The Planet Mars

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9.5.1.1 The Story of the Search for Life

Apart from its distance from the Sun, Mars is, after Earth, the planet that is best placed to be searched for signs of life, and numerous past scientists and philosophers have raised the possibility. In 1877, the Italian astronomer Schiaparelli announced a discovery that caused a sensation: the presence on Mars of linear structures called 'canali' (Fig. 9.14). Although he himself was extremely cautious about the interpretation of his results, other astronomers, beginning with Percival Lowell, saw them as traces of an intelligent civilization. Despite the reservations of other astronomers such as Eugene Antoniadi (Fig. 9.15), the myth of intelligent life on Mars persisted until the 1960s. The first images returned by spaceprobes showed that the linear structures were no more than optical illusions.

In 1975, NASA launched Viking, an ambitious mission to Mars, the main objective of which was search for life on Mars. Two spaceprobes were placed in orbit around the planet, and continued to function for several years, while two descent modules landed on the surface of Mars, to carry out in-situ research (Fig. 9.16). A set of three biological experiments aimed to search for signs of biological activity by means of various tests, including analysis of the degassing of the soil in the presence of a nutrient medium, and search for potential photosynthetic activity of any possible carbon compounds. In terms of these investigations, scientists have concluded that biological activity is absent from the surface of Mars, at the two locations where the modules landed. The total absence of organic molecules on the surface of Mars has been attributed to the irradiation by ultraviolet solar radiation. Note that these experiments do not exclude the possible existence of past or present life at sites that have been protected from destructive solar radiation.

9.5.1.2 The History of Water on Mars

Following the Viking era, space missions to Mars multiplied. The most recent had the prime objective of searching for signs of past liquid water on the surface. We know, in fact (Chap. 4), that current temperature and pressure conditions do not permit liquid water to flow on Mars today. But several indications, acquired from the Viking images, bear witness to its presence in the past: branching networks of valleys in ancient terrains (Fig. 9.17); outwash valleys near the Vallis Marineris canyon system; craters with lobate ejects, possibly indicating the presence of subsurface liquid, etc. New, decisive evidence has been acquired during the course of recent years. In 2002, the Mars Odyssey probe, detected the presence of a significant quantity of ice beneath the poles. The result has been confirmed by the infrared spectrometer on the European Mars Express probe, which detected the spectral signature of water ice beneath the perennial CO2 ice cap at the poles. In another remarkable result, the same instrument has detected, near Vallis Marineris, various types of sulphates, whose formation appears to have required the presence of liquid water. Similarly, the American robot Opportunity has discovered spherules consisting of sulfates at the Meridiani landing site.

Water therefore flowed on Mars in the distant past, but the central question is: in what abundance, and until what date? Did it last sufficiently long to have allowed the appearance and development of life? Scientists are reticent on this point. The

Fig. 9.14 Map of the surface of Mars, drawn by Schiaparelli in 1879. The linear structures, called 'canali' were interpreted as signs of an intelligent civilization

Fig. 9.15 The region of Elysium Planitia on Mars, as drawn by Schiaparelli between 1877 and 1890 (left) and by Antoniadi between 1909 and 1926 (right). Antoniadi's drawings showed that the quasi-linear structures seen by Schiaparelli were nothing like that when spatial resolution was increased (After Antoniadi, La planete Mars, 1930)

Fig. 9.15 The region of Elysium Planitia on Mars, as drawn by Schiaparelli between 1877 and 1890 (left) and by Antoniadi between 1909 and 1926 (right). Antoniadi's drawings showed that the quasi-linear structures seen by Schiaparelli were nothing like that when spatial resolution was increased (After Antoniadi, La planete Mars, 1930)

High-gain antenna pointing at Earth

Camera calibration target

Seismometer

UHF relay antenna

Low-gain antenna

Mass spectrometer

Roll-control thruster

Propellant tank

Camera calibration target

Mass spectrometer

High-gain antenna pointing at Earth

Seismometer

UHF relay antenna

Low-gain antenna

Roll-control thruster

Propellant tank

Meteorology sensors

Retro rockets Articulated arm Scoop

Fig. 9.16 Schematic diagram of the structure of the Viking landers. Once on the surface, the probe used the energy from a plutonium-based radioactive source. The parabolic antenna allows communication with the orbiter or directly with Earth. The two descent probes carried identical equipment, including cameras, meteorological instruments, mass spectrometers and X-ray fluorescence spectrometers, a seismometer, and the biological experiments (NASA)

Meteorology sensors

Retro rockets Articulated arm Scoop

Fig. 9.16 Schematic diagram of the structure of the Viking landers. Once on the surface, the probe used the energy from a plutonium-based radioactive source. The parabolic antenna allows communication with the orbiter or directly with Earth. The two descent probes carried identical equipment, including cameras, meteorological instruments, mass spectrometers and X-ray fluorescence spectrometers, a seismometer, and the biological experiments (NASA)

absence of hydrated minerals on the northern plains seems to question the theory that they were covered in liquid water and the theory of a possible northern ocean. Future Martian exploration missions will continue to give priority to the search for signs of liquid water, to measure the quantity of sub-surface water by the use of radar, as well as drilling into the surface.

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