An estimated mean thickness of about 1 km of frost in the Martian north polar cap summer remnant, if vaporized, would yield about 1000 g/sq cm of atmosphere over the planet, higher global temperatures through the greenhouse effect, and a greatly increased likelihood of liquid water. Vaporization of such cap remnants may occur twice each equinoctial precession, and Martian organisms may now be in cryptobiotic repose awaiting the end of the long precessional winter. The Viking biology experiments can test this hypothesis

Sagan, C.

English

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CORNELL UNIVERSITY
Center for Radiophysics and Space Research
ITHACA, N.Y.
N72-12837 (NAS A-CR- 124639)
MARTIAN BIOLOGY:
(Cornell Univ.)
THE LONG WINTER MODEL OF
A SPECULATION C. Sagan
Aug. 1971 12 p CSCL 03B
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CENTER FOR RADIOPHYSICS AND SPACE RESEARCH
LABORATORY FOR PLANETARY STUDIES
CORNELL UNIVERSITY
ITHACA, NEW YORK
August 1971
CRSR 455
THE LONG WINTER MODEL OF MARTIAN BIOLOGY:
A SPECULATION*
Carl Sagan
Laboratory for Planetary Studies
Cornell University, Ithaca, New York 14850
*Presented at the Viking Annual Science Seminar, NASA Langley
Research Center, 20 April, 1971.
Abstract
An estimated mean thickness - 1 km of frost in
the Martian North Polar Cap summer remnant, if vap-
orized, would yield - 10 3 g cm - 2 of atmosphere over
the planet, higher global temperatures through the
greenhouse effect, and a greatly increased likeli-
hood of liquid water. Vaporization of such cap
remnants may occur twice each equinoctial precession,
and Martian organisms may now be in cryptobiotic re-
pose awaiting the end of the long precessional winter.
The Viking biology experiments can test this hypo-
thesis.
Some scientists, including some biologists, have been
dismayed when confronted with the apparent inclemency of the
present Martian environment. With global mean temperatures
- 210 K, diurnal temperature fluctuations in excess of 100 K°
at equatorial latitudes, mean surface pressures hovering unr
comfortably near the triple point of water, no detectable
oxygen, an ultraviolet flux which delivers the mean lethal
dose to typical unprotected terrestrial microorganisms in
seconds, and a surface at one time widely advertised as "moon-
like," some initial reserve about the habitability of Mars
does seem to be in order. While it has been argued (Vishniac,
et al., 1966; Sagan, 1970) that none of these environmental
parameters provides insuperable physical or chemical obstacles
to an indigenous biology, others have contended that the like-
lihood of such an indigenous contemporary biology on Mars is
very small; and in some circles numbers such as 0.1% or less
are fashionable. It is sometimes held that even if the pro-
bability of an indigenous biology is as low as 10- 3, the sig-
nificance of finding such a biology is so enormous that the
product of probability and significance still implies the
importance of searching for Martian organisms. In fact a
priori estimates of the probability of life on a particular
1
2planet are not possible in our present state of ignorance, and
such probabilities are merely scepticism indices, calibrating
the frame of mind of the speaker. Nevertheless such arguments
have the entertaining consequence that they introduce into the
field of discourse models of the Martian environment which have
probabilities of 0.1% or less of being correct. One category of
alternative model is contemporary microenvironments (Lederberg
and Sagan, 1962); that is, locales on Mars which may have an
integrated area < 10
-
2 the total surface area, but which har-
bor much more clement conditions. I wish here to propose a
different model--a microenvironment, not in space, but in time.
While the Martian south polar cap (SPC) is reported to disappear
in southern summer, the Martian north polar cap (NPC) has never
been known to vanish. The remnant summer NPC, offset from the
aereographic north pole, has an area - 10 km2. Since the
remnant cap remains at the frost point, frost evaporating at
the edge of the cap must largely be redeposited on the cap it-
self, and the thickness of the NPC may be considerable. Be-
cause of the obliquities and orbital geometries of Mars and
the Earth, the north pole is averted away from Earth and is in
darkness at favorable oppositions. It has also never been
observed from spacecraft, and is unlikely to be viewed by the
1971 Mars Orbiters, unless their operational lifetimes con-
siderably exceed the expectation value of 90 days.
The SPC has been observed in southern spring by Mariners
6 and 7--the thinner of the two frost covers, viewed when the
frost was partially dissipated. From observations of craters
partly obscured by frost, and other geomorphological criteria,
the Mariner 6 and 7 experimenters concluded that frost thick-
nesses as great as many tens ofmeters were in evidence, and
that regions of still greater thickness, perhaps partly buried
under a dust overburden, were likely (Sharp, Murray, Leighton,
Soderblom, and Cutts, 1971). Photographic evidence for a
depositional interface, as well as the appearance of the par-
allel arcuate ridges near the south pole, led Belcher, Veverka
and Sagan (1971) to suggest the presence of a large glacial
morraine in the vicinity of the south pole, and the presence
of ice ages on Mars.
The thickness of the NPC remnant is difficult to estimate
reliably, but must be much deeper than these estimates for the
SPC at some distance from the pole in local spring. I will
assume that the appropriate value for the remnant NPC thick7
ness is - 1 km, although an order of magnitude revision down-
ward, and any plausible revision upward, will not significantly
affect the argument. The area of the NPC remnant is - 1% that
of Mars. If we consider the frost to have a local density
3 _
~ 1 g cm , an arresting conclusion emerges: if the NPC remnant
could be vaporized, it would yield - 103 g cm -2 of gas over the
whole planet. There would be as much atmosphere on Mars as on
the earth.
With the Martian NPC remnant completely evaporated the
widespread presence of liquid water on the planet would be much
more likely than it is today. Furthermore, through the green-
house effect the planetary surface temperature would be con-
siderably above present values. An atmosphere of 70 m-atm
C02, pressure-broadened by a neutral gas at 1 bar pressure,
plus 1 g cm-2 of water would increase the mean temperature by
> 30 K0, and therefore greatly increase the fraction of the
disk above the freezing point of water each day. In the expec-
tation that the polar cap is largely frozen C02, with more
than 0.1% water, the incremental greenhouse effect would be
larger yet. The proposal (Broida, et al., 1970) that ozone
and other gases are trapped in the Martian polar caps raises
still more interesting possibilities. The presence of CO in
the Martian atmosphere implies that the net conditions may be
reducing. It is therefore not out of the question that small
quantities of such gases as methane and ammonia may also be
-5trapped in the polar cap. If as much as 10 ammonia and
methane is present, an additional 10 K ° greenhouse results.
It seems apparent that epochs of major dissipation of the NPC
remnant may dominate Martian biology.
In this view, we happen to be observing Mars at an in-
propitious moment--when almost all the atmosphere is frozen
out in the NPC. Under what conditions might this atmosphere
return to the gas phase? Leighton and Murray (1966) have
5pointed out that an effective 5 x 104 year period of equinoctial
precession applied to Mars. At the present time the NPC rem-
nant exists in the north because the northern hemisphere is
averted from the Sun at perihelion. Fifty thousand years from
now the situation will be reversed, and a permanent SPC rem-
nant can be expected. But what is the situation at an inter-
mediate point in time, when the line of equinoxes is along the
line of apsides of the Martian orbit and both poles are heated
equally? The solar constant at Mars and the latent heat of
vaporization of solid CO2 imply an upper limit - 1 cm of frozen
CO2 vaporized per day. We neglect conduction and atmospheric
transport. Thus substantial quantities of polar cap material
can be vaporized in times very much shorter than the preces-
sional cycle--indeed, in times not much longer than one Martian
year. Midway through the precessional cycle the situation
may be uninteresting: roughly equal amounts of cap material
at each pole. But, considering the greenhouse and other insta-
bilities inherent in the problem, it is also possible that a
major fraction of the present NPC remnant is then converted
into a denser Martian atmosphere, and that there is a preces-
sional spring of a few thousand years duration in which condi-
tions on Mars are far more clement than they are today.
The biological consequences of this hypothesis are
straightforward. The bulk of Martian organisms shut up shop
for the (precessional) winter. Spores, vegetative forms, and--
6for all we know--hibernators abound, but only a few or no active
organisms. At first sight, the Viking biology experiments will
have arrived ten thousand years early. But on Earth hibernators
are aroused, spores germinate, etc. because of a complex set
of vernal cues--the principal of which being higher temperatures
and increased availability of liquid water. The Viking biology
package is, entirely by accident, ideally configured to test
this hypothesis. Two experiments are designed to be performed
with little or no liquid water (Horowitz, Hubbard, and Hobby,
1972; Oyama, 1972) and two in the presence of liquid water
(Vishniac, 1972; Levin, 1972). Indeed the latter two experi-
ments have been criticized on the grounds that liquid water
does not exist on Mars and that any Martian microorganisms
introduced into these experiments will drown. Instead, these
experiments may provide local microbial waterholes, coaxing
cryptobiotic forms into precocious biological activity. Spores
and vegetative forms on the Earth are known to survive under
very harsh conditions for hundreds of years and probably longer;
there should be no difficulties with the survival of Martian
cryptobiotic forms for - 104 years.
A mean planetary cover of a few cm of liquid water pre-
sent for, say, 10% of the precessional cycle would be entirely
adequate for a complex biology, and would provide a very useful
source of additional erosion on Mars (see, e.g., Chapman et al.,
1968; Hartmann, 1971). It would not introduce observational
7contradictions such as abundant estuaries and river deltas.
Indeed, small amounts (several tens of pA) of liquid water on
the Martian surface today have been inferred from the Martian
microwave spectrum (Sagan and Veverka, 1971), and may be a
contemporary vestige of the waters of the precessional spring.
With the low Martian exospheric temperature there is no problem
that such an intermittent atmosphere would escape in times short
compared with the lifetime of Mars.
If there is a large residue of condensed volatiles in the
NPC remnant, estimates of the mass budgets of Martian outgass-
ing during the history of the planet will have to be revised
upwards. I do not here broach the question of whether condi-
tions in the very earliest history of Mars were suitable for
the origin of life, but merely note that conditions appropriate
for the origin of life on primitive Mars are exhibited in at
least some scenarios (Sagan and Mullen, 1971).
Finally it is just conceivable that, in time, human en-
deavors could, by volatilizing the present NPC remnant, and
taking advantage of the hypothesized instabilities, introduce
much more clement conditions on Mars, in times considerably
shorter than the precessional period. As with a related sug-
gestion for Venus (Sagan, 1961), such planetary engineering
should be considered seriously only after the most thorough
and ecologically responsible investigation of the planet is
performed.
8The critical .link in this chain of hypotheses is the
assumption that adequate instabilities exist during Martian
precessional springs to release in vapor phase the materials
now trapped in the Martian North Polar Cap remnant, and that
it is possible to recondense such a vaporized polar cap later
in the equinoctial precession (i.e., that the instabilities
are reversible). I cannot estimate the probability that such
instabilities exist, but I guess on intuitive grounds -- the
same grounds utilized for a priori estimates of the habitability
of Mars -- that these probabilities are not insignificant, and
are perhaps even > 10
-
3
Acknowledgement:
I am indebted to C.B. Farmer, M. Noland, and J. Veverka
for stimulating conversations; to Andrew T. Young and J.
Veverka for a critique of these ideas; and to Linda Sagan
for suggesting that the proper recipe for detecting Martian
biology is "Add water." This research was supported by NASA
Grant 33-010-098.
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analogies. Icarus 15,
H.P. Broida, O.R. Lundell, H.I. Schiff, and R.D. Ketcheson
(1970). Is ozone trapped in the solid carbon dioxide
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C. Chapman, J.B. Pollack, and C. Sagan (1968). An analysis of
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The long winter model of Martian biology: A speculation

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