Astrobiology 863
after surface conditions became inhospitable. Liquid water
could be provided by the heat of geothermal or volcanic
activity melting permafrost or other subsurface water
sources. Gases from volcanic activity deep in the planet
could provide reducing power (as CH 4 ,H 2 ,orH 2 S) per-
colating up from below and enabling the development of
a microbial community based upon chemolithoautotrophy.
An example is a methanogen (or acetogen) that uses H 2
and CO 2 in the production of CH 4. Such ecosystems have
been found deep underground on the Earth consuming H 2
produced by the reaction of water with basaltic rock, a plau-
sible reaction for subsurface Mars. However, their existence
is neither supported nor excluded by current observations
of Mars. Tests for such a subsurface system involve locating
active geothermal areas associated with ground ice or de-
tecting trace quantities of reduced atmospheric gases that
would leak from such a system. It is interesting to con-
sider the recent reports of CH 4 in the atmosphere of Mars
at the tens of ppb level. If these reports are confirmed, it
may be that this CH 4 may be related to subsurface biologi-
cal activity. However nonbiological sources of CH 4 are also
possible.
While it certainly seems clear that volcanic activity on
Mars has diminished over geological time, intriguing evi-
dence for recent (on the geological time scale) volcanic ac-
tivity comes from the young crystallization ages (all less than
1 Gyr) of the Shergotty meteorite (and other similar mete-
orites thought to have come from Mars). Volcanic activity by
itself does not provide a suitable habitat for life; liquid water
that may be derived from the melting of ground ice is also
required. Presumably, the volcanic source in the equatorial
region would have depleted any initial reservoir of ground
ice and there would be no mechanism for renewal, although
there are indications of geologically recent volcano/ground
ice interactions at equatorial regions. Closer to the poles,
ground ice is stable. It is conceivable that a geothermal heat
source could result in cycling of water through the frozen
ice-rich surface layers. The heat source would melt and
draw in water from any underlying reservoir of groundwa-
ter or ice that might exist. [SeeMeteorites.]
Another line of reasoning also supports the possibility
of subsurface liquid water. There are outflow channels on
Mars that appear to be the result of the catastrophic dis-
charge of subsurface aquifers of enormous sizes. There is
evidence based on craters and stratigraphic relations that
these have occurred throughout Martian history. If this is
the case, then it is possible that intact aquifers remain. This
would have profound implications for exobiology (as well as
human exploration). Furthermore, it suggests that the de-
bris field and outwash regions associated with the outflow
channel may hold direct evidence that life existed within
the subsurface aquifer just prior to its catastrophic release.
The collection of available water on Mars in the po-
lar regions naturally suggests that summer warming at the
edges of the permanent water ice cap may be a source of
meltwater, even if short lived. In the polar regions of Earth,
complex microbial ecosystems survive in transient summer
meltwater. However, on Mars the temperature and pres-
sures remain too low for liquid water to form. Any energy
available is lost from the sublimation of the ice before any
liquid is produced. It is unlikely that there are even seasonal
habitats at the edge of the polar caps. This situation may be
different over longer timescales. Changes in the obliquity
axis of Mars can significantly increase the amount of insola-
tion reaching the polar caps in summer. If the obliquity in-
creases to over about 50◦, then the increased temperatures,
atmospheric pressures, and polar insolation that result may
cause summer liquid water meltstreams and ponds at the
edge of the polar cap.
The polar regions may harbor remnants of life in another
way. Tens of meters beneath the surface, the temperature
is well below freezing (<− 70 ◦C) and does not change from
summer to winter. These permafrost zones likely have re-
mained frozen, particularly in the southern hemisphere,
since the end of the intense crater formation period. In this
case, there may be microorganisms frozen within the per-
mafrost that date back to the time when liquid water was
common on Mars, over 3.5 Gyr ago. On Earth permafrost of
such age does not exist, but there are sediments in the polar
regions that have been frozen for many millions of years.
When these sediments are exhumed and samples extracted
using sterile techniques, viable bacteria are recovered. The
sediments on Mars have been frozen much longer (1000
times) but the temperatures are also much colder; it may
be possible that intact microorganisms could be recovered
from the Martian permafrost. Natural radiation from U, Th,
and K in the soil would be expected to have killed any or-
ganisms but their biochemical remains would be available
for study. The southern polar region seems like the best
site for searching for evidence of ancient microorganisms
since the terrain there can be dated to the earliest period of
Martian history as determined by the number of observed
craters.6.3.4 METEORITES FROM MARS
Of the thousands of meteorites known, there are over 30
that are thought to have come from Mars. It is certain that
these meteorites came from a single source because they all
have similar ratios of the oxygen isotopes—values distinct
from terrestrial, lunar, or asteroidal ratios. These meteorites
can be grouped into four classes. Three of these classes
contain all but one of the known Mars meteorites and are
known by the name of the type specimen; the S (Shergotty),
N (Nakhla), and C (Chassigny) class meteorites. The S, N,
and C meteorites are relatively young, having crystallized
from lava flows between 200 and 1300 million years ago
(see Fig. 4). Gas inclusions in two of the S type meteorites
contain gases similar to the present Martian atmosphere as
measured by theVikinglanders, proving that this meteorite,