http://en.wikipedia.org/wiki/Extraterrestrial_life wrote:
Extraterrestrial life in the Solar System
Many bodies in the Solar System have been suggested as being capable of containing conventional organic life. The most commonly suggested ones are listed below; of these, five of the ten are moons, and are thought to have large bodies of underground liquid (streams), where life may have evolved in a similar fashion to deep sea vents.
* Mars — Life on Mars has been long speculated. Liquid water is widely thought to have existed on Mars in the past, and there may still be liquid water beneath the surface. Methane was found in the atmosphere of Mars. By July 2008, laboratory tests aboard NASA's Phoenix Mars Lander had identified water in a soil sample. The lander's robotic arm delivered the sample to an instrument which identifies vapors produced by the heating of samples. Recent photographs from the Mars Global Surveyor show evidence of recent (i.e. within 10 years) flows of a liquid on the Red Planet's frigid surface.
* Mercury — The MESSENGER expedition to Mercury has discovered that a large amount of water exists in its exosphere.
* Europa — Europa may contain liquid water beneath its thick ice layer. It is possible that vents on the bottom of the ocean warm the ice, so liquid could exist beneath the ice layer, perhaps capable of supporting microbes and simple plants, just like in Earth's hydrothermal vents.[4]
* Jupiter — Carl Sagan and others[who?] in the 1960s and 70s computed conditions for hypothetical amino acid-based macroscopic life in the atmosphere of Jupiter, based on observed conditions of this atmosphere. These investigations inspired some science fiction stories.
* Ganymede — Possible underground ocean (see Europa).
* Callisto — Possible underground ocean (see Europa).
* Enceladus — Geothermal activity, water vapor. Possible under-ice oceans heated by tidal effects.
* Titan (Saturn's largest moon) — The only known moon with a significant atmosphere. Data from the Cassini-Huygens mission refuted the hypothesis of a global hydrocarbon ocean, but later demonstrated the existence of liquid hydrocarbon lakes in the polar regions - the first liquid lakes discovered outside of Earth. Analysis of data from the mission has uncovered aspects of atmospheric chemistry near the surface which are consistent with – but do not prove – the hypothesis that organisms there are consuming hydrogen, acetylene and ethane, and producing methane.
* Venus — Recently, scientists have speculated on the existence of microbes in the stable cloud layers 50 km above the surface, evidenced by hospitable climates and chemical disequilibrium.
Numerous other bodies have been suggested as potential hosts for microbial life. Fred Hoyle has proposed that life might exist on comets, as some Earth microbes managed to survive on a lunar probe for many years. However, it is considered highly unlikely that complex multicellular organisms of the conventional chemistry of terrestrial life (i.e. animals and plants) could exist under these living conditions.
Even if microbial extraterrestrial life were found on another body in the Solar System, it would still need to be proven that such life did not originate from Earth in the recent or distant past. For example, an alternate explanation for the hypothetical existence of microbial life on Titan has already been formally proposed — theorizing that microorganisms could have left Earth when it suffered a massive asteroid or comet impact (such as the impact that created Chicxulub crater only 65 mya), and survived a journey through space to land on Titan 1 million years later. The Living Interplanetary Flight Experiment, developed by the Planetary Society and due to be launched in 2011, has been designed to test similar theories.>>