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OCR for page 88
4
Future Programs
PROPOSED MISSIONS
Programs for Planetary Geosciences
Types of Missions
The goad of planetary exploration are met through observa-
tions and missions in which the levels of investigation are generally
progressive. Earth-based observations provide limited, but impor-
tant data that allow the formulation of first-order questions.
As the first level of investigation, reconnaissance by flyby rn~s-
sions attempts to reveal the major characteristics of a planet,
such as its radius, mass, rotation rate, and the existence of mag-
netic fields, an atmosphere, oceans, satellites, and the like. The
exploration phase follows and has the goal of describing and un-
derstanding the state of a planet and the general processes that
have influenced its environment. This phase is carried out by long-
lived orbiters equipped with a variety of cameras and other remote
sensing instruments and may include entry probes to measure the
chemical composition of the planet's atmosphere or surface. Such
missions can image the surface of the planet; provide a global map
of the distribution of elements and minerals on the planet's surface;
88
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89
determine a planet's global properties including topography, grav-
ity, and magnetic fields and density distribution; and characterize
the atmospheric and ionospheric structure and dynamics.
The intensive phase of investigation addresses the highest-
order questions revealed by the earlier phases and involves sophis-
ticated and complex missions. Detailed study of the properties
of surface materiab, interaction of surface and atmosphere, strati-
graphic and depositional history, and biologic questions requires in
situ measurements by soft-landed automated laboratories, mobile
laboratories (rovers), and networks of instruments.
"Network science" is defined as geophysical measurements
made over correlatively long time (>1 earth year) at several lo-
cations over a planet's or satellite's surface. Although network
science can be accomplished with a minimum of 3 to 4 stations,
ideally a system would include an array of 6 to 12 stations with
instruments to measure seismic events, physical properties of the
surface, heat flow, and (where applicable) meteorological parame-
ters. The stations must provide simultaneous measurements from
the seismic and meteorological experiments, and may also make el-
emental chemical and mineralogic measurements with appropriate
instruments. Network science can address geophysical questions
on local, regional, or global scales. For example, most knowledge
of the interior of the Earth has been derived from seismic data.
Networks of seismometers on a global scale yield information about
the properties of the core, mantle, and lithosphere; arrays of seis-
mometers on a regional scale can provide detail about systems
of active faults; and local arrays (tens of kilometers or less) can
provide information on the presence and configuration of rock lay-
ers within the lithosphere. Network stations can be emplaced by
penetrators, semihard or soft landers, or a rover.
The measurement of heat flow is fundamental for understand-
ing the interior characteristics of planets and satellites, yet obtain-
ing valid data remains a major technological problem. All present
means for emplacing instruments in the subsurface (e.g., penetra-
tors, drilled holes) disturb the thermal regime for a period of time
that exceeds the lifetime of most missions. These problems must
be overcome, or alternative means found for measuring heat flow.
Sample return missions can provide fundamental data that
can be acquired in no other way about the composition, history,
and evolution of other worlds. At the same time, sample return
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go
missions present exciting technical challenges; they require so-
phisticated robotic spacecraft systems and complex operational
capabilities on planetary surfaces.
Both terrestrial and extraterrestrial rocks preserve evidence
of past events, thereby providing essential clues to understanding
the planet's origin, history, and evolution. The fabric of crystals
that compose a rock (or a piece of solid ice from a comet) reflects
both the original formation and subsequent evolution. One can
distinguish between lava flows that cooled quickly at the surface
and deep-seated crustal rocks that cooled slowly tens or hundreds
of kilometers down. The different minerals in a rock reflect further
details of their formation the formation temperature, the cool-
ing rate, the nature and abundance of volatiles, and the genetic
relations between minerals formed at different times. The detailed
chemical character of a rock can thus be the key to identifying
global planetary processes core formation, crustal separation, or
episodes of widespread volcanism. The measurement of radioac-
tive parent and daughter elements in a rock provides independent
information about the timing of major planetary events origin,
major meteorite impacts, and volcanism.
Analyses and studies of samples returned to the Earth are
unique in that they: (1) can be performed by a variety of scien-
tists with current state-of-the-art technology; (2) permit iterative,
imaginative experiments that can be based on prior results, in-
cluding unexpected ones; (3) allow effective separation and con-
centration of mineral phases, based on the specific properties of the
sample; (4) permit many different analyses on the same sample;
and (5) permit the deferral of certain experiments, if necessary,
until better analytical technology or understanding is available.
The flexibility of laboratory sample analyses, and the resulting
confidence in results, is largely due to the high analytical precision
and to the greater control of experimental parameters possible in
a laboratory.
Studies of the mineralogy, mineral chemistry, texture, and
bulk chemical composition will define the physical and chern~cal
history of rocks. Evidence for processes ranging from crustal for-
mation to volcanism or chemical weathering at the surface will
be addressed through detailed comparison of textural properties,
mineral composition, and the distribution of elements and isotopes
within the rocks.
A wide variety of signatures have been identified among trace
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91
elements as tracers for terrestrial and lunar geochem~cal processes.
Analyses of these various types of elements, either in groups or as
pairs, will yield evidence on the nature of the bulk starting materi-
ab with regard to differentiation, the degree of that differentiation,
the internal heat sources, the temperatures and pressures of inter-
nal processes, and the nature of meteoritic material impacting the
body during the geological past. Experience shows that these anal-
yses can be combined with other geological information to unravel
the complex evolutionary history of planetary surface materials.
Precise isotopic analyses will allow us to solve a wide variety
of chronologic and geochemical problems. Long-lived radioactive
species and their products (U-Th-Pb, K-Ar, R~Sr, Nd-Sm) pro-
vide isotopic ages for rocks and are the only means of establishing
an absolute chronology. Stable isotopes (H. O. N. Si, C, S) pro-
vide very powerful geochemical tracers that can be used with the
chronological data to explore past states of the interior as well as
more recent surface processes. Anomalies left by the decay of ex-
tinct, short-lived radioactive isotopes can be expected to provide
evidence for preaccretion conditions and time scales in the early
solar system, before formation of the planets.
Analyses for He, Ne, Ar, Kr, Xe, and their isotopes are es-
sential for understanding the internal differentiation history, its
interaction with cosmic radiation, and the evolution of its atmo-
sphere. Such data from surface materials will provide powerful
tools for understanding the evolution of the planet's surface and
interior. From our experience with lunar samples, meteorites, and
terrestrial rocks, Xe isotopes are expected to be the most versa-
tile, as the isotopic patterns may reflect several processes—extinct
short-lived isotopes, fission of long-lived and extinct isotopes of U
and Pu, and the mixing effects of various reservoirs of gas.
Samples will be examined for evidence of remanent magneti-
zation related to any past magnetic fields, as well as for a variety
of physical properties, such as grain distribution, density, porosity,
thermal conductivity, capacity for retaining volatiles, and seismic
wave velocity. These measurements will provide basic data for
various physical models.
Planned Missions
The rn~ssions proposed to study the solar system during the
period 1995 to 2015 follow the balanced approach recommended
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92
by the National Research Council's Committee on Planetary and
Lunar Exploration (COMPLEX). By 1995, the exploratory and
reconnaissance phases should have been completed for the inner
planets, with the important exception of Mercury, and the pro-
posed missions will involve the intensive study phase. The Voyager
spacecraft will have performed the initial reconnaissance study of
the satellites of Jupiter in 1979, of Saturn in 198~1981, of Uranus
in 1986, and of Neptune in 1989; the Galileo spacecraft will have
continued exploration of Jupiter's satellites in the 1990s.
During the early part of the study period, the task group rec-
ommends that Galileo-like missions explore the Saturn, Uranus,
and Neptune systems. These spacecraft, with an extensive array
of remote-sensing instruments, would carry out repeated flybys
to characterize the major satellites. Because of scientific inter-
est in its atmosphere, Titan would receive more intensive study,
with radar investigations and a probe to measure the atmospheric
· —
composition.
Our own Moon remains a body of the highest scientific im-
portance. The task group recommends that the global survey by
the Lunar Geoscience Orbiter be followed by deployment of rovers
and a geophysical network, as well as resumption of sample return
from selected locations.
The task group also recommends a Mercury orbiter, with a
landed transponder if possible, to complete the basic character-
ization of the inner planets. Mercury's composition, mass, and
magnetic field provide key tests to many theories on the evolution
of the solar system. The mission will not only involve planetary
science, but will include solar studies and some tests of relativity
physics by careful tracking of the planet's motion. The discovery
of new trajectories has made this mission possible with existing
propulsion systems.
The major mission recommended for the initial decade of the
study period is a Mars rover and sample return program consist-
ing of linked missions launched in 1996 to 1998. This is envisioned
as a very comprehensive mission one with a capable rover to
collect selected samples for return to Earth and carry out ex-
tensive observations over the surface of the planet. The return
of unsterilized martian materials could provide unique data on
the absolute chronology of martian rock units, on detailed de-
tection and characterization of contemporary or fossilized life, on
surface-atmospheric interaction processes and rates, and on the
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93
composition and evolution of Mars' crust and mantle. A sample
return should, if possible, have the capability to provide rationally
chosen samples from a number of carefully selected areas in order
to maximize the value of the samples.
Study of returned martian samples on Earth provides an ex-
cellent opportunity to look for any evidence of martian life, past
or present. To protect the geochem~cal and biological integrity of
the returned sample, sterilization by any method must be avoided.
Because any martian organisms included with the returned sample
might be killed by exposure to the high pressure, high water con-
tent, and high oxygen content of the terrestrial atmosphere, the
most promising life detection Experiments may be those based on
chemistry and morphology, rather than on metabolism. Such ex-
periments would include ~m~cropaleontology~ examinations, per-
haps using stains that are reactive with carbon compounds. If
any living systems should be detected in the returned sample-
whether viable, dormant, recently dead, or fossil—the direction of
our future exploration of Mars would be completely changed and
there might be an early reexamination of manned missions to the
planet.
A rover is necessary as a mobile sampling device in order to
ensure that a wide enough variety of samples is collected to meet
the mission objectives. The rover should have those capabilities
necessary for sample collection, examination, and characteriza-
tion. In general, the rover capabilities needed are roughly those
of a human geologist collecting samples in the field. Like a geol-
ogist, the rover should obtain multispectral, stereoscopic images
at a variety of scales and resolutions, process them, and interpret
them by comparison to images derived from previous experience.
It should lift samples to examine their details closely and to esti-
mate their weight and density, thereby evaluating the amount of
weathering. It should carry out simple chemical tests analogous
to those made with a geologist's traditional Geiger counter and
acid bottle. It should provide this information to Earth so that a
decision can be made either to collect the sample or to discard it
and move on to another.
Although the rover's prime objective is to support sample
collection, it is important to note that its capabilities, and the
data it collects to characterize possible samples, would also be
scientifically unportant during an extended traverse to analyze
and characterize martian surface materials up to a significant
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94
distance from the landing site. After the rover has completed
its sampling traverses (first in the vicinity of the landing site
ant! then, if possible, at greater distances), it could carry out an
important and exciting surface traverse of Mars, making the same
observations over long distances. Such a post-sampling traverse
would provide an important regional context for the sample suite
and would also help understand better the complex processes that
have taken place on the martian surface. Eventually, manned
missions will offer the most complete and comprehensive execution
of the intensive phase of planetary exploration.
During the latter part of the study period the task group
proposes to continue geoscience investigations with ongoing inves-
tigation of Mars as well as with a number of missions that are
now less well defined and of somewhat lower priority, or that will
require new technological developments before they can be carried
out. It Is important to deploy a network of stations on the surface
of Mars to study its interior structure with seismic techniques and
to understand the global meteorology. Sensor networks should
eventually be emplaced on all the inner planets for continued seis-
mic and other studies over a long period of time. Venus has the
highest priority, but the high surface temperatures will make this
mission very difficult.
Detailed study of returned samples will continue to have high
priority throughout the study period. These studies will include
samples from new locations on Mars, including the polar regions,
and on the Moon, including the far side. Eventually, samples
should be returned from Mercury and Venus.
A mission of special irrportance is the investigation of the
resurfacer of Titan, but the nature of this surface—solid or liquid-
will not be known until radar investigations have been made from
a spacecraft. A similarly important mission ~ a lander on To to
investigate the composition of the materials emitted by volcanoes,
both active and inactive.
Programs for the Outer Solar System
Types of Missions
Addressing the goals set forth in Chapter 2 will demand new
missions. Required missions to the outer solar system include
long-lived orbiters, atmospheric probes, deep atmospheric probes,
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95
and a ring rendezvous spacecraft. These new missions will build
on the first look provided by Voyager, which will have observed all
the giant planets: Jupiter, Saturn, Uranus, and Neptune.
Orbiters provide multiple looks for remote sensing of plane-
tary surfaces and atmospheres, and for direct measurements of
the magnetosphere at many locations. They also provide a long
tune scale to study dynamic phenomena. New discoveries can be
investigated in greater depth by changing the later stages of an
orbiter mission to respond to new findings.
Although remote sensing has important virtues of its own,
some kinds of measurement can only be made from within an at-
mosphere, and others can be made much more accurately there.
Notable examples are the abundances of noble gases the principal
clue to the possible presence and nature of a primary atmosphere—
and the abundance of nitrogen, which can be remotely measured
only under favorable circumstances. In general, in situ abun-
dance measurements can be much more accurate than those made
remotely, and have given us most of our current isotopic data.
Further, a descending probe can be tracked to give a vertical wind
profile, whereas remote tracking of clouds gives a nearly global
view, but only at one or a very few heights. Measurement of cloud
properties and radiation balance are also best carried out from a
descending probe.
Special techniques will be required to study Saturn's rings di-
rectly. A proposed ring rendezvous mission would use low-thrust
propulsion to orbit Saturn in a plane just above the equatorial
(ring) plane. By slowly decreasing the semimajor axis and alti-
tude above the ring plane, a spacecraft conic} make multiple, low-
relative-velocity encounters with ring particles. This would allow
direct measurement of particle physical and chemical properties,
and direct observations of inter-particle collisions.
Planned Missions
As described in the recent publication, A Strategy for Explo-
ration of the Outer Planets: 1986-1996 (National Academy Press,
1986), the most immediate priority is intensive study of the Saturn
system. To address the scientific questions concerning this system
will require a long-lived orbiter and probe investigation of the at-
mosphere of Titan, and perhaps Saturn as well. The orbiter would
study the planetary atmosphere, the magnetosphere, the rings,
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96
the surfaces of the satellites, and the surface of Titan by radar.
The probe would be similar to the Galileo probe, making in situ
measurements and perhaps unages. Important scientific objectives
of this mission are to determine the composition and structure of
the atmospheres of Saturn and Titan; make detailed, long-term
studies of Saturn's rings; investigate Saturn's small satellites and
the magnetosphere; and measure the physical nature and state of
Titan's surface.
In addition to the Saturn system, the Uranus and Neptune
systems will also become important subjects for study after 1995.
Prior to 1995, the Voyager 2 spacecraft will have made a prelim-
inary reconnaissance of these two systems. This will provide a
basis for planning future intensive studies of these planets. The
task group expects that many of the same questions about their
atmospheres, magnetospheres, satellites, and rings may arise. This
will lead to space missions similar to Galileo and the Saturn orbiter
with Titan probe. An orbiter will provide exploration of the sys-
tem, multiple looks at the planet and its satellites, and a long time
base for study of meteorology, ring dynamics, and magnetospheric
interactions. The probe will make direct measurements of the
composition, cloudiness, and vertical structure of the planetary
atmosphere (or perhaps the atmosphere of a satellite).
Future Missions and Progeny for Primitive Bodies and the
Origin of the Solar System
We can expect that, by 1995, reconnaissance missions to comet
nuclei and asteroids will have transformed our view of these bodies
from unresolvable points of light into planetary bodies of distinct
shape and surface morphology. It is hoped that a good start will
have been made in understanding their chemical and mineralogical
structure, and their relationship to the small fragments of these
bodies sampled on Earth in the form of meteorites and interplane-
tary dust. Making full use of the potential information obtainable
from these bodies will require initiation of a new phase of detailed
study.
Multiple flyby and rendezvous missions to asteroids will be
needed to address questions of the variety and spatial distribution
of asteroidal bodies. In situ data must be related to earth-based
spectrophotometric data, and to physical theories of asteroid col-
lisional fragmentation and evolution into earth-crossing meteoritic
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97
fragments. Such mussions can also identify the range of mor-
phological and structural characteristics of asteroids, and permit
addressing the question of separating the record of early solar
system history from subsequent collisional processing. With the
background and understanding obtained from such studies, it will
be possible to wisely select samples for return to laboratories on
Earth for detailed chemical, mineralogical, and isotopic investiga-
tions of asteroidal material. Measurements of this kind are not
only essential to interpretation of early solar system events in a
context of planetary geology, but also may provide the key for sim-
ilar interpretation of the large quantity of data obtainable from
meteorites.
The study of comets is in a way easier than that of asteroids,
because cometary activity releases large quantities of material from
the nucleus into the atmosphere from which it can be sampled
without actually landing and collecting samples from the surface.
Valuable chemical and isotopic information can be obtained from
a returned sample collected in this way, even if some mineralogical
structure of the material is destroyed during the collection. A
sample return mission of this kind could be of much importance
in itself, as well as a valuable forerunner of collection and return
to Earth of Pristine samples of a cometary nucleus.
The earth-approach~ng Apollo and Amor objects are a special
class of primitive bodies. Some of these are likely to be ~l-km-
diameter asteroidal fragments derived from main-belt asteroids
and transferred to near-Earth orbits by the same resonance mech-
anisms responsible for the transfer of smaller meteorite-size frag-
ments from the asteroid belt. In fact, objects of this kind are likely
to be the more immediate parent bodies of many of the mete-
orites in our collections. Understanding the physical structure of
this fragmented material should be of much value in understand-
ing the evolution of the steady-state collisional size hierarchy and
the effects of this collisional history on the natural sampling of
asteroidal material in the form of meteorites.
Other Apollo-Amor objects exhibit orbits and physical char-
acteristics suggesting they are likely to be the devolatilized residue
of cometary cores. These can therefore provide an opportunity to
sample cometary material that may not be readily available at
the surface of an active comet. By displaying the end product
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98
of cometary evolution, these bodies can provide better under-
standing of the active processes of comets, and thereby facilitate
interpretation of remote observational data on comets.
The task of developing the instrumentation and technical
means of successfully effecting these future measurements and
remote sample returns will be a challenge. The variety and num-
ber of rendezvous and sample return opportunities to primitive
bodies are now limited by existing ballistic propulsion systems.
Full exploitation of the potential offered by such studies will re-
quire development of low-thrust propulsion systems that permit
selection of targets primarily because of their scientific interest
rather than on the basis of their accessibility.
As discussed in Chapter 3, the fundamental question of the
origin Of the solar system will not be answered simply by future
missions, but will require highly disciplined yet imaginative inte-
gration of theory and observation. Adequate support of such inves-
tigations, including availability of advanced computing systems,
will be needed for this work to proceed apace. Investigations of
this kind will depend heavily on the understanding obtained from
studies of primitive bodies. However, in the long term, evidence
for the existence and characteristics of other planetary systems
should prove to be of comparable, or even greater importance to
resolving this ancient and fundamental subject of human thought.
It is essential that we conduct a comprehensive search for other
planetary systems.
A variety of observational techniques can and should be
brought to bear on this problem. Before 1995, ground-based spec-
troscopic and astrometric studies will provide a preliminary survey
of some of the nearby stars that may harbor planetary systems we
can detect. The Hubble Space Telescope can be expected to make
important contributions in this area, as can the Space Infrared
Telescope Facility. A comprehensive search for and study of other
planetary systems will, however, require an astrometric telescope
in earth orbit. The need to have a long-term (10 to 20 years) sys-
tematic observational program indicates that the telescope should
be on the Space Station. If this activity is initiated in 1995, at the
beginning of the era under consideration, a full survey should be
complete by 2015.
Results from this type of survey, which needs to be conducted
with a system capable of 5- to lO~arcsec accuracy in terms of
its ability to determine the relative positions of stars, would be
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102
polar laminar terrain is a compelling reason to study Mars' climate
change in detail. further knowledge would provide models for
atmospheric evolution under a distinctly different situation from
Earth.
Accumulating geological and climatological evidence suggests
that early Mars may have been suitable for the development of life
during the first billion years of its history. Earth and Mars may
have been quite similar during that period when algae flourished on
Earth. Although the conditions on Mars currently are very difficult
for the survival of life forms as we know them, and although the
Viking missions found no evidence for life or organic material,
the possibility of finding traces of past life provides a scientific
objective for more detailed study. The Mars near-surface rock
layers possibly could contain m~crofossils remaining from this early
period, or records of chemical change on the earlier surfaces caused
by early life. Sedimentary deposits that date back to the first
billion years in Mars' history may be exposed in the walls of the
great equatorial canyons. Analysm of the geologic history and
detailed study of such deposits would have a significant impact on
our understanding of the origin of life.
Scientific Objectives for a Mare Focus
The scientific utility of studying a second terrestrial planet
in sufficient depth that the past and present processes can be
identified and compared with similar ones on the Earth leads
naturally to the recommendation of a major focus on intensive
exploration of Mars. Current understanding of the origin, early
history, and present state of Mars motivates a set of scientific
goals whose accomplishment defines the scope of the recommended
campaign to understand Mars:
1. Characterize the internal structure, dynamics, and bulk
composition of the planet.
2. Characterize the chemical composition, structural features.
and mineralogy of surface materials on a regional and global scale.
3. Determine the chemical composition, mineralogy, and aW
solute ages of rocks and soil for the principal geologic provinces.
4. Characterize the processes that have produced the land-
forms of the planet.
5. Determine the chemical and isotopic composition, distri-
bution, and transport of volatile compounds that relate to the
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103
formation and chemical evolution of the atmosphere, and their
incorporation in surface rocks and polar ice.
6. Characterize the planetary magnetic field and its interac-
tion with the upper atmosphere, solar radiation, and the solar
wind.
7. Determine the extent of organic chemical and possible
biological evolution of Mars, and explain how the history of the
planet constrains these evolutionary processes.
Some of these objectives can be addressed by NASA's planned
missions, including the Mars Observer and the Mars Aeronomy
Orbiter. These will provide a global map of the distribution of
elements and possibly minerals on its surface; determine its global
properties including topography, gravity, and magnetic fields; and
characterize the atmospheric and ionospheric structure and dy-
namics. However, more sophisticated and complex missions are
required to answer the higher order questions. The necessity for
detailed study of the properties of surface materials, interaction
of surface and atmosphere, and stratigraphic and depositional his-
tory, and the need for addressing biological questions require in
situ measurements by complex, automated laboratories or rovers,
and networks of instruments.
The measurement of heat flow is fundamental for understand-
ing the interior characteristics of planets and satellites; yet obtain-
ing valid data remains a major technological problem. All present
means for emplacing instruments in the subsurface (e.g., penetra-
tors, drilled holes) disturb the thermal regime for a period of time
that exceeds the lifetime of most missions. These problems must
be overcome, or alternative means found for measuring heat flow.
The next major scientific objectives beyond those addressed
by current missions could be met with a capable rover to col-
lect selected samples for return to Earth and to carry out ex-
tensive observations over the surface of the planet. The return
and study of pristine martian materials could provide data on the
absolute chronology of martian rock units, on detailed detection
and characterization of possible contemporary or fossilized life, on
surface-atmospheric interaction processes and rates, and on the
composition and evolution of Mars' crust and mantle. A sample
return mission should provide rationally chosen samples from a
number of carefully selected areas in order to maximize the value
of the samples.
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104
Detailed study of returned samples will continue to have high
priority long after their initial return. These samples should be
from many carefully chosen locations on Mars, including the polar
regions. The collection of these samples involves increased com-
plexity with far-traveling, highly capable rover laboratory/obser-
vers and drill stations capable of drilling deep holes. Instruments
to measure seismic waves, composition and physical properties of
the surface, heat flow, and meteorological parameters should be
emplaced in a network of at least 3 to 4 stations; 6 to 12 stations
would be preferable.
Potential landing sites can be selected from high-resolution
Viking orbiter mosaics. Along with previous knowledge, this infor-
mation allows the identification of locations where future surface
exploration would provide substantial scientific return. Several
interesting sites have been identified that contain a rich diversity
of geologic ages and rock types.
Voicanic deposits are exposed in the southeast portion of the
scarp surrounding Olympus Mans. Evidence of tectonic activity is
visible in the layered deposits in Candor Chasma and in the mesas
at the bottom of the fault trough. Polar layered deposits, which
may represent climatic change, are available near the north polar
cap, which is, itself, of great interest. A wide variety of geologic
features are available in the Mangala Valley, including stream
deposits and very young lava flows. For each of these proposed
sites, high-resolution geologic and topographic maps are available.
Role of Humans in Intensive Mare Melioration
The intensive exploration of Mars envisages} here will require
substantial technical capabilities on the Mars surface. Long paths
must be traversed and explored geologically in detail. The scien-
tific goad require samples to be collected, along with preliminary
in situ analysis. To meet the sampling requirements, holes must
be bored and drill cores extracted. Depths of up to 2 km would
be desirable near the poles. The network of seismic and meteoro-
logical stations will require maintenance. Achieving a planet-wide
understanding of the diverse provinces and processes will require
extensive exploration over many parts of the surface. Although
autonomous robot vehicles may perform many of the initial ex-
ploratory activities, the later, more comprehensive exploration will
require capabilities that now are possessed only by humans.
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105
The detailed exploration of Mars and its comparison to Earth,
which is the objective of the proposed campaign to understand
Mars, can be likened to the Apollo exploration of the Moon. The
lunar astronauts successfully landed spacecraft in difficult terrain,
selected a wide variety of samples, used simple tools to enhance
sampling, and applied ingenuity and strength to overcome opera-
tional problems. These technical achievements were accomplished
in relatively short stays. The crews traveled up to 25 km at speeds
up to 10 km per hour in rovers, and selected and documented a
wide variety of samples. Further, the crews have contributed to
post-m~ssion data analysis extending over the past 15 years.
The ability to use simple took can probably be reproduced
by autonomous vehicles. Time pressures would be considerably
less for Mars surface exploration: robotic devices would have
substantially more time to carry out these tasks. However, the
intelligent and interactive selection of appropriate samples, and
the concurrent and later provision of contextual information about
them is well beyond the capability of current automated systems.
Mars shows a much more complex variety of geologic processes
than the Moon. Since the martian surface is also weathered, dis-
cernible differences based on mineralogy, texture, and resistance
to erosion may be important to interpretation. This greater diver-
sity requires subtle judgments and interactions among observation,
analysis, and interpretation. The total context of any sample will
be very important in its scientific interpretation.
Two conclusions follow from these considerations. First, ge-
olog~sts, properly equipped, learning by observation, improvising
as necessary, and advised from Earth, can be fast and effective
at exploring the surface of Mars. Second, it is difficult for au-
tonomous devices, even remotely linked to Earth by cameras, to
achieve the scientific goals set forward above. Ultimately, to re-
solve the important questions and to compare Mars in detail with
the Earth will require exploration capabilities on the surface of
Mars possessed now only by humans.
A Phased Approach
The task group recommends an intensive study of Mars to
be implemented by phases that begin with currently envisioned
missions, progress to newly developed robotic and propulsion sys-
tems later, and culminate in manned missions to Mars. The Mars
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Observer and Mars Aeronomy Observer are missions central to the
broad program of solar system exploration. They will provide the
global overview from which to mount and target later efforts at
the Mars surface. An intensive Mars campaign would begin with a
suite of unmanned rn~ssions that would establish surface measure-
ment networks and return geologic samples that had been selected
and gathered by automated techniques. We know from earth field
studies and from the Apollo experience that the scientific utility
of these returned samples will be limited by lack of information
about their context. The intensive study of Mars must ultimately
be extended to incorporate certain capabilities that currently only
humans can supply: human intelligence is needed for reasoned
sample selection, to provide contextual documentation, and to am
sist in the interpretation of sa~nple analyses. In the meantime we
should develop robotics and artificial intelligence, but these devel-
opments will not obviate the requirement for a human presence
on Mars' surface to support the later phases of the Mars intensive
study.
RECOMMENDATIONS
The goals of planetary exploration are achieved primarily
through the analysis of data returned from spacecraft missions.
Complementary remote observations are obtained using telescopes
in the vicinity of Earth. Physical information on planetary ma-
terials is acquired from terrestrial prototypes, laboratory investi-
gations, and earth orbital observations. In addition, theoretical
modeling furthers scientific understanding. A viable planetary
program must contain all five elements spacecraft missions, tele-
scope observations, field investigations, laboratories, and models-
and the task group's recommendations speak to each.
Melioration of the Solar System
Figures 2.2, 2.3, and 2.4 in Chapter 2 show the status of plane-
tary exploration expected by 1995, and the missions recommended
for the period 1995 to 2015. The scientific objectives of these m~s-
sions are outlined in Table 2.1. It must be emphasized that the
near-term and the far-term planetary exploration projects are pro-
posed in a logical order, following the sequence of reconnaissance,
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exploration, and intensive study. However, no priorities are im-
plied by the ordering. Thus, if the status of planetary studies in
1995 is not as projected, then those studies that have not been
completed should still have priority over the longer-term projects.
The proposed missions follow the phased approach for solar
system study. By 1995, the exploratory and reconnaissance phases
should have been completed for the inner planets with the impor-
tant exception of Mercury. This planet lies deep in the Sun's
gravitational well, so that it is difficult for spacecraft to reach,
particularly with approach velocities small enough to allow cap-
ture into an orbit. Nevertheless, the unique properties of Mercury,
especially the high mean density, which is probably indicative of
strange internal chemistry and mineralogy, make its study of the
greatest interest for comparative planetology. A Mercury orbiter
mission wall be needed to complete the reconnaissance stage of
that planet's study.
The other proposed inner planet mission concepts involve the
intensive study phase. For the Moon, the near-term geophysical
studies from orbit wall undoubtedly raise questions requiring ad-
ditional sample return for their resolution. Some of these samples
will be needed from the far side of the Moon, and one contemplated
project would select and return such samples. Complementary to
the sample return will be global geophysical studies of the planet,
for which the establishment of a network of stations will be re-
quired. In due course we expect that there will be numerous
practical reasons for establishing a permanent base of operations
upon the Moon, and the course of lunar research outlined here will
prepare the way for such a manned lunar base.
For Mars, the stage of intensive study began with the Viking
mission and will continue with the Phobos mission. The next ma-
jor steps, which are of the greatest scientific importance, will be
Mars rovers, the establishment of a global sensor network, and the
selection and return of samples for analysis in terrestrial labora-
tories. When the sphere of human habitation in space enlarges to
encompass Mars, the establishment of a manned base there will
become desirable, and the studies outlined here will be necessary
precursors.
In the case of Venus, a good map is partially in hand; com-
pletion is expected with the planned radar mapper mission (Mag-
elIan). Current lack of this map inhibits detailed projections for
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future mussions. An initial set of geochem~cal and mapping infor-
mation has been obtained from Soviet investigations. The hostile
environment of the planet requires much more technological devel-
opment for future missions than is the case for the other terrestrial
planets. Nevertheless, the kind of geophysical and geochemical in-
formation desired from Venus is similar to that desired from the
other terrestrial planets, and the means needed to acquire this will
include probes, the establishment of a global network, and sample
returns. Accomplishing these objectives will provide interesting
technological challenges.
Detailed planning for the intensive study of Mercury must
await the unaging of the unseen hemisphere and the geophysical
and geochemical mapping that will be done in an orbiter mission.
With those data in hand it will be possible to plan the kind of
surface investigations a lander will allow and to plan the return of
samples for laboratory analysis. It is desirable that these follow-on
missions should be done within the contemplated time period of
this study.
For the four giant planets of the outer solar system the recon-
naissance stage of planetary study requires an orbiting spacecraft
and an atmospheric entry probe. By the 1995 to 2015 period this
should have been accomplished for Jupiter by the Galileo mission.
Orbiter and probe missions for Saturn, Uranus, and Neptune are
then of the highest priority for the outer solar system. In the case
of Jupiter, a different kind of orbiter one in a polar orbit- is
needed as a follow-on to Galileo in order to study the rich inner
magnetosphere of the planet In detail.
The satellites of the giant planets are of great interest, espe-
cially the larger ones. The Galileo mission Is expected to complete
the reconnaissance of the Galilean satellites of Jupiter. Titan, the
large satellite of Saturn, has a substantial obscuring atmosphere
and thus should have a dedicated orbiter-probe mission of its own.
The Saturn orbiter, after delivery of its probe, may become ded-
icated to studies of the rings, and thus would be unavailable for
reconnaissance of the more distant regular satellites. In this case,
a separate orbiter would be desirable to examine these satellites.
The system of Pluto, consisting of an icy planet of low mass
with a satellite of high relative mass, is of great intrinsic interest.
By 1995 it will not have undergone exploration from space. It is
therefore recommended that, owing to the very long flight time
to the planet, the exploration and reconnaissance stages of study
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be combined, and performed by a dedicated orbiter. Extensive
earth-based observations of the system, using the Space Telescope
and other major instruments, will be needed to plan this mission.
Once the outer planet missions outlined above have been car-
ried out, the stage of intensive study can begin. For the gas giant
planets deep probes will be needed! in order to study the atmo-
sphere to greater depths and to refine the measurements made
with the first generation of entry probes. Such deep probes are
recommended for Jupiter, Saturn, and Uranus by 2015.
Intensive study of the satellites of the giant planets will require
surface investigations by means of landers, which should be able
to emplace networks. It Is recommended that several selected
satellites of the Jupiter and Saturn systems be investigated in this
way by 2015. A lander of special design will be needed for Titan
in view of its atmosphere; such a lander mission should be a rich
source of information both about the atmosphere and whatever
kind of surface exists, even if covered by liquid, but the design of
such a lander must await results from the Titan orbiter-probe.
The reconnaissance of comets should have commenced by
1995 with a comet rendezvous mission. The material constituting
comets is likely to be the most primitive form of matter preserved
from the environment of the early solar system that will ever be
available to us. Thus sample return is vitally important for further
studies of comets. It is recommended that a fragmented sample
be returned by means of a fast comet flyby spacecraft that can
easily return to the Earth. This should be followed by a more
advanced mission involving a rendezvous in which special care will
be taken to obtain and maintain several samples of the comet in
their pristine form, and to continue to maintain them in that state
during the return to the vicinity of the Earth.
It is likely that a small number of asteroids will have been
examined in a flyby mode by 1995. However, these bodies are
highly diverse in their properties, and the early glimpses should
be followed by a multiple rendezvous mission, in which a variety of
surface analyses can be made, including some interactive analyses.
These would pave the way for a later mission that would return
samples from asteroids.
The techniques required for the search for other planetary
systems will continually improve. The task group recommends
a dedicated telescope associated with the space station for this
purpose. Even larger facilities, perhaps space-based and perhaps
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lunar-based, will be valuable for this purpose as well as for many
other uses that require high-resolution imaging of the solar system
and beyond.
Earth-based studies are envisioned as a continuous program
throughout the 1995 to 2015 period. Included are observations
from Earth and near-Earth orbit of solar system objects ~d the
search for other planetary systems, laboratory experiments related
to planetary processes, and analyses of meteorites.
Most of the missions shown in Figures 2.3 and 2.4 can be
achieved with existing or near-term technology. The study of
Uranus, Neptune, and many of the small bodies will require the d~
velopment of low-thrust propulsion. The intensive study of Venus
cannot begin without extending the high-temperature survival of
electronics; in addition, the return of Venus samples will require
significant developments in propulsion. The task group recom-
mends that development efforts in these areas of technology be
initiated as soon as possible.
Telescopes on Earth and in earth orbit complement space
probes by providing observational data that, while usually of lower
spatial resolution, can be synoptic in scope and quickly respon-
sive to phenomena. The task group recommends support of a
vital program of planetary astronomy and particularly encourages
preparations to use new generations of astronomical telescopes for
planetary observations.
An integral part of any rn~ssion is adequate support for analy-
sis and interpretation of the data returned from it. Studies in the
field, in earth orbit, and in laboratories are required to provide cor-
roborating aground truth, calibrations, and fundamental physi-
cal constants. The task group recommends continual upgrading of
laboratory instrumentation and of computing equipment used for
data analysis and theoretical modeling.
The missions and activities outlined here would address the
objectives of solar system exploration outlined earlier. We have
learned a great deal in the past 20 years, and the next 30 can be
even more productive. We can also expect to support the exten-
sion of the sphere of human habitation into space by improving
our knowledge of planetary environments. The task group recom-
mends that the program be implemented as vigorously as allowed
by economic and national goals.
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Melioration of Mare
The task group strongly recommends a continuing scientific
exploration of the entire solar system, primarily using instruments
on automated spacecraft. There Is much to be done to complete
even a first-order look at our planetary neighborhood. That in-
vestigation must continue. But a major Mars campaign could be
carried out concurrently, in the same way that the first decade
of planetary exploration was carried out by Mariner spacecraft in
parallel with the Apollo Moon program.
Mars is the most Earth-like of the other planets, displaying
the bull range of terrestrial phenomena (except, possibly, life),
although frequently in greatly modified form. Mars is an ideal lo-
cation for the study of a long geological history parallel to that of
the Earth, including both volcanic and tectonic activity; for exam-
ining the chemical evolution of an atmosphere and its interaction
with a surface, for investigating a complex meteorology includ-
ing cyclic transport of volatiles between surface, atmosphere, and
polar caps; and for exploring evidence of climatic cycles. The
presence of channels of a variety of ages indicates episodic flow of
large amounts of water. Mars may also have preserved evidence of
prebiotic chemical evolution, or even possibly of the development
and evolution of an indigenous biota.
In the extensive exploration of Mars envisaged here, humans
will play an essential role. Thousands of kilometers of the sur-
face will need to be traversed and explored geologically in detail.
Samples must be collected and given preliminary in situ analysis,
holes bored and cores extracted, and automatic stations to mon-
itor meteorological activity, se~smicity, and heat flow emplaced
and maintained. In some areas, particularly on and near the
polar caps, extensive drilling and perhaps excavation should be
undertaken. In order to reduce costs and increase the efficiency
of the operation, equipment for the manufacture of rocket fuel
and of essential water and oxygen for personnel must be estate
fished. It is difficult to imagine that such an extensive, detailed,
planet-wide exploration program conic! be carried out effectively
by autonomous robot vehicles alone. The direct application of
human knowledge and ingenuity to the detailed exploration of a
new world is likely to lead to the maximum return and the deepest
level of understanding.
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Representative terms from entire chapter:
sample return
