It has long been thought that Jupiter's icy moon Europa has an ocean beneath its surface, kept liquid by the heat generated by the tidal forces excerpted by Jupiter's gravity, and that this may possibly be as much as 160 km deep, on a moon with a radius of slightly under 1600 km (though not all scientists agree with this theory). Since life is found everywhere on Earth that water is also found, astrobiologists (scientists who look for life on other planets) consider finding water the most important step in finding life. As such the possible subterranean sea of Europa is considered the most likely place to look for non-terrestrial life in our Solar System.
Diagrammatic representation of a possible ocean on Europa. Most of the moon is rock, with a metallic core, and a surface ocean covered by a layer of ice. NASA.
In a paper in the 10 February edition of the journal Astrobiology, Matthew Pasek of the Department of Geology at the University of South Florida and Richard Greenberg of the Department of Planetary Science at the University of Arizona present a new study of the chemistry of Europa, which suggests that any ocean beneath its surface might be quite hostile to life.
Pasak and Greenberg theorize that the constant bombardment of Europa's surface with high energy particles from Jupiter will break up water molecules (H₂O) in the ice at the surface of the moon forming hydrogen peroxide (H₂O₂) and Oxygen (O₂), both powerful oxidants. In time these will work their way down through the ice at the surface to the water beneath, where they could potentially react with sulphur (S) compounds, forming sulphuric acid (H₂SO₄).
Europa is constantly bathed in high energy particles from Jupiter. NASA.
If this is the case then any ocean on Europa is likely to be acidic, with a pH of about 2.6 (roughly the same as most fizzy drinks) which would present quite a challenge for life. If there are no sulphur compounds in the sea then the situation is even worse; oxidizing compounds are toxic to most life. Life on Earth survived for billions of years before photosynthetic bacteria filled our atmosphere with oxygen. This is thought to have killed of much of the diverse microbial life living at the time, with the few organisms that had learned to cope with (or even use) the oxygen going on to dominate the Earth's later biosphere. Pasek & Greenberg theorize that Europa's much smaller ocean would have been oxidized within 50 million years of its formation, making it much harder for life to adapt in time.