Most of the planets have been visited by spacecraft, humans have walked on the Moon and stay continuously on the International Space Station (ISS) . Many details regarding the climate, radiation and particle environment, meteorology, atmospheric chemistry, geology, habitability and atmospheric evolution of other planets and their satellites are now better understood, so that the search of extant and extinct primitive alien life forms in our solar-system can begin.
The best candidate within our own solar system for finding microbial life forms is planet Mars. There is much geological evidence that liquid water has been present on Mars throughout much of its history. Liquid water and environmental conditions on early Mars similar than on Earth enhance the possibility that life developed independently both on Earth and Mars.
Fig. 1: Mars Global Surveyor Photo of Martian surface features indicating ancient flow of a liquid.
Fig. 2: Detail of the previous Mars Global Surveyor Photo of Martian surface features indicating ancient flow of a liquid.
Results of the Galileo probe have been interpreted as indication that a salty subsurface water ocean or water/ice slush may be present on Jupiter's satellite Europa, which marks it as a second candidate for a possible habitat of alien life forms in our solar system.
Saturn's huge satellite Titan provides
a unique milieu to study, insitu, the products of fundamental physical
and chemical interactions driving planetary organic chemistry and
serves as a natural reference laboratory to study by default, the role
of liquid water [link to role_of_water] in exobiology.
Read more about: Europa, Titan, or Mars