Planetary and Space Science 48 (2000) 181±202
An ESA study for the search for life on Mars
Frances Westall a, 1, Andre Brack b,*, Beda Hofmann c, Gerda Horneck d, Gero Kurat e,
James Maxwell f, Gian Gabriele Ori g, Colin Pillinger h, Franc° ois Raulin i,
Nicolas Thomas j, Brian Fitton k, Paul Clancy l, Daniel Prieur m, Didier Vassaux n
a
DIPROVAL, University of Bologna, Bologna, Italy
Centre de Biophysique Moleculaire, CNRS, Rue Charles-Sadron, 45071, OrleÂans, France
c
Naturhisorisches Museum, Bern, Switzerland
d
Institute of Aerospace Medicine, DLR, KoÈln, Germany
e
Naturhistorisches Museum, Wien, Austria
f
Department of Chemistry, Bristol University, Bristol, UK
g
IPSPS, UniversitaÁ d'Annunzio, Pescara, Italy
h
Planetary Sciences Institute, Open University, Milton Keynes, UK
i
LISA, CNRS, UniversiteÂs de Paris 7 et 12, CreÂteil, France
j
MPI fuÈr Aeronomie, Lindau, Germany
k
Mailbox 161, Gabriel Miro 5, Altea, Spain
l
ESA/ESTEC, Noordwijk, The Netherlands
m
Institut Universitaire EuropeÂen de la Mer, Brest, France
n
Direction de la Prospective Spatiale, CNES, Paris, France
b
Received 25 May 1999; received in revised form 14 September 1999; accepted 11 October 1999
Abstract
Similarities in the early histories of Mars and Earth suggest the possibility that life may have arisen on Mars as it did on
Earth. If this were the case, early deterioration of the environment on Mars (loss of surface water, decrease in temperature) may
have inhibited further evolution of life. Thus, life on Mars would probably be similar to the simplest form of life on Earth, the
prokaryotes. We present a hypothetical strategy to search for life on Mars consisting of (i) identifying a suitable landing site
with good exobiological potential, and (ii) searching for morphological and biogeochemical signatures of extinct and extant life
on the surface, in the regolith subsurface, and within rocks. The platform to be used in this theoretical exercise is an integrated,
multi-user instrument package, distributed between a lander and rover, which will observe and analyse surface and subsurface
samples to obtain the following information:
1.
2.
3.
4.
environmental data concerning the surface geology and mineralogy, UV radiation and oxidation processes;
macroscopic to microscopic morphological evidence of life;
biogeochemistry indicative of the presence of extinct or extant life;
niches for extant life.
Lastly, the rationale for human exploration of Mars will be addressed. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction
* Corresponding author. Tel.: +33-2-38-25-55-76; fax: +33-2-3863-15-17.
E-mail address: [email protected] (A. Brack).
1
Present address: SN2, NASA-Johnson Space Agency, Houston,
USA
0032-0633/00/$20.00 # 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 3 2 - 0 6 3 3 ( 9 9 ) 0 0 0 9 0 - 2
In this paper we summarise the ®ndings of a European Space Agency (Manned Space¯ight and Microgravity Directorate)-supported study of the search for
life on Mars, with special emphasis on in situ analysis
during robotic missions and a brief analysis of manned
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F. Westall et al. / Planetary and Space Science 48 (2000) 181±202
missions. The principles of the exploration strategy
were de®ned as:
1. to increase the chances of detecting chemical or
morphological biosignatures by carefully selecting a
set of exobiologically-interesting landing sites;
2. to provide for sampling at the landing site in locations where the e€ects of surface oxidation processes are thought to be minimised;
3. to subject the samples obtained to an integrated set
of measurements which, taken together, will reduce
the chance of ambiguity in the interpretation of potential biosignatures;
4. to analyse the potential contributions of human
missions to Mars and the precautions that need to
be taken.
This study concerns a hypothetical mission which is
not subjected to any particular mass or ®nancial constraints. Rather, it addresses the search for life on
Mars from a purely scienti®c perspective, so that the
scienti®c requirements will serve to stimulate the
necessary technological developments. Our ®ndings
and recommended exobiological lander/rover package
are designed to act only as guidelines in the planning
of future missions having an exobiological component.
One such mission is the 2003 ESA Mars Express
which will hopefully carry with it the Beagle II Lander
comprising components of the package recommended
here.
We consider, in the ®rst place, general aspects of life
on Mars. We then address the scienti®c objectives of
an exobiological robotic mission. Finally, we propose
an ideal strategy to search for life on Mars.
2. Life on Mars
Man's curiosity about the possibility of life on Mars
spans more than a century. It started when the Italian
astronomer Schiapparelli (1898) believed that he saw
lineations on the surface of the planet. Percival Lowell
(1908) then reinterpreted the lineations as Martiantended irrigation channels, after which the human imagination took over. Serious investigation of life on
Mars started with the Viking lander experiments in
1977, certain results of which are still suscitating
heated debate (Levin, 1997; Levin and Levin, 1998).
2.1. Requirements for the development of life (on Mars
and Earth)
What evidence is there to suggest that life on Mars
might have developed? The premise for the appearance
of life on Mars is based on similarities between the
early histories of Mars and the Earth (i.e. a warmer,
wetter environment, McKay and Davis, 1991; Pollack
et al., 1987) and, especially, on geomorphological evidence for the presence of liquid water on its surface at
various times in the past (Baker et al., 1991; Parker et
al., 1989, 1993). The fundamental requirements for
life, as we know it, are the presence of liquid water,
carbon compounds and a source of energy. The evidence for water on Mars has already been mentioned
(op. cit.) and the presence of carbon on the planet can
be inferred from the existence of Martian carbon in
the Martian (SNC) meteorites EETA79001 (Wright et
al., 1989) and ALH84001 (Grady et al., 1994; Wright
et al., 1998). Other important sources of carbon on
Mars (and Earth), including prebiotic molecules from
comets, meteorites and interstellar dust particles, have
been invoked (Kissel and Krueger, 1987; Kvenvolden
et al., 1970; Maurette, 1998; OroÂ, 1961). Finally,
energy would have been available in the form of sunlight, from chemical redox reactions, or hydrothermal
sources. Life developed on Earth from these prerequisites (whether from purely organo-chemical beginnings
and/or mineralogical interactions) and there seems to
be no reason why life should not have developed on
Mars. Also to be taken into account is the possibility
of cross-contamination between the Earth and Mars
(Melosh, 1988; Roten et al., 1998), a hypothesis
encouraged by the possible evidence of fossil life in
Martian meteorite ALH84001 (McKay et al., 1996).
One aspect of the evolution of life on Earth, which
has implications for life on Mars, concerns the timing
of its initiation. The earliest morphological fossils
occur in 3.3±3.5 Ga rocks from South Africa (Fig. 1)
and Australia (Schopf, 1993, 1998; Westall et al., 1999)
but there is isotopical evidence of bacterial carbon
fractionation in even older rocks from Isua, Greenland, dated about 3.8 Ga (Mojzsis et al., 1996; Rosing,
1999; Schidlowski, 1988). Lunar crater dating suggests
that the inner planets were subjected to heavy bombardment, some of which may have been planet-sterilising in e€ect, until about 3.8 Ga (Maher and
Stevenson, 1988). This has led some scientists to
believe that life could not have evolved until after that
date, or at least if it evolved before, it could not have
survived until after the end of heavy bombardment.
However, recent advances in gene sequencing indicate
that life may have passed through a hydrothermal
`bottleneck' suggesting that life could have taken
refuge in hydrothermal vents during periods of surface
environmental stress, such as large-scale impacts (Nisbet and Fowler, 1996). An alternative theory proposes
that life originated in a hydrothermal environment
(Maher and Stevenson, 1988). Given this scenario,
there is no reason why life could not have developed
at any time after the consolidation of the Earth and
the ponding of liquid water on its surface (a date of
4.2 Ga is estimated by Compston and Pidgeon (1986),
although it may have been even earlier). The 3.9 Ga
F. Westall et al. / Planetary and Space Science 48 (2000) 181±202
183
Fig. 1. Fossilised terrestrial prokaryote (Early Archean 3.3±3.4 Ga), South Africa.
age (Borg et al., 1998) of the secondary carbonates of
ALH84001 in which possible signs of life have been
found (McKay et al., 1996), therefore, ®ts in with an
interesting period in terms of the origin of life on
Earth. This is precisely the period on Earth for which
we have no terrestrial record owing to plate tectonics
which has destroyed all but a few remnants of the terrestrial surface older than 3.8±3.9 Ga and all but a few
fragments older than about 3.2 Ga are badly metamorphosed and strained. With its apparent lack of plate
tectonics (Carr, 1981; Sleep, 1994), Mars' surface is a
museum piece for what is lacking on Earth. If life
arose on Mars (Brack and Pillinger, 1998; Davis and
McKay, 1996), it is the ideal site for determining what
may have happened on Earth from the prebiotic stage
to the appearance of the ®rst organisms (Brack, 1997).
The following exercise will therefore focus on the
search for extinct life without, however, excluding the
search for extant life.
Presuming that life did arise on Mars, how did it
develop? How long did it survive? Is life still extant?
At some early period, perhaps about 3.5 Ga, the histories of Mars and the Earth diverged, as Mars lost
most of its atmosphere, cooled down and became, to
all intents and purposes, dry. Like Earth, initially early
Mars probably had a CO2-dense atmosphere (Pollack
et al., 1987) which, through a greenhouse e€ect (Kasting and Ackerman, 1986; Moroz and Mukhin, 1977;
Owen et al., 1979), kept surface temperatures above
freezing despite the fact that the luminosity of the sun
is estimated to have been 30% lower than today's sun
(Sagan and Mullen, 1972). Loss of its atmosphere has
been attributed to a variety of reasons ranging from
carbonate formation (Kahn, 1985; Pollack et al., 1987)
to erosion from high velocity impacts (Melosh and
Vickery, 1989). However, orbital surveying has not yet
found deposits of carbonate and there is also little evidence of signi®cant impact erosion (Carr, 1989). The
timing of the loss of atmosphere and subsequent cooling is dicult to pinpoint but it may be earlier than
the originally estimated 3.5 Ga (Carr, 1989).
Without permanent liquid water at the surface, potential life that evolved on the surface of the planet
would have retreated to protected environments, much
as do endolithic organisms (i.e. organisms living within
the outer few millimeters of a rock) in Antarctic rocks
(Friedmann and Ocampo, 1976; Friedmann, 1982).
The discovery of organisms deep below the Earth's
crust (Cragg et al., 1996; Parkes et al., 1994; Stevens
and McKinley, 1995) has stimulated the hypothesis
that microorganisms could remain in a dormant state
within the Martian permafrost and become re-animated, for instance, by heating from hydrothermal/
volcanic activity related to a meteorite impact. Also, if
dormant cells are preserved in former lake bed sediments, an in¯ux of liquid water due to such an event
would reactivate them. The previously dormant cells
would then be able to metabolise until conditions
became once again too stressful, when they would
revert to dormancy. There is geomorphological evi-
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F. Westall et al. / Planetary and Space Science 48 (2000) 181±202
dence, supported by the latest Mars Global Surveyor
data for seepage, ground creep, surface water ¯ow and
standing bodies of water (Head et al., 1999) at various
periods in the past after 3.5 Ga. Relatively long
periods of time are required for standing bodies of
water to leave a geomorphological record of strandlines. How was this possible with only a thin CO2 atmosphere? One hypothesis is that the periodic releases
of subsurface water instigated temporary climatic
changes leading to a thickened atmosphere caused by
methane originating from bacteria in the groundwater
(Baker et al., 1991). Icehouse conditions followed on
from the unstable ``greenhouse'' e€ect. There is also
possible evidence for water on the Martian surface at
present: some of the ice at the poles may be water ice
(Carr, 1998) and the Viking data indicate the present
formation of frost at night (Farmer et al., 1977; Levin
and Levin, 1998). Moreover, mineralogical evidence
for past or present water on the surface of Mars
comes from the re-evaluated Mariner 7 IRS spectra
which indicate an hydrous weathering product, probably goethite (Kirkland and Herr, 1998).
An alternative scenario for the development of life
on Mars is that it could have been a completely subsurface phenomenon, related to hydrothermal activity
(Maher and Stevenson, 1988), and that it never
appeared on the surface of the planet. In this case, the
loss of sur®cial liquid water would be of less critical
importance than the gradual freezing up of the subsurface aquifers. In this scenario, intermittent `reawakening' of frozen spores due to impact-caused heating
could lead to metabolic turnover. It is however, dicult to imagine how such an underground life could
have originated in the absence of large amounts of
renewable organic molecules.
2.2. The nature of life on Mars?
Just as the similarities between the nature and histories of early Mars and Earth suggest a similar environment for the possible development of life on
Mars, so these similarities (including the extraterrestrial delivery of the same prebiotic molecules) suggest
that early life forms on both planets might have been
similar (the possibility of cross-contamination would
increase the similarity). Thus, it may be expected that
early life on Mars was similar to prokaryotic life on
Earth and whatever preceded the prokaryotes. However, life on Earth evolved further into more sophisticated, multicellular organisms. The oldest known
eukaryotes (acritarchs) only appear in the rock record
about 1.5 billion years after the oldest known prokaryotes (Han and Runnegar, 1992; Knoll, 1996),
although organic biomakers suggesting the presence of
life forms with some eukaryotic characteristics have
been analysed from 2.7 Ga carbonaceous shales from
the Hamersley Basin, Australia (Brock et al., 1999).
They are associated in time with the earliest, signi®cant
O2 levels in the atmosphere (about 15% the present atmospheric level, Holland and Beukes, 1990). The paucity of terrestrial rocks covering this critical period of
evolution is a hinderance to any detailed understanding of the process, but the fact remains that further
evolution from simple prokaryotes to the more sophisticated eukaryotes apparently required a period spanning at least 1±1.5 billion years.
Mars, on the other hand, lost its atmosphere and
liquid water early in its history and the environment
became extreme, if not completely inhospitable, for life
(the subsurface environment perhaps less so). Is it
possible that continued evolution could have produced
more sophisticated, multicellular life on Mars under
such conditions? A priori, it is more likely that Martian life could have remained at a primitive level of
evolution, as represented by the terrestrial prokaryotes.
It should be remembered, though, that the prokaryotes
are incomparable as survivors and in adapting to seemingly hostile situations.
We, therefore, conclude that Martian life forms
would be similar to simple terrestrial organisms, such
as prokaryotes (Fig. 1).
2.3. Evidence of life on Mars?
2.3.1. Martian meteorite ALH84001
The only present (tenuous) evidence of possible
extinct life on Mars that we have, is encapsulated in
secondary carbonates of Martian origin, deposited in
cracks in the SNC meteorite ALH84001 (McKay et
al., 1996). McKay et al. (1999) have recently found
bacteriomorph structures in younger Martian meteorite Nackhla, 1.3 Ga. The 3.9-Ga old carbonates (Borg
et al., 1998) were deposited in Martian igneous rock at
a period of Mars' history in which there may have
been life. Organisms are not usually associated with
igneous rocks but it has been shown recently that bacteria (N.B. in the following text, the term bacteria is
taken to include all the prokaryotes, i.e. eubacteria
and archaeobacteria) can survive at depth in basaltic
aquifers (Stevens and McKinley, 1995). The lines of
evidence suggestive of the presence of fossil life within
the carbonates include the presence of Martian carbon
(Grady et al., 1994) and PAHs (Clemett et al., 1998),
d 13C values in a range similar to those produced by
terrestrial organisms (Sparks et al., 1990), biominerals
such as parallepiped magnetite (formed on Earth only
by bacterial magnetosomes, Thomas-Keprta et al.,
1998a), and morphological structures (Fig. 2) which
may represent fossilised bacteria or parts of bacteria
(Thomas-Keprta et al., 1998b). There are many counter arguments to these interpretations (Bradley et al.,
1997; Manilo€, 1997; Nealson, 1997) and none of the
F. Westall et al. / Planetary and Space Science 48 (2000) 181±202
185
Fig. 2. Filamentous-shaped bodies within the carbonate globules of SNC meteorite ALH84001. They are interpreted as possible fossil bacteria
(Thomas-Keprta et al., 1998b).
lines of evidence used by McKay et al. (1996) is, on its
own, conclusive of life (Gibson et al., 1998). Further
work is needed to determine the origins of the various
biological signatures in the meteorites. For instance,
there is clear evidence of Antarctic contamination
(Steele et al., 1999) and yet, on the other hand, some
of the organics are clearly Martian in origin (Grady et
al., 1994). A greater understanding of terrestrial biosignatures, themselves, is imperative.
2.3.2. The Viking missions
This leads to the discussion of the possibility of
extant life on Mars. Life on Earth is seemingly ubiquitous, even in apparently inhospitable environments.
Bacteria have adapted to survive many kinds of chemical and physical stresses, such as low nutrient levels
(Novistky and Morita, 1977), low temperatures and
water availability (Friedmann and Ocampo, 1976;
Friedmann, 1982), high temperatures (Stetter, 1996,
1998) (the ®rst organisms may originally have been
hyperthermophile organisms, Woese, 1987), high pressure (e.g. deep sea, Ivanov 1988; Jones et al., 1983),
high salinity (Norton et al., 1993), high or low pHs
(Holt et al., 1994), and high radiation (Allen, 1998;
Pierson et al., 1993). The initial tests made of Martian
soil by the Viking landers at two locations on opposite
sides of the planet provided contradictory results
about which there is still ongoing discussion. A 14Clabelled release experiment indicated biological activity
(Levin and Straat, 1977). The gas chromatograph±
mass spectrometer (GC±MS) on the same lander, however, measured no organic carbon, thus, precluding the
presence of active biology (Biemann, 1979; Biemann et
al., 1976) and suggesting that the positive labelledrelease experiment result was due to purely chemical
reaction. There are counter-arguments concerning the
level of sensitivity of the GCMS (Levin and Levin
1998; Levin and Straat, 1981) and no de®nitive conclusion can be reached. Neither the Viking nor the
Path®nder landers provided visual images of macroscopically visible life (Klein et al., 1972), although
Levin and Straat (1977) think that they see colour
changes on the time-lapse Viking images suggestive
perhaps of lichen-like growth.
Looking at the possibility of extant life in Martian
soil, modelling by Levin and Levin (1998) have shown
that the diurnal frost could form a thin ®lm of water
during the day, sucient for microbial activity on the
basis of analogy with terrestrial desert situations. They
also note that terrestrial microorganisms have evolved
enzymes to cope with H2O2, the presumed atmospheric
oxidant. In the Antarctic, microorganisms have
evolved an ingenious strategy for surviving the harsh
conditions of little water, cold temperatures and high
levels of UV by living protected within the topmost
few millimeters of a porous rock surface (Friedmann,
1982). On Mars UV levels are between 55 and 84%
higher than UV levels on Earth, owing to the lower
amount of O3 in the atmosphere. UV radiation does
not penetrate more than a few millimeters beneath the
surface of the regolith, unless it is cracked. However,
UV penetration of CO2 and water ice, on the other
hand, depends upon the contamination of the ice.
Radiation from the decay of natural radio nuclides has
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F. Westall et al. / Planetary and Space Science 48 (2000) 181±202
been calculated at less than 1% of that resulting from
cosmic rays.
At the present state of uncertainty, we have to consider the possibility of extant life on Mars, either metabolising on a daily basis at the surface of the planet,
or dormant but still viable in a subsurface permafrost
reservoir. Our observational and experimental objectives in the following exercise are, therefore, focussed
on the search for extinct life, without excluding the
search for extant life.
2.3.3. The preservation of organisms
Assuming that life arose on Mars, how could its
record be preserved? As noted above, Martian organisms are likely to have been/be of a simple type, such
as the terrestrial prokaryotes or bacteria. The following, therefore, refers to the preservation of terrestrial
microorganisms. There are four main processes of
preservation.
1. The organic components comprising terrestrial
organisms may be preserved if they are buried
rapidly in ®ne sediments in a reducing environment,
such as lake or sea bottoms. The same procedure
applies to the preservation of soft-bodied organisms
such as bacteria (e.g. Butter®eld et al., 1988). With
time the organic matter undergoes a complex process of degradation and condensation (humi®cation)
to give chemically stable macromolecules known as
`kerogens' (mainly aliphatic and aromatic molecules,
de Leeuw and Largeau, 1993). Kerogens form
>90% of sedimentary organic matter on Earth and
their composition depends largely on the thermal
history they have undergone. In addition, stable,
lipid-rich biopolymers and metalloporphyrin pigments can survive degradation to contribute directly
to the kerogens. Such organic biomarkers occur in
ancient terrestrial sediments from the Precambrian
(Brocks et al., 1999; Imbus and McKirdy, 1993;
Summons et al., 1999). The preservation of organic
matter on Mars would be in¯uenced by oxidising
agents at the surface but these agents are presumed
to have limited depth penetration. Furthermore, not
all organic molecules are equally susceptible to oxidation (Levin and Levin, 1998) and intractable
kerogen may survive at or near the surface.
2. `Permineralisation' is a method of preserving soft
bodied organisms and organic polymers whereby
the structure is permeated by silica which nucleates
to the organic template (Knoll, 1985). The organic
matter is thus `locked into' the silica matrix. This
process is common in structures with a large
amount of robust macromolecules, such as the polymer-rich sheaths of cyanobacteria or the polymerrich bio®lms produced by microorganisms.
3. Complete replacement of the organic structure by
minerals such as silica (Fig. 1), calcium phosphate
and carbonate, siderite, and iron and manganese oxides (Fig. 3) also takes place in a variety of environments (Westall, 1999). Bacteria which are not
characterised by a large amount of associated polymer seem to be preserved by mineral-replacement
rather than permineralisation although bio®lms can
be also completely replaced by minerals. Minerallyreplaced microorganisms contain no measurable organic carbon (at least using present instrumentation).
4. Bacteria may be also preserved as empty moulds in
a mineral matrix (Westall et al., 1999)
Permineralised organic structures, minerally-replaced
organisms and bacterial moulds have a strong potential for their morphological survival, especially if they
are buried beneath the sur®cial impacted layer. Impact
breccias could, on the other hand, contain fragments
of otherwise buried, potential fossil-containing rocks.
Apart from leaving morphological or organic fossils,
life can also leave a signature in the form of etch pits,
reaction product deposits, or biocrusts on rock surfaces or beneath a surface in micropores or cracks.
These biomarkers can occur in igneous rocks as well
as sedimentary rocks, and even in micrometeorites
(Callot et al., 1987).
3. The scienti®c objectives in the search for extant or
fossil life on Mars
Life manifests itself in a variety of ways. The visible aspects of life relate to the morphology of an
organism, characteristic structures built by organisms
or etched markings left by their enzymatic acids,
but most life detection methods use biogeochemical
techniques for determining the biomolecules that
make up an organism, for identifying the kind of
metabolic processes it uses and its metabolic products, or for determining its isotopic fractionation.
Moreover, there are other, indirect indications of
the presence of life related to the interaction of
organisms with their immediate environment (biodegradation and biomineralisation). Distinctive crystallographies, morphologies and/or isotopic ratios
occur when minerals are actively precipitated by
organisms (usually within an organic template); for
example magnetite in magnetotactic bacteria, amorphous silica in diatoms, or sulphate/carbonate/phosphate exo±endosketeltons. Biogenically-precipitated
minerals are characteristically out of equilibrium
with the extracellular environment. Biogenicallymediated minerals formed extracellularly are, however, more dicult to distinguish from those formed
inorganically.
F. Westall et al. / Planetary and Space Science 48 (2000) 181±202
The search for life should, therefore, be based
on a coordinated strategy to search for visible,
biochemical and geochemical signs of life in a
suitable environment. All analyses designed to
187
look for life will need to be placed in a geomorphological, geological and environmental (in terms
of UV radiation and oxidant presence) framework.
Fig. 3. Micron-thick ®laments of probable microbiological origin encrusted with chalcedony (Tertiary basalt, Faeroe Islands). Bottom: Enlargement showing ®ne ®lamentous cores of chalcedony rods (courtesy Beda Hofmann).
188
F. Westall et al. / Planetary and Space Science 48 (2000) 181±202
3.1. Exobiologically-interesting environments
Orbital information from the surface of Mars documents its geomorphological diversity. On Earth, life
exists in the most diverse environments, even apparently inhospitable ones. The same may have been (still
be?) the case with Mars. However, for the purposes of
exploration, the aim would be to choose a landing site
in an environment which would be both conducive to
life and which would preserve a record of past life.
Suitable locations would include both surface and subsurface environments (extant organisms at the surface
are most likely to reside in near-surface protected
niches, such as pore space within a rock or beneath
the regolith), such as lacustrine deposits (Fig. 5), evaporite deposits, duricrusts, hydrothermal spring deposits, possibly glacial deposits and in subsurface
environments including the ground ice or permafrost.
3.1.1. Lacustrine deposits
Gilbert-type deltas and terracing along the walls of
basins (often craters) are evidence of standing bodies
of water on Mars (De Hon, 1992; Mosangini and
Komatsu, 1998; Ori and Baliva, 1998). The ®ne sediments and possible evaporites deposited in such environments would be suitable repositories for fossil life
(either minerally-replaced or as carbonaceous compressions or molecular fossils).
3.1.2. Evaporites/duricrusts
Evaporites are typical of sebkha-type deposits. Sebkhas are periodically-¯ooded dry lakes which abound
with microbial life. Life would have a high preservation potential in such precipitated sediments; however, no such deposits have yet been discovered on the
surface of Mars, although high albedo sites which
could be evaporites have been observed (O'Connor
and Giessler, 1996; Rothschild, 1990, 1995). Somewhat
similar to sebkha deposits are duricrusts formed by the
downward in®ltration of surface ¯uids and solutes into
sediments in arid environments. Crusts such as desert
varnish are associated with mineralised microbial life
(Dorn and Oberlander, 1981).
3.1.3. Hydrothermal springs
There is much interest in life associated with terrestrial thermal springs, either marine, shallow water or
subaerial, because of the possible association they
seem to have with the last common ancestor. (Bock
and Goode, 1996; Russell and Hall, 1997; Walter and
Des Marais, 1993). Moreover, they may have been/be
a last refuge for life on Mars. The mineral-rich solutions emanating from hot springs are excellent for the
preservation of micoorganisms (Cady and Farmer,
1996; Folk, 1993). Furthermore, rocks derived from
hot spring deposits may be rich in ¯uid inclusions
which can retain original liquid and gas, as well as organic molecules.
3.1.4. Glacial deposits
Glacial deposits have been interpreted from the
Argyre Basin in the south polar region of Mars and at
several locations in the northern plains (Kargel and
Strom, 1992). However, although microbial life occurs
in terrestrial glacial deposits, their preservation as fossils in such sediments does not have high potential.
3.1.5. Subsurface environments
Subsurface environments on Mars are perhaps even
more important than surface environments owing to
the inhospitability of the Martian surface (Boston et
al., 1992; Hofmann and Farmer, 1997; Kretzschmar,
1982; Ostroumov, 1995). Microorganisms are able to
resist in subsurface environments on Earth, such as in
deep, basaltic aquifers (Stevens and McKinley, 1995),
in deep-sea sediments to depths of more than 1000 m
(Cragg et al., 1996; Parkes et al., 1994), and in ice
sheets (Abyzov et al., 1998). On Mars, it is hypothesised that there is a layer of permafrost beneath the
surface (Carr, 1996; Cli€ord, 1993), which could be an
ideal niche for extant life. Microorganisms could
metabolise and reproduce in the vicinity of energy-giving (and ice liquifying) sources of hydrothermal heat
and may be ¯ushed onto the surface of Mars from
their permafrost refuge during catastrophic releases of
water following an impact. The life-containing water
would become ponded in depressions and the organisms could continue to metabolise until the water
became completely frozen. The organisms would then
become dormant or, ®nally, extinct (Friedmann and
Koriem, 1989).
The possibility of the preservation of sub-surface
organisms as fossils on Mars is high because of presumed importance of the sub-surface biosphere. Terrestrial subsurface environments in which fossilised
microbes have been putatively identi®ed include hydrothermally altered rocks of various kinds (especially volcanics), oxidation zones in sulphide deposits and
vesicular impactites. Such rocks could be brought to
the surface by impact erosion or other erosional processes including mass wasting phenomena. Terrestrial
sub-surface microbial fossils are typically enclosed in
microcrystalline silica (chalcedony, agate), Fe-hydroxides of clay minerals and have been found at depths of
up to 800 m. Thus, whilst fractured basalt may normally be considered of low priority when searching for
life, it may nevertheless habour rich sub-surface fossil
remains.
3.2. Visible evidence of life
Extant or extinct life on or near the surface of Mars
F. Westall et al. / Planetary and Space Science 48 (2000) 181±202
could leave macroscopic to microscopic signatures of
their presence either as morphological bodies or fossils,
or as microbial buildups and associated biomediated
minerals. The visual search for living organisms would
need to be combined with the biochemical testing for
biomolecules and isotopic fractionation to prove that a
particular structure is biogenic in origin.
3.2.1. Organism morphology
Analysis of the Viking images brought to light patchy colour changes on boulders which were interpreted
initially as possible evidence for lichens on the surface
of rocks (Levin and Straat, 1977). However, the recent
Path®nder mission underlined the diculties of imaging on Mars, especially in relation to colour changes,
because of the in¯uence of the optical properties of the
dust-laden atmosphere (Thomas et al., 1999). Moreover, it is not expected that such large, sophisticated
eukaryotic microorganisms could survive at present at
the surface of Mars. As noted above, given the early
initiation of harsh conditions on Mars, it is not
expected that potential life could have developed
beyond the prokaryotic stage. Most terrestrial prokaryotes are characterised by small size (less than 2 mm
diameter, cf. Fig. 1), although sizes can range up to
tens of micrometers (Fig. 3) (Holt et al., 1994) and
down to R0.1 mm (Kajander and C° iftc° ioglu, 1998;
Uwins et al., 1998). If extant organisms do exist, they
may even leave traces of their presence in fossilised
form because living or recently dead microorganisms
may be fossilised within a matter of days (Westall et
al., 1995).
The interpretation of life forms on the grounds of
morphology will be based on the criteria used for identifying terrestrial microorganisms and microfossils
(Holt et al., 1994; Schopf and Walter, 1983; Westall,
1999; Westall et al., 1999). Criteria related to morphology (visible with an optical or high magni®cation
microscope, such as a scanning electron microscope,
SEM, or atomic force microscope, AFM) include: (i)
size and shape; (ii) evidence of reproduction via cell
division, budding or sporulation (iii) cell wall features
(e.g. the de¯ated surface of a lysed cell, wrinkled or
smooth surfaces); (iv) colony formation; (v) the presence of consortia; and (vi) association with bacterially-produced polymer (bio®lm). A common
morphological distinction between prokaryotes and
eukaryotes is that prokaryotes generally do not possess
membrane-bound internal structures, such as a nucleus
or chloroplast. These are features, however, that
require highly sophisticated instrumentation and
sample preparation (for the transmission electron
microscope) in order to be observed. Many of the
characteristic morphological features of prokaryotes,
especially in some of the larger prokaryotes (e.g. cyanobacteria), can, on the other hand, be observed by
189
optical microscopy, for example cell size and shape,
complexity, colony/consortium presence and association with bio®lm.
3.2.2. Microbial markings and constructions (etch
marks, build-ups, crusts)
Apart from the direct evidence of body fossils, bacteria can leave indirect evidence of their presence. For
instance, with their enzymatic secretions they can create etch marks (often associated with biomediated precipitates) (Barker and Ban®eld, 1996; Feldmann et al.,
1997; Friedmann, 1986; Friedmann and Weed, 1987;
Monty et al., 1995); or they can catalyse mineral precipitation either on their bodies or in their surroundings which form characteristic structures and edi®ces,
such as crusts and bioherms (stromatolites and mudmounds), ranging in size from the microscopic up to
many meters (e.g. Dorn and Oberlander, 1981; Gerdes
et al., 1993; Southgate, 1986; Walter, 1976, 1983). Microbial buildups and crusts are formed when bacteria
secrete `sticky' polymers which become the loci for
mineral nucleation and particle trapping. As laminar
bio®lms, these mineralised polymer sheets can form
thick packets of tabular laminations (Westall et al.,
1999), or they can form three-dimensional vertical
structures such as columnar stromatolites and bioherms (Beukes and Lowe, 1986; Pelechaty and Grotzinger, 1988), hence the term microbial `build-up'.
Microbial structures may be formed in many di€erent
environments, ranging from stromatolites in shallow to
deep water environments (Feldmann and McKenzie,
1997; Rasmussen et al., 1993; Williams and Reimers,
1983), hot spring travertines and sinters record (Cady
and Farmer, 1996; Folk, 1993), Mn/Fe nodules forming in deep-sea environments (Burnett and Nealson,
1981), carbonate mudmounds (Monty et al., 1995), to
desert varnish in dry environments (Dorn and Oberlander, 1981). These mineralised microbial structures
are associated with microbial fossils, etch marks and
biomediated precipitates.
3.2.3. Biominerals
Microorganisms may actively precipitate minerals
and may also passively mediate mineral precipitation.
Microorganisms which actively precipitate minerals
are, however, eukaryotes, whereas mineral precipitation associated with prokaryotes, on the other hand,
appears to be passive (Kajander and C° iftc° ioglu, 1998;
Westall et al., 1995). The most common minerals precipitated through passive microbial activity include calcium carbonate, oxalate, phosphate, silica, siderite,
barite and Fe/Mn oxides and Fe sulphides (Fig. 4).
The biomediated minerals may form small localised
aggregates around and in the vicinity of microorganisms or they can form biocrusts on rock surfaces or
beneath a surface in micropores or cracks. However,
190
F. Westall et al. / Planetary and Space Science 48 (2000) 181±202
Fig. 4. Biominerals: spherical and rectangular pyrite crystals coated by fossilised bacterial polymer (from Westall et al., 1999).
since many biomediated minerals may be also formed
inorganically, concerted e€orts are now underway to
document the di€erences between biogenic and abiogenic mineral formation.
3.3. Biochemical and geochemical signatures of life
Biochemical signatures of life are recorded on the elemental to the molecular scale. They include the presence and abundance of biologically-important
elements, biogenically-in¯uenced isotope ratios, biogenic molecules and chirality (one-handedness). The
geochemical composition of microbial fossils, biomediated minerals and microbial build-ups and crusts
constitute also signatures important for life.
3.3.1. Biologically-important elements
Biologically important elements include C, H, N, O,
S, and P and measurement should be made of their
partition between inorganic and organic components.
Moreover, these analyses need to be done in a stratigraphic sequence in order to correlate changes in
mineralogy, geochemistry and organo-chemistry with
depth from the weathered Martian/rock surface in
order to model abundance changes with increasing
depth. The search for and presence of these elements
on Mars has been extensively discussed by Banin and
Mancinelli (1995), Mancinelli (1996) and McKay
(1997).
3.3.2. Biologically-important isotopes
The isotopic ratios of C, H, N, and S are a very
valuable set of biomarkers (Schidlowski et al., 1983;
Watanabe et al., 1997). 13C depletion is produced by
enzymatic CO2 ®xation and a d 13C 20±30- below
that of inorganic carbon has been maintained for most
of the history of life on Earth. d 13C measurements of
organic carbon in terrestrial rocks indicate that photosynthesis probably developed early during the evolution of life on Earth (Mojzsis et al., 1996; Nisbet et
al., 1995; Schidlowski, 1983), although the earliest evidence for organisms using oxygenic photosynthesis is
in rocks 2.7 Ga old (Brocks et al., 1999; Summons et
al., 1999). The H/D ratio can also be changed by biogenic fractionation with H preferred over D by methanogenic microorganisms. 15N/14N and 34S/32S are
other fractionations occurring biogenically, although
the sulphur isotopic changes are variable and it is
necessary to study the isotopic composition of both
oxidised and reduced sulphur species (Schidlowski et
al., 1983; Strauss, 1993, 1997). There are, however,
nonbiogenic in¯uences on isotope ratios and these
need to be fully evaluated.
3.3.3. Biologically-important molecules
Both inorganic and organic molecules are important
in the search for life. With regards to extant life, the
presence of H2O in the samples collected, as well as
the hydrated state of the mineral phases, is of paramount importance. Other biologically-signi®cant inorganic molecules include CO2/NO2/NO3/NxOy (nitrate
content), SO2/SO3 (sulphates), and phosphates.
Organics in the Martian surface and near surface environment would indicate either a biotic source or an
imported meteoritic source, since the oxidising atmosphere of Mars precludes the present formation of pre-
F. Westall et al. / Planetary and Space Science 48 (2000) 181±202
biotic organics. Molecular analysis would be able to
distinguish between prebiotic meteoritic organics and
indigenous biogenic organics.
Based on an understanding of terrestrial biogenic organic matter, potential organic biomarkers of Martian
origin should contain:
1. compounds with structures characteristic of a biosynthetic origin (e.g. acyclic isoprenoids, terpenoids,
steroids);
2. a 13C depletion in the isotopic composition of the
bulk organic matter (providing that the organisms
had evolved photosynthesis), fractions there from,
stable organic compounds or compounds released
upon pyrolysis;
3. chiral components (see below) with a biological con®guration, present as such, or released by pyrolysis
(e.g. steroids or terpenoids);
4. an homologous series of components, present as
such or released by pyrolysis, with a non-random
carbon number distribution.
Organic molecular signs of life in the near-surface environment include volatile low molecular weight organics such as alkanes. The survival of more complex
organic molecules will depend upon their chemical
stability under Martian conditions (Kanavarioti and
Mancinelli, 1990). Some important biomolecules,
including amino acids (Bada and McDonald, 1995)
and purines, are stable in an oxidising environment,
whereas some others, such as nucleotides, ribose, deoxyribose and pyrimidines, are not (Larralde et al.,
1995).
Thus, the organics to be sought in Martian material
are, in order of importance: (i) volatile low molecular
weight compounds: hydrocarbons, alkanoic acids, and
peroxy acids present in the oxidising atmosphere; (ii)
medium molecular weight compounds: hydrocarbons,
alkanoic acids and alcohols, amino acids, purines; and
(iii) macromolecular components: kerogens (Helfer,
1990), oligo- and polypeptides.
3.3.4. One-handedness or homochirality
Does life necessarily need to be one-handed? Pasteur was probably the ®rst person who realized that
biological asymmetry could best distinguish between
inanimate matter and life. A racemic life (from
racemus, raisin, to commemorate the historical Pasteurian separation of left- and right-handed tartaric
acid) which would simultaneously use the right- and
left-handed forms of the same biological molecules
appears, in the ®rst place, very unlikely for geometrical reasons. Enzyme beta-pleated sheets cannot
form when both L- and D-amino acids are present
in the same chain. Since the catalytic activity of an
enzyme is intimately dependent upon the geometry
of the chain, the absence of beta-pleated sheets
191
would impede, or at least considerably reduce the
activity spectrum of the enzymes. The use of onehanded biomonomers also sharpens the sequence information of the biopolymers. For a polymer made
of n units, the number of sequence combinations
will be divided by 2n when the system uses only
homochiral (one-handed) monomers. Taking into
account the fact that enzyme chains are generally
made of hundreds of monomers, and that nucleic acids
contain several millions of nucleotides, the tremendous
gain in simplicity o€ered by the use of monomers
restricted to one handedness is self evident. Orgel
(1992) elegantly demonstrated chemical, non enzymatic, replication of RNA and showed that these
chemical replications work only when o€ering a pool
of homochiral nucleotides to the template. Eschenmoser (Bolli et al., 1997) showed recently that short segments of complementary pyranosyl-RNA (RNA
analogs which might have preceded RNA) are capable
of self-association and polymerisation only if they are
homochiral.
Life, as we know it, uses homochiral left-handed
amino acids and right-handed sugars. A mirror-image
life using right-handed amino acids and left-handed
sugars is perfectly conceivable and might have developed on the primitive Earth in parallel with life, as we
know it, before being defeated. To date, no signatures
of such a life `behind the mirror' have been found on
Earth.
Thus, homochirality can be a crucial signature for
life. Although homochiral molecules gradually revert
to racemic mixtures with time, in a dry environment
such as that on Mars, the homochirality of molecules
3±4 billion years old may still be detected (Bada and
McDonald, 1995). The ratio of enantiomeric molecules
in a Martian sample can be used as a test for extant or
extinct life: pure enantiomeric molecules would indicate extant life, whereas di€erentiated enantiomeric
mixtures would indicate extinct life (amino acids, for
instance, lose their one-handedness at di€erent rates).
3.4. Mineralogical/geochemical analyses
The study of the microbial fossils and minerals
which may be biomarkers, such as sulphides (WaÈchtershaÈuser, 1988, 1998) and oxides, bio-phosphates, oxalates, silica, biogenic magnetite, barite, should include
mineralogical analysis as well as elemental analyses to
provide quantitative data for major, minor and trace
elements. The oxidation states of certain elements,
such as Fe, Mn, S, N could provide redox information
about the sampled material. These investigations will
also reveal the presence of biologically important elements such as C, H, N, O, S and P.
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F. Westall et al. / Planetary and Space Science 48 (2000) 181±202
3.5. The geological and environmental framework
Any search for life on Mars needs to take place
within the framework of the geological and UV/oxidant environment of the potential microorganisms,
(nb. the latter will have less importance if life originated and remained in the subsurface), microfossils,
molecular fossils or biomarker minerals. The texture
and composition of the rocks and soil analysed is as
important as the analysis of a presumed oxidant in the
Martian soil and the penetration of UV radiation. As
noted above, UV penetration only a€ects the top few
millimeters of the surface but it is not known to what
depth the e€ects of oxidation are felt. This aspect
needs to be modelled in order to determine the depth
of drilling/burrowing necessary to obtain non-oxidised
samples. Oxidant (H2O2, O2, O3) concentration with
depth is predicted to vary inversely with organic concentration, although potential Martian organisms that
lived on the surface of the planet may have evolved
oxidant resistant compounds or strategies. Recent
modelling suggests that the average depth of oxidant is
no more than a few meters (Zent, 1998). In an historical sense, depth measurements of the ratio of organic
to inorganic carbon, and the abundance of nitrogen
and its oxidation state in the Martian soil, will aid
understanding the evolution of the early atmosphere.
4. An exploration strategy for the search for life on
Mars
The strategy we present here is focussed on a
hypothetical mission, making use of both a lander and
a rover. Our strategy should be taken as a broad
guideline of what could be accomplished and is not
meant to represent any single mission. Parts of it may
be used in a number of di€erent missions as is, in fact,
going to be the case with the ESA 2003 Mars Express
mission (Orbiter and Beagle II exobiology lander) and
the NASA/CNES 2003 mission (orbiter and Athena
lander). The strategy commences with the choice of a
landing site, followed by an initial lander-based site
survey. On the basis of the preliminary survey, samples
will be obtained, observed and analysed. We suggest
an ideal instrumentation package to cover the scienti®c
objectives outlined above.
4.1. Landing sites
The selection of suitable landing sites is based on
two philosophies. The ®rst concerns a landing site
in or near a speci®c environment which could have
contained life. In this case there would be biogeochemical as well as visual testing for the presence
of life. The second philosophy is to locate the land-
ing in an area where there is a large variety of
rocks which lend themselves well to petrological/
mineralogical and palaeontological study, but may
not be suitable for geochemical testing. The possibility of microbial/molecular fossils in the Martian
subsurface needs also to be taken into account.
Suitable fossil-life containing subsurface environments would include porous sediments or sediments
with early diagenetic cements, voids in impact breccias and impact melts, vesicles in volcanic rocks,
and fractured rocks of any type (viz. meteorite
ALH84001). Such subsurface rocks would be
exposed on the surface in the chaotic terrains, on
steep canyon walls, on eroded channel ¯oors, and
impact ejecta.
The basic requirements of the potential landing site
are as follows.
1. Age of rocks: the warmer, wetter early Mars hypothesis would favour Early Noachian-age terrain.
However, a large number of lakes, channels and
other life-suitable environments existed up to the
middle Amazonian (Carr, 1996; Head et al., 1999).
2. Concentration: lithologies or environments most
likely to have contained abundant life are water-lain
sediments, evaporites, hydrothermal spring deposits.
Evidence of life may be present in less abundance in
other environments such as duricrusts and glacial
deposits.
3. Preservation: both organic molecular fossils and microbial fossils are likely to be preserved in rapidlyburied, anaerobic, ®ne-grained sediments (e.g. lake
¯oors), although they are also preserved in degraded
form (as kerogen) in permineralised fossils. Otherwise microbial fossils (and, to a certain extent, molecular
fossils)
are
preserved
as
mineral
replacements, especially in shallow water carbonates, evaporites and hydrothermal spring deposits
(either subaerial or subaqueous). Microbial buildups and crusts will also occur in the latter environment. Moreover, microbial crusts could be preserved in subsurface environments.
4. Thin dust cover: the Martian surface is coated with
a pervasive aeolian cover, thus, a potential landing
site should be chosen where this cover is thin.
5. Area of target: a speci®c environment, e.g. lake bed,
sebkha or hydrothermal spring should be large
enough to fall within the area of uncertainty of the
landing area and should be within rover range.
Additional variables to take into account include the
physical requirements (not too high elevation, equatorial latitude) as well as the surface roughness. The following ®ve possible landing sites are only some
examples of the many potential sites. However, they
cover the range of environments discussed above:
F. Westall et al. / Planetary and Space Science 48 (2000) 181±202
1. Marca Crater (10.48S/158.28W; Fig. 5) and adjoining lacustrine craters;
2. SE Elysium basin (38N/1858W) with channel and
lacustrine;
3. Apollinaris Patera (8.68S/187.58W) with possible hydrothermal systems associated with a volcano and
rock debris of various ages (Gulick, 1998);
4. Gusev Crater (148S/184.58W) with lacustrine deposits and a large out¯ow channel (Grin and Cabrol,
1997);
5. Capri Chasma (158S/478W) with chaotic terrain and
sediments.
The sites listed are on or near the equator, one of the
most likely places on Mars where liquid water was
stable, and therefore might have contained fossils of
the most recent Martian microbes. The fact that the
region bounded by 408 latitude is probably extremely
dry (Cli€ord, 1993) Ð even deep below the surface Ð
is expected to have helped preserving the records of
the most recent extinct life. Older sedimentary terrains
in the southern hemisphere may contain older fossils
193
and perhaps traces of an origin of life but they probably su€ered heavy perturbations from meteoritic
impacts.
4.2. General site survey
Before any search for extant or extinct life can take
place, a careful survey of the landing site is necessary.
This survey will provide general environmental data
and should aim at:
. identifying the position of the landing site;
. providing climatic data (UV radiation, temperature,
visibility, humidity);
. providing an assessment of the rock density at the
site;
. determining the rock distribution and its heterogeneity;
. determining the rock size distribution and assess the
evidence for bi- or multi-modal distribution caused
by di€erent processes;
. studying the evidence for macroscopic environmen-
Fig. 5. Marca Crater on Mars (10.48S/158.28W) with possible lacustrine deposits. Viking image, NASA.
194
F. Westall et al. / Planetary and Space Science 48 (2000) 181±202
tal in¯uences on the site (¯ooding, impact etc.);
. investigating aeolian e€ects (e.g. wind tails, ventifacts) for comparisons with global circulation
models;
. studying the surface photometric function;
. investigating the bulk mineralogy of rocks and soils;
. searching for spectral inhomogeneity of soils and
comparing this to the local morphology;
. searching for spectral inhomogeneity of rocks and
investigating evidence for sedimentation and an in¯ux of external material;
. searching for potential targets for the rover, including identi®cation of unusual morphology, the di€erentiation between weathered and unweathered rock,
the study of signi®cant colour di€erences, and the
identi®cation of potential dangers to navigation.
Experience from the Path®nder mission demonstrated
the necessity of mobility in being able to select the correct sample for detailed analysis: although rock distribution at the Path®nder landing site was homogenous
on the large scale, at a meter scale it was inhomogenous. Sampling, therefore, requires a mobility of at
least 20 m from the landing site. Thus, assuming that
some form of mobile device (such as a rover) is
deployed, the survey must also:
. assess the morphology of disturbed soil and determine its consistency;
. compare the colour of the rover tracks with undisturbed soil and assess compaction parameters
needed to produce this colour change;
. assess the rate at which the disturbed soil re-equilibrates with its surroundings;
In addition, the survey must:
. make an assessment of the damage to the site caused
by the landing (including contamination resulting
from the landing strategy employed);
. determine the orientation of the landing vehicle;
. determine the optical depth at the site for purposes
of power management.
The instrumental requirements for the general survey
include a sophisticated panoramic camera similar to
the Imager for Path®nder with a variety of spectral ®lters and the capacity for azimuth and elevation articulation.
4.3. Sample acquisition, distribution and preparation
In the ®rst place the samples need to be free of dust
and oxidisation products. They therefore, need to be
obtained from beneath the weathering rind of the sur®cial rocks (McSween et al., 1999) and from the subsurface. The sample processing routine commences
with observation of the potential sampling site, prep-
aration of a rock surface for observation (removal of
dust and weathering rind), sample acquisition at a suitable depth (surface or sub-surface), sample preparation, microscopic analysis, and transfer for other
types of analysis.
At least two types of material will be encountered,
hard rocks and regolith which may be partially indurated (duricrust) or loose. Unconsolidated samples
may be acquired by burrowing into loose regolith
(below a boulder, for example, to escape the e€ects of
oxidation). These samples would need particle sizesorting and grain size analysis with the ®ne grained
split (<600 microns) being analysed as is and the coarser split being further crushed. Rocks/boulders and
consolidated regolith would be sampled by drilling
(producing a core) or grinding (producing granular
material) with stratigraphic sampling from the crust
into the interior.
Thus, the requirements for acquiring and preparing
a sample are:
. a corer drill or a mole capable of penetrating 1.5 m
into consolidated regolith and a smaller drill for
probing the interiors of surface rocks up to 15 cm;
. a grinding device to remove the dust and weathered
crust from surface rocks for observation;
. a scoop for collecting soil samples;
. a sawing device to smoothen surface cores for optical investigation and micro-analysis;
. a grinding device to crush core aliquots;
. a sample transfer device (manipulator) for transferring prepared samples and selected grains for further
analysis;
. a magnetic grain separation device for separating
particles of di€ering magnetic susceptibilities.
There are a number of general considerations to take
into account in the inspection of surface and subsurface aliquots. It is very important to obtain clean,
unweathered samples. Furthermore, contamination of
samples during sample preparation should be stringently avoided (for example, use of a diamond saw for
samples to be analysed by GC±MS is obviously not
appropriate).
4.4. Sample observation
Surface and subsurface inspection has three main
objectives:
1. to document the mineralogy, petrography, secondary soil components and mobile phases, their compositional changes, and their changes with depth;
2. to search for evidence of extant/extinct microbial
life at all scales down to 0.01 microns;
3. to determine the dust particle characteristics, radiation and any potential danger to human life.
F. Westall et al. / Planetary and Space Science 48 (2000) 181±202
The importance of sample inspection at all stages and
magni®cations is underlined by the Path®nder experience in which it cannot be determined whether the
APX measurements are those of a silica-bearing rind
coating the rock or the rock itself (McSween et al.,
1999). Also, imaging of particular rocks at various
times of day showed that surfaces that appeared to be
scoured of dust were, in fact, not scoured (Thomas et
al., 1999). Thus, it is of the utmost importance to
remove contaminating surface dust and rind for correct imaging and analysis.
As noted above, imaging from the macroscopic to
the microscopic level is important for obtaining relevant background information in the search for biogenic material. The preliminary observations will form
the background for the search for visible and chemical
signs of life. A catalogue of visible signs of terrestrial
life will serve as a database for comparison with the
Martian situation. Visible signs of life range from the
macroscopic to the microscopic scale and may be present on the surface and/or subsurface. Although extant
life on the surface of Mars is not expected, it is necessary to be prepared for the unexpected.
For mineralogical, petrological and palaeontological
purposes, sample observation needs to take place at all
levels of magni®cation possible, down to the submicron scale. In aiming for a useful interpretation, it is
imperative to be able to relate any interesting structure, texture, mineral observed to its overall geological
and geomorphological environment (this includes correlation with the biogeochemical data obtained with
other methods). Thus, the steps in sample observation
will commence with the ®rst observations of the
panoramic camera of areas of regolith, rocks or dunes
suitable for more detailed inspection. Once a target
has been chosen and preliminarily cleaned (i.e. dust
and weathering rind removed if it is a rock), low magni®cation microscopic observation will be used to
screen samples for further high magni®cation and biogeochemical analysis. The low magni®cation microscope can be used to study the gross mineralogical and
textural features of the rock in order to try to identify
its type (igneous or sedimentary) and to try to estimate
the relationship of the target to its environment (i.e.
allochthonous Ð transported by ¯uvial means or mass
wasting from afar, or in situ rubble from impacting).
Structures of microbial origin are visible at many
scales. Macroscopic structures, such as (unlikely)
lichen ¯ecks, large microbial ®laments, and fossil microbial build-ups, may be visible to the panoramic
camera. Many microbial structures, such as ®nely
laminated microbialites and the larger, microbial ®laments can be identi®ed at low microscope magni®cations. The visible identi®cation of most microbes,
however, will rely on higher powers of magni®cation.
Most terrestrial bacteria, for example, are smaller than
195
2 mm. A series of microscopes with overlapping powers
of magni®cation can be used for more detailed investigations, such as a high power optical microscope and
a scanning electron microscope (SEM) and/or atomic
force microscope (AFM). However, there are still
serious limitations to the use of high magni®cation
micoscopes in robotic mode (very short working distance, problems of ®ne focussing, small ®eld of view)
which reduce the chances of ®nding fossil organisms.
However, we consider that considerable e€orts must
be undertaken to ®nd robotic solutions to these problems. In our opinion, a Martian sample return mission will greatly bene®t from in situ robotic high
magni®cation inspection of the samples.
The optical microscope envisaged for the robotic
search for life will probably be a re¯ecting microscope,
given the extra equipment and, therefore, weight
needed for making thin sections. This implies preparing a polished surface for presentation perpendicular
to the microscope.
Loose material, such as regolith and atmospheric
dust can be observed as scatter particles using low and
high magni®cation, re¯ective microscopy. The particulate nature of the material may, however, produce too
much roughness for observation with an AFM.
The instruments recommended for sample observation are:
. a panoramic camera;
. a low and high magni®cation optical microscope
(probably re¯ective);
. an atomic force microscope (bearing in mind the
limitations discussed above).
4.4.1. Recommendation
In order to ensure the most ecient interpretation
of the images that will be obtained with the camera
and various microscopes, scientists should undertake
exercises in the petrological and biological interpretation of images under as realistic `Mars' conditions as
possible, including dirty, uncleaned ®eld material.
4.5. Biogeochemical analyses
The search for life on Mars will take place on
di€erent macroscopic, microscopic and biogeochemical levels, and all levels of observation and analysis
are important in understanding the environmental
framework of any signs of life. The abundance and
depth distribution of oxidant and UV at the surface
will be necessary for the search for extant life and
for the biochemical investigations. Geochemical analyses of the rock and regolith samples will complement the mineralogical and petrological data
obtained from the microscopes. If an SEM is used,
further compositional detail can be obtained by
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F. Westall et al. / Planetary and Space Science 48 (2000) 181±202
using it in backscatter or cathodoluminescence
mode. Instrumentation for obtaining energy, molecular or mineralogical data include an alpha/proton/Xray (APX) spectrometer (all elements above He), a
MoÈssbauer spectrometer (Fe-bearing minerals, Feoxidation state and ratios), and laser Raman spectrometry together with infrared spectrometry (for
the molecular analysis of minerals). Pyrolytic gas
chromatography/mass spectrometry (Pyr±GC±MS)
will also provide analyses of inorganic compounds.
These geochemical analyses provide not only the
necessary mineralogical framework of the samples,
but also they will aid the identi®cation of biominerals.
The biogeochemical analyses of any carbon found
will be necessary for distinguishing between abiogenic and biogenic carbon. This can be accomplished using instruments, such as Laser Raman
and IR spectroscopy, and pyrolytic GC±MS. Moreover, pyrolytic GC±MS will also give the isotopic
ratios and an indication of the chirality of the molecules.
Thus, the analyses needed for biogeochemical analysis of minerals and organics include the following.
1. Raman spectroscopy of single grains or smooth surfaces for molecular analysis of minerals and organics (with IR).
2. APX spectroscopy of ¯at surfaces for bulk and
minor elements (above He).
3. MoÈssbauer spectroscopy for quantitative analysis of
Fe-bearing minerals.
4. Pyrolytic GC-MS for isotopic, elemental, organic
compounds (low and high molecular weight, macromolecular compounds), inorganic molecular composition, and chirality measurement.
5. Electrochemical sensor for H2O2 and other oxidants.
6. The atomic force microscope used in sample observation could also image selected particles for the
identi®cation of condensates and the investigation
of radiation damage.
7. IR spectroscopy for molecular analysis of minerals and organics (with Raman). This instrument
can also test for on-going hydrothermal activity.
Appropriate technologies for some of the instrumentation still need to be developed further. These include a
turbo molecular pump, an LA±ICP ion source, an
H2O2 speci®c sensor, GC detectors (nano thermal conductivity detectors, chiral), and a grinding/sample
preparation system.
A number of other instruments (some already miniaturised) and methods are available which could be of
further use in biogeochemical analyses.
1. Ion/electron probe analysis: this would provide sur-
2.
3.
4.
5.
face analysis (of coatings, hydrogenated or carbonated surfaces or rock-forming minerals), major,
minor and trace elements of rock-forming minerals,
secondary minerals and hard and soft biogenic matter, isotope analysis of minerals and organic matter
(H/D, Li, B, C, N, O, S etc.), impact glasses, ice,
salts, cements.
X-ray analysis of powdered samples for mineral
identi®cation.
Di€erential thermal analysis for H2O-, OH-, CO2bearing minerals.
UV spectroscopy to determine the ¯uorescence of
single grains or grains in a polished mount.
Bulk chirality analyser which could measure the
bulk homochirality of a sample as well as enantiomeric separations obtained from chiral GC columns.
5. Preliminary considerations on human explorations of
Mars
The following considerations and recommendations
were prompted by the question ``If humans could land
on Mars, what could their presence do to aid exobiology research?''. The bene®t of human presence will be
the exercise of judgement, based on extensive professional and personal experience, coupled with ¯exibility and ingenuity to adapt and to improvise in real
time.
A manned base should be located close to sites of
exobiological interest within reach of manned rovers.
Human exploration would be integrated with the use
of unmanned rovers. The latter can be used for a
radar-based search for permafrost water and ice, for
IR scans from balloons of potential hydrothermal
sites, and for the inspection of chaotic terrains (crater
ejecta, cli€s, cli€ falls). It is essential that, at least,
some of the crew on Mars have relevant professional
scienti®c training in order to eciently choose
sampling sites and selectively screen samples for
further analysis and/or extraction in a suitablyequipped laboratory for return to Earth-based laboratories. The presence of humans on Mars will make
deep drilling possible. Sucient time should be allocated to the mission to enable the Mars crew to make
the most ecient use of their e€ort and expertise.
Humans on Mars would be able to better search for
the presence of extant life than robots. For example, a
trained eye would be able to recognise subtle changes
in colour, humidity, granulometry and porosity which
may indicate the presence of life. Potential life-containing samples could be cultured using suitable media in
a base laboratory by a microbiologist.
Of great importance is the risk of biological con-
F. Westall et al. / Planetary and Space Science 48 (2000) 181±202
tamination. There are three aspects to this risk. Firstly,
the risk of Mars explorers bringing Martian life into
the base from outside. Secondly, the risk to Earth
from returned Martian samples. Thirdly, the risk to
Mars from terrestrial organisms. Although terrestrial
organisms would probably not survive in the harsh
Martian environment, there is always the chance that
some of them may ®nd relatively protected habitats
and propagate and, eventually, mutate. Both live and
dead organisms would progressively contaminate the
Martian environment from an exobiological point of
view. It is recommended that studies on survival,
mutagenesis and growth of microorganisms in a simulated Martian environment (atmosphere, surface, subsurface) be undertaken.
Planetary protection issues are covered by an international treaty (UN Doc.A/6621, 17 December 1966)
and agreement (UN Gen. Ass. Resol. A/34/68, 5
December 1979) with the intent of protecting the
planet being explored from contamination by terrestrial microbes and organics, and to protect the Earth
from potential hazards imported from extraterrestrial
material. With respect to Mars, the e€orts at
planetary protection should be targeted towards
identifying and minimising contamination of the
local and global environment associated with humans
in order to safeguard the scienti®c (exobiogical)
exploration of Mars whilst in its pristine condition
(DeVincenzi and Stabekis, 1984; Rummel, 1989;
Space Studies Board, 1992).
The ®nal recommendation is that any Science de®nition team commissioned to prepare a human mission
to Mars should include an exobiologist.
6. Conclusions
The three fundamental requirements for a satisfactory search for life on Mars are a landing site with
exobiological potential, sampling requiring mobility,
and an integrated instrument package providing observational, spectroscopic and analytical techniques to
reduce the potential for ambiguous interpretation of
the results.
6.1. Landing site
Suitable landing sites include sedimentary terrain
with low aeolian sand cover, or hydrothermal sites.
The mineralogy of the landing sites would need to be
adequately mapped from orbit. The ®ve sites chosen
are a small but representative selection from a number
of potentially interesting sites and later information
from future missions could increase this number. The
sites, all with good exobiology potential, are: (i) Marca
Crater (10.48S/158.28W), or adjoining possible crater
197
lakes, with lacustrine deposits in several craters; (ii) SE
Elysium basin (38N/1858W) with channel and lacustrine; (iii) Apollinaris Patera (8.68S/187.58W) with
possible hydrothermal systems associated with a volcano and rock debris of various ages; (iv) Gusev Crater (148S/184.58W) with lacustrine deposits and a large
out¯ow channel; (v) Capri Chasma (158S/478W) with
chaotic terrain and sediments.
6.2. Sample acquisition, distribution and preparation
The samples need to be free of dust and oxidation
products and, therefore, need to be obtained from the
subsurface, or from beneath the weathering rind of
sur®cial rocks. Mixing of the regolith over geological
time by impacting will need to be studied by sequential
oxidation analysis. Subsurface samples satisfying these
requirements may be obtained by core drilling to
between 1 and 1.5 m sub-surface depth. The size of the
drill will necessitate its location on the lander rather
than the rover. Successive cores will need to be
observed by a low resolution, colour camera and then
temporarily stored during successive, synchronised
analyses. The core will have to be sawn for analysis
into three fractions: one for optical and AF microscopy, Raman/APX/MoÈssbauer spectroscopy, and
ion probe analysis; one split into two parts for pyrolysis/gas chromatography/mass spectroscopy and oxidant analysis; and one for storage. Surface rock
samples from suitable sites imaged by the close-up
camera may be drilled from a drill located on the
rover. Penetration will probably be up to 15 cm. Soil
samples will be collected by a scoop and then sieved
before analysis.
6.3. Visible signs of life
Visible signs of fossil prokaryote terrestrial life occur
at the macroscopic level down to microscopic levels.
Microbial build-ups and microbial mats leave macroscopic signatures whereas the individual fossil organisms have a wide size range going down to the submicrometer level. Visible signs of possible (although
unlikely) extant life also have a wide size range, from
macroscopic microbial buildups and biominerals to
sub-micrometer-sized organisms. Observation on Mars,
therefore, needs to make use of a macroscopic system,
low to high resolution optical microscopes and an
atomic force microscope (although there are still limitations associated with the latter). The optical and
camera systems will provide the necessary background
of mineralogical and petrological information for making interpretations regarding signs of life.
198
F. Westall et al. / Planetary and Space Science 48 (2000) 181±202
6.4. Geochemical and organo-chemical analyses
Mineralogical and petrological analyses and observations will characterise the samples and identify potential biominerals. Geochemical studies of the same
samples will provide their bulk elemental composition
and help de®ne the geological history of the site.
Analysis of the oxidation state of elements such as Fe,
Mn, S, N is necessary, as is a knowledge of the relative
abundances and organic/inorganic partitioning of biologically signi®cant elements such as C, H, N, O, S,
and P. A study of nitrogen and its oxidation state in
Martian soil will aid understanding of the fate of initial atmospheric nitrogen. The stable isotope ratios of
13
C/12C, 15N/14N, and 34S/32S may provide evidence
for biological fractionation. Geochemical analyses will
determine the presence of potential biominerals (e.g.
oxalates, carbonates, phosphates, Fe/Mn oxides and
sulphides, etc.). The presence and depth pro®le of
water and oxidant beneath the surface will be
measured.
Of great importance will be the measurement of the
hitherto-elusive organics of biological origin, as
opposed to those of igneous or meteoritic origin. Biomarker organics to be searched for include: (i) low
molecular weight compounds including hydrocarbons
(methane), alkanoic acids and peroxy acids; (ii)
medium molecular weight compounds, including
hydrocarbons (straight and branched chain, isoprenoids, terpenoids, steroids, and aromatics); (iii)
macromolecular compounds (kerogens, oligo- and
polypeptides).
Instrumentation for undertaking the elemental, isotopic, and molecular determinations outlined above
include:
. Alpha/Proton/X-ray spectrometer for elemental
analysis (except H and He),
. MoÈssbauer spectrometer for Fe-bearing minerals, Fe
oxidation state and ratios,
. Laser Raman Spectrometer (200±3500 cmÿ1 and
8 cmÿ1 resolution) for molecular analysis of organics and minerals,
. Infrared spectrometer (0.8±10 mm, spectral resolution 100, spatial resolution 200 mm) for molecular
analysis of minerals and organics,
. Pyrolytic gas chromatograph and mass spectrometer.
6.5. A possible exobiology experiment package and
operating arrangement
The above described equipment should be distributed between a large Lander and a Rover. The Rover (7
kg) would be used for initial imaging, sample taking
and possibly simple analyses, whereas the Lander (26
kg) would act as the centre for subsurface sampling
and in situ sample preparation and analysis.
Acknowledgements
This study was undertaken on the initiative of ESA
Manned Space¯ight and Microgravity Directorate. G.
G. Ori acknowledges the following people: Lucia Mariangeli, Ignasi Casanova, Gow Komatsu. The manuscript was improved by the instructive comments of
the referees.
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