It is also possible that geological activity plays an important role on the persistence of life on Earth: the tectonics of the plates on our planet is at the source of the mountains which then erode, allowing a return to the oceans of the chemical elements of the rocks they are composed of. This recycling of chemical elements is indispensable to life. It is also encouraged by the planet’s hydrothermal processes.

As we can see, what constitutes the habitability of a planet or a moon is complex and certainly multidimensional. In addition habitability depends on time: a planet can be habitable for a given period but be no longer so afterwards. Habitability is also different following a spatial scale as it can concern an entire planet or a more restricted habitat (forest, desert, etc.) or even a micro-habitat (pores between grains of sand, etc.).

An understanding of the concept of habitability took a giant stride with the discovery and study of extremophiles, the organisms which live and even only survive in extreme conditions. The latter can be physical (pressure, temperature, radiation) or geochemical (salinity, pH, concentration of metals, the presence of gas, etc.). The environments of these adventurers of the extreme can be close to the conditions which prevail or have prevailed on the primitive Earth or on Mars, astrobiology’s primary target.

Mars, then, habitable or not? It is an age since the Red Planet lost its magnetic field and its atmosphere was eroded by the solar wind. Moreover Mars’ crust is a monoplate: the planet does not have plate tectonics even though it has been geologically active, as is notably borne out by the largest volcano of the solar system, Mount Olympus. If these characteristics are a bad presentiment  for present life, traces of liquid water dating from the era when the planet still possessed an atmosphere and a magnetosphere leave open the question of life in the past: if Mars is not inhabited today, it could have been habitable in its past for a short period.