Is there life in Mars ?
As you know, these days Mars is back in fashion. On August 6th, we have seen the Mars Science Laboratory (MSL) of the NASA  landing successfully on the red planet, and now the Curiosity rover is walking to see what is there. This robot of about 900 kg, the largest that has been managed to land safely on Mars, is very well equipped. It contains 10 scientific instruments, including infrared laser teledetector, mass spectrometer, gas chromatograph, X-ray diffraction, microscopic camera, radiation detector, weather stations, and more than 17 photographic cameras .
These equipments are designed to detect any trace of water, to analyze accurately the rocks, to study the minerals in the Martian surface, to measure the chirality of the molecules detected (you know, L-or D-, such as L-amino acids typical of living beings) and to take pictures in high resolution.
The Curiosity rover, of Mars Science Laboratory mission, NASA
With these tools, the main objective of the MSL is determining whether there have been conditions conducive to life in Mars. Therefore, rather than looking directly for finding living beings, the goal is to try to find if there are any signs of past or present life, and also for looking the possibilities for a possible human settlement.
Life as we know it on Earth is based on the chemical elements of the basic biochemistry, namely carbon, oxygen, hydrogen, nitrogen, phosphorous, sulphur, and other trace elements. While these elements are found in many parts of the outer space, including Mars, it is necessary to quantify them to see if their proportions are indicative of a possible life, present or past, on Mars.
Of course, a key point in the search for life is to find liquid water, since on Earth there are living beings wherever there is liquid water, regardless of other conditions, aerobic or not, and extreme pHs and other inhospitable conditions. As you know, there are organisms on Earth everywhere, and among the so called extremophiles, either archaea or bacteria, there are some living at sea depths with enormous pressures, some others living at pH below 1 and temperatures near the boiling point, others a few hundred meters underground, others in the upper atmosphere, others endure radiation, others in a high osmotic pressure (e.g. saline environments). But in all cases, and even sometimes with very few nutrients, organisms live always in the presence of liquid water, even at low concentration. On Earth, wherever there is liquid water, always living beings are found, mainly microorganisms.
So well, is there liquid water in Mars ? It seems highly unlikely, but not impossible at all. There is water, certainly, especially ice at the poles (see picture) and in other places, but the low atmospheric pressure (the highest on the Martian surface is 0.6% of the Earth’s one), makes ice sublimating directly to water vapour, which is hardly retained by the atmosphere and escapes the planet. The Martian atmosphere contains 95% CO2, 3% N2, and traces of oxygen and water, as well as many particles in suspension. These give a tawny colour to the atmosphere, similar to the aspect of the planet as we see, due to abundance of iron oxide. Surface temperatures oscillate from about -140 °C at the poles to about 35 °C, occasionally at the equator. Therefore, even temporarily, there can be liquid water.
True-colour view of Mars seen through NASA’s Hubble Space Telescope
Despite the current hostile conditions of Mars, there is geological evidence of the existence of liquid water in the past, like the geomorphology of valleys and channels, etc., and also by the presence of some minerals that only can be originated with water (e.g. hematite).
On the other hand, small amounts of methane and formaldehyde have been detected there. These compounds are mostly generated biologically on Earth, but conditions of the Martian atmosphere make them short-lived . However, their geological origin seems also possible.
The habitability of Mars has been tested in the laboratory reproducing its conditions, by placing polar and alpine lichens at these conditions . Surprisingly, it has been shown that these lichens can resist them.
The question of possible life on Mars had a peak in 1996, when scientific staff from NASA published in Science magazine  the microscopic images of a fragment of ALH84001, a meteorite from Mars. This stone seems to be of Martian origin, it was launched into space by the impact of a meteorite on Mars about 15 million years ago, and then it travelled till the Earth reaching the Antarctica, where it was found in 1984. The structures seen there recall bacterial chains, and so, it was proposed that they would be martian microfossils. However, there is no other evidence of the biological origin of these structures and afterwards an inorganic origin of the meteorite minerals has been proposed .
Electronic microscopy of a fragment of ALH84001 meteorit (image of NASA)
Well, we wait with expectation the results of the tests done by the Curiosity. If evidence of past or present life on Mars is found, there will be very much to think about, especially in the sense that terrestrial organisms would not be alone in the universe. And in spite of the evidence that would be very simple microbial life, we should conclude that life is a feature very widespread in the universe.
What do you think ? Will Curiosity find evidence of life on Mars ?
Earth and Mars, at the same scale, two Bio-planets ? (image from NASA)
 NASA: http://www.nasa.gov/mission_pages/msl/
 Mahaffy, P. (2009) Sample analysis at Mars: Developing analytical tools to search for a habitable environment on the red planet. Geochemical News (Geochemical Society), 141, oct. 2009
 Krasnopolski, V.A. et al. (2004). Detection of methane in the Martian atmosphere: evidence for life?”. Icarus 172, 537–547
 DLR (German Aerospace Center) Surviving the conditions on Mars: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-3409/
 McKay, D.S. et al. (1996) Search for past life on Mars: Possible relic biogenic activity in Martian meteorite ALH84001. Science 273, 924–930
 Golden, D.C. et al. (2004) Evidence for exclusively inorganic formation of magnetite in Martian meteorite ALH84001. American Mineralogist 89, 681–695
[ ] … and Wikipedia:
– Mars Science Laboratory: http://en.wikipedia.org/wiki/Mars_Science_Laboratory