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OCR for page 8
2
Exobiology
WHAT IS E:XOBIOLOGY?
Throughout history, humanity's creation myths appear to re-
flect each culture's perception of the dimensions of its universe
and its place within it. Today, the scope of those perceptions has
expanded beyond the reaches of the solar system to the stars and
the interstellar clouds that populate the seemingly limitless ex-
panse of space. We see life as the product of countless changes in
the form of primordial stellar matter wrought by the processes of
astrophysical, planetary, and biological evolution. The science of
exobiology attempts to reconstruct the natural history of processes
and events involved in the transformations of the biogenic elements
from their origins in nucleosyntheses to their participation in Dar-
winian evolution in the solar system on planet Earth. Prom this
reconstruction will emerge a general theory for the evolution of
living systems from inanimate matter.
The goal of exobiology is to increase knowledge of the origin,
evolution, and distribution of life in the universe. This is a mul-
tidisciplinary science, and the conceptual and experimental tools
of virtually all scientific disciplines and branches of learning are
relevant. In seeking answers to such questions as how the devel-
opment of the solar system and its planets led to the origin of
8
OCR for page 9
9
life on Earth, how planetary evolution subsequently influenced the
course of biological evolution, and where else life may be found
in the solar system and beyond, exobiology brings together life
scientists and physical scientists in a common interest.
Exobiology is concerned with four evolutionary epochs: (1)
cosmuc evolution of biogenic elements and compounds; (2) pre-
biotic evolution; (3) early evolution of life; and (4) evolution
of advanced life. Each of these epochs, briefly described below,
represents a major arena of research. Between now and the mid-
l990s, the task group expects this conceptual framework to become
widely acknowledged and to act as a stimulus for interdisciplinary
attacks on exobiological research problems.
1. Cosmic Evolution of Biogenic Elements and Compounds.
The first epoch encompasses galactic time and distance scales
and involves the death and birth of stars. It begins with the
synthesis in stars of the biogenic elements the elements that
make up all life and their ejection into the interstellar medium; it
ends with the distribution of these elements and their compounds
throughout our solar system within the planetoids, which became
building blocks of planets. Discoveries in carbonaceous meteorites
strengthen this perspective, as organic and mineral matter made
up of carbon, nitrogen, hydrogen, and oxygen has been found that
retains properties traceable to its origins in interstellar clouds and
stars.
How cornrnonly the aggregation of interstellar dust and gas
into small primitive bodies occurs during star formation is not
known. What transformations were undergone by the biogenic
elements and their compounds during this process remain poorly
understood, as are the ways in which the physical and chemical
properties of these elements and their compounds may have in-
fluenced the course of events during the formation of the solar
system.
Answers to these questions will develop, however, as astro-
physicists and astrochemists take advantage of the capabilities
for large-scale modeling, and make use of sensitive space-borne
astronomical telescopes with high spatial and spectral resolution
to make observations of condensed matter in protosteliar regions.
In ground-based laboratories and by remote spacecraft, studies of
interstellar dust and samples of other relict material—meteorites,
comets, asteroids, and interplanetary dust- will continue to help
OCR for page 10
10
reconstruct the nature and chronology of the processes that took
place at the time of the solar system's formation.
2. Prebiotic Evolution. This epoch begins with the accretion
of planets and ends, in the case of Earth, with the emergence of
living organisms after nearly 1 billion years. This is the epoch for
which there has been found no geological record on Earth and,
therefore, no direct basis on which to reconstruct the conditions of
the environment. Yet it is the period in which life emerged from
inanimate matter. For any planet this ~ the period of most rapid
environmental change as the energy of accretion and radionuclide
decay dissipates, and the planet undergoes a transition to either
geological inactivity or to a steady state. Prebiotic chemical evo-
lution is inseparable from planetary evolution, and the path that
leacis to the origin of life may be terminated if the development of
a planet takes the wrong turn.
In what ways and how fast were conditions changing on Earth
in its first billion years when life originated? Were conditions
initially similar on Mars or Venus? What types of geological
settings were maintained far from equilibrium and conducive to
the origin of life? What processes and reactions involving the
biogenic elements were important for the origin of life?
Answers to these questions can come from several sources:
the application of planetary geophysics and geochemistry to the
development of models for Earth's earliest history, the deciphering
of the existing geological record among the terrestrial planets, and
laboratory simulations of prebiotic reactions.
3. Early Evolution of Life. The third epoch begins with
the emergence of living systems prior to 3.5 billion years ago,
and carries through to the evolution of multicellular organisms,
which appear in the fossil record about 1 billion years ago. The
history of life in this period is largely the history of microscopic
unicellular organisms. This period is also a time of continued
change in environmental conditions, although less intensive or
rapid than In the previous epoch, as the physical evolution of Earth
moved into a more moderate stage. The Sun gradually became
more luminous, the frequency of large-scale accretionary impacts
declined, the Moon's separation from Earth increased, volcanism
declined, the continents grew in volume and wandered across the
face of the planet, oxygen began to appear in the atmosphere, and
the geomagnetic poles reversed.
OCR for page 11
11
In this epoch the major questions concern the relationship be-
tween the evolution of unicellular life and the climate and changing
geology of the Earth. What was the path by which the ancestors
of modern microbes evolved from the first living organisms? In
what ways did physical and chemical changes in the environment
influence the rate and direction of microbial evolution? Over
what geological time scales did major events in mucrobial evolu-
tion occur? How did the evolving biota modify and modulate their
environment over time? What was the nature of the earliest forms
of life, and in what sequence were new attributes acquired? What
are the supplest biochemical mechanisms and biophysical struc-
tures that can fulfill the functions of living systems, and what
irreducible combinations of these constitute a living entity? Did
life arise on Mars or on other planets during this time, and if so
what changes in those extraterrestrial environments would have
led to its extinction?
The answers to these questions hinge on our ability to iden-
tify and obtain inorganic and organic fossils, and to decipher the
record of geological and biological evolution through the layers of
alteration and mutation accumulated over several billion years.
This means that the right rocks and organisms must be studied;
the phylogeny of microbial life must be tied to the chronology
of geological change; and the time resolution with which we can
discern changes in these two records must become more finely
tuned.
4. Evolution of Advanced Life. The fourth epoch deals with
the most recent billion years of the history of life, in which mul-
ticellular plants and animals and intelligent species evolved. Exo-
biological interest in this epoch grew out of the realization that
collisions of asteroidal-sized objects with Earth produce global
changes in its surface environment. That these changes would
perturb the biosphere cannot be denied; whether they can ac-
count for major extinctions in the course of biological evolution,
particularly events that may have placed advanced life on its evo-
Jutionary track to intelligence, remains to be determined. What IS
clear, however, Is the importance of gaining more knowledge about
the relationship between biological evolution and changes in the
environment.
OCR for page 12
12
PLANETARY E::gP[ORATION AND TEE NEED FOR
SPACE DATA
In this search for knowledge about the origin of life as a natural
process, NASA must explore as many extraterrestrial bodies as
possible for the relevant information they can supply. The search
should include bodies totally devoid of organic chemicals, those
conceivably undergoing (or having underdone) organic chemical
evolution, and those possibly harboring life.
The study of lifeless planets provides examples of environ-
ments where chemical or biological evolution ended. On them it
may be possible to find the remnants of chemical evolution or of
past life and to learn how planetary changes may have broken
the thread of chemical or biological evolution. The discovery that
there is no life, extant or extinct, or no organic matter on a planet
is of high interest because the conditions on the planet and what
we can learn of its past history constitute basic data pertinent to
a general theory of the origin of life.
Mars continues to be an extremely interesting site for explo-
ration and study for exobiology. Although the Viking mission
fount} no evidence of extant life, the search was Ignited and not
directed at optimum sites. In recent years, dicoveries about the
earliest history of life on Earth have reemphasized the need to
examine the geological record of Mars. Compelling evidence for
flourishing communities of microorganisms has been found in 3.5-
billion-year-old sedimentary rocks of Australia and South Africa.
Even if no martian life exists, the evidence of liquid water and a
more clement epoch in the first billion years of martian history, at
the time when Earth already had a thriving microbial biosphere,
has important implications. The possibility that life arose on Mars
early on and subsequently became extinct must be kept open and
investigated whenever the opportunity to send missions to Mars
arises. In addition, Mars exploration may provide a geological
record of the first billion years of planetary evolution, for which
little trace has yet been found on Earth.
The Mars Orbiter Mission (MOM) will be invaluable for pro-
viding data that will aid in identifying sedimentary and other
types of environments that have resulted from the interaction of
liquid water with the planetary surface. Further characterization
of such sites should be carried out with landers or penetrators
capable of making in situ measurements both at the surface and
OCR for page 13
13
at depth. These measurements should provide further analysis of
the chemistry and mineralogy of the surface and subsurface rocket
They should determine the nature and abundances of the forms
in which the biogenic elements occur. These explorations should
provide a sound basis for site selection for future sample return
· ~
mlmlons.
The unusual chemistry of the martian surface soils, manifested
in the biom~metic responses elicited by the Viking biology experi-
ments, continues to be intriguing and inadequately understood; it
may contain important clues to the role of minerals and inorganic
chemistry in prebiotic evolution. Elucidation of this chemistry
may be attained in part by conducting experiments with landers
and penetrators, but a full understanding will probably require
detailed chemical, mineralogical, and geochronological study in
earth laboratories of a sample returned from Mars.
It Is conceivable that a sample return mission could be
mounted early in the first decade of the twenty-first century, in
which case it should be given highest Anion priority for exo-
biology. By the m-1990, planning for integrated exobiological
and geological field investigations should get under way in the
event that sample return missions will involve human rather than
robotic activity on the martian surface. Microscale methods of
analysis for characterizing and determining the origin of chemical
and mineral phases composed of the biogenic elements should be
developed for use in both remote and terrestrial laboratories.
Comet sample return is high on the mission priority list.
Earth-based astronomical observations of comets, together with
the Halley encounters, have provided a wealth of data on the com-
position of volatile gases in the coma and the properties of the
dust. From these observations conclusions may be drawn about
what comets are composed of and how they behave while under the
influence of the Sun's radiation, gravitation, and magnetic fields.
But major questions about comets remain unanswered- their ori-
gin, the identity of higher molecular weight organic compounds,
the nature of material contributed by interstellar and solar new
ular sources, the ages of their components, and their histories of
accretion and thermal and dynamical evolution.
All of these questions are pertinent to exobiology, insofar
as comets are composed largely of water, organic matter, and
other materials containing the biogenic elements. Comets provide
a "fossils record of the materiab and processes involved in the
OCR for page 14
14
transition from the diffuse realm of gas and dust represented by
interstellar clouds to the highly condensed realm of planetary
objects. As with carbonaceous meteorites, which are thought to
have been derived from either primitive asteroids or "burnt out"
comets (or both), this record can best be studied with samples.
The maximum scientific return will be obtained from samples that
have been collected and preserved in a state as close as possible to
that of their original storage in a comet.
Comets and asteroids are the remnants of star formation in
a system in which planets formed and life arose. Detection of
planets orbiting other stars would represent a major milestone in
establishing the generality of planetary solar systems and increas-
ing the probability that life could have arisen elsewhere in the
galaxy. Searches for cometary and asteroidal matter associated
with other stars or with protostelIar objects will provide a basis
for determining the frequency of occurrence of preplanetary mat-
ter containing the biogenic elements. By the turn of the century,
capable astrometric and infrared telescopes should be available,
and searches for planetary and preplanetary objects should have
broadened. In the outer solar system, Titan will continue to be a
target of interest for exobiology; it represents a natural laboratory
for the study of planetary organic chemistry. Titan provides a
Urey-Miller (in contrast to a Rubey) type mode! for Earth's prebi-
otic atmosphere, one which contains nitrogen and hydrogen and in
which methane is the predominant carbon source. The variety of
minor atmospheric constituents identified by the Voyager missions
and earth-based astronomical observations already testifies to the
organic chemistry taking place in Titan's atmosphere. However,
the origin of the major gases in the atmosphere, their relation-
ship to the accretionary and outgassing history of the satellite,
the full range of complexity in the organic chemistry occurring
in the atmosphere, and what process is responsible for which oW
served minor compounds will remain poorly understood. A Titan
atmospheric probe and a lander, capable of both characterizing
the molecular and isotopic compositions of materials composed of
the biogenic elements as well as measuring the abundances and
isotopic compositions of the noble gases, would address many of
these questions.
v ~ 7
Venus, with its atmospheric water, and the Galilean satel-
lites Callisto, Ganymede, and Europa, with their water ice, are
OCR for page 15
15
objects that contain clues to the history of water in planetary en-
vironments. Whether Venus has lost most of its initial aqueous
endowment, which may have been comparable in size to that of
Earth, or whether it was always strongly depleted in water is an
issue pertinent to understanding the mechanisms that distributed
and preserved the biogenic elements among the terrestrial plan-
ets and perhaps made Earth unique among them. The Galilean
satellites, if built up from planetesimals resembling carbonaceous
chondrites and comets, may also contain a frozen record of the
organic matter that survived planetary accretion. Venus and the
Galilean satellites wall be pertinent targets of exobiological interest
weD into the next century.
RESEARCH TOPICS
Formation and Evolution of Biogenic Elements and Compound
The biogenic elements are those that compose the bulk of life
and are generally thought to be essential for all living systems.
Primary emphasis is placed on the elements hydrogen, carbon,
nitrogen, oxygen, sulfur, and phosphorus. The compounds of
major interest include water and those normally associated with
organic chemistry. The essential elements usually associated with
inorganic rather than organic chemistry are also included- e.g.,
iron, magnesium, calcium, sodium, potassium, and chlorine—but
they are given secondary emphasis. Water plays a central role
in the development of life as we know it, and therefore in the
environments in which chemical evolution could have occurred.
Thus, special importance is attributed to the cosmic history of
water and its interaction with other substances of either organic
or inorganic nature.
It is useful to identify six stages in the cosmic history of
the biogenic elements and compounds: (1) nucleosynthesis and
ejection to the interstellar medium, (2) chemical evolution in the
interstellar medium, (3) protosteliar collapse, (4) chemical evolu-
tion ~ the solar nebula, (5) growth of planetesimals from dust, and
(6) accumulation and thermal processing of planetoids. In each of
these stages there are major scientific questions to be answered.
OCR for page 16
16
Nucleosynthesis and Ejection to the Interstellar Medium
This stage begins the cosmic evolution of the biogenic ele-
ments. Not only ~ it responsible for the origin of the elements, it
also initiates the condensation of solid matter out of the gas phase.
Astronomical observations should be made of supernovae, no-
vae, late type giant stars, circumstelIar shells, and planetary new
ulae where biogenic elements are being produced and ejected into
the interstellar medium. Whereas emphasis in the past has been
placed on gases, future studies should focus on characterizing the
dust and grains and determining the extent to which these primor-
dial solid condensates survive the transit from sites of stellar ori-
gin to interstellar clouds. Especially useful will be high-resolution
measurements at millimeter and infrared wavelengths. These mea-
surements will require the Large Deployable Reflector (LDR) and
the Space Infrared Telescope Facility (STRTF), which should be in
operation by the turn of the century.
Observations of planetary nebulae and the circumstelIar sheds
of late type carbon stars indicate the growth of carbonaceous
grains from the gas phase. The mechanisms by which the con-
densation processes occur are unknown and require elucidation.
Although future theoretical simulations of these processes will be
undertaken, the Space Station should offer microgravity condi-
tions highly suitable for experimental investigations. Provisions
for m~croparticle research should be included in the Space Sta-
tion's capabilities.
Chemical Evolution in the Interstellar Medium
Interstellar clouds serve both as the collectors of atomic and
dusty debris from stars in terrn~nal stages of evolution and as the
spawning grounds of new stars. In the course of cosmic evolution
they provide the first environments in which gas-gas and gas-
solid interactions occur between water, organic, and inorganic
compounds.
By the mid-1990s, the gas phase chemistry of {ow-molecular-
weight compounds in interstellar clouds should be reasonably well
known. For lack of observational tools, however, the chemistry and
the role of grains in interstellar processes will still be poorly under-
stood. The telescope facilities mentioned in the preceding section
should be used to characterize the biogenic elemental composition
OCR for page 17
17
and organic chemical content of interstellar dust and, thereby, to
begin closing this large gap in knowledge. These same facilities
should also be employed to search for interstellar methane and car-
bon dioxide, key starting materiab for organic chemical evolution
in the solar system.
Knowledge of the properties of high-molecular-weight organic
compounds in the gas phase will continue to be important in
the next century. This information is necessary to determine the
complexity of interstellar chern~stry and to allow mechanisms for
the formation of these molecules to be determined. With the
availability of suitable microwave spectra, sensitive searches for
glyc~ne and adentne, which are among the supplest molecular
building blocks of proteins and nucleic acids, can be carried out.
Computer modeling of interstellar grain growth and ga~grain
interactions should be used to explore the organic chemistry that
could occur on or in various types of grains. For experimental
approaches to such research, the microgravity conditions attain-
able on the Space Station could over advantages over terrestrial
environments. The occurrence of molecules predicted on the bash
of model studies could be tested by telescopic searches.
The acquisition of intact interstellar grains for detailed labo-
ratory studies of their structure and composition is a very exciting
prospect, and should be set as a goal for the turn of the century.
Especially important would be application of dating techniques to
determine the chronology of the presolar processes that produced
the interstellar grains. Throughout this time, substances of inter-
stelIar origin should continue to be sought in comets, interstellar
dust particles (IDP), and carbonaceous meteorites to establish
more firmly the continuity of cosmic evolution from interstellar
cloud to solar nebula.
Protosteliar Collapse
This stage in cosmic evolution encompasses the transition
from interstellar cloud to the nascent solar system. During proto-
stellar collapse, while temperatures remain at or below 20K, the
concentration of gas and dust undergoes enormous change over
approx~nately 7 orders of magnitude from the highly diffuse con-
ditions of interstellar clouds to the considerably denser state of the
solar nebula.
OCR for page 18
18
As more and more regions of protostar formation are oW
served, biogenic compounds e.g., CO, CS, H2CO, and HCN-
should come into increasing use as molecular probes to reveal the
physical conditions and the variations in chemical composition
that occur in these environments. Future observational studies of
small-scale structures in dark interstellar clouds should be made
with instruments currently unavailable such as a millimeter-wave,
very-large-array (VLA) radio telescope, or an orbiting LDR. Fur-
ther developments In astrophysical theory coupled with these oh
servations are expected to yield self-consistent models of collapse
dynamics. These models should be used for assessing the ex-
tent to which observed inhomogeneous distributions of the organic
compounds and water in interstellar clouds are preserved during
collapse. As knowledge of collapse kinetics and conditions grows,
calculations of increasing sophistication and reliability should be
carried out to determine the accompanying chemical and isotopic
fractionations (redistributions) that occur between the gas phase
and dust. These calculations could provide theoretical bounds on
the amount and distribution of interstellar matter composed of
biogenic elements and compounds that become incorporated into
solar system dust and which survive homogenization processes in
the solar nebula.
Chemical Evolution in the Solar Nebula
The solar nebula corresponds to the terminal state of proto-
stelIar collapse from an interstellar cloud. ~ this stage, temper-
atures increase, gas-solid interactions occur readily, energy fluxes
increase, turbulent mass transport of matter between environ-
ments that differ in temperature and composition can occur, and
solid objects larger than interstellar grains begin to accumulate.
Observations and theoretical understanding of protosteliar
systems in the mid-199Os should yield more tightly constrained
models of the solar nebula. Computer models should be devel-
oped to simulate the processes that may have contributed to the
chemistry and governed the distribution of biogenic elements and
compounds within the nebula over time. Among the processes
yielding organic compounds and carbonaceous grains that should
be studied are photochemistry due to starlight and the early Sun,
large-scare electric discharges, ion-molecuTe reactions in a partially
ionized nebula, and reactions of gases on grains.
OCR for page 48
48
Using just the first three criteria, the SSEC selected Mars
and a comet nucleus for their candidate missions, to be initiated
prior to the year 2000. These two targets nicely satisfy the fourth
criterion as well.
As the SSEC report states in the case of Mars, "Important
bioscience goals could also be achieved through a sample return.
The presence or absence of indigenous life especially fossil life
and the reasons for the surprising absence of organic matter in
the martian soil could be determined directly by the laboratory
examination of an unsterilized sample returned to Earth." The
report points out that comets ". . . are an entirely new and vir-
tually unstudied class of objects which, because of their primitive
nature, are fundamentally important to understanding the origin
and earliest history of the solar system. With a returned sample,
"a complete characterization of cometary organic material would
be possible, with exciting implications for understanding the origin
of life...." Mars and the comets are at the top of the task group's
list, too, and it strongly endorses the SSEC recommendations that
these be the first of the sample return missions.
Given that the task group's study of future missions extends
15 years beyond the limit accepted by the SSEC, the task group
can legitimately add some missions to the SSEC list. These might
include precursors to sample return missions such as soft lanclers
and penetrator networks. Titan is certainly a prime candidate for
a soft lander mission, similar to the Viking project that studied
Mars in 1976. We will need the results from the Cassini rn~ssion
before such a mission can be planned in detail. At that point we
will know whether we must use a lander that floats or whether a
more conventional spacecraft will suffice. It will also be possible
to address such issues as the location of the landing site and the
type of surface sampling devices.
There has also been great interest in a possible mission to
Europa, the smallest of Jupiter's Galilean satellites, which may be
covered with an ocean of liquid water whose surface is shielded by
an icy crust. Here, the next step in exploration after the Galileo
mission will be the deployment of some type of seismic network to
determine whether the subsurface ocean really exists. The pene-
trators that carry the seismometers could also be equipped with
devices to make some compositional analyses of their immediate
surroundings. Thus, it would be possible to determine, for ex-
ample, if the dark material associated with some of the ~cracks"
OCR for page 49
49
on the surface indeed connote of organic compounds as some have
suggested. Plans for a sample return minion or a subsurface probe
could then be developed after the results from this precursor mid
sion were analyzed.
It should be evident that the specific mmsions being consid-
ered here involve techniques for gathering data that can easily
be applied to other targets as well. Thus a comet nucleus sam-
ple return ITussion, once it has been successfully carried out for a
given comet, will allow the study of many other comets with the
same payload and mission strategy. Modifications in the payload
would permit the use of the same or similar spacecraft to carry
out sample return missions to selected asteroids.
The penetrator network described for Europa could be de-
ployed on asteroids and other satellites. Phoebe and the dark side
of lapetus would both be fruitful targets for such rn~ssions. Both
Of these satellites in the Saturn system have surfaces that seem to
be covered with carbon-rich material. The discovery by Voyager
2 that the dark material on the satellites of Uranus has distinctly
different optical properties from the material found in the Saturn
system invites direct exploration of the Uranus system as well.
Given the low temperatures in this part of the solar nebula, this
organic material could be even less modified than that found in
the carbonaceous chondrites.
The next two sections of this report wiD concentrate on the
Mars and comet nucleus sample return missions. The SSEC stud-
ies will form the background for this discussion, which win em-
ph~ize those aspects of these two missions that are especially
interesting to the life sciences. The final section will cover the soft
lander and penetrator missions to outer planet satellites.
Sample Return from Mars
In addition to the high priority awarded it by the SSEC, a
sample return mission from Mars was also implicitly endorsed by
the Space Science Board of the National Research Council 1978:
The study of Mars is an essential basis for our understanding of
the evolution of the Earth and the inner solar system.... We
recommend that intensive study of Mars be achieved within the
period 1977-1987~ (Strategy for Exploration of the Inner Planets:
1977-1987, National Academy Press, 1978~.
It is now clear that this recommendation will not be satisfied
OCR for page 50
50
in the stated time interval. As Table 2.1 indicates, the first U.S.
mission to follow the success of the Viking project will be the
Mars Orbiter Mission (MOM). Nevertheless, sentiment is building
to support a Mars-intensive program that would culminate in the
return of one or more samples. This might be done as a cooperative
project with the Europe or the U.S.S.R., continuing a welcome
international participation in planetary exploration. The sample
return missions can also be viewed as precursors for the still more
advanced idea of manned Mars missions, ultimately leading to the
establishment of a base on the martian surface.
The return of samples from another planet is obviously a very
expensive proposition. In the case of Mars, a large number of
different scientific inquiries can be aided by the ability to carefully
study preserved samples that are successfully returned to terres-
trial laboratories. This means that great care must be exercised
in the choice of landing site and sampling strategy in order that
the greatest number of scientific objectives can be met. It will
be necessary to return a variety of carefully chosen samples taken
from sites that have been selected with specific objectives in mind.
This means that the sample identification and selection devices
must be mobile; some type of intelligent rover will be required.
Even with careful planning, there will be conflicts. From a
geological point of view, the key units for sampling in order of
decreasing priority are young volcanic units, intermediate-age vol-
canic units, ancient cratered units, layered units, polar units. The
last two have the highest priority from a biological perspective.
Areas where major sedimentation has occurred may provide the
best opportunity for collecting samples that contain evidence for
past life on Mars. Of special interest are those regions where
standing or sIow-moving water is thought to have covered the sur-
face. It may well be possible to combine a sampling expedition to
an area of sedimentation with one of the higher ranking geological
objectives. But investigation of the polar regions will require a
· —
separate mission.
The poles are important for several reasons. The north pole
is already known to be covered by a permanent water ice cap.
The long-term availability of water even in the form of ice at the
planet's surface could have been essential for any surviving indige-
nous organisms. Both poles serve as cold traps on the planet. They
are known to be regions of aeolian deposition; layered terrains have
been observed in both locales. These are therefore regions where
OCR for page 51
51
we Alight expect primitive organic material to be preserved, pro-
tected from the hostile surface environment by layers of ice. To
reach this material, it will be necessary to drill through the per-
manent cap. The missions being designed for Mars sample return
include the capability of landing anywhere on the planet. The
rover will be equipped with a drill, a core tube, and a sampling
arm, all of which could be put to good use on a polar cap mission.
Finally, it is unportant to stress that many of the geological
studies, especially those aimed at an understanding of the history
of water and surface-atmosphere interactions on the planet, will
be of great interest to the biological community.
The troublesome problems of contamination of Mars and back
contamination of Earth require additional study. Both biologists
and geologists agree that the samples returned to Earth should
not be sterilized, since the required heat or chemical treatment
could have a seriously deleterious effect on the samples. The
Space Station might play a role here by providing a specialized
environment in which preliminary analyses of unsterilized samples
could be carried out before shipping them to Earth. One way of
using this facility would be to sterilize a small, carefully selected
portion of a sample that could then be shipped to Earth for
detailed study. The major portion of the sample would remain
on the Space Station, presumably undergoing tests for dangerous
biological or chemical activity.
This approach will require careful thought and international
agreement on the kinds of tests to be run before the samples are
considered "safe." Biological safety could be assured in a two-step
process in which a preliminary set of tests are run in orbit, while
a more sophisticated set would be carried out in laboratories on
Earth. This two-tiered procedure would reduce the cost associated
with a fully-equipped laboratory on the Space Station.
It Is not possible to look beyond sample return missions with
much certainty. The discovery of viable organisms in returned
martian samples would have a profound impact on strategies for
future exploration of the planet. In the more likely case that the
samples are sterile, it still seems inevitable that momentum for a
manned mission to Mars will continue to grow. The maintenance of
the people sent on such a long mission, including their survival on
Mars, is another aspect of the problems confronted by specialists
in space medicine, as described elsewhere in this report.
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Comet Sample Return
Comets potentially offer the leasLaltered samples of solid ma-
terial still available in the solar system. This is demonstrably true
of substances whose volatility is equal to or less than that of carbon
dioxide, and probably holds for still more volatile compounds as
well. A comet nucleus has been likened to a dirty snowball, where
the snows are now known to consist predominantly of ices of water
and carbon dioxide, and the dirt includes carbon-rich compounds
in addition to silicates. Here then is an opportunity to examine
some of the solid materials from which the planets accreted, un-
changed since the solar nebula first formed. Comets represent the
solar system's starting conditions, providing examples of the kind
of chemistry that went on before there were planets. This includes
examples of materials that have been continually delivered to the
surfaces of planets sometimes catastrophically—but sometimes
yielding starting materials for new chemical reactions. It is by no
means established that organic material from comets contributed
to the famous prunordial soup from which life on our planet arose.
Yet this is one of several current hypotheses regarding the first
steps for the origin of life, and it is certain that comets must have
contributed some fraction of the volatile elements that dominate
the chemistry of life as we know it: the hydrogen, oxygen, carbon,
and nitrogen from which we are mainly composed.
These considerations make the investigation of comets of great
interest to the life sciences. Accordingly, the task group enthusia~-
tically endorses the recommendation of the SSEC that "the return
of a sample from the nucleus of a comet is one of the highest pri-
orities for an augmentation mission and should be undertaken as
soon as possible."
It is only by bringing samples back to our laboratories on
Earth that we can perform the full array of experiments with the
finesse required to tell us what we want to know. What compounds
are present and in what proportions? Are they really pristine, or
is there evidence of some processing during the comet's lifetime?
How does this suite of organic compounds compare with those
found in carbonaceous chondrites? in the interstellar medium? in
laboratory experiments? Are there clues from isotope ratios or rare
gas abundances that can let us decide what fraction of terrestrial
volatiles were actually brought to Earth by these icy messengers?
Do comets represent frozen primordial soup? That is, would a
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melted comet constitute a good starting point for further chemical
evolution leading ultimately to life? These are just some of the
questions we would like to answer.
The Choice of a Suitable Comet. As the news from the Halley
observations by spacecraft and Earth-based techniques trickles
in, it ~ clear that early ideas suggesting that comets contain
substantial amounts of organic material are indeed correct. The
special attraction of comets arriving in the inner solar system for
the first tone Is that they may have been maintained at very low
temperatures (less than 20K) since the solar system began. Even a
comet like Halley's, which has been trapped in a short-period orbit
that forces repeated visits to the vicinity of the Sun, displays the
full range of phenomena and the variety of molecular fragments
that are found in "fresh comets.
In choosing a comet for a sample return, however, we will
have to pick from less active objects that are in orbits of still
shorter periods than Halley's majestic 76 years. That means they
are, on average, much closer to the Sun and wait therefore have
lost some of their most volatile constituents. Nevertheless, we
know from ground-based spectroscopic observations that there are
comets even in this category that are actively emitting gas in which
molecules such as ON, C2, and Cat can be identified.
Furthermore, the icy nuclei of all comets whose nuclei have
been directly observed are found to be very dark. They do not
exhibit the high reflectivity associated with the ices we know they
contain. For example, the nucleus of Halley's Comet was found to
be nearly twice as large as the size calculated from its assumed
reflectivity, because it was actually much darker than had been
thought. Evidently, these comet nuclei are coated with dark ma-
terial, which apparently becomes concentrated as the ices sublime
away, much as lag gravels are left behind during aeolian erosion.
With reflectivities below 5 percent, this material must contain a
large amount of carbon. Hence the surface layers of a short-period
comet represent a concentrated sample of the organic material
that ad comet nuclei presumably contain. Thus, it may be con-
cluded that we should be able to answer many of our most pressing
questions by sampling one of the small, short-period comets that
are relatively common and easy to reach. To make this enterprise
worthwhile for scientific purposes, however, we must require that
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(1) the comet IS still actively giving a* gas, and (b) the gas shows
the presence of C3, C2, CH, and CN radicals.
Mission Description. "It is now technologically within our reach
to collect pristine samples directly from active comets and to
return them to Earth for intensive laboratory study.
This bold statement from the SSEC report sets the tone for
the mission. While bringing back samples from the surface of a
comet nucleus is obviously of great interest and must be done,
we also want to obtain a deep core sample. This offers the best
opportunity to obtain material that has undergone only minimal
heating, outgassing, and exposure to radiation that would lead to
alteration of the original state of the material.
The extent to which this is possible will depend on the par-
ticular comet that ~ sampled. It is likely that we shall have to
sample several nuclei of different comets that are at various stages
of disintegration (devolatilization) before we will know just how
primitive (or representative) is a particular sample. But even if
the most volatile substances such as carbon monoxide, molecular
nitrogen, methane, and argon are deficient or absent, we shall be
very pleased to be able to obtain samples containing cyanides,
aidehydes, and the other more complex substances that are re-
sponsible for the dark coatings on these icy nuclei.
The basic mission profile as currently envisaged is as follows:
1. Rendezvous with the comet.
2. Spend some time observing the comet's behavior and se-
lecting appropriate sampling sites.
3. Obtain at least two separate core samples approximately 1
m long and 6 cm in diameter.
4. Deploy a long-lived surface lander to be left on the nucleus.
5. Return to Earth with the samples contained in an environ-
mentally controlled capsule.
This may sound straightforward, but there are a number of
serious technical concerns to address before such a mission can
be realized. The first of these is the propulsion system. This
mission requires a low-thrust capability that is currently (1986) not
available. An example is the solar powered ion drive, under study
in both the United States and West Germany. An engine of this
general type could provide a low, but continuous propulsion that
would allow the spacecraft to carry the heavy payload required
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ss
on the round trip to the nucleus and to carry out the necessary
rendezvous and reconnaissance maneuvers at the target comet.
A second major problem is the hazard presented by dust that
is expelled by the comet nucleus along with the subliming gas.
Both the Vega and the Giotto spacecraft were damaged by dust
from Halley's Comet; this was a much greater problem than had
been anticipated. The comets selected as targets for sample return
will be far less active than Halley, however, and this problem needs
to be reevaluated after the CRAP mission has successfully visited
one of these better behaved candidates.
Finally, the questions of sample handling and contamination
problems will have to be studied carefully. Despite familiarity
with extraterrestrial samples through the Apollo program, the low
temperature, highly reducing, volatile-rich environment of a comet
nucleus will provide new challenges that should be anticipated as
part of the mission planning. It would be ironic indeed if the effort
and expense of a comet nucleus sample return mission were wasted
in the end because of an inability to examine the sample properly
on Earth.
In Situ Studies of Titan
On the assumption that the Cassini mission (or some surrogate
that accomplishes the same objectives) will have made a successful
study of Titan by the year 2005, we can ask what the next stage
of exploration of this satellite would be. This assumption implies
that we will know the following:
1. The location and extent of lakes, seas, or oceans of liquid
hydrocarbons (principally ethane) on the surface of Titan.
2. The composition of the atmosphere to the level of 1 ppm
(better in some cases), including isotope ratios for abundant ele-
ments and identifications and abundances of volatile organic com-
pounds.
3. A first-order characterization of the chemical composition
of aerosols collected during descent.
4. The temperature and pressure profile of the atmosphere
along the descent trajectory and the vertical distribution of clouds
along this same path.
5. A crude characterization of the surface at the probe's im-
pact site. (This may be no more than a test of whether the surface
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is solid or liquid, but it might include some information about the
composition of that solid or liquid).
6. A variety of other information about atmospheric winds,
the inorganic composition of the surface, constraints on internal
structure, and so on of less immediate relevance to the task
group's specific concerns.
The next step in exploration will require some type of soft
lander with a capability to move around on the surface. Depending
on the nature of that surface, this could be either a rover, a boat,
or some type of amphibious device. An alternative approach would
be the use of a powered dirigible with the capability of sampling
the surface by means of one or more instrument packages lowered
on a tether.
With so much uncertainty remaining about the nature of the
environment we wish to explore, it is not possible to be much
more specific than this. It seems highly probable from our current
perspective that Titan will have some liquid hydrocarbons on its
surface. We will surely want to examine these—determining their
composition, perhaps looking for evidence of optical activity in the
organic molecules dissolved in them, scouting their shores for con-
centrations of material deposited from the atmosphere. If there
is a global ocean the need for surface mobility ~ less obvious, but
we would still like to be able to investigate the atmosphere over a
wide range of latitudes. The dark polar cap and the hemispherical
asymmetry in the reflectivity of the smog layer observed by Voy-
agers ~ and 2 imply that different chemical reactions are occurring
at different locations in the atmosphere. This is borne out by the
spectroscopic evidence for latitudinal gradients in the abundances
of hydrocarbons and nitrites.
At the present time, we have no specific information about the
chemical composition of Titan's aerosols. We only know that they
must be a combination of condensates of the volatiles detected
spectroscopically and polymers produced by the irradiation of
these polymers. The Cassini probe is being designed to have some
capability for collecting and analyzing these aerosols, but only
along the single probe entry trajectory. To investigate latitudinal
differences in chemistry, a very ambitious mission will be called
for, but it is simply too early to define it.
The same impediment interferes with efforts to specify a pay-
load. The main thrust of this mission will be to understand the
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chemical reactions taking place on Titan. What processes are
involved and what are they producing? Are there any preferred
pathways toward complexity? Are any catalysts available and
what role do they play? Do the molecules produced show any
specific optical activity? Have there been any changes in these
processes over geologic time? Are further reactions occurring at
the surface in addition to those in the atmosphere? What rele-
vance, if any, does this history have to the organic chemistry on
the primitive Earth?
Attempts to answer these questions (and others likes them) will
require an array of sophisticated instruments operating in a benign
environment that is maintained in spite of ambient temperatures of
94K or less. This will be a formidable challenge, but one that surely
can be met. The opportunity to explore this natural laboratory
(and repository) already seems well worth the effort. The task
group anticipates that the results from the Cassini mission will
simply increase the appeal of this next step in the exploration of
Titan.
CONCLUSIONS AND ~ CO~IENDATIONS
~ The set of missions described in Table 2.1 implies that
significant advances in our understanding of the various members
of the solar system will be achieved by the year 1995.
The Task Group on Life Sciences particularly endorses
the new missions to Jupiter, to comets, to the martian satellite
Phobos, to Mars itself, and to Titan. But the task group is
concerned that most of these missions are being planned with
little concern for the detection and analysis of organic compounds
with high molecular weights. The task group strongly urges the
spacefaring countries of the world to give greater consideration to
this issue.
~ Mars, comets, and the satellite Titan are currently the
prime targets for intensive exploration from the perspective of
the life sciences. The task group strongly supports efforts to learn
more about the progress and history of chemical evolution on these
bodies. The dark asteroids and the dark material found on the
surfaces of icy satellites in the Uranus and Saturn systems also
merit further investigation from this same point of view.
~ Some specific recommendations for continued planetary
exploration are as follows:
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1. Mars: The task group recommends intensive exploration
of the surface to define regions most suitable for subsequent in
situ analysis. These areas include layered terrains such as those
exposed on canyon walls, areas of sediment deposit such as the
floors of ancient basins that held standing water, and the ground
around and under the polar caps. This exploration phase Is then
to be followed by the deployment of highly instrumented rovers or
the return of samples for analysis on Earth. The choice of samples
to be brought back from Mars must include an assessment of their
relevance to the problem of life's origin.
2. Comets: The task group recommends further characteri-
zation of the compositional and behavioral heterogeneity of comet
nuclei and evaluation of the dust hazards in their immediate vicini-
ties. This should be followed by sample return missions that will
bring unaltered samples to Earth for detailed chern~cal analysis.
Concurrently, there must be development of facilities to handle
these low-temperature, volatile-rich samples.
3. Titan: The task group recommends that the initial orbit-
er-probe mission (Cassini or its surrogate) should be followed by
a mobile lander (or a floater with surface-sampling capability)
that can carry out sophisticated analyses of the organic materials
accumulating on Titan's surface and define the processes that lead
to their production.
Representative terms from entire chapter:
biogenic elements
