The Messenger Mission to Mercury

643 Pages·2007·48.548 MB·English

by  Sean C. Solomon, Ralph L. McNutt Jr., Robert E. Gold (auth.), D. L. Domingue, C. T. Russell (eds.)

D.L Domingue (cid:12)9 C.T. Russell
srotidE
The MESSENGER Mission
to Mercury
Foreword by D.L. Domingue and C.T. Russell
published Previously ni Space Science Reviews Volume ,131
Issues 1-4, 2007
regnirpS
D.L. Domingue C.T. Russell
The Johns Hopkins University Institute of GeOphysics & Planetary Physics
Applied Physics Laboratory University of California
Laurel, MD, USA Los Angeles, CA, USA
Cover illustration: Artist's rendition of the MESSENGER spacecraft orbiting Mercury.
Copyright (cid:14)9 2007 The Johns Hopkins University / Applied Physics Laboratory.
All rights reserved.
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(cid:14)9 2007 Springer Science+Business Media, LLC
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Contents
Foreword
D.L. Domingue (cid:12)9 C.T. Russell 1
MESSENGER Mission Overview
S.C. Solomon (cid:12)9 R.L. McNutt (cid:12)9 R.E. Gold (cid:12)9 D.L. Domingue 3
The Geology of Mercury: The View Prior to the MESSENGER Mission
J.W. Head (cid:12)9 C.R. Chapman (cid:12)9 D.L. Domingue (cid:12)9 S.E. Hawkins (cid:12)9 W.E. McClintock (cid:12)9
S.L. Murchie (cid:12)9 L.M. Prockter (cid:12)9 M.S. Robinson (cid:12)9 R.(3. Strom (cid:12)9 T.R. Watters 41
MESSENGER and the Chemistry of Mercury's Surface
W.V. Boynton (cid:12)9 A.L. Sprague (cid:12)9 S.C. Solomon (cid:12)9 R.D. Starr (cid:12)9 L.(3. Evans (cid:12)9 W.C. Feldman (cid:12)9
J.I. Trombka (cid:12)9 E.A. Rhodes 85
The Geophysics of Mercury: Current Status and Anticipated Insights from the
MESSENGER Mission
M.T. Zuber (cid:12)9 O. Aharonson (cid:12)9 J.M. Aurnou (cid:12)9 A.F. Cheng (cid:12)9 S.A. Hauck (cid:12)9 M.H. Heimpel (cid:12)9
(3.A. Neumann (cid:12)9 S.J. Peale (cid:12)9 R.J. Phillips (cid:12)9 D.E. Smith (cid:12)9 S.C. Solomon (cid:12)9 S. Stanley 105
MESSENGER: Exploring Mercury's Magnetosphere
J.A. Slavin (cid:12)9 S.M. Krimigis (cid:12)9 M.H. Acufia (cid:12)9 B.J. Anderson (cid:12)9 D.N. Baker (cid:12)9 P.L. Koehn (cid:12)9
H. Korth (cid:12)9 S. Livi (cid:12)9 B.H. Mauk (cid:12)9 S.C. Solomon (cid:12)9 T.H. Zurbuchen 133
Mercury's Atmosphere: A Surface-Bounded Exosphere
D.L. Domingue (cid:12)9 P.L. Koehn (cid:12)9 R.M. Killen (cid:12)9 A.L. Sprague (cid:12)9 M. Sarantos (cid:12)9 A.E Cheng (cid:12)9
E.T. Bradley (cid:12)9 W.E. McClintock 161
The MESSENGER Spacecraft
J.C. Leary (cid:12)9 R.E Conde (cid:12)9 (3. Dakermanji (cid:12)9 C.S. Engelbrecht (cid:12)9 C.J. Ercol (cid:12)9
K.B. Fielhauer (cid:12)9 D.(3. Grant (cid:12)9 T.J. Hartka (cid:12)9 T.A. Hill (cid:12)9 S.E. Jaskulek (cid:12)9 M.A. Mirantes (cid:12)9
L.E. Mosher. M.V. Paul. D.E Persons. E.H. Rodberg. D.K. Srinivasan (cid:12)9
R.M. Vaughan (cid:12)9 S.R. Wiley 187
MESSENGER Mission Design and Navigation
J.V. McAdams (cid:12)9 R.W. Farquhar (cid:12)9 A.H. Taylor (cid:12)9 B.(3. Williams 219
The Mercury Dual Imaging System on the MESSENGER Spacecraft
S.E. Hawkins. J.D. Boldt. E.H. Darlington. R. Espiritu. R.E. Gold. B. Gotwols (cid:12)9
M.E Grey (cid:12)9 C.D. Hash (cid:12)9 J.R. Hayes (cid:12)9 S.E. Jaskulek (cid:12)9 C.J. Kardian (cid:12)9 M.R. Keller (cid:12)9
E.R. Malaret (cid:12)9 S.L. Murchie (cid:12)9 EK. Murphy (cid:12)9 K. Peacock (cid:12)9 L.M. Prockter (cid:12)9 R.A. Reiter (cid:12)9
M.S. Robinson (cid:12)9 E.D. Schaefer (cid:12)9 R.G. Shelton (cid:12)9 R.E. Sterner (cid:12)9 H.W. Taylor (cid:12)9
T.R. Watters (cid:12)9 B.D. Williams 247
The MESSENGER Gamma-Ray and Neutron Spectrometer
J.O. (3oldsten. E.A. Rhodes. W.V. Boynton. W.C. Feldman. D.J. Lawrence.
J.I. Trombka. D.M. Smith. L.(3. Evans. J. White-N.W. Madden-P.C. Berg-
(3.A. Murphy. R.S. (3urnee (cid:12)9 K. Strohbehn. B.D. Williams-E.D. Schaefer-
C.A. Monaco (cid:12)9 C.P. Cork (cid:12)9 J. Del Eckels (cid:12)9 W.O. Miller (cid:12)9 M.T. Burks (cid:12)9 L.B. Hagler (cid:12)9
S.J. DeTeresa (cid:12)9 M.C. Witte 339
The X-Ray Spectrometer on the MESSENGER Spacecraft
C.E. Schlemm (cid:12)9 R.D. Starr (cid:12)9 (3.C. Ho (cid:12)9 K.E. Bechtold (cid:12)9 S.A. Hamilton (cid:12)9 J.D. Boldt (cid:12)9
W.V. Boynton. W. Bradley. M.E. Fraeman. R.E. Gold. J.O. (3oldsten. J.R. Hayes-
S.E. Jaskulek (cid:12)9 E. Rossano (cid:12)9 R.A. Rumpf (cid:12)9 E.D. Schaefer (cid:12)9 K. Strohbehn (cid:12)9 R.(3. Shelton (cid:12)9
R.E. Thompson (cid:12)9 J.I. Trombka (cid:12)9 B.D. Williams 393
The Magnetometer Instrument on MESSENGER
B.J. Anderson (cid:12)9 M.H. Acufia (cid:12)9 D.A. Lohr (cid:12)9 J. Scheifele (cid:12)9 A. Raval (cid:12)9 H. Korth (cid:12)9
J.A. Slavin 417
The Mercury Laser Altimeter Instrument for the MESSENGER Mission
J.F. Cavanaugh (cid:12)9 J.C. Smith (cid:12)9 X. Sun (cid:12)9 A.E. Bartels (cid:12)9 L. Ramos-Izquierdo (cid:12)9 D.J. Krebs (cid:12)9
J. McGarry- R. Trunzo- A.M. Novo-Gradac. J.L. Britt. J. Karsh. R.B. Katz.
A.T. Lukemire (cid:12)9 R. Szymkiewicz (cid:12)9 D.L. Berry (cid:12)9 J.E Swinski (cid:12)9 G.A. Neumann (cid:12)9
M.T. Zuber (cid:12)9 D.E. Smith 451
The Mercury Atmospheric and Surface Composition Spectrometer for the
MESSENGER Mission
W.E. McClintock (cid:12)9 M.R. Lankton 481
The Energetic Particle and Plasma Spectrometer Instrument on the MESSENGER
Spacecraft
(3.B. Andrews (cid:12)9 T.H. Zurbuchen (cid:12)9 B.H. Mauk (cid:12)9 H. Malcom (cid:12)9 L.A. Fisk (cid:12)9 (3. (31oeckler (cid:12)9
(3.C. Ho (cid:12)9 J.S. Kelley (cid:12)9 P.L. Koehn (cid:12)9 T.W. LeFevere (cid:12)9 S.S. Livi (cid:12)9 R.A. Lundgren (cid:12)9
J.M. Raines 523
The Radio Frequency Subsystem and Radio Science on the MESSENGER Mission
D.K. Srinivasan (cid:12)9 M.E. Perry (cid:12)9 K.B. Fielhauer (cid:12)9 D.E. Smith (cid:12)9 M.T. Zuber 557
Launch and Early Operation of the MESSENGER Mission
M.E. Holdridge (cid:12)9 A.B. Calloway 573
The MESSENGER Science Operations Center
H.L. Winters (cid:12)9 D.L. Domingue (cid:12)9 T.H. Choo (cid:12)9 R. Espiritu (cid:12)9 C. Hash (cid:12)9 E. Malaret (cid:12)9
A.A. Mick (cid:12)9 J.P. Skura (cid:12)9 J. Steele 601
ecapS icS veR (2007) 131:1-2
IOD ls/7001.01 2-5729-700-4121
Foreword
D.L. Domingue (cid:12)9 C.T. Russell
dehsilbuP :enilno 12 September 7002
(cid:14)9 regnirpS ssenisuB+ecneicS Media .V.B 7002
Fifteenth and sixteenth century explorers conquered the oceans of this world with wooden
sailing ships, reaching every corner of the globe by relying on the precarious nature of the
winds and their strength of will. These were long, difficult journeys oftentimes in harsh
environments. Success meant great rewards both financially, by opening new trade routes,
and scientifically, by making discoveries that still benefit us today. Twentieth and twenty-
first century explorers now sail the vast emptiness of space, making new discoveries amongst
the stars their ancestors used for navigation. These ventures are difficult, and they are just as
costly as they were to the coffers of seafaring nations five centuries ago. Yet we still pursue
them, driven to expanding the boundaries of our world and trusting that these voyages will
bring home scientific riches, not least of which is a new and deeper understanding of our
planetary ancestral roots.
Ancient explorers would return home with wondrous tales and artifacts from exotic ports
of call. Our spacecraft make ports of call at the planets themselves, returning tales and
wonders in the information and data they send home. Like ancient Mariners before it, the
MESSENGER spacecraft braves its own set of harsh environments to visit such ports of
call as Venus and Mercury, the innermost and most forbidding of the terrestrial planets.
As a second-generation explorer of this region, MESSENGER does not simply pass by its
ultimate target, Mercury, but it establishes a long-term presence in orbit, perhaps paving the
way for more ambitious settlement later.
This volume describes the MESSENGER mission to Mercury and our present under-
standing of this exotic, alien land beginning with an overview of the mission by the princi-
pal investigator (S.C. Solomon et al.). It is followed by articles on the geology (J. Head, III
et al.), surface geochemistry (W. Boynton et al.), surface and interior geophysical prop-
erties (M.T. Zuber et al.), the magnetosphere (J.A. Slavin et al.), and the atmosphere
D.L. eugnimoD
ehT Hopkins Johns ytisrevinU Physics Applied ,yrotarobaL Laurel, DM 20723, ASU
.T.C Russell (t~)
ytisrevinU of ,ainrofilaC soL Angeles, AC 90095, ASU
:liam-e ude.alcu.ppgi@llessurtc
1~ regnirpS
2 .L.D ,eugnimoD .T.C llessuR
(D. Domingue et al.). The mission to Mercury is no less intriguing than the target. The
spacecraft has to operate in extremely harsh thermal and solar environments and the navi-
gation of the interplanetary trade winds involves as much art as science .J( Leary et al. and
J. McAdams et al., respectively). While the brains and brawn of such missions are in the
spacecraft, the heart and soul reside within the payload. The payload is comprehensive, as
befits the multifaceted nature of Mercury and its environment. The dual imaging system
(S. Hawkins et al.) will return images of the surface never before seen by a spacecraft. The
gamma ray and neutron spectrometer .J( Goldsten et al.) along with the X-ray spectrome-
ter .C( Schlemm et al.) will provide the first information about the elemental chemistry of
the Mercurian surface. The MESSENGER magnetometer (B. Anderson et al.) will map the
magnetosphere only glimpsed by Mariner .01 The laser altimeter has been designed .J( Ca-
vanaugh et al.) to provide topographic information that will be used to help unravel the
mysteries of Mercury's surface evolution. The atmosphere and surface composition spec-
trometer .W( McClintock and M. Lankton) will provide the first in situ measurements of
the atmosphere and the first high spatial mineral maps of the surface. The energetic particle
and plasma spectrometer (G. Andrews et al.) will provide insight into the space environment
and the intricate connections between solar particles, magnetosphere, atmosphere, and sur-
face properties. And, as is traditional, the last science system to be described is the radio
system (D. Srinivasan et al.) that provides the gravity science needed to understand the evo-
lution of the planet's interior. The operation of this mission (M. Holdridge and A. Calloway)
is a complex balancing of subsystem operations and constraints that guide the spacecraft
through the harsh environment to its final destination and goal. Science operations .H( Win-
ters et al.) describes how the glorious tales of the journey, captured through the observations
and measurements of the spacecraft payload, will be disseminated and retold for generations
to come.
The success of this volume is due to many people, but first of all the editors wish to
thank the authors who had the difficult job of distilling the thousands of documents and the
millions of facts such missions produce into highly readable documents. The editors also
benefited from an excellent group of referees who acted as a test readership, refining the
manuscripts provided by the authors. These referees included: .T Armstrong, R. Arvidson,
.W Baumjohann, M. Bielefeld, D. Blewett, D. Blaney, D. Byrnes, A. Cheng, U. Christensen,
.T Cole, A. Dombard, W.C. Feldman, K.H. Glassmeier, J. Green, .S Joy, K. Klaasen, A.
Konopliv, .J Longuski, .W Magnes, A. Matsuoka, .T McCoy, L. Nittler, .T Perron, T.H. Pret-
tyman, M. Ravine, G. Schubert. M. Smith, H. Spence, .P Spudis, V.C. Thomas, E Vilas,
J. Witte, and D. Yeomans. The MESSENGER PI, .S Solomon, also provided excellent re-
views and helped to mold this issue into a consistent view of the mission. Equally important
has been the strong support this project received at Springer and the extra effort expended
by Fiona Routley, Randy Cruz, and Harry B lom. At UCLA we were skillfully assisted by
Marjorie Sowmendran who acted as the interface between the editors, the authors, and the
publishers.
regnirpS
Space Sci Rev (2007) 131" 3-39
DOI 10.1007/s11214-007-9247-6
MESSENGER Mission Overview
Sean C. Solomon. Ralph L. McNutt, Jr. (cid:12)9
Robert E. Gold. Deborah L. Domingue
Received: 9 January 2007 / Accepted: 31 July 2007 / Published online: 5 October 2007
(cid:14)9 Springer Science+Business Media B.V. 2007
Abstract The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MES-
SENGER) spacecraft, launched on August ,3 2004, is nearing the halfway point on its voy-
age to become the first probe to orbit the planet Mercury. The mission, spacecraft, and
payload are designed to answer six fundamental questions regarding the innermost planet:
(1) What planetary formational processes led to Mercury's high ratio of metal to silicate?
(2) What is the geological history of Mercury? (3) What are the nature and origin of Mer-
cury's magnetic field? (4) What are the structure and state of Mercury's core? (5) What
are the radar-reflective materials at Mercury's poles? (6) What are the important volatile
species and their sources and sinks near Mercury? The mission has focused to date on com-
missioning the spacecraft and science payload as well as planning for flyby and orbital
operations. The second Venus flyby (June 2007) will complete final rehearsals for the Mer-
cury flyby operations in January and October 2008 and September 2009. Those flybys will
provide opportunities to image the hemisphere of the planet not seen by Mariner ,01 obtain
high-resolution spectral observations with which to map surface mineralogy and assay the
exosphere, and carry out an exploration of the magnetic field and energetic particle distri-
bution in the near-Mercury environment. The orbital phase, beginning on March ,81 2011,
is a one-year-long, near-polar-orbital observational campaign that will address all mission
goals. The orbital phase will complete global imaging, yield detailed surface compositional
and topographic data over the northern hemisphere, determine the geometry of Mercury's
internal magnetic field and magnetosphere, ascertain the radius and physical state of Mer-
cury's outer core, assess the nature of Mercury's polar deposits, and inventory exospheric
neutrals and magnetospheric charged particle species over a range of dynamic conditions.
Answering the questions that have guided the MESSENGER mission will expand our un-
derstanding of the formation and evolution of the terrestrial planets as a family.
S.C. Solomon (~)
Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015,
USA
e-mail: scs @ dtm.ciw.edu
R.L. McNutt, .rJ (cid:12)9 R.E. Gold (cid:12)9 D.L. Domingue
The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
Springer
4 S.C. Solomon et al.
Keywords Mercury (cid:12)9 MESSENGER (cid:12)9 Planet formation (cid:12)9 Geological history (cid:12)9
Magnetosphere. Exosphere
1 Introduction
Mercury is the least studied of the inner planets. A substantially improved knowledge of
the planet Mercury is nonetheless critical to our understanding of how the terrestrial planets
formed and evolved. Determining the surface composition of Mercury, a body with a ratio
of metal to silicate higher than any other planet or satellite, will provide a unique window on
the processes by which planetesimals in the primitive solar nebula accreted to form planets.
Documenting the global geological history will elucidate the roles of planet size and solar
distance as governors of magmatic and tectonic history for a terrestrial planet. Character-
izing the nature of the magnetic field of Mercury and the size and state of Mercury's core
will allow us to generalize our understanding of the energetics and lifetimes of magnetic
dynamos, as well as core and mantle thermal histories, in solid planets and satellites. De-
termining the nature of the volatile species in Mercury's polar deposits, atmosphere, and
magnetosphere will provide critical insight into volatile inventories, sources, and sinks in
the inner solar system.
MESSENGER is a MErcury Surface, Space ENvironment, GEochemistry, and Rang-
ing mission designed to achieve these aims. As part of the Discovery Program of the
U.S. National Aeronautics and Space Administration (NASA), the MESSENGER space-
craft will orbit Mercury for one Earth year after completing three flybys of that planet
following two flybys of Venus and one of Earth. The Mercury flybys will return signif-
icant new data early in the mission, while the orbital phase, guided by the flyby data,
will enable a focused scientific investigation of the innermost planet. Answers to key
questions about Mercury's high density, crustal composition and structure, volcanic his-
tory, core structure, magnetic field generation, polar deposits, atmosphere, overall volatile
inventory, and magnetosphere will be provided by an optimized set of seven miniatur-
ized scientific instruments. In this paper we first describe the rationale for and scien-
tific objectives of the MESSENGER mission. We then summarize the mission imple-
mentation plan designed to satisfy those objectives. Companion papers in this issue pro-
vide detailed descriptions of the MESSENGER spacecraft (Leary et al. 2007) and mis-
sion design (McAdams et al. 2007), mission (Holdridge and Calloway 2007) and sci-
ence operations centers (Winters et al. 2007), payload instruments (Anderson et al. 2007;
Andrews et al. 2007; Cavanaugh et al. 2007; Goldsten et al. 2007; Hawkins et al. 2007;
McClintock and Lankton 2007; Schlemm et al. 2007), and radio science (Srinivasan et al.
2007), as well as more expansive summaries of the principal scientific issues to be addressed
by a Mercury orbiter mission (Boynton et al. 2007; Domingue et al. 2007; Head et al. 2007;
Slavin et al. 2007; Zuber et al. 2007).
2 Context for MESSENGER Selection
The selection of MESSENGER as a NASA Discovery Program mission was a decision
rooted in a 25-year history of Mercury exploration and strategic planning for improving our
understanding of the inner planets.
The only spacecraft to visit Mercury to date was Mariner .01 In the course of three
flybys of the planet in 1974 and 1975, Mariner 01 imaged about 45% of Mercury's surface
Springer
MESSENGER Mission Overview 5
Fig. 1 Mosaic of images of
Mercury obtained by the Mariner
01 spacecraft on the incoming
portion of its first flyby of
Mercury (Robinson et al. 1999)
(Fig. )1 at an average resolution of about 1 km and less than %1 of the surface at better than
500-m resolution (Murray 1975). Mariner 01 discovered the planet's internal magnetic field
(Ness et .la 1974, 1975); measured the ultraviolet signatures of H, He, and O in Mercury's
tenuous atmosphere (Broadfoot et al. 1974, 1976); documented the time-variable nature of
Mercury's magnetosphere (Ogilvie et al. 1974; Simpson et al. 1974); and determined some
of the physical characteristics ofM ercury's surface materials (Chase et al. 1974).
Immediately following the Mariner 01 mission, a Mercury orbiter was widely recog-
nized sa the obvious next step in the exploration of the planet (COMPLEX 1978). Further,
the primary objectives of such an orbiter mission were defined: "to determine the chemical
composition of the planet's surface on both a global and regional scale, to determine the
structure and state of the planet's interior, and to extend the coverage and improve the reso-
lution of orbital imaging" (COMPLEX 1978). In the late 1970s, however, it was thought that
the change in spacecraft velocity required for orbit insertion around Mercury was too large
for conventional propulsion systems, and this belief colored the priority placed on further
exploration of the innermost planet (COMPLEX 1978).
In the mid-1980s, about a decade after the end of the Mariner 01 mission, multi-
ple gravity-assist trajectories were discovered that could achieve Mercury orbit insertion
with chemical propulsion systems (Yen 1985, 1989). This finding stimulated detailed stud-
ies of Mercury orbiter missions in Europe and the United States between the mid-1980s
and early 1990s (Neukum et al. 1985; Belcher et al. 1991). During the same time in-
terval there were important discoveries made by ground-based astronomy, including the
Na and K components of Mercury's atmosphere (Potter and Morgan 1985, 1986) and the
radar-reflective deposits at Mercury's north and south poles (Harmon and Slade 1992;
Slade et al. 1992). A re-examination of the primary objectives of a Mercury orbiter mis-
sion during that period affirmed those defined earlier and added "that characterization of
Springer
6 S.C. Solomon et al.
Mercury's magnetic field be [an additional] primary objective for exploration of that planet"
(COMPLEX 1990).
In the early 1990s, after re-examining its approach to planetary exploration, NASA ini-
tiated the Discovery Program, intended to foster more frequent launches of less costly,
more focused missions selected on the basis of rigorous scientific and technical compe-
tition. Mercury was the target of a number of early unsuccessful proposals to the Dis-
covery Program for flyby and orbiter missions (Nelson et al. 1994; Spudis et al. 1994;
Clark et al. 1999). The MESSENGER concept was initially proposed to the NASA Dis-
covery Program in 1996, and after multiple rounds of evaluation (McNutt et al. 2006) the
mission was selected for flight in July 1999.
In parallel with the selection, development, and launch of MESSENGER, the European
Space Agency (ESA) and the Institute of Space and Astronautical Science (ISAS) of the
Japan Aerospace Exploration Agency (JAXA) have approved and are currently developing
the BepiColombo mission to send two spacecraft into Mercury orbit (Grard et al. 2000;
Anselmi and Scoon 2001). BepiColombo was selected by ESA as its fifth cornerstone
mission in 2000, and ISAS announced its intent to collaborate on the project that same
year. The two spacecraft, scheduled for launch on a single rocket in 2013, will be in
coplanar polar orbits. An ESA-supplied Mercury Planetary Orbiter will emphasize obser-
vations of the planet, and an ISAS-supplied Mercury Magnetospheric Orbiter will em-
phasize observations of the magnetosphere and its interactions with the solar wind. Pay-
load instruments on the two spacecraft were selected in 2004 (Hayakawa et al. 2004;
Schulz and Benkhoff 2006).
3 Guiding Science Questions
The MESSENGER mission was designed to address six key scientific questions, the answers
to which bear not only on the nature of the planet Mercury but also more generally on the
origin and comparative evolution of the terrestrial planets as a class.
1.3 What Planetary Formational Processes Led to the High Ratio of Metal to Silicate in
Mercury?
Mercury's uncompressed density (about 5.3 Mg/m3), the highest of any planet, has long
been taken as evidence that iron is the most abundant contributor to the bulk composition.
Interior structure models in which a core has fully differentiated from the overlying silicate
mantle indicate that the core radius is approximately 75% of the planetary radius and the
fractional core mass is about 60% if the core is pure iron (Siegfried and Solomon 1974);
still larger values are possible if the core has a light element such as sulfur alloyed with the
iron (Harder and Schubert 2001). Such a metallic mass fraction is at least twice that of the
Earth (Fig. 2), Venus, or Mars.
Calculations of dynamically plausible scenarios for the accretion of the terrestrial planets
permit a wide range of outcomes for Mercury. Given an initial protoplanetary nebular disk
of gas and dust, planetesimals accrete to kilometer size in 10 4 years (Weidenschilling and
Cuzzi 1993), and runaway growth of planetary embryos of Mercury- to Mars-size accrete
by the gravitational accumulation of planetesimals in 501 years (Kortenkamp et al. 2000).
During runaway growth, Mercury-size bodies can experience substantial migrations of their
semimajor axes (Wetherill 1988). Further, each of the terrestrial planets probably formed
from material originally occupying a wider ange in solar distance, although some correlation
Springer