Based on our knowledge of life on Earth, the presence of liquid water is considered to be one of the essential requirements for the existence of extraterrestrial life. Although water is very common in the solar system, liquid water is much less common and is present only over a limited temperature range. Therefore the presence of liquid water on another body in the solar system would appear to shorten the odds of life elsewhere.
The Galileo Mission
The Galileo mission was the first to probe the atmosphere of Jupiter directly, and it carried out long term measurements of the Jovian system after its launch in 1989. In 2003, the satellite was sent on a collision course with Jupiter's atmosphere to avoid affecting Europa and its possibility of harboring life. A future mission to probe for the existence of life on Europa is currently being studied by NASA.
The Composition of Europa
Europa is the 4th largest satellite of Jupiter, and slightly smaller than the Earth's moon.
Based on data from the Galileo satellite, Europa is believed to consist of a small metallic core, surrounded by a large rocky layer. In turn this is covered by a liquid water ocean and an icy crust.
At such a distance, the sun does not provide much heat. Instead, the ocean is maintained by the heat generated by friction associated with tidal forces from Jupiter.
Liquid Water on Europa
In a paper published in Nature on 16 November, observations from the Galileo spacecraft reveal that heated water causes fissures in the Europa crust, allowing the water to rise. Although it was previously thought that an ocean existed 10-30 km (6-18 miles) below the crust, the latest data show that a large water reservoir is in places only 3km below the surface.
In principle, recovery of the water by future missions might answer the question directly of whether primitive Europan (not to be confused with European) life forms exist. It is plausible that the deep water is being mixed with the shallow water and transferring nutrients for life in the process.
As pointed out by Martin Siegert for BBC News, similar geological processes also occurred in Antarctica, with warm water uplifting and forming cracks in the rocks on freezing and expansion. Don Blankenship, a coauthor of the Nature paper, pointed out in a NASA article that the improved understanding of Europa would not have been possible without observations of the Earth's ice sheets and floating ice.
The Prospects for Liquid Water Elsewhere in the Solar System
It seems increasingly likely that the place to search for liquid water is not the inner planets (Mercury and Venus are too hot, and Mars is too cold) but rather amongst the entourage of the gas giants. Their satellites typically have rocky cores and ice. In addition, tidal forces in some cases provide the heat necessary to counteract the low sunlight. This is the likely process on Europa, in the Jovian system, while in the Saturnian system, the moon Enceladus provides prospects of harboring life of some kind: see the special issue of Science magazine, Cassini at Enceladus.
References
Carpenter, J., Jupiter moon Europa 'has shallow lakes', BBC News on line, 16 November 2011.
Lemonick, M.D., Oceans, a Lake — and Life? — on One of Jupiter's Moons, Time Science on line published Nov 17, 2011.
Schmidt, B.E. et al., Active formation of 'chaos terrain' over shallow subsurface water on Europa, Nature, published on line 16 November 2011.
