Astrobiology 861
pressures and low temperatures make it unlikely that water
will exist as a liquid on Mars. Because of seasonal transport,
the available surface water on Mars is trapped as ice in the
polar regions. In the locations at low elevation where the
pressures and temperatures are sufficient to support liq-
uid water, the surface is desiccated. Even saturated brine
solutions cannot exist in equilibrium with the atmosphere
near the equator. The absence of liquid water on the sur-
face of Mars is probably the most serious argument against
the presence of life anywhere on the surface of the planet.
A second significant hazard to life on the Martian surface
is the presence of solar ultraviolet light in the wavelengths
between 190 and 300 nm. This radiation, which is largely
shielded from Earth’s surface by atmospheric oxygen and
the ozone layer, is highly effective at destroying terrestrial
organisms. Wavelengths below 190 nm are absorbed even
by the present thin Martian CO 2 atmosphere. Compound-
ing the effects of UV irradiation, and perhaps caused by it,
are possible chemical oxidants that are thought to exist in
the Martian soil. Such strong oxidants have been suggested
as the causative agent for the chemical reactivity observed at
theVikingsites. [SeeMarsAtmosphere:History and
Surface Interaction.]
6.3.2 EARLY MARS
There is considerable evidence that early in its history Mars
did have liquid water on its surface. Images from the many
orbiters show complex dendritic valley networks that are
believed to have been carved by liquid water. These val-
leys are predominantly found in the heavily cratered, hence
ancient, terrains in the southern hemisphere. This would
suggest that the period of liquid water on Mars occurred
contemporaneously with the end of the last stages of heavy
cratering, about 3.8 Gyr ago, the same epoch at which life
is thought to have originated on Earth (see Fig. 4). [See
Mars, Surface and Interior.]
Figure 8 shows part of Nanedi Vallis on Mars. The canyon
snakes back and forth, which is characteristic of liquid flow.
On the floor of the canyon appears a small channel, which
presumably was the flow of the river that carved the canyon.
It would have taken considerable flow, although not nec-
essarily continuous flow, for this river to have carved the
much larger canyon. This image provides what is perhaps
the best evidence from orbit that liquid water flowed on
the surface of Mars in stable flow for long periods of time.
Figure 9 shows evidence for liquid water form the surface
rover missions. The “blueberries” seen at the Meridiani Site
are interpreted as concretions formed in liquid water.
The presence of liquid water habitats on early Mars at
approximately the time that life is first evident on Earth sug-
gests that life may have originated on Mars during the same
time period. Liquid water is the most critical environmental
requirement for life on Earth and the general similarity
between Earth and Mars leads us to assume that life on
FIGURE 8 Liquid water on another world. MarsGlobal
Surveyorimage showing Nanedi Vallis in the Xanthe Terra
region of Mars. The image covers an area 9.8 km by 18.5 km; the
canyon is about 2.5 km wide. This image is the best evidence we
have of liquid water anywhere outside the Earth. (Photo from
NASA/Malin Space Sciences).Mars would be similar in this basic environmental require-
ment. More exotic approaches to life on Mars cannot be
ruled out, nor are they supported by any available evidence.
It is interesting to consider how evolution may have pro-
gressed on Mars by comparison with the Earth. The history
of Earth and Mars are compared in Figure 4, which shows
that the period between 4.0 and 3.5 Gyr ago is the time
when life is most likely to have evolved on both planets.
On Earth, life persists and remains essentially unchanged