The current understanding of planetary habitability favors planets that have liquid water on their surface. This most often requires that the orbit of a planet lie within the habitable zone, which for the Sun currently extends from just beyond Venus to about the semi-major axis of Mars. During perihelion Mars dips inside this region, but the planet’s thin (low-pressure) atmosphere prevents liquid water from existing over large regions for extended periods. The past flow of liquid water, however, demonstrates the planet’s potential for habitability.
At present there cannot exist water in liquid state on martian surface, due to very low atmospheric pressure. It is very likely that under the martian frozen soil, there are lot of water reservoirs present, especially under martian poles.
Animation of possible water under the martian ground::
There can be found also extensive evidence of recent existence of seas, oceans and rivers which formed the current landscape on Mars. Also this is one of reasons why there is detail research of any signs of past or present life (bacteria or primitive forms of life).
At the Johnson space center lab, some curious shapes have been found in the Martian meteorite ALH84001. Some scientists propose that these geometric shapes could be fossilized microbes extant on Mars before the meteorite was blasted into space by a meteor strike and sent on a 15 million-year voyage to Earth. However, an exclusively inorganic origin for the shapes has also been proposed.
Under the scanning electron microscope structures were revealed that may be the remains—in the form of fossils—of bacteria-like lifeforms. The structures found on ALH 84001 are 20-100 nanometres in diameter, similar in size to the theoretical nanobacteria, but smaller than any known cellular life at the time of their discovery. If the structures are in fact fossilized lifeforms, as proposed by the so-called biogenic hypothesis of their formation, they would be the first solid evidence of the existence of extraterrestrial life, aside from the chance of their origin being terrestrial contamination.
View under the microscope:
sources and image credits: JSC NASA