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Status of Planetary Science in 1995
OVERVIEW
This chapter begins with two sections summarizing the ac-
complishments of solar system exploration over the three decades
from 1965 to 1995, and the expected scientific questions as of the
end of that period. The remaining sections constitute a much
more detailed status report for individual classes of objects, with
further discussions of open questions. All this material supports,
and leads to, the program of missions for the period 1995 to 2015
presented in Chapter 4.
State of Planetary Exploration as of 1995
Among the high points already attained or anticipated for the
first three decades of planetary study ending in 1995 are:
. Mercury: Characterization of physiographic provinces for
half the surface; discovery of a planetary magnetic field.
19
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. Venus: Establishment of atmospheric and cloud composi-
tion; characterization of the high-temperature surface environ-
ment; preliminary elemental analysis of surface material from
landers; study of solar wind interaction; determination of global to-
pography and gravitational field; characterization of physiographic
provinces from radar images.
. Moon: Determination of detailed geological history, chro-
nology, and geochemistry of major geological provinces; detailed
study of selected samples of surface material; investigation of cra-
tering, regolith formation, and interaction of the surface with the
solar wind for an airless body; discovery of remanent magnetic
fields; seismic characterization; measurement of heat flow; de-
termination of composition of the solar wind, both present and
ancient. (By 1995, global surface mapping should be achieved or
under way.3
. Mars: Near-global mapping of topography, gravity field,
and thermal properties; establishment of geological diversity (vol-
canoes, canyon lands, polar terrains, etch; discovery of evidence
for former extensive surface water (e.g., valley and channel net-
works); preliminary surface chemical analysis from landers; estate
lishment of structure and chemical and isotopic composition of the
atmosphere; determination of geological processes and a relative
chronology; study of local and global meteorology over three mar-
tian years from landers and orbiters; search for microbial life and
organic compounds (yielding negative results). (By 1995, global
characterization- morphology, elemental distributions, and some
mineralogy—of surface units is expected.)
. Jupiter system: Study of atmospheric composition and
circulation; detailed composition and structure of atmosphere and
clouds from direct entry probe; discovery of atmospheric lightning
and auroras; detailed characterization of the magnetic field and
the magnetosphere (sources and sinks, plasma processes); study of
the lo plasma torus and of the interactions between this satellite
and the magnetosphere; discovery and characterization of the To
volcanoes and interior heat flow; discovery of the ring and several
small satellites; comparative studies of icy and rocky planetary oh
jects. (The Galileo orbiter will carry out detailed global mapping
of the large Galilean satellites and continue efforts in many of the
other areas mentioned above, especially the torus and magneto-
sphere. The probe will carry out a detailed sounding of Jupiter's
atmosphere and clouds.)
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~ Saturn system: Initial global study of Saturn and its mag-
netosphere; establishment of atmospheric composition differences
between Jupiter and Saturn; detailed study of the ring system and
investigation of new dynamical phenomena; discovery of several
new satellites, including previously unknown orbital configura-
tions; measurements of the composition and structure of the at-
mosphere and clouds of Titan; low-resolution mapping of satellite
surfaces, except Titan.
~ Uranus: Results of Voyager flyby (1986~. (Initial discov-
eries include a strong magnetic field with a large inclination and
remarkably diverse geology on several of the satellites.)
Neptune: Results of Voyager flyby (1989~.
Comets: Results of Halley flybys (1986), including imaging
of the nucleus, and exploration of the proximate environment.
Also, deployment of planned comet rendezvous missions.
~ Asteroids: Results of Galileo flyby of a selected asteroid
and of planned flybys by the Comet Rendezvous mission.
. Meteorites: Evidence for early magnetic field, late addi-
tions of material with differing nucleosynthetic histories, wide-
spread high-temperature events In the solar nebula; many exam-
ples of core formation In small bodies, basaltic volcanism, extrater-
restrial synthesis of arn~no acids; discovery of meteorites from the
Moon and possibly Mars.
Other Planetary Systems: Discovery that many stars are
surrounded by dust clouds or disks, and imagery of one such disk;
discovery of a star with a planet-like companion. (Many follow-up
studies are expected by 1995.)
Applications of these results to the study of planetary origin
and evolution include:
~ Establishment of the age of the solar system as 4.6 bil-
lion years by analysis of radioactive decay products in the Earth,
meteorites, and lunar samples.
~ Dating of the late stages of accretion of the Moon (and
presumably the other terrestrial planets) as 3.7 billion years ago,
although most of the mass was probably accumulated within the
first 107 or 108 years.
~ Determination of a geological chronology for the Moon,
with the final major stages of lunar volcanism measured at 3 billion
years ago; establishment of the current rate for impact cratering
in the Earth-Moon system.
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~ Comparative studies of geological processes on the terres-
trial planets and the icy satellites of the outer solar system, includ-
ing impact cratering, volcanic and tectonic activity, and erosional
and depositional processes.
~ Preliminary study of the development and evolution of
planetary crusts in planets of different compositions and internal
structures, with insight into the role of tectonics and magmat~m
in the formation of the crust and interior of the Earth and other
planets.
~ Inference that the great bulk of the atmospheres of Earth,
Mars, and Venus are all secondary, that is, degassed from the
interior or acquired late in accretion, and not remnants of the gas
from the solar nebula.
.
Discovery of unique and as yet unexplained abundances of
noble gases (total amounts, relative amounts, and isotopic ratios)
on Earth, Mars, and Venus.
Discovery of a large (100 times) enrichment of deuterium
on Venus compared with Earth. Venus must have started out with
much more water (or vapor) than it has now, and a Runaway
greenhouses may have caused most of it to be lost.
Discovery that all terrestrial bodies have experienced dif-
ferentiation, with accompanying volcanism and tectonics, but wit
differences in history from one planet to another.
~ Discovery of the uniquely high levels of volcanic activity
on To, and preliminary characterization of volcanism based on
different physical-chemical systems than had been encountered
in the terrestrial planets. In the Saturn system, resurfacing on
Enceladus represents yet another example of such volcanic activity.
. Discovery of unexpected complexity in the rings of Saturn
and Uranus (e.g., the presence of shepherd satellites, of spiral
density waves, and of bending waves), providing important insights
into the dynamics of self-gravitating spinning disks.
~ In situ investigation of plasma processes of wide astrophy~
ical application in the huge magnetospheres of Jupiter and Saturn.
. The determination of the composition of Jupiter's atmo-
sphere, which is expected to be representative of the composition
of the solar nebula, especially for hydrogen and the noble gases.
The abundances that will be determined by the instruments on
the Galileo probe will probably become the standard for solar
composition.
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23
In supporting future investigations, an essential contribution
will be made by theorists who endeavor to mode! the natural evm
lution of gas-dust disks into stars and their associated planetary
bodies. Theoretical investigations of the early stages of this evolu-
tion begin with numerical and analytic modeling of star formation,
in particular, the conditions under which single stars like the Sun
can form. Study of the later stages of this evolution emphasizes
modeling the manner and time scale for the accumulation of dust
into planetesimals, and the subsequent accumulation of these plan-
etesimals into planetary cores of silicates, metal, and ices. In the
case of at least Jupiter and Saturn, the final stage of formation
involved the gravitational capture of massive envelopes from the
gas of the disk.
Between now and 1995 we can expect that continuing progress
will be made in this field, most likely without the help of crucial
observations or sudden theoretical breakthroughs. However, in
the absence of a new generation of observational facilities that
permit higher resolution imaging of other protostelIar systems, it
is quite possible that in the next decade we will not address the
first-order questions required to make substantive progress. On
the other hand, we can look forward to a significant refinement
ant] enhancement of theoretical understanding concerning many
aspects of nebular evolution. Much of this progress in theoretical
understanding is contingent upon the availability of computational
resources of continually greater power.
If the first asteroidal flybys occur during the next decade, we
can expect to begin to be able to place the great wealth of me-
teoritical data into a planetological context. We can also expect
that basic information regarding early solar system history will
continue to flow from laboratory study of meteorites and strato-
spheric collection of interstellar particles. In this connection, it
should be pointed out that, to a large extent, the current laW
oratory instrumentation used in this work was obtained during
lunar sample analysis during the 1960s and early 1970s, and that
attention must be given to modernizing the laboratories in which
this work is done.
Scientific Questions as of 1995
Fundamental questions in planetary science will remain much
the same in 1995 as they are today, but new knowledge and new
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capabilities will alter our view of how to approach them. First,
the reconnaissance and exploration of the solar system will by no
means be completed. Saturn and Titan are already ripe for in situ
investigation and study of interactions among the magnetosphere,
rings, and other satellites. Investigation of comets and asteroids
will have begun, but intensive study and exploration of the wide
diversity of asteroids will remain. In this area we will want to
know the following: the overall structure of the asteroid belt and
its radial variations of composition and physical characteristics,
which are expected to reveal clues about the structure of the pro-
toplanetary nebula; the mechanisms that powered the evolution of
differentiated asteroids; and the chemical composition and phys-
ical character of comet nuclei, in order to determine under what
conditions these most primitive planetesimals formed.
Internal structure of terrestrial bodies is a broad field for
which, apart from the Earth, we still will have only the limited
data for the Moon from Apollo, and the even more limited data
for Mars from Viking. Even such basic information as crustal
thickness will still be lacking. The absolute history of planetary
bodies will not be understood without an unambiguous chronology
based on radioactive clocks. For example, it is suspected that the
martian channels and volcanoes were formed over a protracted
period, even though the time scale is based only on crater counts
and is very uncertain. There is little prospect of obtaining dates by
other means than laboratory analysis of returned samples. Such
samples remain valuable long after their acquisition and return to
Earth: improved techniques can (and do for the Moon) continue
to be applied to the original samples.
Only one side of Mercury will have been imaged from space-
craft, but all the other terrestrial planets are known to be asym-
metric In the distribution of geological provinces. While the
Galilean satellites of Jupiter will have been studied in some detail,
only the most rudimentary reconnaissance will have been made
of the other outer planet satellites. Only single flybys of Saturn,
Uranus, and Neptune will have taken place, and the Pluto system
will remain unvisited.
Our ideas about the origin of this solar system lead us to
believe that planet-forming processes occur commonly during star
formation. We will want to determine the prevalence and the prop-
erties of planetary systems around other stars accurately enough
to compare them with one another, as well as with our own system.
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We will want to carry on detailed studies of protostars in order to
ascertain the physical character of their accretion disks, thought
to be the sites of planet formation.
It seems likely that Earth is the only site of organic life in
the solar system, but there is no dearth of organic molecules on
or in such objects as meteorites, Titan, the jovian planets, and
giant molecular clouds located in other parts of the galaxy. Mars,
formerly the object of greatest interest, is now seen to be the
site of destruction of organic compounds by an intensely oxidizing
atmosphere and soil. Conditions, however, may have been more
benign in the remote past. There is still much to be learned about
the origin of life by study of the objects mentioned above, and
perhaps others such as comets. If other planetary systems exist,
they may be seats of organic evolution.
PLANETARY GEOSCIENCES
During a relatively short period of time, studies of planets
made by earth-based telescopes have advanced to detailed in situ
measurements from spacecraft of the planets' surfaces and at-
mospheres. A complex view of the planets and their satellites
continues to emerge.
In late 1962, Mariner 2 the first interplanetary spacecraft-
flew by Venus: the journey of Voyager 2 is still in progress. The
203-kg Mariner 2 had only six instruments, whereas the 81~kg
Voyager 2 has two color TV cameras and ten other advanced
instruments. These two spacecraft represent the simple beginning
and the sophisticated continuation of solar system exploration.
In the early years of exploration, missions were selected more
by technical feasibility than by scientific priority. So little was
known that any mission greatly increased our knowledge. Now,
comparative study of the planets is a significant scientific en-
deavor. Great advances in understanding the origin and evolution
of the planets and properties of the solar system will come from
comparisons of all planetary objects. Common features such as
atmospheres, magnetic fields, and geologic processes can be un-
derstood best by such comparison. In turn, these comparative
planetary studies provide insight about the history and evolution
of the Earth. Nevertheless, exploration has shown that each planet
is unique and interesting in its own right.
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Scientific Objectives for Planetary Geosciences
The following topics in planetary geosciences contribute to an
understanding of the solar system: formation; interior structure,
dynamics, and physical state; crustal evolution; and planet mor-
phology and surface processes. These topics, and the measurement
objectives for them, are discussed below.
Formation
One key to understanding the formation of the planets is the
determination of their chemical and isotopic compositions and the
timing of their accretion. The results can be compared for all
the planets, satellit es, and meteorites in order to place constraints
on models of chemical difl.erentiation as a function of heliocentric
or planetocentric distance. The results also shed light on the
potential for heat sources important for considerations of internal
activity and to assess models of planetary accretion.
Interior Structure, Dynamics, and Physical State
Measurements of the seismic behavior of planets, the strength
and nature of their magnetic and gravity fields, and the heat flow
from their interior are critical for determining the characteristics
of planetary interiors. When combined with knowledge of mass
and composition, the results permit assessment of the nature of
possible interior differentiation (core/mantIe/crust) and the pos-
sibilities for an internal dynamo.
Crustal Evolution
A principal objective in planetary exploration is the determi-
nation of the age, composition, and distribution of crustal mate-
rials, including volatiles. The results allow refinement of models
relating to planetary accretion, differentiation, and degassing. In
addition, such determinations allow assessment of the style and
timing of volcanism and tectonism and their relation to other geo-
logical events, as well as the role of volcanism in the evolution of
possible atmospheres.
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Planet Morphology and Surface Processes
The types and distributions of landforms and other geological
units on planetary surfaces can be determined through geological
mapping using remote-sensing data. The results allow assessment
of the processes, such as volcanism and tectonism, that have led
to the formation and modification of planetary surfaces. Some
landforrns, such as dunes and valleys, are indicative of processes
associated with wind and water, and thus contribute to models
of atmospheric evolution. Assessments must therefore be made
of the distribution and exchange of volatiles among the crust,
regolith, poles, and atmosphere. Knowledge of the geological
processes- volcanism, tectonism, impact cratering, and surficial
modifications can be combined with relative and radiometric age
determinations of the features associated with those processes to
derive geological histories of the planetary surfaces.
An important aspect of comparative planetology relates to the
origin and evolution of life. Knowledge of the geological environ-
ments permits assessment of the likelihood for the evolution and
sustenance of organic life, at least in comparison to Earth. The
images of Earth taken from space with its thin skin of oceans and
clouds help us begin to appreciate the uniqueness of our planet
and the fragile balance that makes life here possible.
An additional impetus for planetary exploration is the poten-
tial for using space resources. In a period when natural resources
are being depleted rapidly on Earth, no detailed assessment has
been made of the resources that exist in space. The Moon and
asteroids may hold significant potential as sources of metals and
minerals for utilization in space. The initial utilization of such re-
sources may be to support space missions that would travel farther
into space, or permanent bases on the Moon or Mars.
Measurement Objectives
The goals outlined above guide the definition of a set of general
scientific objectives as follows:
~ Characterize the internal structure, dynamics, physical
state, and bulk composition of the planet of interest;
. Characterize the planet's chemical composition and min-
eralogy of surface materials on a regional and global scale;
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~ Determine the planet's chemical composition, ~runeralogy,
and absolute ages of rocks and soil for the principal geologic
provinces;
~ Characterize the processes that have produced the land-
forms of the planet;
~ Determine the chemical and isotopic composition, distri-
bution, and transport of probative compounds that relate to the
formation and chemical evolution of the planet's atmosphere, and
their incorporation in surface and crustal rocks and polar ice;
~ Characterize the planetary magnetic field and its interac-
tion with the upper atmosphere, solar radiation, and the solar
wind;
~ Determine the extent of organic chemical and possible bio-
logical evolution on Mars and Titan, and explain how the history
of the planet constrains these evolutionary processes.
The Inner Solar System
The inner planets- Mercury, Venus, Earth and its Moon, and
Mars range from 0.4 to 1.5 AU in distance from the Sun and are
smaller and denser than the outer planets. These terrestrial plan-
ets are composed chiefly of rock and metal, are poor in volatiles,
and have few satellites. Their densities range from S.4 g/cm3 for
Mercury to 3.9 g/cm3 for Mars. The variation in density with so-
lar distance has been discussed in the context of a thermodynamic
mode} for the proto-solar nebula in which temperature and pres-
sure decrease with distance from the nebular center and control the
chern~stry of condensed material. However, it may be that primary
differences in planetary density are due to accidental variations in
fractionation and reaggregation from collisions. After their for-
mation, all inner planet surfaces were significantly modified by a
wide variety of internal and external processes. Nevertheless, each
planet has followed its own evolutionary path.
By exploring this diverse family of planets and by comparing
their features with those of the Earth, we seek to characterize the
evolution of the inner solar system and the causes of the unique
aspects of each planet. We also seek to gain insights into the
history, as well as the future, of Earth and the life that has evolved
on it. Further insights into the terrestrial planets will come from
study of the large satellites of the outer solar system as well.
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The Moon
Apollo yielded an enormous advance in the understanding of
planets by providing samples of the Moon and a wealth of other
information. The tune scale of the Moon's evolution has been
established, and several first-order questions have been answered.
Equally important, a basis has been established for interpreting
the evolution of other planetary bodies, including the Earth. Dur-
ing the accretionary phase of continuous planetesimal in-fall, the
Moon appears to have melted to depths of at least a few hundred
kilometers. The ancient crust developed during tints maelstrom,
with segments repeatedly fragmented and reincorporated into the
evolving magmas until a thickness was established that could with-
stand the waning bombardment.
The larger craters on the Moon record a period of intense bom-
bardment that ended about 3.7 billion years ago, a phenomenon
that presumably affected all of the inner planets at about the same
fume. This bombardment provides a chronological reference, ac-
curately measured in the case of the Moon by radioisotope dating
techniques, that ~ the basis for constructing the geologic history
of Mars, Mercury, and (presumably) Venus.
The evolution of the crust of the Moon is known from remote
sensing, from instrument data provided by landers, and from study
of returned samples. Remote-sensing data show that two major
provinces constitute the lunar crust: young (sparsely cratered),
low-albedo mare terrains, and old (heavily cratered), high-albedo
highlands. The oldest reliably dated rocks on the Moon (from the
highlands) are radiometrically dated at about 4.5 billion years old.
The youngest mare basaltic lava flows are estimated to be about
2.3 billion years old. The lunar highlands appear to be the result
of differentiation at 4.5 billion years and consist of minerals that
floated in the melt. However, there is controversy as to whether
the upper crust of the Moon was generated in a "magma ocean,"
whether the whole planet was molten, or whether local areas were
successively molten over a long period of time.
Geophysical data show that the Moon had a strong magnetic
field early in its history, but the field has since disappeared. Al-
though most scientists consider the Moon to have a small, partly
molten core, its presence is the subject of intense debate due to
differences in interpretation of the seismic record.
Because all the data provided by Surveyor, Apollo, Soviet
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images of about 20 km resolution at 2 AU from the Earth and
will determine the shape of many asteroids. In addition, the high
spatial resolution will allow reflectance spectra to be taken at
many spots to test for surface heterogeneity. The capabilities of
ST will also allow reflectance spectra to be taken in the ultraviolet
region, and perhaps in the future, in more of the infrared region.
These regions may show more diagnostic features than have been
available from ground-based studies.
Many asteroids have been grouped according to spectral re-
fiectance features, and many of these features have been related to
known meteorite types. One difficulty in the link between asteroids
ant} meteorites is that the reflectance spectra of the most common
meteorites, the ordinary chondrites, are not precisely matched by
the main-belt asteroid spectral reflectance groups. The closest
match, some of the S asteroids, may actuary be the source of the
ordinary chondrites if the surficial features of the asteroids have
been modified by exposure to the space environment. By 1995
these processes may be better understood as a result of labora-
tory studies, facilitating a closer observational link between the
meteorites and asteroids.
There are good dynarn~cal reasons for believing that most
ordinary chondrite meteorites are derived from a limited (~0.05
AU) region of the asteroid belt in the vicinity of the 3:1 Kirkwood
gap at 2.5 AU. (Asteroids with values of their sern~major axes in
this region wiD have periods in resonance with the orbital period
of Jupiter). Except for the questions of spectrophotometric inter-
pretation mentioned above, the known larger S asteroids, in the
vicinity of 2.5 AU are prone candidate sources for ordinary chon-
drites. Most of the meteorite population is clearly derived from
the main-belt asteroids, and their study provides our most detailed
understanding of the nature and properties of the asteroids.
Because of the diversity of studies on meteorites, it is dif-
ficult to predict the state of knowledge in 1995. Prior to 1995,
we can expect much new information will be obtained through the
NSF-sponsored Antarctic meteorite collection program and the de-
velopment of new analytical techniques. This collection represents
a les~biased sample of the small meteorite end of the terrestrial
meteorite flux and a substantial increase in the number of stony
meteorites available for study. For this reason it may provide a
look at meteorites derived from the asteroid belt at velocities so
high that a significant yield of large meteorites ~ not expected.
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This possibility appears to have already been realized by iden-
tification of several meteorites of lunar origin In the Antarctic
collection.
A large group of meteorites, the achondrites, have clearly
undergone igneous differentiation processes dated at 4.5 billion
years nearly contemporaneous with the formation of the solar
system. Besides giving evidence that at Idast some asteroids were
once geologically active, the meteorites allow the study of igneous
processes that occurred under a set of conditions unlike those of
Earth or Moon, and thus broaden our understanding of the effects
of different parameters that cannot otherwise be studied in the
laboratory. In addition, most of the iron meteorites formed very
early in solar system history in cores of many (up to 50) different
asteroids. These data provide strong evidence for the presence of
a substantial heat source capable of producing primordial melting
of the asteroids (and possibly planetary planetesimals as well).
Many of the meteorites are breccias (made up of fragments of
other preexisting rock) and provide evidence for the nature of
collisional processes at an early epoch. The meteorites, and hence
the asteroids, also retain a record of the cosmic-ray intensity in the
past, as well as a record of significant (up to a few gauss) magnetic
fields that were present before the asteroids accumulated. Several
of the carbonaceous chondrites have been found to contain amino
acids and other complicated organic molecules, which are clearly
of extraterrestrial origin. Carbonaceous chondrites also contain
calcium and aluminum-rich inclusions that appear to be among
the earliest objects to have formed in our solar system. These
inclusions provide evidence of very complicated processes in the
early solar nebula involving multiple episodes of high temperature
(~1400K). They also contain a record of the nucleosynthesis of
elements that were added to our solar system just before the
inclusions formed.
A problem associated with all of these meteorite studies, how-
ever, is the lack of a geological context for the interpretation of the
observations. An asteroid sample return mission would go a long
way toward providing this context. It would also provide the in
situ data that would substantially increase the value of the remote
studies of the asteroids. Similarly, the SNC meteorites can answer
a number of questions about Mars, but they are clearly derived
from a lava flow, whose composition is unlikely to represent that
of the surface as a whole.
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Questions in Asteroid Science. Accessing the information that
asteroids contain about the early solar system will require much
more detailed understanding of the asteroids' surface composi-
tions, their internal structure, the degree to which they are het-
erogeneous, and their dynamical and collisional evolution.
At present our knowledge of the composition of specific aster-
oids is almost entirely founded on earth-based spectrophotometric
data. Although we may expect this situation to change some-
what between now and 1995, for almost all asteroidal bodies it
will still prevail. Earth-based data represent a surficial average
over the entire observed disk of the asteroid, and heterogeneity
is at present only crudely exhibited as a consequence of aster-
oid rotation. When looked at carefully enough, every asteroid
appears spectrometrically unique, and different in detail from bad
oratory spectra of meteorites. Are these differences fundamental
or are they primarily the result of heterogeneity or exposure to
the surface environment or regolith phenomena? Making these
distinctions requires remote sensing data of higher resolution. In
addition, corroborating "ground truth" data are lacking, and will
be required to ensure that unanticipated mineralogical differences
are not overlooked or misinterpreted.
It is likely that asteroidal collisions have exposed interior re-
gions of asteroidal bodies. When combined with higher resolution
spectral and imaging data, this affords an opportunity to overcome
the apparent irritation to surficial composition. This may facil-
itate addressing such questions as the fragmentation history and
internal structure of differentiated asteroids, and the identification
of specific asteroids as sources of particular meteorite classes.
A better understanding of this asteroid collisional evolution is
also needed In order to learn which asteroids are primitive objects,
as opposed to collision products of larger bodies, and whether they
are best thought of as "rubble piles," megaregoliths, or simply as
solid rocks. In this same connection, present observations of Hi-
rayama families serve as a key source of information concerning as-
teroidal collision phenomena. Their apparent spectrophotometric
heterogeneity calls into question the basic assumptions supporting
these inferences.
With these more detailed compositional data it will also be
possible to address the relationship between the heliocentric dis-
tance of asteroids and their chemical composition. Understanding
this possible relationship is central to examining the conventional
1
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assumption that the formative solar system had a marked radial
temperature dependence, related to the composition of planetes-
imals and planets formed at different locations in the early solar
system.
COMETS
General Characteristics
Comets are thought to be small conglomerates of rock, ice,
and dust several kilometers in diameter and 10~5 to 10~8 g in
mass formed during the early years of the solar system's history.
Today most reside in the so-called Oort cloud, in loosely bound
orbits at tens of thousands of astronomical units from the Sun.
Perturbations induced by the gravity of passing stars and inter-
stelIar clouds occasionally alter a comet's orbit and send it near the
Sun, where solar heat evaporates the ice. The subsequent outflow
of gas and entrained dust, illuminated by sunlight, produces the
comets. Some of these comets that venture into the solar system
are further influenced by the gravity of planets, becoming trapped
in periodic orbits near comets. Some of these comets that ven-
ture into the solar system are further influenced by the gravity of
planets, becoming trapped in periodic orbits near the Sun. Such
periodic comets appear regularly for thousands of years until their
volatile material is depleted.
Several schools of thought hold that comet-like objects were
among the fundamental building blocks of some larger planetary
bodies. As a result of their small sizes and their large average
distances from the Sun, evolutionary processes that differentiated
the planets are thought to have been insignificant for many comets.
They may have played a role in later states of planetary evolution,
perhaps by providing volatile constituents for some atmospheres.
It has been speculated that some of these cometary constituents
were essential to the origin of life.
A bright comet appears as a roughly spherical coma or at-
mosphere composed of comparable quantities of dust and volatile
species such as neutral gases and ions. The curved, relatively
featureless dust tad! directed almost exactly away from the Sun
shows considerable temporal and spatial structure. Depending on
a comet's distance from the Sun and the light in which it is oW
served, its coma can be quite large, in the range of 104 to 107 km
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when a comet ~ at 1 AU. The plasma and dust tails are even larger
in the case of a bright comet, some 107 to 108 km in extent. All of
these phenomena result from the gas and dust that emanate from
the nucleus; the nucleus itself is small, of the order of 1 to 10 km
in diameter for typical comets.
The ice and snow in comet nuclei are composed of condensed
gases and other volatile materials, including water and probably
carbon monoxide, carbon dioxide, HCN, CH3CN, and uniden-
tified complex organic molecules. These species are the parent
molecules of the molecules and ions observed in the coma and tail.
The nonvolatile material is in the form of grains ranging from
subrn~crometer-sized dust to sand grains and perhaps pebbles and
boulders, containing silicates and possibly metals, oxides, sulfides,
and organic compounds. When solar heat vaporizes the volatile
material, the outflowing gas carries smaller solid particles with it.
Since the comet's gravity is finite, though weak, any larger peW
bles and boulders are likely to remain bound on the surface-of the
nucleus possibly leading to the formation of an "extinct comet of
asteroidal appearance. Some of the earth-approaching "asteroids"
may be highly evolved comets of this kind. Since many comets
show evidence of directional emission of gas and dust, it appears
that the surfaces of their nuclei are inhomogeneous and may have
localized active regions.
A tentative estimate of the volatile fractions of four recent
comets has been made from their apparent production rates of
carbon, oxygen, and nitrogen. Although the parent molecules are
uncertain, they seem to be composed mainly of hydrogen, carbon,
nitrogen, and oxygen. The mass ratio of the dust to gas liberated
from a nucleus has been estimated in two cases to be 0.5 and
1.7 within a factor of 2. By comparison, the ratio of volatile to
nonvolatile components is about 100 for solar material. These
results imply that hydrogen and helium are depleted in comets.
Nevertheless, comets seem to contain 3 to 10 times as much volatile
material as the most volatile-rich meteorites. Thus comets appear
to have formed from material at temperatures much lower than
that characteristic of meteorites, at about 150K as opposed to
more than 400K. This suggests that cometary material is the least-
differentiated and best-preserved product of the preplanetary solar
nebula that is known to remain in existence.
It is speculated that some fraction of comet dust may be
unaltered interstellar material. It may be possible to illuminate
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this question by establishing some elemental isotopic ratios. Rel-
ative isotopic abundances of the elements reflect their formation
processes. Carbon Is a good example. Bodies within the solar
system, including the Sun, Moon, terrestrial planets, meteorites,
and Jupiter, exhibit a common value of about 90 for the i2C/~3C
isotopic ratio. Red giant stars show a range from 12 to 50 for this
ratio. In carbon stars the ratio falls in a wide range from 2 to 100.
Although observed values in the interstellar medium also span a
wide range from 13 to 105, some investigators have argued that
a value of 40 Is representative. Many of the so-called "BrownIee
particles," which are collected in the stratosphere, are suspected
to originate In comets. At best, however, they are necessarily
deprived of their most volatile components.
State of Enowledge In 1995
Our ideas about comets are constructed from lirn~ted remote
observations as well as from our more general notions about solar
system bodies. Significant advances in our knowledge of comets
are expected during the decade preceding 1995, as a result of ini-
tial spacecraft missions carried out by a number of different space
agencies. The NASA fly-through of Comet Giacobini-Zinner con-
ducted the first in situ measurements of a comet. The instrument
suite carried by that spacecraft was designed for solar wind and
magnetospheric studies. The primary contribution of the mission
was to characterize the particle and field distribution around the
comet and to establish the basic features of that comet's ~nter-
action with the solar wind. Several important physical problems
were addressed by the Giacobini-Zinner mission, including the
medium-energy, nonthermal particle distribution, the morphology
of the magnetic field, the character of the interaction between
cometary gas and the solar wind plasma, and plasma instabil-
ities excited by the interaction. Although the Giacobini-Zinner
investigation was the first in situ comet investigation, neither the
encounter orbit nor the instruments were chosen with the comet
in mind. The same is true of the Pioneer Venus orbiter, which
obtained images in radiation scattered by atomic hydrogen.
The three scientific spacecraft that encountered Halley's
Comet were specifically designed for the purpose. The ESA Giotto
project aimed for the closest approach to the comet's nucleus-
passing as close as 500 km. Giotto passed through all of the outer
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comet/solar wind interaction layers and into the inner cometary
coma. The instruments carried out a first-order characterization
of the solar wind interaction, a crude characterization of the den-
sity and composition of cometary gases, and a crude analysis of
the elemental composition of cometary dust. In addition, a limited
number of medium-resolution pictures of the comet's nucleus were
obtained.
The Soviet Halley project sent two spacecraft through the
outer regions of the comet's interaction with the solar wind. Gen-
erally speaking, the information from the Soviet mission was sim~-
lar to that from Giotto. The Japanese Suisei and Sakigake missions
provided coordinated information on the solar wind flow upstream
of the comet.
Altogether, the investigations of Comets Halley and Giacobini-
Zinner achieved a gross characterization of the morphology of
large-scale cometary phenomena. As valuable as these investiga-
tions were, most of the highest-priority questions that challenge
our understanding of comets and that promise to reveal clues
about the early solar system remain unanswered.
Detailed understanding of comets will require more intimate
and extensive measurements than are accessible to flyby investi-
gations. Comet rendezvous and comet nucleus sample return will
be the means by which the major objectives of cometary science
will be realized. The United States will carry out a comet ren-
dezvous mission by the middle of the l990s, which will follow a
comet through most of its inner-solar-system passage. Analyses
of cometary gas and dust, energetic particles, and magnetic fields
and plasmas, as well as detailed investigations of the structure and
gross composition of the comet nucleus will be carried out by the
comet rendezvous mission.
Successful completion of the comet rendezvous should an-
swer many outstanding questions about the gross characteristics
of cometary features and phenomena. The next obvious step in
the study of comets will be comet sample return. A crude sample
return may be accomplished by flying a collector, at high velocity,
through a cometary coma. However, the material returned in this
way will retain only the information about its basic elemental and
isotopic composition. In some ways, this situation resembles that
of the stratospheric "BrownIee particles," which do in fact retain
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much of their original structure. Valuable as they are, these par-
ticles cannot be identified with any particular comet, and in fact
may not even arise from comets at all.
By 1995, the next major step in cometary science will be to
return an intact sample of comet nucleus material. This material
can then be analyzed in detail in laboratories in order to carry out
the mineralogical, chemical, and isotopic analyses that are needed
to unravel the formation processes and evolutionary history of
comets.
Questions in Cometary Science
It is expected that as a result of spacecraft missions and earth-
based studies, our knowledge of comets will increase significantly
during the next decade. It ~ clear that those results will represent
only the beginning of our quest for understanding these most
primitive and unevolved aggregates of matter assembled during
the birth of the solar system.
An important set of questions concerns the description of the
present state of cometary nuclei. These include descriptions of the
chemical, isotopic, and mineralogical composition of comets, their
internal structure, and the range of variation of these characteris-
tics between different comets.
Questions of another class address cometary processes. As oW
served, comets are complex systems of neutral gas plasma, dust,
and larger solid bodies. We must understand the physical pro-
cesses that determine the loss of material from the cometary nu-
cleus and the resulting short-term evolution of its structure, and
those that produce the elaborate extended coma and tail of the
comets.
Some of these same physical processes determine the nongrav-
itational evolution of cometary orbits, that is, a Rocket effect"
caused by the asymmetrical emission of gases from the nucleus.
These altered orbits can have a substantial effect on the probabil-
ity that a comet will impact planets and are central to the question
of the evolution of active comets into Apollo objects of asteroidal
appearance.
This knowledge of the present state of comets and the physi-
cal processes to which they are subject is required to understand
the properties of earth-impacting material derived from comets.
It is known that meteors, meteorites, and cosrn~c dust represent
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the impact of cometary and asteroidal materials, but the relation-
ship of this material to their sources is imperfectly understood.
Like returned samples, laboratory study of this material provides
important information regarding these bodies and their origin.
Characterization and identification of possible cometary material
are required.
The resulting knowledge of present-day cometary composition,
structure, and processes is fundamental to understanding where,
when, and how the comet nuclei formed. In particular, we need to
understand the age and any alteration of the various components
of cometary nuclei. To what extent do they represent evolved
solar system material, early solar nebula condensates, or solids
of interstellar origin? This extrapolation back to the time of the
origin of the solar system is also needed to interpret the extent to
which comet-like objects contributed to the formation of the giant
planets and the volatile inventories of the terrestrial planets.
Comet Measurements and Technical Requirements
Addressing the scientific questions cited above will require
detailed investigations of the comet's nucleus and the emitted
dust, as well as the gas, plasmas, and fields in the comet.
Comelary Nucleus. Investigations of the comet nucleus should
be aimed at establishing its composition and its physical and
structural characteristics. Compositional measurements should
determine the atomic, molecular, and mineralogical content of the
refractory and the volatile solids. Together with measurements
of the physical and structural features, this information will help
in ascertaining the history of cometary matter and the processes
responsible for its formation and for the assembly of cometary
nuclei.
Accurate isotopic measurements should be carried out on both
the refractory and volatile constituents to explore the nucleosyn-
thetic history and establish a time scale for major events in the
history of cometary material. Both the compositional and struc-
tural investigations should be extended over the variety of physical
scales that characterize cometary nucleus material; this covers a
range from the microscopic grains to the full size of major macro-
scopic components of cometary nuclei. It is desirable to identify
the major mineral assemblages of the nucleus for those constituents
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that make up 5 percent or more of the comet's composition, and
with a resolution of better than 10 percent of the nuclear diameter.
On the small scales, complete determination of the structure and
composition of cometary material requires detailed analysis of the
dust component to discern the dust's character and origin.
Cometary Atmosphere. Measurements of the gaseous component
of cometary effluent will reveal information about the most volatile
materials in the nucleus. Spacecraft should have the capability to
identify and determine the abundances of all molecular species in
the mass range 1 to several hundred, and to determine the isotopic
ratios for the important species at an accuracy sufficient to identify
deviations from solar composition and other anomalous isotopic
variations. A major purpose of the cometary atmospheric mea-
surements is to ascertain the composition of the so-called parent
molecules of cometary effluent and their evolution as they leave the
comet. Both the neutral and ionized components of the cometary
atmosphere should be analyzed in detail, including the variations
in composition with distance from the comet's nucleus.
Solar Wind Interaction. The structure of the large-scale cometary
phenomena should be deterrn~ned. Measurements of the electro-
magnetic fields, plasma, energetic particles, and neutral gas should
be made in a volume surrounding the nucleus and encompassing
the upstream coma and solar wind interaction region, as well as
a significant volume of the tail. Measurements should be made
with sufficient spatial and temporal completeness to allow identi-
fication of the major dynarn~cal physical processes, including tran-
sients, that play important roles in shaping the overall cometary
structure and providing its sources of energy.
Technical Capabilities. Cometary studies require a sophisticated
complement of spaceborne and laboratory instrumentation that
will not be discussed in detail here. Scientific investigations of
comets need a launch system able to reach a variety of orbits and
to maneuver for an extended period of time near the comet. At
present, low-thrust electric propulsion seems to provide, by a wide
margin, the best propulsion system for such missions.
Achieving the scientific objectives of comet exploration will
require missions of months' duration designed to carry out ex-
tended investigations of a single comet during both its most active
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and its more quiescent times. Return of high-~ntegrity samples of
cometary nucleus material to Earth for detailed analysm in ter-
restrial laboratories will be essential to realize the overall goals
of comet science, as well as of solar system science in general.
However, adequate analyses should be carried out in situ, because
these materials may well undergo significant changes once they are
removed from their natural environment.
Representative terms from entire chapter:
magnetic fields
