Landing sites on Europa identified

Galileo got there first (Image: NASA/JPL/University of Arizona/University of Colorado)

A RIGOROUS analysis of the jagged terrain of Jupiter's moon Europa is helping to identify safe landing strips for future missions.

Europa is thought to have an ocean of water beneath its icy shell. The latest study is the first to use images from the Galileo spacecraft, which orbited Jupiter from 1995 to 2003, to generate measurements of Europa's slopes. "This is the first quantitative sampling that gives hard numbers, real numbers that you can believe," says Paul Schenk of the Lunar and Planetary Institute in Houston, Texas.

Schenk used shadows, plus pictures taken from two different angles, combined into 3D images, to calculate the slopes of various regions of Europa. He examined four different kinds of terrain: ridged plains that make up the majority of the surface; impact craters; so-called "chaos" regions where icebergs appear to float in a frozen soup; and long smooth stripes called dilational bands.

Chaos regions and impact craters are particularly exciting for planetary scientists since liquid water from a subsurface ocean may have burst through at these points, making it possible to search for evidence of life without having to drill below the surface.

These sites are bad news for landers, though. Up to half the landscape in these regions was tilted more than 10 degrees, a similar incline is making life difficult for the Mars rover Spirit. The steepest slopes can reach 20 or 30 degrees. Even the ridged plains have rounded tops that could pose problems for landers.

The only smooth features were the dilational bands, which slope at about 5 degrees or less. These broad tracks, tens of kilometres wide and hundreds of kilometres long, form when cracks in the ice shell open in response to the gravitational pull of Jupiter and the other large moons. The cracks then fill with water and open even further, leaving smooth tracks in between. "It's a little bit like a mid-ocean ridge spreading on Earth," Schenk says.

These areas could be smoother because they didn't form as violently as impact craters, or because the upwelling water smoothed over whatever rough patches were there. Thanks to regular flooding, the cracks could also harbour life. "These bands are one of the places that a future project might decide it wanted to land," Schenk says.

Europa was also recently selected as the target for an orbiting mission. The orbiter will finish mapping Europa's surface, where Galileo left off. "The Galileo antenna malfunctioned," says Schenk. "They could only map about 15 per cent of the moon at resolutions that are worth mapping."

"The issue of topography is very important as we put together the objectives for the Europa orbiter mission," says Bob Pappalardo at NASA's Jet Propulsion Laboratory in Pasadena, who is working on the Jupiter Europa Orbiter scheduled for launch in 2020.