862 Encyclopedia of the Solar System
FIGURE 9 Blueberries. The triplet of connected spheres,
dubbed blueberries, as seen in this MEROpportunityimage, is
a strong indication that they are concretions formed in the
presence of water, not in volcanic eruptions or meteor impacts.
Concretions are spherical mineral structures formed by
groundwater percolating through porous rocks. On Earth, as
concretions grow in close proximity to each other, their outer
edges often intersect each other, producing connected spheres.
(NASA/JPL)
for several billion years until the cumulative effects of O 2
production induces profound changes on the atmosphere
of that planet. On Mars, conditions become unsuitable for
life (no liquid water) in a billion years or less. Thus, is it
likely that if there were any life on early Mars it remained
microbial.
The evidence of liquid water on early Mars, particularly
that provided by the valley networks, suggests that the cli-
mate on early Mars may have been quite different than at
present. It is generally thought that the surface temperature
must have been close to freezing, much warmer than the
present− 60 ◦C. These warmer temperatures are thought
to have occurred as a result of a greatly enhanced green-
house due to a thick (∼1-5 atm) CO 2 atmosphere. How-
ever, CO 2 condensation may have limited the efficacy of the
CO 2 greenhouse but theoretical models indicate that CO 2
clouds or CH 4 could enhance the greenhouse and maintain
warmer temperatures.
If Mars did have a thick CO 2 atmosphere, this strength-
ens the comparison to the Earth, which is thought to have
also had a thick CO 2 atmosphere early in its history. The
duration of a thick atmosphere on Mars and the concomi-
tant warm, wet surface conditions are unknown but simple
climate models suggest that significant liquid water habitats
could have existed on Mars for∼0.5 Gyr after the mean sur-
face temperature reached freezing. This model is based on
the presence of deep ice-covered lakes (over 30 m) such as
those in the dry valleys of the Antarctic where mean annual
temperatures are− 20 ◦C.
If we divide the possible scenario for the history of wa-
ter on the surface of Mars into four epochs, the first epoch
would have warm surface conditions and liquid water. As
Mars gradually loses its thick CO 2 atmosphere, the second
and third epochs would be characterized by low tempera-
tures but still relatively high atmospheric pressures. This is
because the temperature would drop rapidly as the pressure
decreased. During the second epoch, temperatures would
rise above freezing during some of the year and liquid water
habitats would require a perennial ice-cover. However, by
epoch three the temperature would never rise above freez-
ing and the only liquid water would be found in porous
rocks with favorable exposures to sunlight. In epoch four
the pressure would fall too low for the presence of liquid
water.
A point worth emphasizing here is that the biological
requirement is for liquid waterper se. Current difficulties
in understanding the composition and pressure of the at-
mosphere need not lessen the biological importance of the
direct evidence for the presence of liquid water. In fact,
as we observe in the Antarctic dry valleys, ecosystems can
exist when the mean temperatures are well below freezing.
Mars need not have ever been above freezing for life to
persist.
The particular environment on the early Earth in which
life originated is not known. However, this does not pose
as serious a problem to the question of the origin of life
on Mars as might be expected. The reason is that all of
the environments found on the early Earth would be ex-
pected to be found on Mars, including hydrothermal sites,
hot springs, lakes, oceans (that is planetary scale water reser-
voirs), volcanoes, tidal pools (solar tides only), marshes, salt
flats, and others. Thus, whatever environment or combina-
tion of environments needed for life to get started on Earth
should have been present on Mars as well, and at the same
time.
Since the rationale for life on Mars early in its history is
based on analogs with fossil evidence for life on the early
Earth it is natural to look to the fossil record on Earth as
a guide to how relics of early Martian life might be found.
The most persuasive evidence for microbial life on the early
Earth comes from stromatolites as discussed before. The
resulting structures can be quite large: they are macroscopic
fossils generated by microorganisms.6.3.3 SUBSURFACE LIFE ON MARS
Although there is currently no direct evidence to support
speculations about extant life on Mars, there are several in-
teresting possibilities that cannot be ruled out at this time.
Protected subsurface niches associated with hydrother-
mal activity could have continued to support life even