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THERE鈥橲 good and bad news for the prospects of finding alien life on Jupiter鈥檚 moon Europa. The raw materials for life do exist on the moon, which is thought to have a liquid ocean under its icy surface. But we may never get to look for any life there because the frozen crust is at least 19 kilometres thick.

Elements such as carbon are crucial to life on Earth. But nobody knew if Europa had large enough amounts of these elements to make life possible. To find out, Elisabetta Pierazzo of the University of Arizona in Tucson and Chris Chyba of the SETI Institute in Mountain View, California, calculated the numbers of large comets that hit Europa鈥檚 surface since it formed.

Assuming the comets鈥� make-up was similar to that of Halley鈥檚 Comet, the impacts would have dumped between 1 billion and 10 billion tonnes of carbon on the moon鈥檚 surface, along with a billion tonnes each of nitrogen and sulphur. If microorganisms in Europa鈥檚 ocean mopped up all the carbon, they would now populate the oceans to levels of about 1 per cent of their counterparts in Earth鈥檚 oceans.

But could a space probe ever sample the ocean to look for bugs? Some scientists thought that the overlying ice was around a kilometre thick, in which case a lander might one day be able to burrow through. But that was too optimistic, says Paul Schenk of the Lunar and Planetary Institute in Houston, Texas.

Schenk compared Voyager and Galileo images of craters on some of Jupiter鈥檚 moons with craters on the rocky surface of our Moon. On Europa, all the craters more than about 30 kilometres across have concentric rings, which could be 鈥渞ipples鈥� created when a comet or asteroid smashed into the ice powerfully enough to 鈥渇eel鈥� the fluid ocean beneath. If so, the pattern suggests the ocean lies more than 19 kilometres below the ice.