Europa has a thick skin

Europa's craters: "inconsistent with thin-ice-shell models"

By Dr David Whitehouse BBC News Online science editor

Jupiter's moon Europa may have a skin of ice much thicker than had been thought.

A new analysis suggests that the larger craters on Europa's surface are shallower and not as well defined as similar sized impact zones on satellites elsewhere in the Solar System.

Researchers think this may be because a substantial layer of "warm" ice below the surface of Europa is simply not strong enough to support large, deep structures.

Many scientists regard the warm oceans beneath Europa's ice crust as a possible place where primitive forms of life could develop.

Poorly developed craters

This latest study will be depressing reading for those who hoped that we could one day send a space probe to Europa that could melt its way through the ice crust to see if there is life in the warm oceans below.

Beneath the ice there could be life

Instead of being a thin shell as was once thought, the ice appears to be a slab 19 to 25 kilometres (13 miles to 16 miles) thick, Paul Schenk, of the US Lunar and Planetary Institute, writes in the journal Nature.

Schenk analysed images of craters on Europa and two other Jovian ice satellites, Ganymede and Callisto, and compared them with craters on our Moon, which is roughly of the same size and gravity.

He found that small craters looked similar on all three bodies but larger craters were very different.

On the Moon, big craters are deep, with high walls and a prominent central dome. But on Ganymede and Callisto, craters of a similar size were poorly developed.

On Europa, larger craters become smoothed out with gentle concentric rings. They are shallower than their counterparts on other moons.

Mission impossible

Schenk believes that this is because of the influence of a very thick cushion of ice.

Previous attempts to estimate the thickness of Europa's ice shell have suggested that could be as little as three km (two miles) thick.

That gave hope to space mission planners that they could devise a way to land a probe on the surface and have it burrow its way down to the ocean below.

The American space agency has even been working with technologists to trial equipment in the Antarctic on Earth.

The water below Europa's surface, with its minerals and geothermal energy sources, is regarded as a possible abode for life.

But a very thick ice shell would make a burrowing mission virtually impossible.

"The appearances of the craters on Europa are inconsistent with thin-ice-shell models," says Schenk.