Far from being a haven of ice and water and an ideal spot for the search for alien life, Jupiter鈥檚 moon Europa may be a corrosive hotbed of acid and peroxide. That is the conclusion of researchers who met last week to prepare for NASA鈥檚 proposed Jupiter Icy Moons Orbiter, an ambitious mission to study Jupiter鈥檚 moons.
Almost all the information we have about Europa comes from the spacecraft Galileo, which completed its mission to study Jupiter and its moons close up before NASA dramatically crashed it into Jupiter in 2003.
Although the general perception of Europa is of a frozen crust of water ice harbouring a salty subterranean ocean kilometres below, researchers studying the most recent measurements say light reflected from the moon鈥檚 icy surface bears the spectral fingerprints of hydrogen peroxide and strong acids, perhaps close to pH 0, if liquid.
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But they are not sure whether this is just a thin surface dusting or whether the chemicals come from the ocean below. The hydrogen peroxide certainly seems to be confined to the surface, as it is formed when charged particles trapped in Jupiter鈥檚 magnetosphere strike water molecules on Europa.
But parts of the surface are rich in water ice containing what looks like an acidic compound. Robert Carlson of NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, thinks this is sulphuric acid.
Surface bombardment
He says that up to 80 per cent of the surface ice in some spots may be concentrated sulphuric acid, and suggests this may be a veneer formed by surface bombardment with sulphur atoms originally thrown out from volcanoes on Io, another of Jupiter鈥檚 moons.
Other scientists think the results suggest that the acid derives from Europa鈥檚 internal ocean. Tom McCord of the Planetary Science Institute in Winthrop, Washington state, points out that the greatest concentrations of acid seem to be in areas where the disrupted surface suggests that ocean liquid has gushed upward and frozen.
McCord thinks that the acid on the surface began as salts from the ocean underneath largely magnesium and sodium sulphates. Intense surface radiation caused chemical reactions, he says, which left an icy crust containing a high concentration of sulphuric acid as well as other sulphur compounds.
Jeff Kargel of the US Geological Survey in Flagstaff, Arizona, believes the sulphuric acid is coming directly from the ocean. He thinks that Europa鈥檚 heart is rocky, with undersea volcanoes releasing sulphur-containing compounds and oxygen that react with the ocean water to form sulphuric acid.
鈥淓uropa has an Io hiding underneath the ocean,鈥� he says. If the surface sulphates have come from the water deep below, Europa鈥檚 ocean might be an 鈥渁cidic sulphate brine鈥�.
Big hills
That could be bad news for life, as strong acids tend to destroy organic compounds. But it does not rule out the possibility 鈥� some terrestrial species of bacteria thrive in environments with a pH as low as zero.
Another blow to researchers hoping to probe this ocean for life comes from hints that Europa is much hillier than previously thought. Paul Schenk of the Lunar and Planetary Institute reports that a dark spot on Europa鈥檚 surface is a 350-metre depression situated near a 900-metre rise 鈥� a total relief of 1250 metres. To support such a large relief, he calculates that the ice layer must be 10 to 30 kilometres thick, which is a daunting distance for any probe to drill through.
However, researchers are unlikely to find out much more from Galileo鈥檚 data. Its measurements cover only a small fraction of Europa鈥檚 surface, and much of the detail is obscured by background 鈥渘oise鈥� and low resolution.
鈥淚 don鈥檛 think we鈥檙e going to have a definite answer until we get back with better spectrometers, better resolution, and maybe a lander,鈥� McCord told New 杏吧原创. Figuring out how to do that is part of his job as a member of the science team for the Jupiter Icy Moons Orbiter, due to launch in 2012.
鈥淚f the surface is made of sulphuric acid, landing should not be a problem as long as the ice stays frozen,鈥� he says. But if it becomes liquid, the acid could be strong enough to eat through most materials used to make spacecraft. That may give pause to proposals for a probe that melts through the surface to study the ocean below.