves more like
rock at these temperatures. However, the pressures and temperatures on the surface are close to
the triple point of methane and it is thought that
a methane cycle – similar to the water cycle on
Earth – exists on Titan, with evaporation, condensation to form rain clouds, and even the possibility of liquids on the surface. The instruments
onboard Cassini have seen a possible hydrocarbon lake and large numbers of localized clouds,
providing evidence for an active “hydrological
cycle”. There are also surface features that
appear to have been shaped by flowing liquids.
One of the many mysteries about Titan is the
fact that there is so much methane in the atmosphere. Methane forms the basis of many photoA&G • October 2005 • Vol. 46
TEANBY: PLANETARY ATMOSPHERES
: the story so far
1: Titan shown in visible light as viewed by Cassini’s ISS instrument.
Titan’s surface is shrouded by a dense orange haze, which is composed
of organic material formed by photochemical reactions in the upper
atmosphere. (NASA/JPL–Caltech/Space Science Institute)
chemical reactions on Titan, which cause its
depletion over time so that it has a lifetime of
only 10 million years in the atmosphere (Yung
1984). Therefore, unless it is being replenished
somehow, we should not observe it in such high
quantities. Before Cassini arrived, an ocean of
liquid hydrocarbons had been proposed as a
source for methane replenishment. However,
current data from Cassini make large-scale
global oceans unlikely. Volcanoes may provide
an alternative source of methane, and features
resembling cryovolcanic domes have been
observed by Cassini on the surface.
Titan’s dynamic atmosphere
A close flyby in December 2004 revealed a
highly complex haze structure in Titan’s upper
atmosphere. Cassini observed Titan from the
night side, which resulted in stunning images of
numerous detached haze layers illuminated by
scattered sun light (figure 2). These haze layers
extend up to 500 km above the surface, about
150 km higher than the hazes seen by Voyager in
1981. At the time of the Voyager flyby the northern hemisphere was in early spring, but the current season is early winter. It is possible that the
difference in haze altitudes represents seasonal
variations in the haze structure. In another close
A&G • October 2005 • Vol. 46
2: Titan’s complex haze structure is captured in this image of scattered
sunlight taken from the night side. As many as 12 separate haze layers
are visible above the main orange haze, extending to over 500km above
the surface. The layers are thought to be caused by waves in the atmosphere (Porco et al. 2005). (NASA/JPL-Caltech/Space Science Institute)
flyby this April, Cassini passed within 1000 km
of Titan’s surface. This allowed the mass spectrometer to scoop up gas from the upper atmosphere and determine molecular masses of the
constituents (Waite et al. 2005). A complex array
of short- and long-chain hydrocarbons and
nitriles was detected, with masses up to the maximum range of the instrument (100 atomic mass
units). The largest chains had up to seven carbon
atoms – surprisingly complex for this atmospheric level. This soup of organic compounds
eventually combines to form the thick orange
haze in the lower atmosphere. The complex
interaction between photochemically produced
organic compounds and haze production on
Titan is not yet fully understood.
The mass spectrometer has also shown that
Titan’s atmosphere is enriched in the heavy isotope of nitrogen (15N) with respect to the more
abundant lighter one (14N). The isotope ratio
15
N/14N is much greater than on Earth (Waite et
al. 2005). This is also apparent in infrared spectra of the stratosphere taken by the CIRS instrument. Upon orbital insertion, Cassini’s
magnetospheric imaging instrument detected a
vast torus of gas orbiting around Saturn along
with Titan. Titan lies within the magnetosphere
of Saturn and is constantly bombarded by ener-
getic particles from Saturn’s radiation belts.
These particles collide with gas in Titan’s atmosphere and cause gas to be ejected, forming a vast
cloud of gas encircling Saturn and its rings. The
loss of atmosphere caused by this process – and
also by the molecules’ intrinsic thermal energy –
results in enrichment of heavy isotopes because
lighter isotopes move faster and therefore escape
preferentially from Titan’s gravitational field. It
is thought that large amounts of nitrogen must
have been lost from Titan’s atmosphere in the
past to give the large enrichment that we see
today. The exact amount lost is hard to constrain
at present, but it is estimated that in the past
Titan’s atmosphere was between 1.6 and 100
times more massive (Waite et al. 2005).
In contrast to the nitrogen enrichment, the
heavy isotope of carbon is not enriched. In fact,
the ratio 13C/ 12C is slightly lower than it is on
Earth. This is consistent with methane replenishment, which would maintain the same relative abundance of each isotope, and requires a
source of methane on the surface.
Stormy weather
One of the most exciting aspects of Titan is the
prospect of clouds and rain, composed of condensed hydrocarbons. The largest concentra5.21
TEANBY: PLANETARY ATMOSPHERES
The Cassini/Huygens Mission
The Cassini/Huygens mission is an
international collaboration between NASA,
the European Space Agency and the Italian
Space Agency, with a total of 17 countries
taking part. The prime mission objectives are
to study Saturn and its rings, Titan and its
atmosphere, and the icy moons. Cassini has
12 scientific instruments, described below, as
well as the Huygens probe, which descended
into Titan’s murky atmosphere early this year.
The primary mission is planned to last for
four years and includes 45 flybys of Titan and
76 orbits of Saturn.
The Cassini orbiter’s 12 instruments
Imaging Science Subsystem (ISS): Wide- and
narrow-angle imaging cameras covering
0.2–1.1 µm. Several filters correspond to
atmospheric “window” regions, allowing
images of Titan’s surface to be taken.
● Visible and Infrared Mapping Spectrometer
(VIMS): Imaging spectrometer covering
0.35–5.1 µm, allowing composition mapping
of Titan’s surface and atmosphere.
● Composite Infrared Spectrometer (CIRS):
Fourier transform spectrometer covering the
mid- and far-infrared (7–1000 µm). Atmospheric composition and temperature mapping.
● Ultraviolet Imaging Spectrograph (UVIS):
●
tions of clouds seen by Cassini so far accumulate around the south polar region (Porco et al.
2005). These are suspected to be convective
clouds composed of methane and provide some
of the first direct evidence of meteorological
activity on Titan (see figure 3). Cassini has also
seen a feature that could be a hydrocarbon lake
– perhaps fed by rain from the methane clouds.
The south pole is currently in early summer and
the extra heating there is expected to cause
evaporation of surface methane; the hot air then
rises and forms cumulus clouds, just like those
on Earth but made of methane instead of water.
These clouds move between successive images
and during some flybys were not present at all.
The south pole seems to be a region of intense
meteorological activity at present.
Clouds also occur at mid-latitudes, although
they are a lot fainter than at the south pole and
last only for a few hours. Tracking cloud features
in the troposphere has provided estimates of the
wind speed as high as 34 m s–1 (75 mph) from
west to east at mid-latitudes, which is faster than
Titan itself rotates. Such circulations are referred
to as “super-rotation”, and have been predicted
for Titan but never directly observed before (Del
Genio et al. 1993). Venus also has a super-rotating atmosphere and this seems to be a characteristic of slowly rotating planets.
5.22
Imaging spectrometer covering 56–190 nm
designed for investigating atmospheric
aerosol properties, composition and
temperature.
● Radar: Both active and passive modes allow
the radar to measure Titan’s surface elevation,
textures and provide information on
composition.
● Radio Science Subsystem (RSS): Uses
Cassini’s main dish to perform, for example,
radio occultations through Titan’s
atmosphere to determine the vertical
temperature structure.
● Cassini Plasma Spectrometer (CAPS):
Measures energy and electrical charge of
particles in the Saturnian system.
● Ion and Neutral Mass Spectrometer
(INMS): Measures the masses of charged and
neutral particles in the upper atmospheres of
Saturn and Titan, allowing composition and
temperature determinations.
● Cosmic Dust Analyser (CDA): Measures
size and velocity of tiny dust grains in the
Saturnian system.
● Duel Technique Magnetometer (MAG):
Measures the strength and direction of the
magnetic field around Saturn in order to map
the magnetic field and magnetosphere.
● Magnetospheric Imaging Instrument
Perhaps the strangest clouds observed so far
take the form of east-west streaks hundreds of
kilometres long. These clouds seem to originate
from fixed positions and one interpretation is
that they are produced by processes on the surface, and may be gas vented from Titan’s interior, which is then swept along by the
super-rotating winds.
To understand many of the processes described
above requires knowledge of temperature, composition and wind speeds. The CIRS instrument
is ideally suited for this task and some of the
early results from CIRS are discussed later on.
Titan’s mysterious surface
The surface of Titan has long been a mystery and
the subject of much speculation, as it is very hard
to see due to absorption of radiation emitted from
the surface by methane and the thick organic haze
that permeates the atmosphere. Cassini’s instruments were designed with this in mind and so far
have provided a wealth of information on the surface characteristics, although there is still much
speculation at present and future flybys are
eagerly anticipated. The main instruments that
Cassini uses to penetrate the atmosphere are: the
radar, which is not impeded by atmospheric
absorption; and ISS and VIMS, which can see
down to the surface through several atmospheric
(MIMI): Images the particles trapped in
Saturn’s magnetic field.
● Radio and Plasma Wave Science (RPWS):
Measures radio signals from Saturn and the
interaction of the solar wind with the
Saturnian system.
Main instruments for studying Titan
● Atmosphere: CIRS, INMS, ISS, RSS, UVIS,
VIMS.
● Surface: ISS, Radar, VIMS.
● Huygens Probe: six instrument packages for
taking in-situ measurements through Titan’s
atmosphere and on the surface.
Cassini/Huygens timeline
15 Oct 1997
26 Apr 1998
24 Jun 1999
18 Aug 1999
30 Dec 2000
1 Jul 2004
2 Jul 2004
13 Dec 2004
24 Dec 2004
14 Jan 2005
15 Feb 2005
31 Mar 2005
16 Apr 2005
launch
first Venus swing-by
second Venus swing-by
Earth swing-by
Jupiter encounter
Saturn orbital insertion
Titan flyby (339 000 km alt.)
Titan flyby (1200 km alt.)
Huygens probe leaves Cassini
Huygens lands on Titan
Titan flyby (950 km alt.)
Titan flyby (2520 km alt.)
Titan flyby (950 km alt.)
windows – wavelengths where the absorption
from haze and methane is reduced. The descent of
the Huygens probe on 14 January provided valuable information on the surface around the landing site, although data interpretation continues.
So far around 1% of the surface has been
mapped by high-resolution radar and very few
impact craters have been detected (Elachi et al.
2005). Objects that would cause craters less
than 20 km wide would burn up in Titan’s
atmosphere before reaching the surface. However, based on observations of other moons of
Saturn, we would expect to observe hundreds of
craters larger than 20 km in the areas mapped
so far, but there are only a few, rather weathered-looking craters. Figure 4 shows the largest
crater discovered so far, provisionally named
Circus Maximus, which is 440 km wide and
believed to be caused by an object 5–10 km
across (Elachi et al. 2005). The characteristic
central peak from this crater is missing and has
probably been removed by surface processes
ISS and VIMS have also seen very small numbers of craters (Porco et al. 2005 and Sotin et al.
2005). The low crater count tells us that the surface of Titan is geologically very young, possibly
only 130–300 Myr in places (Porco et al. 2005)
and is being reworked by processes such as viscous sagging, tectonics, cryovolcanism, erosion
A&G • October 2005 • Vol. 46
TEANBY: PLANETARY ATMOSPHERES
3: These ISS images of the surface of Titan, taken through one of the atmospheric windows, show distinct light and dark regions. These regions are
suspected to have different surface compositions and one possibility is that the dark areas are rich in organic material, perhaps precipitated from the
atmosphere, and light areas are outcrops of the ice bedrock. Clouds near the south pole are clearly visible in all images (white blobs). These clouds change
with time, indicating that active meteorology is taking place on Titan and provide evidence for a methane-cycle, much like the water cycle on Earth – with
evaporation, cloud formation and rain. The image on the right shows a possible hydrocarbon lake (dark patch) in the south polar region. Enhanced
meteorological activity here, as evidenced by the white methane clouds, could be feeding this lake. Alternatively, the dark patch could be a dried up lake
bed or a concentrated deposit of organic material. Also see article by Porco et al. (2005). (NASA/JPL-Caltech/Space Science Institute)
4: Radar can penetrate through Titan’s thick atmosphere, providing these surface images. Brighter areas correspond to rougher surfaces, different
composition, or slopes facing the radar. The image on the left shows a large impact crater, provisionally named Circus Maximus, which is 440km in
diameter. The crater has no central peak, which may have been removed by erosion or viscous sagging. Titan has very few impact craters compared to
other bodies in the Saturnian system, indicating that its surface is very young and may be reworked by tectonic and erosion processes. On the right is an
image just to the east of the crater, which shows linear features that could be channels where liquid methane has washed rubble from the crater downslope. This rough rubble surface would then appear bright to the radar. (NASA/JPL-Caltech)
by winds or liquids, or possibly a combination
of all of these. Ice pebbles photographed by the
Huygens probe on Titan’s surface have a
rounded appearance, consistent with erosion of
sharp edges by wind or liquids (see figure 5).
All three instruments show a varied and complex surface terrain comprising light and dark
areas (Porco et al. 2005, Sotin et al. 2005 and
Elachi et al. 2005) as seen in figure 3. One very
large bright area, Xanadu Regio, is thousands of
kilometres wide. Light and dark areas may correspond to different materials, textures, or illumination cause by sloped surfaces. There are thin
dark features that extend for many hundreds of
kilometres. Some of these are linear and may be
caused by tectonic activity. Others are more sinuous and may be channels cut by liquids flowing
on the surface. Parallel streaky features reminiscent of wind-blown dunes have been observed
A&G • October 2005 • Vol. 46
near the equator. These are oriented east–west
and have sharp western boundaries and
extended eastern boundaries, which is consistent
with formation by the super-rotating winds from
west to east derived from cloud tracking.
The numerous channel-like features observed
on Titan provide some of the best evidence for
the presence of liquids on the surface. Many
sizes, types and locations of channels have been
observed so far. Figure 4 shows channels originating from the Circus Maximus crater seen by
radar, which are up to 200 km long and may be
composed of debris washed from within the
crater walls. Down at the south pole, where large
numbers of clouds have been observed, there is
an extensive system of channels. One possible
explanation is that these were cut by liquid
methane which fell as rain caused by the
enhanced meteorological activity indicated by
the convective clouds observed there. Indeed,
one such channel is over 1500 km long and could
act as an intermittent river. During its descent,
the Huygens probe captured many images of
smaller channels with dark material in the channel “valley” compared to the surrounding terrain of higher and lighter material (see figure 5).
It may be that the light areas represent outcrops
of the ice bedrock, and the dark areas are organics deposited from the atmosphere and washed
into low-lying regions by methane rain.
It has long been suspected that there were
oceans, or perhaps lakes, of liquid methane on
the surface of Titan. So far there has been no
direct evidence of this from the radar, VIMS, or
ISS, although very recently a dark patch seen by
ISS near the south pole has features remarkably
similar to a shoreline and it could be a hydrocarbon lake – possibly dried up (see figure 5). If
5.23
TEANBY: PLANETARY ATMOSPHERES
indeed there is no large-scale reservoir of liquid
methane on the surface, then an alternative
source of methane is required to explain the
atmospheric composition.
One possible source of atmospheric methane
could be out-gassing from cryovolcanoes, which
might erupt mixtures of methane and water ice
combined with other compounds. The VIMS
instrument recently observed a circular feature,
approximately 30 km in diameter, which may be
the first cryovolcano to be discovered on Titan
(Sotin et al. 2005). Close inspection of the
infrared images show boundaries that may correspond to overlapping flows, along with a central “caldera” (see figure 6). Other possible
interpretations are a cloud or dune. However, the
feature has not been seen to move between successive images and the shape and spectral characteristics are different to clouds already
observed. A circular-shaped dune is also unlikely,
given the strong east–west winds. Interpreting
this feature as a volcano is appealing, not only
because of its appearance, but because cryovolcanoes would help provide the source of methane
required to sustain Titan’s atmospheric composition. The energy source for such a volcano is currently unknown but could be from tidal heating
or radioactive decay in Titan’s interior.
5: Titan’s surface photographed by Huygens Descent Imaging Spectral Radiometer (DISR). The lefthand image was taken as the probe descended through Titan’s atmosphere on 14 January this year.
The image shows what appears to be a complex network of drainage channels, possibly cut by liquid
methane. The dark material in the channels is not thought to be liquid but may be organic material that
has been deposited from the atmosphere and subsequently washed into the “valleys” by methane
rainfall. The right-hand image was taken by DISR on the surface and shows rounded ice pebbles,
which may have been eroded by the wind or fluid flow. (ESA/NASA/JPL-Caltech/University of Arizona)
CIRS atmospheric mapping
The Composite Infrared Spectrometer (CIRS)
measures spectra covering a wide range of wavelengths (7–1000 µm) in order to obtain quantitative information on the temperature and
composition of Titan’s atmosphere (Flasar et al.
2004). This wavelength range overlaps with the
peak in thermal emission from Titan and corresponds to the spectral features of many organic
compounds. This makes CIRS ideal for probing
the chemistry of this complex world.
In order to extract information from the CIRS
spectra, a computer model of the atmosphere
must be constructed. The atmosphere is modelled as a series of layers and each layer is prescribed a temperature, pressure, composition,
haze and cloud properties. The modelled spectrum is then the sum of contributions from all
the layers and should be comparable to the
observed spectra if the model is a good representation of the real atmosphere.
Infrared properties of gases are taken from
laboratory measurements. However, the spectral
properties of hazes on Titan are poorly known,
which introduces some uncertainty into the
models. Spectra measured from laboratory synthesis of the Titan haze particles, made by passing electricity though nitrogen–methane
mixtures, are used as a best guess of the haze
properties. As the mission progresses, the complex hazes on Titan will be better characterized.
To extract temperature and composition information, we start with an initial guess of the
atmospheric temperature and composition,
5.24
10
8
6
–148
–146
–144
–142
30 km
6:OntheleftisaninfraredVIMSimageofasuspectedcryovolcano.Thecircularfeatureat–143.5°W,
8.5°Nisabout30kmindiameter.Ithasremainedstationaryandsoisnotthoughttobeacloud.It
appearstohaveacentralcaldera,similartovolcanoesonEarthandalsoaseriesofoverlappingflows.
Aschematic“geologicalmap”isshownontheright,whichshowsthesefeaturesmoreclearly.Different
coloursrepresentareaswithdifferentbrightnessandtexture.Themagmafromsuchavolcanois
thoughtobeamethane–watericemixwithothercompounds.Cryovolcanoesmaysupplymethaneinto
Titan’satmosphere.AlsoseeSotinetal.(2005).(NASA/JPL-Caltech/UniversityofArizona)
based on measurements from Voyager and telescopes. This gives a synthetic spectrum that can
be compared to the observations. The model is
then perturbed until a good fit to the measured
spectrum is obtained. Different parts of the spectrum can be used to investigate temperature or
gas abundances. An example segment of a spectrum from CIRS is shown in figure 7.
Atmospheric temperature affects the height of
all the spectra peaks. A larger peak could mean
a larger gas abundance or a higher temperature.
Therefore, the most important first step in
analysing the CIRS spectra is to determine the
variation of temperature with altitude. This is
done using the emission features of methane
from 7.4–8.1 µm, which depend only on temperature and methane abundance: if the abundance is fixed, the temperature can be found.
Methane abundance is measured independently
using a different part of the spectrum.
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radiance (µW/cm2 /sr/µm)
TEANBY: PLANETARY ATMOSPHERES
14
C 2H 2
12
10
8
CH 4
HCN
6
C 2H 6
4
C 3H 4
C 4H 2
C 2H 4
2
0
HC 3N
CO 2
x10 –7
K
7
8
9
10 11 12 13 14 15 16 17
wavelength (µm)
7: Example infrared spectrum of Titan taken at
latitude 60°N by the CIRS instrument. The
emission features of many stratospheric gases
occur in this range and some of the more
prominent species are marked on the figure.
The methane feature centred around 7.7µm
can be used to extract atmospheric
temperature as it does not overlap with any of
the other gases and this region of the spectrum
is not affected much by Titan’s haze. Once
temperature is determined the other peaks can
be used to obtain abundances of trace gases.
Currently CIRS has surveyed latitudes
between 90°S and 60°N on Titan. Temperature
varies mostly with latitude and variations with
longitude are very small, giving a banded
appearance to the temperature map shown in
figure 8. The temperature gradient is related to
the wind speed and can be used to determine
wind speeds indirectly. Strong zonal winds have
been inferred, consistent with the high speeds
determined from cloud tracking. The temperature gradient in the northern hemisphere is
much greater than in the south; winds in the
north are therefore stronger (Flasar et al. 2005).
CIRS found that at the south pole, which is
now in summer, the stratosphere is colder than
in the equatorial region by about 4–5 K (Flasar
et al. 2005). This is unexpected because, if the
haze and gases are uniformly distributed across
Titan, the south pole should be about 15 K
warmer, because it is in constant sunlight. This
implies that the gas or haze distribution could
differ with latitude. Alternatively, there is atmospheric circulation with upwelling at the south
followed by transport towards the north. The
rising air in the south would be adiabatically
cooled as it rises and expands, explaining the
colder temperatures. Further analysis is required
and subsequent flybys may give more clues.
Once the atmospheric temperature profile has
been determined, the abundances of trace
organic gases can be found by adjusting the composition in the model to give the best fit to the
measured spectrum. Figure 8 shows maps of the
abundance of two nitriles: hydrogen cyanide
(HCN) and cyanoacetylene (HC3N) (Teanby et
al. 2005). The gradient of the abundance variations from south to north differ for these two
gases, but they both display a banded pattern,
consistent with zonal winds derived from cloud
tracking and temperatures. The abundance of
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140 145 150 155 160
temperature at 3 mbar
1 2 3 4 5 6
HCN abundance at 3 mbar
x10 –10
0.0 0.5 1.0 1.5 2.0 2.5
HC 3N abundance at 3 mbar
8: Maps of temperature, HCN, and HC3N in Titan’s atmosphere at the 3mbar level (140km altitude).
Temperature shows a banded pattern, with most of the variation in the latitude coordinate. The north
pole is experiencing winter, which explains the cold temperatures there. HCN abundance is roughly
constant in the southern hemisphere, and gradually increases towards the winter pole. This is thought
to be due to organic enriched air from the upper atmosphere sinking to lower levels, where it can be
observed. HC3N increases much more steeply than HCN, and is almost undetectable southwards of
40°N. HC3N has a much shorter lifetime in Titan’s atmosphere than HCN and hence does not have time
to become mixed with air at lower latitudes before it dissociates. Maps like these give us clues about
the nature of winds and circulations occurring in Titan’s atmosphere. (Redrawn from Teanby et al.)
both gases increases towards the winter north
pole, probably because cold dense air in the
north subsides, dragging down air enriched in
organics from the high atmosphere where the
organics are produced by photochemistry. A simple numerical model explains this enrichment if
the air is subsiding at around 0.3 mm s–1.
The different gradients observed for HCN and
HC3N are thought to be due to different photochemical lifetimes. HCN has a lifetime of
around 10 years and HC3N has a lifetime of
under a year (Wilson and Atreya 2004). HCN
therefore has time to mix in with air at lower
latitudes, decreasing the gradient, whereas
HC3N dissociates before it has a chance to mix.
Other organic compounds also increase
towards the north pole, for example cyanogen
(C2N2), methylacetylene (C3H4) and diacetylene
(C4H2) (Flasar et al. 2005 and Teanby et al. 2005).
The enrichment of these gases at high northern
latitudes is consistent with subsidence and isolation of the north polar atmosphere from the
atmosphere at lower latitudes. It is possible that
the high winds around the north pole create a
polar vortex, which inhibits the mixing of air
from inside the vortex with that outside. This may
be analogous to the polar vortex that surrounds
Antarctica on the Earth, where cold temperatures,
and subsiding air from the high atmosphere, leads
to the formation of polar stratospheric clouds,
which in turn facilitate chlorine production and
subsequent ozone depletion. Similar strange
chemistry, involving different gases, may happen
at Titan’s winter pole. CIRS data already show
spectral features of organic compounds that are
not detectable at more southern latitudes. More
detailed analysis must wait until Cassini observes
the north polar region later in the mission.
Discoveries for the near future?
Cassini is now just over one year into its fouryear primary mission and already there have
been many discoveries, and even more questions
left to answer. Although a large-scale ocean now
seems unlikely, the search continues for liquid
methane on the surface and methane lakes now
seem like a possibility. Evolution of the south
polar clouds as summer progresses may reveal
seasonal variations in the meteorological
processes that shape the surface. The region
where VIMS observed a possible cryovolcano
has yet to be mapped in detail by the radar, and
with only 1% of the surface mapped, what other
exotic surface formations await discovery?
Titan’s north polar region is just entering winter and has yet to be mapped. Strange organic
molecules and even stranger chemistry are anticipated as organic molecules are accumulated by
atmospheric circulation as winter progresses.
The Cassini mission has only just begun, but
already there is enough data to keep scientists
busy for years. ●
Nick Teanby is a postdoctoral researcher working as
part of the Cassini–CIRS investigation team at
Oxford University. [email protected]
References
Coustenis A and Taylor F W 1999 Titan: the Earth-like
moon (World Scientific).
Del Genio A D et al. 1993 Icarus 101 1.
Elachi C et al. 2005 Science 308 970.
Flasar F M et al. 2005 Science 308 975.
Flasar F M et al. 2004 Space Sci. Rev. 115 169.
Lorenz R and Mitton J 2002 Lifting Titan’s veil (CUP).
Porco C C et al. 2005 Nature 434 159.
Sotin C et al. 2005 Nature 435 786.
Taylor F W and Coustenis A 1998 Planet. Space Sci. 46
1085.
Teanby N A et al. 2005 submitted to Icarus.
Waite J H et al. 2005 Science 308 982.
Wilson E H and Atreya S K 2004 J. Geophys. Res. 109
E06002.
Yung Y L et al. 1984 Astrophys. J. Suppl. 55 465.
More information
Images and movies from the Cassini Mission:
http://saturn.jpl.nasa.gov
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