Perhaps a lander on the icy moon of Jupiter Europe will have to dig at least one foot to find signs

New research suggests that robots will have to dig relatively deep into the frozen moon of Jupiter, Europe, to have a chance to find life signs.

Scientists believe that Europa houses an enormous ocean of liquid water under its icy shell. This ocean appears to be in contact with the rocky nucleus of the 1,940-mile (3,100-kilometre) moon, which makes a range of complex chemical reactions possible. Consequently, Europa is generally considered one of the best bets of the solar system to shelter foreign life.

If life has ever existed in Europe, some signs of it can come out of this ocean buried on the surface, where robots could potentially find it. Well, not to the surface; Europa gets hammered by charged particles, which are captured and accelerated by the powerful magnetic field of Jupiter.

Previous work has suggested that just 8 inches (20 centimeters) of ice could likely shield any biomolecules that might exist on Europa from that punishing radiation environment, even in the hardest-hit regions of the moon.

The new survey, published Monday (12 July) in the journal Nature Astronomy, is somewhat more pessimistic. In it, the researchers modeled how Europa's surface is disturbed by small but frequent impacts - a real issue in a world without a substantial atmosphere to burn up incoming hunks of rock and ice.

They found that such "impact gardening" likely churns the top 12 inches (30 cm) or so of Europan ice significantly, bringing previously buried bits up to the surface, where radiation can zap any interesting molecules into unrecognizable goo.

"If we hope to find pristine, chemical biosignatures, we will have to look below the zone where impacts have been gardening," study lead author Emily Costello, a planetary research scientist at the University of Hawaii at Manoa, said in a statement. "Chemical biosignatures in areas not as deep as this area may have been exposed to destructive radiation."

This new work will probably be of interest to NASA, which expects to launch a Europa spacecraft in 2024. The mission, known as Europa Clipper, will orbit Jupiter but will study the moon and its ocean buried above dozens of nearby passages.

Clipper's many duties also include scouting out potential touchdown sites for a life-hunting lander - a mission that Congress has directed the agency to mount but which remains a concept for now.