scans 28, 291-300 (1976) og pp 236

The Prospects for Life on Mars: A Pre-Viking Assessment

CARL SAGAN

Laboratory for Planetary Sturlies, Cornell University,
ithaca, New York 14253

AND

JOSHUA LEDERBERG

Department of Medical Genetics, Stanford University Medical School,
Stanford, California 94305

Received February 2, 1974; revised December 21, 1375

Mariner 9 has provided a refutation or reinterpretation of several historical
claims for Martian biology, and has permitted an important further characteriza-
tion of the environmental constraints on possible Martian organisms. Four
classes of conceivable Martian organisms are identified, depending on the environ-
mental temperature, 7’, and water activity, a,: Class I, high 7’, high a,,; Class IT,
low 7, high a,,; Class III, high 7, low a,; and Class IV, low 7, low a,. The Viking
lander biology experiments are essentially oriented toward Class I organisms,
although arguments are given for the conceivable presence on Mars of organisms
in any of the four classes. Organisms which extract their water requirements from
hydrated minerals or from ice are considered possible on Mars, and the high ultra-
violet flux and low oxygen partial pressure are considered to be negligible
impediments to Martian biology. Large organisms, possibly detectable by the
Viking lander cameras, are not only possible on Mars; they may be favored. The
surface distribution of Martian organisms and future search strategies for life on
Mars are discussed.

INTRODUCTION tribution to biological studies was rather in
refining our knowledge of Martian habitats
and thereby in specifying boundary con-
ditions that any organisms would have to
meet in order to survive on Mars at the pre-
sent time. In addition, the history of Mars
is now perceived to be one of surprising
complexity, relieving some of the pre-
viously held constraints on the evolution
of life on that planet. Finally, some his-
torical ground-based observations that
had laid claims for relevance to the problem
have now been refuted or drastically re-
interpreted.

The Mariner 9 orbital mission to Mars
obtained 7200 photographs of the entire
planetary surface, hundreds of occultation
profiles and infrared radiometric observa-
tions, and tens of thousands of ultraviolet
and infrared spectra. Many of these data
and a wide range of interpretations of geo-
logical and atmospheric phenomena on
Mars have already been published. Our
present task is to survey the further impli-
cations of these findings for the investiga-
tion of Martian biology in the next decade,
beginning with the Viking mission.

Mariner 9 was not designed to detect life -
on Mars, nor did it. Only the most extra-
ordinary contingencies would have allowed
direct evidence of life to be sensed by the

Mariner 9 anpd HistoricaL CLAIMS
FOR OBSERVATIONS OF MARTIAN BioLocy

photographic and other instrumentation These historical claims, and their refuta-
of this mission. The most important con- tion or reinterpretation, are now. briefly
Copyright © 1976 by Academic Press, Inc. 391

All rights of reproduction in any form reserved.
Printed in Great Britain

 
29% SAGAN AND LEDERBERKU

summarized, A network of fine rectilinear

- markings reported on Mars by Schiaparelli
and Lowell initiated one tradition of
“evidence” for engineering constructions
and an advanced civilization on the
Martian surface. However, the reality of
many of these markings has long been sus-
pect (see, e.g., Antoniadi, 1930). No sign
of their presence was obtained: in the
Mariner 4, 6, and 7 missions (Leighton
et al., 1969). Some smaller rectilinear
features of probable tectonic origin, the
long rift valley Vallis Marineris, and a few
crater chains were uncovered by the
Mariner 9 mission; but no Lowellian canals
were found, and the locales of many of the
classical rectilinear features show no hint
of anomalous linear or other features
(Sagan and Fox, 1975).

The Martian bright and dark areas vary
their relative contrast both annually and
irregularly; a biological interpretation of
these changes also dates back to the 19th
century and has had recent supporters
(see, e.g., Slipher, 1962). The high spatial
resolution and long time base of Mariner 9
have shown that such changes are almost
certainly due to wind-transportable fine
particulates, and no biological interpreta-
tion is required (Sagan et al., 1972, 1973.
Veverka e¢ al., 1974). ,

The apparent secular acceleration of
Phobos, the innermost moon of Mars, inter-
preted as due to drag in the Martian atmos-
phere, led Shklovskii to propose an origin
of Phobos as a hollow artifact of an
advanced Martian civilization (Shklovskii
and Sagan, 1966). High-resolution photo-
graphy of Phobos by Mariner 9 has shown
conclusively that it is an entirely natural
and ancient satellite (Pollack et al., 1972).
In addition, the most recent reduction of
the celestial mechanical data shows that
there may be no large secular acceleration;
and what secular acceleration there is can
be understood in terms of body tides
(Smith and Born, 1976; Pollack, 1976).

At 100m surface resolution, the Earth
displays a remarkable checkerboard pat-
tern due to human agricultural and urban
constructs. Mariner 9 provided the first
opportunity to examine Mars at adequate
coverage and resolution to detect such a

pattern on that planet, should it exist
(Sagan and Wallace, 1971). No such
features were found. Some peculiar geo-
metric features were uncovered, but they
are amenable to abiological interpretations,
particularly when account is taken of the
high Martian winds (ef. Sagan, 1975).

Thus, Mariner 9 was able to exclude four
previous hypotheses on Martian biological
activity. These negative results, however,
leave the question of life on Mars open, as
it was before the Mariner 9 mission. Some
prejudgments and simple hypotheses have
been excluded, but no critical tests have yet,
been performed.

WatTER

The availability of liquid water as a sol-
vent for indigenous molecular biology ap-
pears to be the most stringent limiting
factor for possible Martian biology. How-
ever, the water need be liquid only within
the organism. Even on Earth, organisms
are known which acquire water in the vapor
phase under high humidities from the air
(e.g., spanish moss, lichens), or metabolic-
ally from foodstuffs (e.g., desert rodents).
Growth of terrestrial microorganisms is
known at least down to —15°C and the
germination of spores down to ~25°C: (see,
e.g., Packer et al., 1963). The corresponding
water activities a, (numericall ¥ equivalent
to relative humidities) are about 0.85 to
0.70. Unfortunately, there has been no
systematic search for ultimate limits on
@y, nor any real understanding of the
mechanisms that determine such limits.
Less well-documented claims of growth
down to —20 and —25° appear in the
literature, but they cannot be accepted
without critical reexamination.

From time to time, brines are invoked
as possible eutectic point depressors below
0°C, but many other factors may influence
the microenvironment of an organism to
permit it to flourish in or on bulk ice. For
example, many bacteria do not have a well:
defined outer boundary. They secrete a
mucopolysaccharide gum or capsule in
which water should dissolve even from an
ice interface, and allow the absorption of

ke be ahi encanta neta eniadl

Sohn mes

att OA te tanetenatmntnas Ds aisteries’s «

+

PO mmm sane.
1 Ww hw

n
n

LIFE ON MARS 293

water at subzero temperatures: a kind of
portable tethered antifreeze. These gummy
layers on microbes are sometimes microns
thick, often exceeding the diameter of the
bacterium itself. Similar substances are
employed by antarctic fish to maintain the
fluidity of their circulatory fluids.

In addition to hypothesized environ-
ments for liquid water on Mars, discussed
below, there is direct evidence for water
vapor in the Martian atmosphere, water
ice in the Martian polar caps, and water of
crystallization in the Martian surface
materials. Subsurface permafrost must
also exist because the planet is outgassed
and the mean temperatures are below the
freezing point of water. The distribution of
these various forms of water substance may
be heterogeneous both in space and in time.
Thus, under rather parochial assumptions
about the indigenous biochemistry, the
question is whether there are (7', @,) pairs
suitable for a conceivable Martian biology.

Frost

Now that extensive vulcanism is known
to exist on Mars, as well as probable
thermokarst terrains (Sharp et al., 1971;
Belcher et al., 1971) a previous suggestion
(Lederberg and Sagan, 1962) that there
may be locales of geothermal penetration
of the subsurface permafrost layer and
consequent patches of subterranean liquid
water appears somewhat more plausible.
It has also been suggested (Sagan ¢t al.,
1968; Farmer, 1976) that ice, deposited in
soil interstices, may enter a temporary
liquid phase due to the soil diffusion barrier.
Tf the soil can hincer evaporation long
enough for the ice to reach —10°C, that
would be enough for many terrestrial
microbes. And it is of course plausible
that Mars-adapted microbes would extend
those boundaries further (e.g., by creating
low-albedo microenvironments), especially
as they would not simultaneously have to
be armed against drowning in higher water
activities at above-zero temperatures.

Heterogeneities of water abundance with
time occur on diurnal, annual, and long-
term climatic change time scales. Even at

equatorial latitudes, the surface tempera-
ture drops below the frost point of the
atmospheric water-vapor partial pressure
before dawn each Martian day. The amount
of water that is actually deposited on the
ground is still uncertain, but a few micro-
meters per night might support a steady-

‘state biomass as thick as a cover of moss or

lichens. The problem is to estimate the
frost thickness that will survive tempera-
tures up to the —10°C range; and to seek
means by which colder frost can be
sequestered by the postulated frost eaters,
described below.

The Martian polar caps vary in extent
with the seasons, reaching in winter down
to about 60° latitude. This annually vary-
ing component is probably mostly frozen
carbon dioxide, although some frozen
water will partake of the annual cycle.
Temperatures low enough to freeze carbon
dioxide are certainly low enough to freeze
water. The permanent polar cap is thought
to have considerably larger amounts of
frozen water (Murray ef al., 1972). A local
slope of X° at low latitudes is the tempera-
ture equivalent of an increment of X°
latitude on a smooth surface (see, e.g,
Balsamo and Salisbury, 1973). Thus, a
boulder at 30° latitude with a 30° slope will
have the same temperature for half a
Martian year as a smooth surface at 60°
latitude. In addition, the temperatures
within the boulder shadow are determined
largely by radiation; convective heat ex-
change with the atmosphere and conduc-
tive heat transport in the porous surface
are both very low. Permanent shadows
can be significantly colder than their sur-

roundings and the progressive buildup of

frost in such shadows can be expected. At
polar latitudes, much larger quantities of
water substance change phase. A vertical
polar organism might manage high T and
high ay simultaneously. Because the
Martian year is twice as long as the terres-
trial, very high temperatures can be
achieved in midsummer at: Martian polar
latitudes, including temperatures above the
freezing point of water. Thus the edge of
either polar cap in midsummer is another
potential habitat of interest for conceivable
Martian organisms.

I

wean Re RE OREO EERE, Or
294 SAGAN AND LEDERBERG

PossIRLE CLEMENT EARLIER
ENVIRONMENTS

The Martian polar laminae (Murray
et al., 1972) and particularly the sinuous
dendritic channels (Sagan ef al., 1973;
Milton, 1973; Carr, 1974; Pieri, 1976)
suggest a significant climatic variability of
the Martian environment. For many of
' the channels no plausible alternative to
flowing surface liquid water has been
offered; although some investigators be-
lieve that extensive surface liquid water on
Mars is so unlikely that they would wish to
reserve judgment on the matter. At least
one specific numerical model giving cli-
matic variations from environments more
severe than the present Martian environ-
ment to Martian climates rather close to
the present terrestrial climate has been
proposed (Sagan ef al., 1973). The period
for such climatic variation is of course not
well determined; but the shortest of the
climatic time scales calculated in the pre-
ceding reference are ~10° yr; the ages of
some channels of intermediate size (Pieri,
1976) suggest the longest of these time
scales to be ~10° yr. The time scale for the
origin of life on Earth may have been
significantly shorter than the earliest
clement epoch on Mars. If such more-
clement epochs existed in past Martian
history, extensive liquid water would be
available episodically; and it has been
suggested that some subset of Martian
organisms may be in cryptobiotic repose
awaiting the return of wetter and warmer
conditions (Sagan, 1971).

A CLASSIFICATION OF POSSIBLE
Martian BIoLoOGres

From the standpoint of an unrecon-
structed Earth chauvinist, we must strain
at improbabilities in searching for con-
genial habitats on Mars. But to surrender
the search is to give too little credit to the
adaptive versatility of living organisms,
a, versatility whose boundaries are under-
stood very poorly. Biological evolution
has a major stochastic component, which
makes biology more akin to history than
to planetary physics. We will never be

able to make precise predictive models in
biology that will approach even the most
difficult models of, e.g., planetary differen-
tiation. We do not know, for example, how
many alternative self-replicating mole-
cular systems there might be other than
nucleic acids.

In this spirit of seeking possible alterna-
tive ecological niches, we wish to offer four
possible classes of Martian organisms
associated with temperature/water eco-
logical niches in the external environment
(ef. Fig. 1):

Class I organisms. Organisms requiring
high temperatures (JT > 240K) and high
water activities (a, > 0.7). These require-
ments correspond to those of many known
terrestrial organisms. Such organisms may
flourish only during the presumed high
water phase of Martian climatic varia-
tions; or they may inhabit soil interstices,
which have temporary accumulations of
liquid water arising from the diurnal cycle;
or they may inhabit locales where the
permafrost has been breached by geo-
thermal activity, but the soil overburden
keeps the vapor pressure high enough to
cross the liquidus.

Class II organisms. Organisms inhabiting
niches with low temperatures (7 < 240K)
and high water activities (a, > 0.7). The
low abundance of water vapor on Mars
forces high humidities to occur at typical
nighttime temperatures. It is known
(Pollack et al., 1970) that the hydration
water of ferric oxide polyhydrates or otLer
minerals can be in gas-mineral equilibrium
with water vapor in the Martian atmos-
phere. The water vapor pressure must be
very high within a few mean free paths
of the surface. The Martian regolith can be
regarded as a chemical as well as a physical
buffer for water activity, which allows the
kinetic persistance of favorable (T, a,)
microenvironments with appreciable de-
partures from thermodynamic equilibrium.
(See the cross-hatched region in Fig. 1.)

Class III organisms. Organisms inhabit-
ing ecological niches of high temperature
and low water activity. Such organisms
are unknown on Earth, but there are no
obvious physical impediments to their
existence. Very deliquescent organisms

se
asadad dakiibdebas Pe psa

times be

Tent des aloadal

 
 

oe eid

ME

at

LIFE ON MARS . 295

 

a T T

disequilibrium t

    

g

 

80\-

60}—-

Water Activity , Ow + in percent

 

7
77 A Region of possible water vopor

n Martian surface

1 ' ‘ {
Typical terre strial regime

 

imits of possibl
inetic extension
n Martion surfac
moterial (scheme:

 

 

0
1590 470 190 210 230

  
 
   

 

250 2?

   
   

290 310

Temperature T, in °K

Fic. 1. A highly schematic representation of water activity/temperature space for Earth and Mars.
The general provinces of Class I, II, IIT, and IV organisms, described in the text, are shown, as is the

province of the bulk of the Viking biology experiments. The cross-hatched region shows & possible
kinetic extension of the provinces of Mars on this diagram, due to departures from thermodynamic
equilibrium near soil/atmosphere interfaces (ef. Farmer, 1976). The Martian province continues at

high water activities to very low temperatures. P

ossible Martian excursion to high temperature/high

water activity provinces, as, for example, in subsurface permafrost melted by internal heating, are
not shown. The curve for Mars “dry” atmosphere corresponds to ~1t precipitable zm of water vapor;
Mars ‘‘wet”’ atmosphere, to 100 pm; and Furth “normal” atmosphere, to 1 precipitable em.

are one possibility. The metabolic concel-
tration of rare materials, such as phospho-
rous or vanadium or niobium is well
known among terrestrial organisms. Water
substance is relatively much more avail-
able on Mars than these elements are on
Earth, and concentration mechanisms are
also known to exist. Indeed, the kidney

function of mammals compared to fish is.

testimony to the remarkable evolutionary
adaptations to water scarcity which are
possible.

Class IV organisms. Organisms inhabit-
ing low temperature and low water activity
ecological niches. They are, by ordinary
terrestrial standards, the most exotic. But
technological H. sapiens is on occasion &
Class IV organism. Martian surface miner-
als are known to have 2 bound water con-
tent ~1% (Houck et al., 1973). This is
probably some combination of physically
and chemically bound water. The hydra-
tion bond energy corresponds to a few elec-

tron volts, or the mean energy of a solar
near-ultraviolet photon which penetrates
the Martian atmosphere. Class IV orga-
nisms would have to heat themselves meta-
bolically as well as employ metabolic
energy to extract moisture from surface
minerals or ice. The lifetime for which such
metabolic heating can be maintained varies
directly with the size of the organism.
Because of this argument concerning the
area-to-volume ratio, there may be a strong
selection pressure for Class IV Martian
organisms to have large climensions.

Class III and Class IV organisms may
extract water from minerals or from ice in
the arid Martian environment. Neither
adaptation is known on Earth, but there
has not been strong selection pressure for,
and indeed there may have been evolu-
tionary potential barriers against, their
development on a wet planet. For future
convenience, we propose calling orga-
nisms with such adaptations petrophages
296 SAGAN AND LEDERBERG

(Greek, rock eaters) and crystophages
(Greek, frost eaters), respectively.

Finally, we note that the internal en-
vironment of Class IT, III, or IV organisms
may be a Class I environment for para-
sites or commensual organisms.

ULTRAVIOLET FLUX

Except during a global dust storm, the
ultraviolet flux at the surface of Mars in
the 2000-3000A range is lethal to all
terrestrial organisms. Nevertheless, this
does not strike us as a significant hazard,
and there are three conceivable adapta-
tions, which have recently been discussed
by Sagan and Pollack (1974). First, the
organisms may carry many redundant
copies, as do some terrestrial radiation-
resistant species; or they may. contain a
genetic material which is not photolabile
in near-ultraviolet light. Their genetic
material might be simply nonabsorbing at
these wavelengths or—if, for example,
composed of many aromatic compounds—
it may absorb but no bonds may be broken.

Second, the efficiency of repair of ultra-°

violet damage to Martian genetic material
may be more efficient than ours. Less than
107? of uv lesions escape terrestrial repair
mechanisms (Jaggar, 1967). We cannot
exclude the possibility that on Mars the
number is 107-5 or 107’. Third, Martian
organisms may be shielded from ultra-
violet damage. Significant protection can
be provided by tens of microns of nitro-
geneous bases (Sagan, 1973a); or by a
lem depth of Martian surface particulates
(Sagan and Pollack, 1974), a depth at which
there may still be enough visible light for
photosynthesis. Thus, Martian organisms
may live in a surface habitat or may carry
ultraviolet-absorbing shields around with
them, like the exoskeletons of shellfish
and insects. Such shields might also be
useful for water conservation. If the shields
are inorganic, they may provide an abun-
dant fossil trace.

ATMOSPHERIC COMPOSITION

The primary constituents of the Earth’s
atmosphere are the products of biological

activity. Oxygen is produced by green
plant photosynthesis, and nitrogen by
denitrifying bacteria. Even minor consti-
tuents such as carbon dioxide and methane
are deeply involved in biological activities
on Earth. The major constituent of the
Martian atmosphere is CO;, present in
absolute abundance about 20 times the
abundance of CO, in the terrestrial atmos-
phere. The carbon dioxide equivalent of
the carbonates in the terrestrial sedimen-
tary column is some 50 to 100 bars. The
carbonate content of the Martian litho-
sphere is unknown. An upper limit to the
N, abundance in the Martian atmosphere
is ~10-? (Dalgarno and McElroy, 1970).
Thus, the ratio of atmospheric N, to sedi-
mentary CO, on the Earth is not inconsis-
tent with the ratio of atmospheric N. to
atmospheric CO, on Mars. The detectivity
of N, in the Martian atmosphere should
be greatly improved in the near future by
mass spectrometers on Mars atmosphere
entry probes.

Let us consider the case that the atmos-
pheric N, abundance on Mars proves to be
<107?, or that the crustal carbonate con-
tent proves to be large. Then the N,/CO,
ratio would be significantly below the com-
parable value for the Earth. In the absence
of denitrifying bacteria, nitrate nitrogen
atoms would not be returned to the atmos-
phere as N,. Thus a low N,/CO, ratio on
Mars may only indicate that denitrifying

‘bacteria are not preeminent. This by no

means excludes Martian biology, and bio-
logical exchange processes between nitrates
and ammonia which do not involve gas
phase N, can easily be envisioned. Indeed,
because of the stricture for water conserva-
tion on Mars, Martian organisms may have
major barriers to gas exchange, and the
rarity of nitrogenous gases in the Martian
atmosphere may be only a biological reflec-
tion of the underabundance of gaseous
water in the present Martian atmosphere.

The mixing ratio [0,]/(CO,] is ~10~ in
the Martian atmosphere. Molecular oxygen
is of course not necessary for even rather
complex forms of life, as both obligate and
facultative anaerobes on the Earth demon-
strate. Conceivable electron transfer pro-
cesses feasible for Mars, which do not

 

cee enone ete ate

bien eee ne

cu
M:
mi:
alt
ex
wi
at
Mc
or

tel
we

“

one ceee ttl ee

ae

LIFE ON MARS - 297

involve molecular oxygen, have been dis-
cussed elsewhere (Vishniac et al., 1966).
Martian animals may metabolize organic
material produced by Martian plants; or
alternatively may live out a heterotrophic
existence off carbohydrates produced by
ultraviolet irradiation of the Martian
atmosphere (see Hubbard eé al., 1971).
Molecular oxygen does provide almost an
order-of-magnitude improvement in the
efficiency of the glycolytic pathways in
terrestrial metabolism. Even if the mole-
cular oxygen in the Martian atmosphere
is not utilized for metabolism, we do not
believe that the absence of aerobic metabo-
lism on Mars excludes the presence of larger
organisms.

The Viking biology experiments, with a
terminal heating stage, can test the idea of
Martian biological gas exchange barriers
or directionally selective gas flow inter-
faces. Significant departures from thermo-
dynamic equilibrium in a planetary atmos-
phere may be a strong indication of bio-
logical activity (Lovelock and Hitchcock,
1967; Lippincott ef al., 1967). However, the
observed upper limits on minor consti-
tuents in the Martian atmosphere (Owen
and Sagan, 1972) are far above the values
required to make a significant test of
departures from thermodynamic equili-
brium (Lippincott et al., 1967). The Viking
atmospheric experiments might just con-
ceivably perform such a confrontation.

Micropes AND MACROBES

The widespread impression that if there
is life on Mars it must be confined to micro-
organisms is a misunderstanding. Not all
Martian ecological niches may be filled by
primary photoautotrophs or chemoauto-
trophs: we saw earlier that the heat and
water budgets at low Martian tempera
tures provide a premium for low area-to-
volume ratios, and therefore a selective
advantage for large organisms, which we
here call macrobes. There may be other
vicissitudes of the Martian environments
which can be better managed by macrobes
than by microbes.

Martian macrobes have the advantage
that they can be detected without assump-

tions on their biochemistry. A lander
imaging experiment, for example, can
detect an unusual biogeometry even if the
macrobe’s metabolism is very exotic.
These considerations also suggest that
macrofossils may be a suitable objective of
Martian landed imagery experiments,
especially on a rover. Living and dead
fossils are possible: living fossils being the
dormant forms of macrobes which flourish
during the favorable segment of the pre-
sumed Martian climatic cycle; and dead
fossils being of the usual sort, of organisms
which flourish at any time in these cycles.
We have already mentioned the possibility
of ultraviolet shields as a dominant fossil
form. The hint of geological stratification
in the Martian polar laminae and, for ex-
ample, in the interior cliffs of Vallis Mari-

-neris suggests particular sorts of locales

in searches for fossils.

DIsTRIBUTION OF ORGANISMS

All categories of Martian organisms dis-
cussed above, Classes I through IV, may
have an essentially global distribution.
Even Class I organisms, which require high
temperatures and high water activities,
are conceivable in the summer circumpolar
regions. However, some habitats, such as
underground thermal springs, may have a
very limited distribution. Tf the only life
on Mars lives in such habitats, then Mar-
tian biology is restricted to microenviron-
ments. However, there is observational
evidence that even underground springs
may have a substantial distribution. The
so-called chaotic terrain is most likely
thermokarst produced by melting and
lateral flow of subsurface permafrost (Sharp
ef al., 1971; Belcher e¢ al., 1971). Chaotic
terrain covers more than 1% of the
Martian surface. Indeed, several of the
major sinuous channels are discovered
emanating from chaotic terrain, and there
seems to be a substantial possibility of
extensive underground river systems on
contemporary as well as on ancient Mars.

If Martian biology is ubiquitous (Class IV
organisms, for example), reproduction in
place may be expected. However, there are
many cases where aeolian transport of

tae

(eee ORE EET MERE NE NEE

cere creer
298 SAGAN AND LEDERBERG

dispersules would have a high selective
advantage on as windy a planet as we know

Mars to be—including isolated micro-

environments, and habitats which are un-
stable or unpredictable for relatively short
periods of time. Especially to the extent
that unoccupied ecological niches appear,
due to climatic or tectonic activity, such
aeolian dispersion may be expected. Dis-
persules carried easily by high Martian
winds can have diameters of at least
hundreds of microns (see, e.g., Sagan et al.,
1972). This permits aeolian transport of
macrobial as well as microbial dispersules
(cf., e.g., fern spores or pine tree pollen).
As on Earth, macrobial dispersules can be
generated in apparent extravagance and
still not be a significant fraction of the
biomass.

SPACECRAFT STRATEGIES

Such considerations of course affect
choices of landing sites on Mars in searches
for indigenous biology. If Class IV orga-
nisms exist, or if airborne dispersules are
common, all places on Mars may be of
almost equal value in the search for life.
Alternatively, on the microenvironment
hypothesis, only certain locales, occupying
a small fraction of the Martian surface
area, are suitable for exobiological in-
vestigations, Apart from engineering con-
siderations, Viking landing site selection
attempted to optimize these strategies. A
latitude of 21° was selected for the prime
and backup sites of the A mission; and a
latitude of 44° was selected for the prime
and backup sites of the B mission. A low-
latitude site may optimize the chances of
discovering Class JII organisms; and a
high-latitude site, the chances of finding
Class IT organisms. A latitude of 44° is of
course not polar, and it represents a com-
promise which, however, hopes to take
advantage of the yearly accumulation of
frost.

There is some observational evidence for

‘higher water vapor abundances at such

middle latitudes (Farmer, 1976; Barker,
1976). However, any Martian latitude
poleward of 23° is inaccessible to quasi-
specular radar reflectivity studies. A

significant fraction of potential Viking
landing sites at low latitudes which appear
suitable to Mariner 9 B-frame photo-
graphy have distressingly low radar re-
flectivities (cf. Lipa and Tyler, 1976), due
either to high roughness or to high porosity
and consequent low bearing strength, either
contingency posing serious potential prob-
lems for the Viking landings. Thus, the
B-site landing decision depends on whether
the arguments for liquid water at 44°N
are sufficiently persuasive in a biological
context to offset the additional dangers of
the landing site in an engineering context,
The authors of the present paper find them-
selves in disagreement on this vexing issue.

The prime site of the A mission is in
Chryse near the confluence of several
sinuous channels, where it may be possible
to find dormant or possibly even contem-
porary Class I organisms. However, even a
simple comparison with comparable land-
ing sites on the Earth shows that more than
a few landers are necessary to perform an
adequate characterization of planetary
biology (Sagan, 1973b).

The explicit biology experiments on the
Viking 1976 landers (Klein e¢ al., 1972;
Horowitz et al., 1972; Levin, 1972; Oyama,
1972) are, largely for engineering reasons,
mainly directed to searches for Class I
microbes, either active or dormant; the one
exception, which will search for Class T1I
and IV microbes, is the pyrolytic fixation
option, but this has the disadvantage of a
reiatively short time base. It is conceivable
but unlikely that the range of conditions in
these experiments will overlap the active
range of organisms primarily adapted to
Classes IT, III, or IV. The lander cameras
can detect life of any class; but only if it is
macrobial, nearby, and slow-moving
(Mutch et al., 1972)—or leaves footprints.

Thus, while Viking 1976 represents a
significant first step in the in situ search
for a Martian biology, negative results
would exclude an important subset, but
only a subset, of the possible classes of
Martian organisms. Whether Viking 1976
succeeds or fails, however, subsequent
landed spacecraft on Mars should have a
wider range of biology experiments, able
to search for a broader spectrum of con-

 

 
4

 

eee ef te el

oe ees —

i

(po

li

LIFE ON MARS

ceivable Martian organisms. The design of
instrumentation and mission strategies to
search for Martian microbiota of Classes If,
IIL, and LV could begin even before results
from Viking 1976 on microbes of Class U
are fully analyzed.

ACKNOWLEDGMENTS

This research was supported in part by NASA
Grants NGR 33-010-101, NAS 1-9683, and NGR
05-020-004517. We are grateful to N. H. Horo-
witz for a helpful critique of an earlier draft.

REFERENCES

Awronrapi, E.-M. (1930). La planéte Mars.
Hermann, Paris.

Barsamo, 8. R., anp Sarissury, J. W. (1973).
Slope angle and frost formation on Mars.
Iearus 18, 156-163.

Barker, E.S. (1976). Martian atmospheric water
vapor observations: 1972-74 apparition.
Icarus 28, 247-268.

Bevcuer, D., VeverKa, J., AND Sacan, C.
(1971). Mariner photography of Mars and aerial
photography of Earth: Some analogies. Icarus
15, 241-252.

Carr, M. H. (1974). The role of lava erosion in
the formation of lunar rilles and Martian
channels. Icarus 22, 1-23.

Datcarno, A., AND McE roy, M. B. (1970).
Mars: Is nitrogen present? Science 170, 167—-
168.

Farmer, C. B. (1976). Liquid water on Mars.
Iearus, 28, 279-289.

Horowrrz, N., Hussarp, J. S.. anD Hopssy,
G. L. (1972). The carbon-assimiliation experi-
ment: The Viking Mars lander. Icarus 16,
147-152.

Houck, J. R., Pounack, J. B., Sacan, C.,
Scraack, D., AND Decker, J. A. (1973). High
altitude infrared spectroscopic evidence for
bound water on Mars. Icarus 18, 470-480.

Husearp, J. 8., Harpy, J. P., AND Horowitz,
N. H. (1971). Photocatalytic production of
organic compounds from CO and H,O ina
simulated Martian atmosphere. Proc. Nat.
Acad. Sci. USA 68, 574-578.

trt®g Jaccer, J. (1967). Introduction to Research in

Ultraviolet Photobiology. Prentice-Hall, Engle-
wood Cliffs, N. J.

Kein, H. P., LEDERBERG, J., AND Rieu, A.
(1972). Biological experiments: The Viking
Mars lander. Icarus 16, 139-146.

LEDERBERG, J., AND Sacan, C. (1962). Micro-
environments for life on Mars. Proc. Nat.
Acad. Sci. USA 48, 1473-1475.

- 299

Leicuton, R. B., Horowirz, N. H., Murray,
B.C., SHarp, R. P.. Herrimas, A. H., Youne,
A. T., Ssura, B. A.. Davies, M. E., axpb
Lrovy, C. B. (1969). Mariner 6 and 7 television
pictures: Preliminary analysis. Science 166,
49-67. .

Levin, G. V. (1972). Detection of metabolically
produced labelled gas: The Viking Mars
lander. Icarus 16, 153-166.

Lipa, B., anD TYLer, G.L., (1976). Surface slope
probabilities from the spectra of weak radar
echoes: Application to Mars. Icarus. 28, 301-
306.

Lippincort, E. R., Ecx, R. V., Daygorr, 31. 0.,
axp Sacan, C. (1967). Thermodynamic
equilibria in planetary atmospheres. ip. J.
147, 753-766. ~

Lovetocr, J. E., aND Hircucock, D. R. (1967).
Life detection by atmospheric analysis.
Icarus 7, 149-159.

Miuron, D. J. (1973). Water and processes of
degradation in the Martian landseape. J.
Geophys. Res. 78, 4037-4048.

Morray, B. C., SoDERBLOM, L. A., CUTTS, J.A.,
Suarp, R. P., Mittoy, D. J., AND LEIGHTON,
R. B. (1972). Geological framework of the
south polar region of Mars. Icarus 17, 328-3438.

Murcn, T. A., Brxper, A. B., Hucr, F.0.,
Levixtuat, E. C., Morris, E. C., Sacay, C.,
anp Younc, A. T. (1972). Imaging experi-
ment: The Viking lander. Icarus 16, 92-110.

Owen, T., AND Saas, C. (1972). Minor consti-
tuents in planetary atmospheres: Ultraviolet
spectroscopy from the Orbiting Astronomical
Observatory. Icarus 16, 557-568.

Ovama, V. I. (1972). The gas exchange experi-
ment for life detection: The Viking Mars lan-
der. Icarus 16, 167-184.

Packer, E., SCHER, S., anp Sacas, C. (1963).
Biological contamination of Mars. II. Cold and
aridity as constraints on the survival of terre-
strial microorganisms in simulated Martian
environments. Icarus 2, 293-316.

-Puert, D. (1976). Distribution of small channels

on the Martian surface. Icarus 27, 25-50.

Po.ack, J. B. (1976). The Martian satellites. In
Planetary Satellites (J. Burns, Ed.}. University
of Arizona Press, Tucson. In press.

Pouzacg, J. B., Prrmay, D., Kuare, B. N., AND
Sacan, C. (1970). Goethite on Mars: A labora-
tory study of physically and chemically bound
water in ferric oxides. J. Geophys. Res. 75,
7480-7490.

Pouuack, J. B., VEVERKA, J., Nowaxn, M.,
Sacay, C., Hartaays, W. K., DcxsurRy,
T. C., Bory, G. H., Mizroy, D. J., AND SsuTH,
B. A. (1972). Mariner 9 television observations
of Phobos and Deimos. Icarus 17, 394-407.
300 SAGAN AND LEDERBERG

Sacan, C. (1971). The long winter model of
Martian biology: A speculation. Icarus 13,
511-514.

Sacan, C. (1973a). Landing on Mars. Nature
(London) 244, 61. .

Sacan, C. (1973b). Ultraviolet selection pressure
on the earliest organisms. J. Theoret. Biol. 39,
195-200.

Sacan, C. (1975). The recognition of extraterres-

trial intelligence. Proc. Roy. Soc. London B
189, 143-153.

Sacan, C., anp Fox, P. (1975). The canals of
Mars: An assessment after Mariner 9. Icarus
25, 602-612.

Sacan, C., LEDERBERG, J., AND LEVINTHAL,
E. C. (1968). Contamination of Mars. Science
159, 1191-1196.

Sacan, C., anp Pornack, J. B. (1974). Differen-
tial transmission of sunlight on Mars: Bio-
logical implications. Icarus 21, 490-495.

Sacan, C., Toon, O. B., anp Grierascn, P. J.
(1973). Climatic change on Mars. Science 181,
1045-1049.

Sacan, C., Veverka, J., Fox, P., Dusiscu, R.,
Frencu, R., Grerascu, P., Qoam, L., LEDER-
Berc, J., Levintuat, E., Tucker, R., Eross,
B., and Poxnack, J. B. (1973), Variable
features on Mars. II. Mariner 9 global results,
J. Geophys. Res. 78, 4163-4196.

Saaan, C., VEvERKa, J., Fox, P., Dusiscu, R.,
Lepersers, J., Levinvnar, E., Quam, L.,
Tucker, R., Poutack, J. B., anxp SMITH,

B. A. (1972). Variable features on Mars:
Preliminary Mariner 9 television results.
fearus 17, 346-372.

Sacan, C., anD WaLLace, D. (1971). A search for
life on Earth at 100 meter resolution. Icurus
15, 515-554.

Saarp, R. P., Sopersiom, L. A., Murray, B.C:,
anD Curts, J. A. (1971). The surface of Mars.
II. Uncratered terrain. J. Geophys. Res. 76,
331-342.

SHrtovsxu, I, §., anp Sacan, C. (1966). Intel-
ligent Life in the Universe. Holden—Day, San
Francisco.

SLru|er, E. C. (1962). Mars. Lowell Observatory
Flagstaff, Ariza.

Suira, J., AnD Born, G. (1976). Secular
acceleration of Phobos and Q of Mars. Icarus
27, 51-54. on

Veverga, J., Sacay, C., Quam, L., Tucker, R.,
AND Eross, B. (1974). Variable features on
Mars. III. Comparison of Mariner 1969 and
Mariner 1971 photography. Jcarus 21, 317-
368.

Visuyiac, W., ATwoop, K. C., Bocr, R. M.,
GAFFRON, H., Juxes, T. H., McLaren, A. D.,
Sacay, C., anD Sprnpap, H. (1966). A model
of Martian ecology. In Biology and the Explora-
tion of Mars (C. 8. Pittendrigh, W. Vishniac,
and J. P. T. Pearman, Eds.), pp. 229-242.
National Academy of Sciences-National Re-
search Council, Washington, D.C.

a ted ei

 

 

ee ete te te eee we ode et

oe eee nets ey ee

fee te ene ee foe