[meteorite-list] Plausibility of Martian Microbes
baalke at zagami.jpl.nasa.gov
Tue Apr 27 14:02:05 EDT 2004
Plausibility of Martian Microbes
April 27, 2004
Summary: The National Academy of Sciences took up the question of
whether dormant microbes might be found today or in the distant past
on Mars. Their plausibility criteria include climate and water as key
factors, but also address whether life originated on Earth or Mars
first in some scenarios of meteor transfer.
Plausibility of Martian Microbes
based on National Academy of Sciences report
The surface environment of Mars may not always have been so hostile to life.
Early in the planet's history, the average temperature almost certainly was
warmer and the atmosphere more dense, and liquid water may have existed at
the surface. Evidence for the presence of surface water on early Mars comes
from interpretation of the geomorphology of the planet's surface. A
substantial fraction of the surface of Mars is older than about 3.5 billion
years, based on the number of impact craters, which provide a window into
the planet's early history.
Two aspects of these older surfaces suggest that the climate prior to about
3.5 billion years ago was different from the present climate. First, impact
craters smaller than about 15 kilometers in diameter have been obliterated
on these older surfaces, and impact craters larger than this have undergone
substantial degradation, whereas younger impact craters have not been
altered significantly. This suggests that erosion rates were up to 1,000
times larger early in martian history. The style of erosion that is seen on
some of the remaining larger impact craters is indicative of water runoff,
and water erosion is considered to be responsible for removing the smaller
craters. Second, many of the same older surfaces contain networks of valleys
that form dendritic patterns similar to terrestrial water-carved stream
There is continuing debate as to exactly how these valleys were formed the
process may have involved runoff of precipitation, seepage of subsurface
water in a process termed "sapping," or erosion by water-rich debris flows.
Independent of the exact process, their formation must have involved the
presence of liquid water at or very near the surface during these earlier
Thus, geological evidence suggests that the martian climate prior to about
3.5 billion years ago was somehow warmer than the present climate and that
liquid water flowed on the surface in a way that is not observed today.
Unfortunately, the observations do not allow a unique determination of what
the temperature, atmospheric pressure, or partitioning of liquid water
between the subsurface, surface, and atmosphere were at that time. Evidence
from measurements of martian stable isotopes suggests that a large fraction
of the volatiles from early Mars may have been lost to space, causing the
surface environment to become cooler and drier and to evolve into the state
The climate on early Mars may have been similar to the climate on Earth at
that time. Although martian erosion rates undoubtedly were substantially
lower than terrestrial erosion rates, suggesting less widespread water,
liquid water certainly was present on both planets. Both planets probably
had a mildly reducing atmosphere, containing substantial quantities of
carbon dioxide. Given that life arose on Earth, it seems possible and even
plausible that life could have arisen on Mars under similar conditions and
at roughly the same time. If such were the case, a significant community of
microorganisms may have existed on early Mars.
Interestingly, an alternative source for life on Mars may have been Earth
itself. Asteroid impacts are capable of ejecting rocky material from planets
into space. Once in space, close encounters with their planet of origin
would alter the orbits of such material. The orbits of material ejected from
Mars could evolve to the point that they would cross the orbit of Earth;
similarly, ejecta from Earth could evolve to the point that their orbits
would cross the orbit of Mars. At that point, collisions could occur,
providing a mechanism for transferring mass from one planet to the other.
Meteorites have been discovered on Earth that are identified as having come
from Mars, indicating that this process actually does occur. A martian
origin for these meteorites is indicated by their young age, by the presence
of oxygen isotopes that rule out an origin on Earth or the moon, and by
gases trapped within them that are identical in composition to the martian
atmosphere and distinct from any other known source of gas in the solar
system. Some of the material ejected by an impact is not heated or shocked
substantially, and bacteria or bacterial spores may be able to survive the
ejection event. If organisms or spores could survive within a rock during
interplanetary transit and find a satisfactory environment on a new planet,
they could possibly survive and multiply. This would allow living organisms
on one planet to be transferred to another. Indeed, one can ask the
following questions: On which planet did life originate? Could life have
originated on Mars and been transferred to Earth or vice versa?
If life forms ever existed on Mars, either by having been formed in an
independent origin or by having been transferred there from Earth, it is
possible that they have continued to exist up to the present time. Such life
forms could survive in occasional localized ecological niches. Such niches
could be liquid water or hot springs associated with extrusive and intrusive
volcanism or liquid water buried deep beneath the surface where it is
stable. It is important to note, however, that biological material may not
stay confined in such locations; organisms conceivably might produce dormant
propagules (spores) that could be dispersed more widely.
Did results from the Viking mission in the late 1970s not suggest that Mars
was probably devoid of life? That was the accepted interpretation at the
time, based on the results of three experiments that tested for biological
activity and the absence of organic molecules in the surface materials.
However, this conclusion may be open to some debate based on recent advances
in our understanding of biology.
The Viking experiments were able to test for only a couple of the possible
mechanisms by which putative martian organisms might obtain energy; these
involved the utilization of either carbon dioxide or extant organic
molecules as a source of carbon in the production of organic molecules.
Putative martian biota might employ other mechanisms to obtain energy and
might do so under physical conditions quite different from those of the
Viking biology experiments. Martian life also might reside in the interior
of rocks (which were not sampled by Viking), where liquid water might occur.
Finally, if life exists only in isolated oases where liquid water exists,
such as recent volcanic vents or fumaroles, the Viking experiments might
have been the right ones but carried out at the wrong location.
The surface of Mars is inhospitable to life as we know it, although there
may be localized environments where life could exist. Conditions on Mars may
have been conducive to the formation of life, either during an earlier epoch
when the climate was likely more clement or in hydrothermal systems and hot
springs that may have existed on Mars throughout geological time. Therefore,
it is possible that life arose on Mars. It is also possible that living
organisms from Earth could have been delivered to Mars by impact transfer,
and, if so, such organisms might have chanced upon the occasional oasis in
which they could survive and multiply. If life arose on Mars or was
delivered to Mars from Earth, it is possible that it has survived in
localized environments that may be more hospitable than the general surface.
Thus, there are plausible scenarios in which a sample returned from Mars
could contain living organisms, either active or dormant.
The 2009 Mars Science Laboratory is planned as the first set of biological
experiments in the current exploration strategy. As the NASA Office of Space
Science noted however, there has been considerable debate about when to time
a sample return: "We note with concern that there appears to be a growing
division within the Mars community between scientists seeking early Mars
Sample Return and those who believe it is best to delay it."
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