12 Searching for Life on Mars: A Brief Summary
117
12.2 The Exploration of Mars
Since the first detailed images of the Martian surface were acquired by automated
probes, in the early seventies of the last century, it became obvious that the planet
surface was sculpted by the action of water [2]. Imponent features such as valley
networks and outflow channels were already visible in images from Mariner 9. The
former are dendritic network of channels resembling a terrestrial fluvial system, while
the latter are large stretches of terrain that appear to have been scoured by the flow
of huge quantities of water. Other landforms, such as rampart craters and polygonal
terrain, hinted at the present of ground ice.
These findings are at odds with the current climate of Mars. The current atmospheric pressure and temperature do not allow the survival of liquid water for any
extended period, but the planet does have polar caps that appear to be constituted
mostly by water ice [3]. Attempts to reconcile today’s conditions on Mars with the
evidence of past liquid water on the surface led to the hypothesis that Mars had to
have a much denser CO 2 atmosphere in the past that was lost over the ages due to
the weak gravitational pull resulting from a mass that is about one-tenth that of the
Earth. Such dense atmosphere would produce a greenhouse effect capable of raising
the mean surface temperature above the melting point of water [2].
The realization that Mars once had climatic conditions similar to those of the Earth
led to the speculation that life could have been developing in the planet’s past, and
that it could exist even today. As a consequence, NASA launched the two twin Viking
spacecrafts, each carrying a lander endowed with a set of experiments meant to detect
life through the analysis of Martian soil. Most of the Viking experiments, which
were based on the capability of living microorganisms to activate in the presence
of liquid water or to use carbohydrates in their metabolism, produced no evidence
for life [4], but some still debate whether one of them recorded signs of chemical
activity that could be attributed to biotic activity [5]. In hindsight, this outcome
could have been expected even if life still survived on Mars because its surface,
due to the thin atmosphere, is constantly bombarded by ultraviolet radiation and
high energy particles, which would make the survival of even the most tenacious
terrestrial microorganisms extremely challenging.
These negative results led to the abandonment of Mars robotic exploration for
more than a decade, during which the copious harvest of data collected by the Viking
probes was slowly elaborated into a more detailed and consistent picture of the evolution of Mars through the ages. The geologic history of the planet was divided into
three main eras, the Noachian, the Hesperian and the Amazonian. The Noachian,
lasting a few to several hundred million years, was the age in which Martian climate
was more favourable to life, while the Hesperian was characterized by intense volcanic activity, leading to the formation of the Tharsis plateau on which most of the
main volcanoes of the planet are located, and by the presence of massive ice sheets.
The Amazonian, the modern era of Mars, is the current arid and glacial age of Mars
[2] (Fig. 12.2).
117
12.2 The Exploration of Mars
Since the first detailed images of the Martian surface were acquired by automated
probes, in the early seventies of the last century, it became obvious that the planet
surface was sculpted by the action of water [2]. Imponent features such as valley
networks and outflow channels were already visible in images from Mariner 9. The
former are dendritic network of channels resembling a terrestrial fluvial system, while
the latter are large stretches of terrain that appear to have been scoured by the flow
of huge quantities of water. Other landforms, such as rampart craters and polygonal
terrain, hinted at the present of ground ice.
These findings are at odds with the current climate of Mars. The current atmospheric pressure and temperature do not allow the survival of liquid water for any
extended period, but the planet does have polar caps that appear to be constituted
mostly by water ice [3]. Attempts to reconcile today’s conditions on Mars with the
evidence of past liquid water on the surface led to the hypothesis that Mars had to
have a much denser CO 2 atmosphere in the past that was lost over the ages due to
the weak gravitational pull resulting from a mass that is about one-tenth that of the
Earth. Such dense atmosphere would produce a greenhouse effect capable of raising
the mean surface temperature above the melting point of water [2].
The realization that Mars once had climatic conditions similar to those of the Earth
led to the speculation that life could have been developing in the planet’s past, and
that it could exist even today. As a consequence, NASA launched the two twin Viking
spacecrafts, each carrying a lander endowed with a set of experiments meant to detect
life through the analysis of Martian soil. Most of the Viking experiments, which
were based on the capability of living microorganisms to activate in the presence
of liquid water or to use carbohydrates in their metabolism, produced no evidence
for life [4], but some still debate whether one of them recorded signs of chemical
activity that could be attributed to biotic activity [5]. In hindsight, this outcome
could have been expected even if life still survived on Mars because its surface,
due to the thin atmosphere, is constantly bombarded by ultraviolet radiation and
high energy particles, which would make the survival of even the most tenacious
terrestrial microorganisms extremely challenging.
These negative results led to the abandonment of Mars robotic exploration for
more than a decade, during which the copious harvest of data collected by the Viking
probes was slowly elaborated into a more detailed and consistent picture of the evolution of Mars through the ages. The geologic history of the planet was divided into
three main eras, the Noachian, the Hesperian and the Amazonian. The Noachian,
lasting a few to several hundred million years, was the age in which Martian climate
was more favourable to life, while the Hesperian was characterized by intense volcanic activity, leading to the formation of the Tharsis plateau on which most of the
main volcanoes of the planet are located, and by the presence of massive ice sheets.
The Amazonian, the modern era of Mars, is the current arid and glacial age of Mars
[2] (Fig. 12.2).