Planetary and Space Science 49 (2001) 1395–1407
www.elsevier.com/locate/planspasci
Returns to Mercury: science and mission objectives
R&ejean Grarda;∗ , Andr&e Baloghb
a Space
Science Department of ESA, European Space Research & Technology Center, Keplerlaan 1, 2200 AG Noordwijk, Netherlands
b The Blackett Laboratory, Imperial College, London SW7 2BZ UK
Abstract
As the inner end-member of the planetary system, Mercury plays an important role in constraining and testing dynamical and
compositional theories of planetary formation. With its companions Venus, Earth and Mars, it forms the family of terrestrial planets, a
category of celestial objects in which each member holds information essential for retracing the history of the whole group. For example,
knowledge about the origin and evolution of these planets is one of the keys to understanding how conditions to support life have been
met in the Solar System and, possibly, elsewhere. This quest is all the more important as terrestrial-like objects orbiting other stars are
not yet accessible; our own solar system remains the only laboratory where we can test models that are also applicable to other planetary
systems. The exploration of Mercury is therefore of fundamental importance for answering questions of astrophysical and philosophical
c 2001 Elsevier Science Ltd.
signi8cance, such as: ‘Are terrestrial bodies a common feature of most planetary systems in the Galaxy?’ All rights reserved.
1. Introduction
Mercury is the least known planet in the inner solar system. It is di>cult to observe from the Earth, because of
its proximity to the Sun (Mendillo et al., 2001; Warell and
Limaye, 2001). The best observations were made in 1974
–75, during the three ?ybys of NASA’s Mariner 10 spacecraft, the only space mission so far dedicated to Mercury.
Mariner 10’s three ?ybys provided images of about 46% of
the planet’s surface and another set of observations of which
the discovery of the planetary magnetic 8eld proved the
most important and lasting achievement (Ness et al., 1975).
These data have formed the basis for Mercury studies over
the past quarter century, although other observations from
the Earth have since provided evidence for the variability of
the exosphere and given important clues about the unknown
regions of the planet. It appears now likely that volatiles
have been deposited in deep craters near the poles, and that
a large, possibly volcanic dome may dominate the unseen
side of the planet.
In spite of the lack of data to constrain su>ciently the
models of the interior, magnetic 8eld, exosphere and magnetosphere, it is remarkable that so much progress has been
made in understanding the planet, as well as in formulating
the questions and de8ning the observations which are most
∗
Corresponding author: Tel.: +31-71565-3596; fax: +31-71565-4697.
E-mail address: [email protected] (R. Grard).
crucial for deciding among competing models of its origin,
history and present state. It is clear that the limited set of
observations by Mariner 10 has been put to very good use.
The renewed interest in Mercury which we have witnessed
during the past years stems from the recognition that the
history of the terrestrial planets, the Earth, Venus, Mars and
Mercury itself, needs to be understood, as a whole, to make
sense of the formation of planetary systems in general. These
planets represent a unique family, a group of relatively small
objects, close to our star, the Sun; such a planetary system
is unlikely to be discovered around any other star in the
foreseeable future. Each of these planets has its own history,
resulting in apparently large diFerences that can be seen
today; but Mercury is, among the terrestrial planets, the least
known and least understood of the family (Fig. 1).
The space agencies have at last responded positively to
the mission proposals to Mercury made repeatedly by the
international planetary community. Although the European
Space Agency was the 8rst to initiate the in-depth study of
a mission to Mercury in 1993, and approved it as the planetary cornerstone of its Horizons 2000 programme in 1996,
NASA’s precursor Messenger mission was the 8rst to have
a concrete schedule con8rmed, in 1999. Messenger is to be
launched in 2004 and it will get into Mercury orbit in 2009.
In the meantime, Japan’s Institute for Space and Astronautical Sciences (ISAS) also examined the feasibility
of a small, mostly magnetospheric Mercury orbiter. In the
autumn of 2000, ESA selected its Mercury mission as the
c 2001 Elsevier Science Ltd. All rights reserved.
0032-0633/01/$ - see front matter PII: S 0 0 3 2 - 0 6 3 3 ( 0 1 ) 0 0 0 8 1 - 2
1396
R. Grard, A. Balogh / Planetary and Space Science 49 (2001) 1395–1407
Table 1
Messenger mission summary (courtesy Messenger Team)
MESSENGER
MEcury Surface, Space ENvironment, GEochemistry, and Ranging
Fig. 1. The terrestrial planets. They share a common origin and each of
them must be properly understood before one can make sense of all four
of them.
8fth cornerstone of its programme, and ISAS decided to
cooperate with Europe in this mission. This joint ESA–
ISAS mission, due for launch in 2009 and expected to
arrive at Mercury in 2012 is called BepiColombo, in
memory of the great Italian dynamicist who 8rst explained
the 3 : 2 ratio between the durations of Mercury’s sidereal
year and day (Colombo, 1965, 1966). The Mercury resonant
orbit that enabled Mariner 10 to make its three ?ybys was
also implemented by NASA on Giuseppe (Bepi) Colombo’s
advice.
The collection of papers in this issue of Planetary
and Space Sciences was gathered following the session
on Returns to Mercury held at the 25th general assembly of the European Geophysical Society in April 2000
in Nice. In scienti8c terms, it is a representative sample of preoccupations with several aspects of Mercury,
the planet’s interior and its surface, as well as its exosphere and magnetosphere. This issue also presents the
two missions to Mercury, Messenger and BepiColombo.
The scienti8c objectives of Messenger (Solomon et al.,
2001), its payload (Gold et al., 2001) and the mission and
spacecraft (Santo et al., 2001) are described in some detail. Similarly, the technical aspects of BepiColombo are
described by Anselmi and Scoon (2001), Novara (2001)
and Racca (2001). The scienti8c objectives of BepiColombo and its model payload are summarised in this
paper.
Overviews of the NASA and ESA–ISAS missions are
given in Tables 1 and 2. Messenger is at present under development and BepiColombo entered its de8nition phase in
May 2001. The information contained in Table 2 is therefore tentative and alternative scenarios (e.g., a single launch
with Ariane 5) are possible.
Scienti8c objectives
Chemical composition of Mercury’s surface
Mercury’s geological history
Nature of Mercury’s magnetic 8eld
Size and state of the Mercury’s core
Volatile inventory at Mercury’s poles
Nature of Mercury’s exosphere and
magnetosphere
Science payload
Mercury dual imaging system (MDIS)
Gamma-ray and neutron spectrometer
(GRNS)
X-ray spectrometer (XRS)
Magnetometer (MAG)
Mercury laser altimeter (MLA)
Mercury atmospheric and surface composition
spectrometer (MASCS)
Energetic particle and plasma spectrometer
(EPPS)
Radio science (RS)
Transfer to Mercury
March 2004 launch (20-day window);
12-day backup window in May 2004
Ballistic interplanetary cruise with two
Venus and two Mercury gravity
assists (unpowered) plus two deep-space
maneuvers.
Arrival April 2009; capture with Mercury
Orbit Insertion burn into 12 h
◦
orbit: initial inclination 80 , initial periapsis
◦
latitude 60 N, initial periapsis
altitude 200 km.
Spacecraft module
Stabilization
Orientation
MESSENGER is a single, integrated unit
3-axis
Sunshade toward Sun, instruments toward
planet as allowable
450 W orbit phase from 5:0 m2 solar array
(1:5 m2 cell area)
X-band
Dual 18 × 76 cm2 phased arrays; 2 fanbeam;
4 low-gain antennas
200 × 15193 km altitude
1 Earth year
9:8 Mbyte=day (orbit phase)
3:1 kb=s
Power or energy
TM band
Antenna
Deployment
Operational lifetime
Data volume
Equivalent aver. bit
rate
Propulsion
Dry mass
Propellant type
Propellant mass
Nominal thrust
Number of thrusters
Total mass at launch
Mass margin
Launch
Chemical dual-mode propulsion
485 kg, full spacecraft (including margin)
N2 H2 and N2 H2 + N2 O4
608 kg
645 N; 317 s Isp
One 645 N Leros-1b biprop; four 22 N+
ten 4 N monoprop
Cruise duration
Ground station
1093 kg
25.3%
Delta 2925H-9.5, Eastern Test Range, Cape
Canaveral Air Force Station, Florida
5 years
34 m NASA deep space network (DSN)
Priority technology
R&D
Uses current state-of-the art; no developmental
R&D required.
R. Grard, A. Balogh / Planetary and Space Science 49 (2001) 1395–1407
1397
Table 2
BepiColombo mission summary (October 2000 baseline)
BepiColombo Mercury cornerstone
Scienti8c objectives
Origin and evolution of a planet close to the parent star
Mercury as a planet: form, interior structure, geology, composition and craters
Origin of Mercury’s magnetic 8eld
Mercury’s vestigial atmosphere (exosphere): composition and dynamics
Mercury’s magnetized envelope (magnetosphere): structure and dynamics
Test of Einstein’s theory of general relativity
Detection of potentially Earth-threatening asteroids
Three payloads
Mercury Planetary Orbiter (MPO): imagers, spectrometers (IR, UV, X-ray, (-ray, neutron),
Ka-band transponder, accelerometer, altimeter, and asteroidal telescope
Mercury Magnetospheric Orbiter (MMO): magnetometer, ion spectrometer, electron energy
analyser, cold and energetic plasma detectors, plasma wave analyser, and imager
Mercury Surface Element (MSE): thermal and physical properties analyser, alpha X-ray
spectrometers, seismometer, magnetometer, and imagers
Transfer to Mercury
Two launches: MPO and MMO-MSE composite
Interplanetary cruise with Solar Electric Propulsion Module and gravity assists of the
Moon, Venus and Mercury; Solar Electric Propulsion Module to be jettisoned upon arrival
at Mercury
Mercury capture, MPO and MMO insertion in polar orbits, and MSE descent with
Chemical Propulsion Module; MSE soft landing with airbags
Spacecraft modules
Stabilisation
Orientation
Mass
Power or energy
TM band
Antenna
Deployment
Operational lifetime
Data volume
Equivalent average bit rate
MPO
3-axis
Nadir pointing
357 kg
420 W
X=Ka
1:5 m diameter
400 × 1500 km altitude
¿ 1 yr
1550 Gb=yr
50 kb=s
Propulsion modules
Dry mass
Type of propellant
Propellant mass
Electrical power @ 1 AU
Nominal thrust
Number of thrusters
Solar Electric Propulsion Module
365 kg
Xenon
230=238 kg
5:5 kW
0.17 or 0:34 N
3 (1 or 2 in operation)
Total mass at launch
Mass margin
Launches
Typical cruise duration
Ground station
1229=1266 kg
23=19%
Vehicles: Soyuz–Fregat-Range: Baikonur - Date: August 2009
3:5 yr (¡ 2:5 yr without Moon ?yby, but mass margin ∼ 5%)
Perth, 35 m antenna, 8 h=day
MMO
15 rpm spin
◦
Spin at 90 to Sun
165 kg
185 W
X
1 m diameter despun
400 × 12 000 km
¿ 1 yr
160 Gb=yr
5 kb=s
MSE
NA
NA
44 kg
1:7 kWh
UHF
Cross dipole
◦
±85 latitude
¿ 1 week
75=138 Mb=wa
128=228 b=sa
Chemical Propulsion Module
71 kg
N2 O4 -MMH
156=334 kg
NA
4 kN
1
Priority technology research
and development
High-temperature thermal control materials
High-intensity and high-temperature solar generators
High-temperature X=Ka band antenna
High-temperature antenna pointing and despin mechanisms
Highly integrated control and data systems
Vision-based landing system
a Note: 8rst and second 8gures refer to MPO and MMO-MSE launches, respectively, unless speci8ed otherwise, with relay on MMO or MPO,
respectively.
Both missions to Mercury address the key questions that
have been identi8ed since Mariner 10. They will provide
observations that will constrain the models of planetary formation and evolution, and study the closely linked Hermean
space environment, magnetosphere and exosphere, and its
coupling with the surface. The scienti8c objectives of the
missions and their relation with our current knowledge of
the planet are discussed in the following.
1398
R. Grard, A. Balogh / Planetary and Space Science 49 (2001) 1395–1407
2. Mercury’s interior, evolution and magnetic eld
Fig. 3. The correlation between the magnetic dipole and quadrupole
moments of Mercury, determined from diFerent analyses of the Mariner
10 ?yby data (after Connerney and Ness, 1987 and references therein).
The diFerent determinations cannot independently determine these two
terms, but provide diFerent values for diFerent assumptions concerning
the location of the dipole and the interpretation of magnetospheric models.
The most striking feature of Mercury, its very large iron
core, is inferred from its density. Fig. 2 shows that the density of Mercury is signi8cantly larger for its radius when
compared to those of the other terrestrial planets and the
Moon. A simple, two-component model of the interior gives
a core radius of about 0.75 that of the planet, compared
to 0.49 for Venus, 0.54 for the Earth and 0.44 for Mars.
The composition of Mercury is also anomalous, the density
implies an iron=silicon ratio about twice that of the other
terrestrial planets. The state of the interior is not known;
however, the discovery of the planetary magnetic 8eld
opened up the possibility that at least the outer part of the
core is still molten, to provide the dynamo for the magnetic
8eld. This is contrary to what had been expected prior to
Mariner 10 and has led to an increasingly sophisticated
modelling of the thermal evolution of the planet to explain
the current state of the interior (Schubert et al., 1988; Spohn
et al., 2001b).
The existence of the magnetic 8eld provides a strong
constraint on Mercury’s interior. The dipole moment, about
300 nT R3M , where RM is Mercury’s radius, is relatively
weak. Energy considerations about the strength of the
dynamo, based on current interior models (Spohn et al.,
2001b), would imply a larger magnetic 8eld, maybe by as
much as one order of magnitude or more. Although some
care must be exercised in interpreting this conclusion, it
is clear that neither Mercury’s internal structure, nor the
hydrodynamic dynamo are su>ciently well understood
to explain the observations. One of the limitations of the
Mariner 10 data set is that spatial aliasing, inherent to the
?yby geometry, makes the interpretation of the structure
of the magnetic 8eld ambiguous. In particular, it is not
possible to distinguish uniquely the relative magnitudes
of the dipole and quadrupole terms of the planetary 8eld.
The diFerent interpretations of the observations, dependent
on assumptions concerning, in particular, the contribution
of magnetospheric currents (see below), are illustrated in
Fig. 3 (Connerney and Ness, 1988).
It is clear that the models of Mercury’s interior must be reinforced by more comprehensive measurements of both the
magnetic 8eld and the gravitational 8eld. A full mapping of
the magnetic 8eld is required, to derive not only the dipole
and quadrupole terms, but also higher order terms for discriminating among possible source mechanisms. High order
terms should indeed be measurable, given the likelihood that
the magnetic 8eld originates in either the outer layers of the
core or at the core-mantle boundary, as in the thermoelectric
dynamo model proposed by Stevenson (1987). A simulation based on a simple current model in Mercury’s interior
indicates that the magnetosphere and its current system affect the accuracy with which the internal 8eld is measured
from an orbiting spacecraft (Giampieri and Balogh, 2001).
Determining the coe>cients of the magnetic 8eld to order
10 or higher, for example, requires that the orbiting altitude
do not exceed a few 100 km.
The mass distribution is constrained by the ratio between
the moment of inertia around the principal axis C and the
mass M . For a homogeneous sphere, the ratio C=MR2M is 0.4;
the value deduced from the Mariner 10 ?ybys is 0.34, but
with a very large uncertainty. The quantitative constraints
on the interior structure come from the lower order terms
of the spherical harmonic expansion of the gravitational potential (Spohn et al., 2001b; Milani et al., 2001). These determine not only the diFerent moments of inertia, but also
yield, when combined with the accurate determination of
the rotation state, the Love number k2 . This complex number is de8ned by the ratio of the amplitudes and the phase
Fig. 2. Absolute density of the terrestrial planets and the Moon.
R. Grard, A. Balogh / Planetary and Space Science 49 (2001) 1395–1407
Fig. 4. Hemisphere of Mercury imaged by Mariner 10 (composite image,
no data from blank areas).
angle between the time-dependent deformation of the gravitational potential of the planet and the external potential that
leads to the deformation; in eFect, this number measures the
tidal response to the solar gravitational potential. The Love
number yields, in turn, information about the thickness of
the ?uid layer of the core and the viscosity of the mantle.
The higher orders of the gravitational potential will reveal
the presence of inhomogeneities in the mantle and at the
core-mantle interface (Milani et al., 2001).
3. Mercury’s surface
Super8cially Moon-like, Mercury’s surface is marked by
cratering and (possibly) early volcanism; evidence for early
shaping and evolution of the surface features has remained
in the absence of the kind of tectonic activity that modi8ed
the surfaces of Venus and the Earth. Only 46% of Mercury’s
surface was imaged by Mariner 10 (Fig. 4), and conclusions concerning its formation and evolution are awaiting a
complete survey. In fact, radar imaging from Earth of the
unseen side of Mercury has provided a tentative evidence
of a large volcanic dome (Harmon, 1997); no comparable
features exist on the side viewed by Mariner 10.
The Mariner 10 images have been extensively studied to
establish the cratering history (Strom and Neukum, 1988;
Neukum et al., 2001). The late heavy bombardment (LHB)
following the formation of the planets, between about 4.5
and 3:8 Ga ago, left Mercury’s surface marked by a distribution of craters similar to that of the Moon, but with notable
diFerences that remain largely unexplained, or can be explained by more than one scenario. The nature of the intercrater plains, the most frequent type of terrain on Mercury,
1399
is uncertain, although they are generally considered to be
of volcanic origin, rather than basin ejecta. Their estimated
age, 4.2–4 Ga, coincides with the period of LHB; the state
of Mercury at that time was likely to facilitate widespread
volcanism and would explain the age and extent of the intercrater plains (Spudis and Guest, 1988).
The Caloris basin is a good example (Fig. 5), when comparing the cratering (and related) features of Mercury and
the Moon. If not the largest, it is the best-preserved and
most complex impact-generated structure seen by Mariner
10. Its diameter is about 1300 km, although only about half
of it was imaged by Mariner 10. A strong similarity has
been found between the Caloris ring system and that of the
Imbrium basin on the Moon; both have the same morphology of six concentric rings, and both have smooth plains
surrounding the main basin rim. This similarity has led to
the suggestion that the smooth plains on Mercury, as the lunar maria, are of volcanic origin, 8lling in the features left
by the impacts that led to the formation of the basins. The
Caloris impact is considered to be the most important such
event in the history of Mercury. It was probably the last
major impact, su>ciently violent to have led to widespread
seismic activity and to the formation of the disturbed terrain
on the opposite side of Mercury (Spudis and Guest, 1988,
and references within).
A particular feature of Mercury is the presence of
the lobate scarps that intersect pre-existing features
(Watters et al., 2001). The lobate scarps are thought to
be thrust faults, evidence of tectonic activity and planet
contraction, as the core cooled down. It is generally considered that Mercury’s radius was reduced by more than
1 km (estimates range from 1 to about 5 km) in that process (Solomon, 1977). Although tectonics has not played a
major part in Mercury’s history, signs of activity, considered in the global history of the formation and evolution of
the surface, provide important clues to the details of that
history and also shed light, if indirectly, on processes that
occurred as the crust and mantle reached their present state
(e.g. Thomas, 1997).
Mercury’s regolith is likely to be more mature, with
smaller grain sizes and a larger proportion of glassy particles
(Langevin, 1997) than that of the Moon. All the processes
that are known in weathering planetary regoliths, such as
on the Moon, are likely to be more intense at Mercury.
These include solar radiation, occasionally impacting solar
wind particles and solar energetic particles; both the ?ux
of solar photons and energetic particles is up to an order of
magnitude higher than at the orbit of the Moon. The depth
of the regolith is unknown, but may be comparable to that
on the Moon, i.e., 5 –10 m. In the absence of an ionosphere
the role played by the regolith in closing magnetospheric
current loops is unclear, but it may still ful8l an important
function in the electric coupling between the surface and
the space environment.
The discovery of volatiles near the poles by ground-based
radar observations is a remarkable but not surprising
1400
R. Grard, A. Balogh / Planetary and Space Science 49 (2001) 1395–1407
Fig. 5. Mariner 10 image of part of Caloris Basin. The ?oor is covered with smooth plains. Ridges form concentric rings around the centre of the basin
(courtesy NASA).
fact, given the results of the Lunar Surveyor spacecraft
on the presence of water ice on the Moon. It is likely
that some craters near Mercury’s poles remain in permanent shadow and retain volatiles deposited by impacting
comets.
Remarkable progress has been achieved from the 2500
images returned by Mariner 10. New and sophisticated image processing techniques have led to interesting results in
classifying surface features but the limits of that venerable
archive have now probably been reached. The lack of spatial
resolution and the absence of spectroscopic discrimination
impede further investigations. The BepiColombo and Messenger missions, with ambitious imaging and spectroscopic
capabilities, will give opportunities to settle outstanding
issues. Geochemical probing with gamma and X-ray
detectors, as well as UV and IR imaging, will widen the
range of observables. A full coverage with overlapping
images under diFerent solar illuminations will provide us
with the 8rst global overview of Mercury’s surface.
4. The exosphere
Mercury, like the Moon, has no stable atmosphere. The
gaseous environment can be described as an exosphere. The
mean free paths of the constituents are large and the total
column density is assumed to be less than 1012 cm−3 . The
existence of six elements, O, H, He, Na, K and Ca has been
established, the 8rst three by Mariner 10, the last ones by
ground-based observations. It is highly probable that other
elements, such as Al; Fe; Mg; Si, are also present. In addition, the possible presence of volatile deposits could also
contribute further constituents, such as S and OH, to this
vestigial atmosphere.
The surface interactions possibly responsible for the
source and loss processes are illustrated in Fig. 6. The
broadening of the Na and Ca emission lines indicates that
the neutrals are hotter than the planet and are emitted with
velocities which can reach a signi8cant fraction of the escape velocity (4:2 km s−1 ). This implies that the density
pro8les of the sputtered particles are governed by scale
heights larger than those expected from a barometric law
and suggests that the emission of Na atoms is governed
by an energetic mechanism, such as ion sputtering (Killen
et al., 1999).
The density and composition of this tenuous atmosphere is
extremely variable. The total exospheric content can change
by a factor of 2 or 3 over a period of several days. The sodium
and potassium brightnesses are strongly correlated and are
more intense at subsolar longitudes. They appear to be a
function of solar activity (Potter and Morgan, 1997; Potter
et al., 1999) and interplanetary magnetic 8eld orientation
(Sarantos et al., 2001).
Other production mechanisms include photosputtering, and impact vapourisation by in-falling particles,
photon-simulated desorption and diFusion through the regolith. Possible loss processes are photoionization and
charge exchange; at high altitude, radiation pressure
also plays a role in transporting exosphere constituents
from the dayside to the nightside. Additionally, the sputtered atoms may have velocities in excess of the escape
velocity.
The formation of electron–ion pairs by photoionization results in densities which do not exceed a few cm−3 .
No ionosphere can develop in these conditions and the
height-integrated transverse conductivity of the exosphere
does not exceed 10−5 S (Lammer and Bauer, 1997),
compared with 10 –20 S in the night sector of the Earth
environment.
A conductive ionosphere provides magnetic 8eld tying
and hinders plasma convection, it also ensures current continuity by closing the upward and downward 8eld-aligned
currents associated with substorms events. The absence of an
ionosphere at Mercury therefore poses a signi8cant problem
for the circulation of the currents which sustain the shape
and dynamics of the magnetosphere.
R. Grard, A. Balogh / Planetary and Space Science 49 (2001) 1395–1407
1401
Fig. 6. Schematic representation of the sources and sinks for the exosphere (after Morgan and Killen, 1997).
Fig. 7. Modulus of the magnetic 8eld observed during the third Mariner 10 ?yby (after Ness et al., 1976).
5. The magnetosphere
The two closest ?ybys (Mercury I and III) clearly revealed the existence of a magnetosphere, the result of the
interaction of the planetary magnetic 8eld with the solar
wind (Fig. 7). The speci8c features identi8ed in the observations included bow shock and magnetopause, a distortion
of the planetary magnetic 8eld, as well as the presence of
energetic particles (e.g., Wurz and Blomberg, 2001), waves,
8eld-aligned currents (Slavin et al., 1997), and the possible
evidence for a substorm-like event (Russell et al., 1988, and
further references within).
Tentative models of the magnetopause and bow shock
have been derived on the basis of the Mariner 10 data
(Fig. 8). The most obvious diFerence between the Earth’s
and Mercury’s magnetospheres is that the relative size of the
latter is much smaller compared to the planet’s radius. This
is partly due to the smaller magnetic moment of the planet,
and partly due to the larger solar wind dynamic pressure
(Burlaga, 2001). The stand-oF distance (from the planet’s
centre) of the magnetopause has been estimated as about 1.3
RM , although a more discriminating study of the data shows
that the stand-oF distance may have been as little as 1.1 RM
during the 8rst ?yby (Engle, 1997).
The existence of the Hermean magnetosphere is not in
question, but a simple scaling of the Earth’s model may be
misleading. Due to its smaller size, the magnetized environment of Mercury is more subject to major recon8guration
and distortion on short timescales compared to the Earth’s
(Luhman et al., 1998) as observed during the two ?ybys.
Given the variability of the Sun’s activity, it is expected
that the Hermean 8eld cannot always prevent the solar wind
from impacting the surface, although MHD simulations indicate that this should be a rare event (Kabin et al., 2000).
However, the very large magnetic 8elds carried by coronal
mass ejections make such an occurrence a certainty, probably several times per solar cycle. The partial collapse of the
magnetosphere may take place more frequently, although
1402
R. Grard, A. Balogh / Planetary and Space Science 49 (2001) 1395–1407
Fig. 8. A model of the magnetosphere of Mercury (after Slavin et al., 1997; Grande et al., 2001).
our current understanding of such a phenomenon can only
be very tentative.
By scaling a model of the Earth’s magnetosphere, that
takes into account the direction of the interplanetary magnetic 8eld, it has been possible to reproduce a particularly
disturbed period in the Mariner 10 observations (Luhman
et al., 1998). It is concluded that the magnetosphere of Mercury responds directly to changing interplanetary conditions,
without the delay that is normally involved at the Earth. As
no signi8cant amount of magnetic ?ux can be stored in Mercury’s magnetotail, contrary to the Earth’s, the dynamics of
the whole system is necessarily driven by the solar wind.
The precipitation of energetic particles on the surface in the
high latitude regions, corresponding to the auroral zones on
Earth, is a potential source of X-ray radiation (Grande et al.,
2001).
A key question is the nature of the magnetospheric current systems at Mercury. The Earth’s magnetopause and tail
currents ?ow through the ionosphere, the electrically conducting layer of the outer atmosphere. In the absence of a
stable atmosphere, no ionosphere can form at Mercury. The
closure of the current loops therefore remains a major unknown; it is likely to involve the surface of the planet and
its exosphere, but it is unclear whether they can support the
rates of change of currents that are implied by dynamical
considerations.
6. Review of the scientic objectives of BepiColombo
The ESA BepiColombo Cornerstone mission consists of
the three elements shown in Fig. 9: the Mercury Planetary Orbiter (MPO), the Mercury Magnetospheric Orbiter
(MMO) and the Mercury Surface Element (MSE). The
MPO is nadir pointing while the MMO is spin stabilized;
both spacecraft are on polar orbits with resonant periods of
2:32 and 9:3 h (Fig. 10). The landing site of MSE is located
◦
at a latitude of 85 near the terminator.
The mean features of the mission are summarized in
Table 2 and the model payload carried by the three elements is recapitulated in Table 3. This con8guration was the
baseline in September 2000 and is still subject to changes.
The MMO design will be reviewed by ISAS; the two other
spacecraft elements and the whole mission concept will be
reassessed by ESA during the de8nition phase.
The overall mass of the model payload carried by the
two orbiters and the lander is 83 kg, but it does not include
the instruments marked with asterisks which have not been
taken into account during the preliminary design exercise.
This list is representative but constitutes by no means the
8nal payload, which is due to emerge from open competitive
selections in Europe and in Japan at the end of 2002 (MPO
and MSE) or in the middle of 2003 (MMO).
6.1. Mercury Planetary Orbiter
The Planetary Orbiter (MPO) carries instruments mainly
devoted to close range studies of the surface and measurements of the gravity 8eld and rotational state.
The imager system performs a global mapping of the surface at better than 200 m resolution (WAC) and explores
selected areas (up to 5% of the total surface) better than
20 m resolution (NAC); the orbital period of 2:3 h provides
a suitable shift in ground track between successive orbits.
The intercomparison of the images will greatly bene8t from
the knowledge of the topography acquired with the laser altimeter (TOP).
Mineralogical mapping is performed over a combined
spectrum which stretches almost continuously from 70 nm
R. Grard, A. Balogh / Planetary and Space Science 49 (2001) 1395–1407
1403
Fig. 9. The three spacecraft elements MPO, MMO and MSE with tethered micro-rover and subsurface penetrator.
Fig. 10. The orbits of MMO and MPO around Mercury.
up to possibly 60 m. The UV spectrometer (UVS) maps
the surface albedo and searches for oxides of elements such
as Al, S and Si, and water frost deposits in permanently
shadowed area. The range of the infrared spectrometer (IRS)
covers the absorption bands of most minerals, with a spatial resolution between 150 m and 1:25 km. Complementary
information on mineralogical composition and polar condensate deposits are obtained from the infrared radiometer
(IRR), whose principal function is to study the thermophysical properties of the surface.
The bombardment of the surface by galactic cosmic rays
and solar X-rays generates a cascade of secondary particle
which are detected by X-ray, gamma-ray and neutron spectrometers (MXS, MGS and MNS). The MXS and MGS measurements yield the surface distributions of elements such
as K, Th, U, O, Mg, Al, Si, Ca, Ti, Fe, H and C. MNS is
sensitive to gradients in the concentration of Fe, Ti and especially H, down to a depth of about 1 m, and will contribute
to the search for H2 O over the polar regions.
The elemental composition of the surface can also be
studied, though less directly and on a more global scale, with
the dust mass spectrometer (DMS) and the neutral particle
analyser (NPA). UVS will observe the limb aiglow, using an
articulated mirror for example, to determine the abundance
of elements such as Al, S, Na and OH in the exosphere. Both
UVS and NPA will study the dynamics of the exosphere, and
NPA will image the magnetospheric plasma ?ow. These two
instruments will therefore support the observations carried
out with MMO (Orsini et al., 2001; Barabash et al., 2001;
Lukyanov et al., 2001a, b).
The radio science experiments (RSE) involve many instruments: NAC, transponder and accelerometer
(Iafolla and Nozzoli, 2001) and spacecraft subsystems:
radio link, startracker, as well as the ground antennas to
determine (1) the libration and obliquity of Mercury (size
and physical state of the core), (2) the global structure of
the gravity 8eld and tidal eFects (internal structure) and (3)
the local gravitational anomalies. Correlations between the
gravity 8eld and the topography observations performed
with TOP will assess the degree of isostatic compensation
of crustal units. The relationship between the gravity 8eld
and the magnetometer (MAGP) data will contribute to a
better understanding of the origin of Mercury’s internal
magnetic 8eld.
Moreover, RSE will determine the orbital parameters of
the planet and study the propagation of electromagnetic
1404
R. Grard, A. Balogh / Planetary and Space Science 49 (2001) 1395–1407
Table 3
Possible instruments for the scienti8c payload of BepiColombo, with
typical dynamic ranges and characteristic parameters
be dedicated to the observation of NEOs provided adequate
resources are available.
Mercury Planetary Orbiter (MPO): 50 kg=60 W
6.2. Mercury Magnetospheric Orbiter
Narrow angle camera (NAC)
Wide angle camera (WAC)
Infrared mapping spectrometer (IMS)
Ultraviolet spectrometer (UVS)
X-ray spectrometer (MXS)
Gamma-ray spectrometer (MGS)
Neutron spectrometer (MNS)
Radio science experiments (RSE)
Laser altimeter∗ (TOP)
Magnetometer∗ (MAGP)
Neutral particle analyser∗ (NPA)
Dust mass spectrometer∗ (DMS)
Infrared radiometer∗ (IRR)
Near-earth object camera∗ (NET)
0:35–1 m
0:35–1 m
0:8–2:8 m
70–330 nm
0:5–10 keV
0:1–8 MeV
0:01–5 MeV
32–34 GHz
1064 nm
±4000 nT
0–100 keV
1–10 000 amu
2–60 m
18 mag
Mercury Magnetospheric Orbiter (MMO): 27 kg=29 W
Magnetometer (MAGM)
Ion spectrometer (IMS)
Electron analyser (EEA)
Cold plasma detector (CPA)
Energetic particle detector (EPD)
Search coil (RPW-H)
Electric antenna (RPW-E)
Positive ion emitter (PIE)
Camera (SCAM)
±4000 nT
50–35 keV
0–30 keV
0–50 eV
30 eV–300 keV
0:1–1 MHz
0:1–16 MHz
1–100 A
0:35–1 m
Mercury Surface Element (MSE): 6 kg=16 W
Descent camera (CLAM-D)
Surface camera (CLAM-S)
Magnetometer (MAGL)
Seismometer (SEISMO)
Alpha X-ray spectrometer (AXS)
Close-up camera∗ (CUC)
MRossbauer spectrometer∗ (MOS)
Heat ?ow and physical properties package (HP3 )
Micro-rover (MDD)
Mole (MMR)
0:3–1 m
0:3–1 m
±4000 nT
0:05–50 Hz
0:9–10 keV
0:3–1 m
5–100 keV
∗ The total mass and average power numbers do not take into account
the instruments marked with asterisks which have not been considered in
the preliminary study.
waves between Mercury and the Earth to solve for fundamental quantities such as the oblateness of the Sun, the general relativity parameters, ; and , and the time derivative
of the gravitational ‘constant’ G, with unprecedented accuracy (Iess and Boscagli, 2001).
The risk of impacts on the Earth by asteroids and comets
has stimulated world-wide eForts to detect near-earth objects (NEOs). One class of NEOs of particular interest for
BepiColombo, the inner earth objects (IEOs) have orbits
that supposedly lie entirely inside that of the Earth. IEOs are
hard to observe from the ground or from a space observatory
near the Earth; in fact no IEO has ever been detected. From
the vicinity of Mercury, on the other hand, IEOs would be
seen against a dark sky background, well lit by the Sun,
A small telescope (NET) with an aperture of 20 cm could
The Magnetospheric Orbiter (MMO) is mostly dedicated
to the study of the 8eld, wave and particles in the environment of the planet and in the solar wind. The objectives and
instrumentation which are presented here formed the basis
for the ESA preliminary study, and do not diFer markedly
from those considered by ISAS.
The magnetometer (MAGM) is an essential component
of the payload since it addresses both the planetary and magnetospheric objectives. Measurements performed simultaneously on the two orbiters with MAGM and MAGP would
help discriminating the internal 8eld of the planet from the
induced 8eld and the 8elds generated in the magnetosphere
by the interaction of the planetary 8eld with the solar wind.
A set of charged-particle detectors covers combined energy ranges of 0–300 keV for electrons (EEA, EPD) and 0–
26 MeV for ions (CPA, IMS, EPD). A positive ion emitter
(PIE) would prevent the spacecraft surface from charging
to positive potential of several tens of V, due to photoemission, and thus facilitate the detection of the low energy ions.
The ion mass spectrometer will contribute to the identi8cation of exospheric species, in synergy with UVS and NPA
on MPO (Mildner et al., 2001).
The wave receiver consists of a tri-axis search-coil magnetometer deployed on a boom (RPW–H) and a one-axis
electric antenna, made of two 35 m monopoles extended radially in the spin-plane (RPW–E).
A camera with a resolution of 10–20 m at periapsis
(SCAM) provides additional imaging information and acts
as a back-up for the imagers carried by MPO.
6.3. Mercury Surface Element
The Mercury Surface Element (MSE) will explore a sample area of the planetary surface with the maximum possible
resolution and perform local measurements against which
the data collected by the orbiters can be validated. The lander includes two tethered facilities, a soil-penetrating device
(MDD) which will reach a depth of several metres in the
regolith and a micro-rover (MMR) which can deploy instruments at selected site several tens of m away from the
lander.
The imaging system on MSE includes a descent camera
(CLAM-D) which will take its last image a few 100 m above
the surface and pinpoint the landing site, a panorama camera
(CLAM-S) to characterize the environment of the lander
and, possibly, a close-up imager (CUC) mounted on the
rover.
The alpha-X spectrometer (AXS) on MMR contains a set
of Cm-244 sources that emit energetic particles which are
backscattered or induce X-ray emission from the sample.
R. Grard, A. Balogh / Planetary and Space Science 49 (2001) 1395–1407
The X-ray mode is sensitive to Na, Mg, Al, Si, K, Ca, Fe,
P, S, Cl, Ti, Cr, Mn and Ni, the alpha mode to C and O. A
MRossbauer spectrometer (MOS), also carried by MMR will
perform a quantitative analysis of Fe-bearing materials.
The heat ?ow and physical properties package (HP3 ) is
mounted on MMR; the sensors consist of thermistors, accelerometer and radiation densitometer. HP3 measures parameters such as temperature, thermal conductivity and diffusivity, bulk density and mechanical hardness as a function
of depth (Spohn et al., 2001a).
The magnetometer (MAGL) will characterize the magnetic properties of the surface and provide a reference for
models of the intrinsic planetary 8eld. It will be possible to
derive the electric conductivity of the ground by simultaneously recording the magnetic 8eld ?uctuations on the two
orbiters and on the lander.
The seismometer (SEISMO) is tentatively considered for
recording tidal deformations and sound waves excited by
quakes in the crust and in the mantle.
7. Concluding remarks
These are some of the questions about Mercury as a planet
that form the central rationale of the Messenger and BepiColombo missions
• What will be found on the uninspected hemisphere of
Mercury?
• How did the planet evolve geologically?
• Why is Mercury’s density so high?
• What is its internal structure and is there a liquid outer
core?
• What is the origin of Mercury’s magnetic 8eld?
• What is the chemical composition of the surface?
• Is there any water ice in the polar regions?
• Which volatile materials compose the vestigial atmosphere?
• How does the planet’s magnetic 8eld interact with the
solar wind?
1405
the two missions will result in an overall return of Mercury science that greatly exceeds the sum of the scienti8c
returns form each mission without communication between
the Agencies.
Several speci8c areas of possible coordination can be
identi8ed
• Timely identi8cation of a landing site for the BepiColombo MSE. The global survey of Mercury made by
Messenger prior to the arrival of the ESA spacecraft will
enable a site to be selected that minimizes landing risk
while maximizing the scienti8c return, including ground
truth comparisons with remote-sensing data from both
missions.
• Complementary measurements of surface features from
phase angles that diFer between the two missions as a
result of diFering orbital geometries.
• Using Messenger as a “precursor” to BepiColombo for
imaging and remote sensing measurements, enabling targeted observations at higher resolution than global observations.
• Extension of the temporal baseline for fundamental
physics measurements by using ranging and other tracking data from both spacecraft.
Giuseppe Colombo (1920 –1984): The Science Programme Committee of the European Space Agency recognized the achievements of the late Giuseppe (Bepi)
Colombo of the University of Padua by adopting his name
for the Mercury cornerstone.
BepiColombo’s other possible objectives go beyond the exploration of the planet and its environment, to take advantage of Mercury’s close proximity to the Sun
• Fundamental science: is Einstein’s theory of gravity correct?
• Impact threat: what asteroids lurk on the sunward side of
the Earth?
The participation of three space agencies, ESA, ISAS and
NASA, in the “Returns to Mercury” evidences the renewed
worldwide interest in the innermost terrestrial planet and
the importance of a concerted approach to its exploration.
Potential synergies between Messenger and BepiColombo
are signi8cant and mutually bene8cial. Cooperation between
Giuseppe Colombo was a Mathematician and Engineer
of astonishing imagination. The Italian scientist explained,
as an unsuspected resonance, Mercury’s peculiar habit of
rotating three times around itself in every two revolutions
around the Sun. He also suggested to NASA how to place
Mariner 10 into an orbit that would enable the spacecraft to
perform three ?ybys of the planet Mercury in 1974 –1975.
1406
R. Grard, A. Balogh / Planetary and Space Science 49 (2001) 1395–1407
Acknowledgements
The authors wish to express their indebtedness to the
members of the BepiColombo Science Advisory Group who
provided their expertise in the multidisciplinary environment
during the study phases of the mission, to distil the scienti8c
rationale for this complex planetary mission. Special thanks
are due to Dr. Y. Langevin whose insights into the mission
design made BepiColombo a realistic mission. Thanks are
also due to Dr. M. Coradini, from the ESA Science Mission Coordination O>ce, for successfully piloting the BepiColombo mission through the complex approval cycle. The
successful System and Technology study for BepiColombo
was led by G. Scoon and M. Novara (ESA), and A. Anselmi
(Alenia Aerospazio). The authors are also grateful to Prof.
Wing Ip, the Convenor of the EGS session that led to this
collection of papers and to the authors and referees who
ensured the quality of the collection. The contributions by
the scientists and engineers of the Messenger mission is specially acknowledged.
References
Anselmi, A., Scoon, G.E.N., 2001. BepiColombo, ESA’s Mercury
cornerstone mission. Planet. Space Sci. (this issue).
Barabash, S., Lukyanov, A.V., Brandt, P.C., Lundin, R., 2001. Energetic
neutral atom imaging of the Mercury magnetosphere 3, simulated
images and instrument requirements. Planet. Space Sci. (this issue).
Burlaga, L.F., 2001. Magnetic 8elds and plasmas in the inner heliosphere:
Helios results. Planet. Space Sci. (this issue).
Colombo, G., 1965. Rotational period of the planet Mercury. Nature 208,
575.
Colombo, G., 1966. Cassini’s second and third laws. Astron. J. 71, 891.
Connerney, J.E.P., Ness, N.F., 1988. Mercury’s magnetic 8eld and interior.
In: Vilas, F., Chapman, C.R., Matthews, M.S. (Eds.), Mercury. The
University of Arizona Press, Tucson, p. 494.
Engle, I.M., 1997. Mercury’s magnetosphere: another look. Planet. Space
Sci. 45, 127.
Giampieri, G., Balogh, A., 2001. Modelling of magnetic 8eld
measurements at Mercury. Planet. Space Sci. (this issue).
Gold, R.E., Solomon, S.C., McNutt Jr., R.L., Santo, A.G., Abshire, J.B.,
Acuna, M.H., Afzal, R.S., Anderson, B.J., Andrews, G.B., Bedini,
P.D., Cain, J., Cheng, A.F., Evans, L.G., Follas, R.B., Gloeckler,
G., Goldsten, J.O., Hawkins III, S.E., Izenberg, N.R., Jaskulek, S.E.,
Ketchum, E.A., Lankton, M.R., Lohr, D.A., Mauk, B.H., McClintock,
W.E., Murchie, S.L., Schlemm II, C.E., Smith, D.E., Starr, R.D.,
Zurbhuchen, T.H., 2001. The MESSENGER mission to Mercury:
scienti8c payload. Planet. Space Sci. (this issue).
Grande, M., Dunkin, S.K., Kellett, B., 2001. Opportunities for X-ray
remote sensing at Mercury. Planet. Space Sci. (this issue).
Harmon, J.K., 1997. Mercury radar studies and lunar comparisons. Adv.
Space Res. 19, 1487.
Iafolla, V., Nozzoli, S., 2001. Italian spring accelerometer (ISA):
high sensitive accelerometer to be used in “BepiColombo” ESA
Cornerstone. Planet. Space Sci. (this issue).
Iess, L., Boscagli, G., 2001. Advanced radio science instrumentation for
the mission BepiColombo to Mercury. Planet. Space Sci. (this issue).
Kabin, K., Gombosi, T.I., DeZeeuw, D.L., Powell, K.G., 2000. Interaction
of Mercury with the solar wind. Icarus 143, 397.
Killen, M.R., Potter, A., Fitzsimmons, A., Morgan, T.H., 1999. Sodium D2
line pro8les: clues to the temperature structure of Mercury’s exosphere.
Planet. Space Sci. 47, 1449.
Lammer, H., Bauer, S.J., 1997. Mercury’s exosphere: origin of surface
sputtering and implications. Planet. Space Sci. 45, 73.
Langevin, Y., 1997. The regolith of Mercury: present knowledge and
implications for the Mercury Orbiter mission. Planet. Space Sci. 45,
31.
Luhman, J.G., Russell, C.T., Tsyganenko, N.A., 1998. Disturbances in
Mercury’s magnetosphere: are the Mariner 10 “substorms” simply
driven? J. Geophys. Res. 103, 9113.
Lukyanov, A.V., Barabash, S., Lundin, R., Brandt, P.C., 2001a. Energetic
neutral atom imaging of the Mercury magnetosphere 2, distribution of
energetic charged particles in a compact magnetosphere. Planet. Space
Sci. (this issue).
Lukyanov, A.V., Umnova, O., Barabash, S., 2001b. Energetic neutral atom
imaging of the Mercury’s magnetosphere: 1, distribution of neutral
particles in an axially symmetrical exosphere. Planet. Space Sci. (this
issue).
Mendillo, M., Warrell, J., Limaye, S.S., Baumgardner, J., Sprague, A.,
Wilson, J.K., 2001. Imaging the surface of Mercury using ground-based
telescopes. Planet. Space Sci. (this issue).
Milani, A., Rossi, A., Vokrouhlicky, D., Villani, D., Bonanno, C., 2001.
Gravity 8eld and rotation state of Mercury from the BepiColombo
Radio Science experiments. Planet. Space Sci. (this issue).
Mildner, M., Wurz, P., Scherer, S., Zipperle, M., Altwegg, K., Bochsler,
P., Benz, W., Balsiger, H., 2001. Measurement of neutral atoms and
ions in Mercury’s exosphere. Planet. Space Sci. (this issue).
Morgan, T.H., Killen, R.M., 1997. A non-stoichiometric model of the
Composition of the atmospheres of Mercury and the moon. Planet.
Space Sci. 45, 81.
Ness, N.F., Behannon, K.W., Lepping, R.P., Whang, Y.C., 1975. Magnetic
8eld of Mercury con8rmed. Nature 255, 204.
Ness, N.F., Behannon, W., Lepping, R.P., Whang, Y.C., 1976.
Observations of Mercury’s magnetic 8eld. Icarus 28, 479.
Neukum, G., Oberst, J., HoFmann, H., Wagner, R., Ivanov, B.A., 2001.
Geology evolution and cratering history of Mercury. Planet. Space Sci.
(this issue).
Novara, M., 2001. The BepiColombo Mercury surface element. Planet.
Space Sci. (this issue).
Orsini, S., Milillo, A., De Anagelis, E., Di Lellis, A.M., Zanza, V.,
Livi, S., 2001. Remote sensing of Mercury’s magnetospheric plasma
environment via energetic neutral atoms imaging. Planet. Space Sci.
(this issue).
Potter, A.E., Morgan, T.H., 1997. Sodium and potassium atmospheres of
Mercury. Planet. Space Sci. 45, 95.
Potter, A.E., Killen, R.M., Morgan, T.H., 1999. Rapid changes in the
sodium exosphere of Mercury. Planet. Space Sci. 47, 1441.
Racca, G.D., 2001. Capability of solar electric propulsion for planetary
missions. Planet. Space Sci. (this issue).
Russell, C.T., Baker, D.N., Slavin, J.A., 1988. The magnetosphere
of Mercury. In: Vilas, F., Chapman, C.R., Matthews, M.S. (Eds.),
Mercury. The University of Arizona Press, Tucson, p. 514.
Santo, A.G., Gold, R.E., McNutt Jr., R.L., Solomon, S.C., Ercol,
C.J., Farquhar, R.W., Hartka, T.J., Jenkins, J.E., McAdams, J.V.,
Mosher, L.E., Persons, D.F., Artis, D.A., Bokulic, R.S., Conde, R.F.,
Dakermanji, G., Gross Jr., M.E., Haley, D.R., Heeres, K.J., Maurer,
R.H., Moore, R.C., Rodberg, E.H., Stern, T.G., Wiley, S.R., Williams,
B.G., Yen, C.-W.L., Peterson, M.R., 2001. The MESSENGER mission
to Mercury: spacecraft and mission design. Planet. Space Sci. (this
issue).
Sarantos, M., ReiF, P.H., Hill, T.W., Killen, R.M., Urquhart, A.L., 2001.
A Bx-interconnected magnetosphere model for Mercury. Planet. Space
Sci. (this issue).
Schubert, G., Ross, N.M., Stevenson, D.J., Spohn, T., 1988. Mercury’s
Thermal History and the Generation of its Magnetic Field. In: Vilas,
F., Chapman, C.R., Matthews, M.S. (Eds.), Mercury. The University
of Arizona Press, Tucson, p. 429.
Slavin, J.A., Owen, J.C.J., Connerney, J.E.P., Christon, S.P., 1997. Mariner
10 observations of 8eld-aligned currents at Mercury. Planet. Space
Sci. 45, 133.
R. Grard, A. Balogh / Planetary and Space Science 49 (2001) 1395–1407
Solomon, S.C., 1977. The relationship between crustal tectonics and
internal evolution in the Moon and Mercury. Phys. Earth Planet. Int.
15, 135.
Solomon, S.C., McNutt Jr, R.L., Gold, R.E., Acuna, M.H., Baker, D.N.,
Boynton, W.V., Chapman, C.R., Cheng, A.F., Gloeckler, G., Head
III, J.W., Krimigis, S.M., McClintock, W.E., Murchie, S.L., Peale,
S.J., Phillips, R.J., Robinson, M.S., Slavin, J.A., Smith, D.E., Strom,
R.G., Trombka, J.I., Zuber, M.T., 2001. The MESSENGER mission to
Mercury: scienti8c objectives and implementation. Planet. Space Sci.
(this issue).
Spohn, T., Ball, A.J., Seiferlin, K., Conzelmann, V., Hagermann, A.,
Koemle, N.I., Kargl, G., 2001a. A heat ?ow and physical properties
package for the surface of Mercury. Planet. Space Sci. (this issue).
Spohn, T., Sohl, F., Wieczerkowski, K., Conzelmann, V., 2001b. The
interior structure of Mercury: what we know, what we expect from
BepiColombo. Planet. Space Sci. (this issue).
Spudis, P.D., Guest, J.E., 1988. Stratigraphy and geologic history of
Mercury. In: Vilas, F., Chapman, C.R., Matthews, M.S. (Eds.),
Mercury. The University of Arizona Press, Tucson, p. 118.
1407
Stevenson, D.J., 1987. Mercury’s Magnetic Field: A Thermoelectric
Dynamo?. Earth Planet. Sci. Lett. 82, 114–120.
Strom, R.G., Neukum, G., 1988. The cratering record on Mercury and the
origin of impacting objects. In: Vilas, F., Chapman, C.R., Matthews,
M.S. (Eds.), Mercury. The University of Arizona Press, Tucson,
p. 336.
Thomas, P.G., 1997. Are there other tectonics than tidal despinning,
global contraction and Caloris related events on Mercury? A review
of questions and problems. Planet. Space Sci. 45, 3.
Warell, J., Limaye, S.S., 2001. Properties of the hermean regolith: I,
global regolith albedo variation at 200 km scale from multicolor CCD
imaging. Planet. Space Sci. (this issue).
Watters, T.R., Robinson, M.S., Cook, A.C., 2001. Large-scale lobate
scarps in the southern hemisphere of Mercury. Planet. Space Sci. (this
issue).
Wurz, P., Blomberg, L., 2001. Particle populations in Mercury’s
magnetosphere. Planet. Space Sci. (this issue).