12 Searching for Life on Mars: A Brief Summary
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by landers and rovers, and other space agencies started launching their missions
to Mars, more and more evidence was found for an ancient environment in which
liquid water persisted on the surface for extended periods of time, the atmosphere
was denser than today and a global dipolar magnetic field similar to Earth’s protected
it from erosion by the solar wind [7]. High resolution images of the surface showed
features that were interpreted as due to the recent occurrence of liquid water on the
surface [8], although only in limited locales and for brief periods during the warm
season. Neutron spectroscopy detected the presence of permafrost in the first meter
of soil, extending from the poles to mid-latitudes [9]. Radar sounding allowed the
identification of water ice as the predominant constituent of the polar caps [10].
Finally, the measurement of the current erosion rate of the atmosphere due to the
solar wind allowed more precise estimates of the total atmospheric loss, including
water vapour, over the age of the Solar System [11].
The discovery of methane in the atmosphere of Mars both from the Earth [12] and
from Martian orbit [13] started a debate on its origin that is still ongoing. Methane is
destroyed by ultraviolet radiation in a relatively short time, and its presence, however
small, implies an active source. On Earth, the main processes releasing methane in the
atmosphere are volcanism and biologic activity, both of which could not be observed
on Mars. A currently operative mission dedicated to the study of trace gases in the
Martian atmosphere has yet to provide a definite answer to the question of methane
origin, but the recent detection of seasonal variability in the quantity of atmospheric
methane seem consistent with sources located at the surface or in the subsurface [14],
rather than in the atmosphere.
Another recent development has been the detection of a system of liquid water
bodies beneath the south polar cap of Mars [15]. In spite of the theoretical difficulties
in reconciling this presence with the very low mean annual temperature at the poles,
requiring at a minimum the presence of dissolved salts depressing the freezing point
of water [16], and perhaps some thermal anomalies in the crust beneath this area
[17], no alternative interpretations have yet been proposed for the radar observations
leading to the detection. These subglacial bodies of water constitute the first potential
habitat on Mars, being thermally stable and protected from the radiation flooding the
surface. However, the lack of any information on the chemical composition and
availability of redox pairs in such an environment prevents any assessment of this
hypothesis.
Accessing the Martian subsurface to search for evidence of past or present life
is considered a high-priority goal in the exploration of the planet, but the technical
challenges for such an endeavour are daunting. The Rosalind Franklin rover, to be
launched in 2022, will carry a drill capable of reaching a depth of two meters [18],
while a recent mission concept for landing a drilling station on Mars foresees a
maximum penetration of the order of a hundred meters [19]. Current and planned
missions for the search of life on Mars focus on the detection and collection of samples
from sedimentary beds or mineral deposits formed in an aqueous environment and
accessible on the surface [20].
The detection of life, on Mars and elsewhere, is a complex problem that cannot
be solved through a single measurement technique [21]. Because of potential differ-
119
by landers and rovers, and other space agencies started launching their missions
to Mars, more and more evidence was found for an ancient environment in which
liquid water persisted on the surface for extended periods of time, the atmosphere
was denser than today and a global dipolar magnetic field similar to Earth’s protected
it from erosion by the solar wind [7]. High resolution images of the surface showed
features that were interpreted as due to the recent occurrence of liquid water on the
surface [8], although only in limited locales and for brief periods during the warm
season. Neutron spectroscopy detected the presence of permafrost in the first meter
of soil, extending from the poles to mid-latitudes [9]. Radar sounding allowed the
identification of water ice as the predominant constituent of the polar caps [10].
Finally, the measurement of the current erosion rate of the atmosphere due to the
solar wind allowed more precise estimates of the total atmospheric loss, including
water vapour, over the age of the Solar System [11].
The discovery of methane in the atmosphere of Mars both from the Earth [12] and
from Martian orbit [13] started a debate on its origin that is still ongoing. Methane is
destroyed by ultraviolet radiation in a relatively short time, and its presence, however
small, implies an active source. On Earth, the main processes releasing methane in the
atmosphere are volcanism and biologic activity, both of which could not be observed
on Mars. A currently operative mission dedicated to the study of trace gases in the
Martian atmosphere has yet to provide a definite answer to the question of methane
origin, but the recent detection of seasonal variability in the quantity of atmospheric
methane seem consistent with sources located at the surface or in the subsurface [14],
rather than in the atmosphere.
Another recent development has been the detection of a system of liquid water
bodies beneath the south polar cap of Mars [15]. In spite of the theoretical difficulties
in reconciling this presence with the very low mean annual temperature at the poles,
requiring at a minimum the presence of dissolved salts depressing the freezing point
of water [16], and perhaps some thermal anomalies in the crust beneath this area
[17], no alternative interpretations have yet been proposed for the radar observations
leading to the detection. These subglacial bodies of water constitute the first potential
habitat on Mars, being thermally stable and protected from the radiation flooding the
surface. However, the lack of any information on the chemical composition and
availability of redox pairs in such an environment prevents any assessment of this
hypothesis.
Accessing the Martian subsurface to search for evidence of past or present life
is considered a high-priority goal in the exploration of the planet, but the technical
challenges for such an endeavour are daunting. The Rosalind Franklin rover, to be
launched in 2022, will carry a drill capable of reaching a depth of two meters [18],
while a recent mission concept for landing a drilling station on Mars foresees a
maximum penetration of the order of a hundred meters [19]. Current and planned
missions for the search of life on Mars focus on the detection and collection of samples
from sedimentary beds or mineral deposits formed in an aqueous environment and
accessible on the surface [20].
The detection of life, on Mars and elsewhere, is a complex problem that cannot
be solved through a single measurement technique [21]. Because of potential differ-