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New missions on drawing board to probe Jupiter's moons

Paul Sutherland, Feature writer
Sep 10, 2013, 7:00 UTC

Sen—Space missions are continuing to investigate Mars to find out whether it was ever habitable. But increasingly it seems that some of the outer moons of the Solar System could be even better places to look for life.

This week, an international conference of planetary scientists in London is being told of fresh plans to explore two major satellites of Jupiter to learn more about oceans of water that lie beneath their surfaces.

Preparations are already advanced for a European mission called JUICE - the Jupiter Icy Moon Explorer - which is due to launch in 2022. This probe, which we have written about before at Sen, will study three of the four largest moons, Callisto, Ganymede and Europa, which were, with Io, first observed by Galileo in 1610. All three are thought to have sub-surface oceans.

NASA, which currently has a probe of its own called Juno on its way to study Jupiter itself, is also now developing ideas for an orbiter mission to take a closer look at Europa. And Russia is considering its own mission to the planet that would plant a lander on the surface of Ganymede. Both concepts are being described in presentations at the European Planetary Science Congress, being held at University College London.

NASA’s concept, called Europa Clipper, would place a spacecraft in orbit around Jupiter to study Europa in detail. The spacecraft, which would be designed to withstand powerful radiation from Jupiter, would make a long, looping orbit around the planet to perform 32 close flybys of the moon, ranging in altitude from 2,700 km to 25 km.

Instruments being considered for its payload include radar to penetrate Europa’s frozen crust and determine the thickness of the ice shell and find evidence for the liquid saltwater believed to lie below. Detailed measurements of the icy surface features would be made using cameras as well as a study of the composition of those features and Europa’s trace atmosphere, using spectrometers.

Europa, Ganymede and Callisto

From left to right, Europa, Ganymede and Callisto are all thought to have underground oceans. Credit: NASA

The Europa study team, based at NASA’s Jet Propulsion Laboratory, say that understanding Europa’s habitability is intimately tied to understanding the three “ingredients” for life - water, chemistry, and energy.

Scientists believe Europa, which is 3,140 km in diameter (1,950 miles), may have all three ingredients - an extensive saltwater ocean beneath an ice shell that is geodynamically active and relatively thin; essential chemical elements including some delivered by asteroids and comets; and the combination of irradiation of its surface and tidal heating of its interior that produce a rich source of chemical energy.

Meanwhile, the Russian Space Research Institute (IKI) is studying the possibility of a future mission, in conjunction with the European Space Agency, that would send an ESA orbiter into the Jupiter system but place a Russian lander close to the equator on its largest moon Ganymede too.

The combination of both probes would allow measurements of how much Ganymede rocks as it orbits Jupiter - called libration - and how much its surface rises and falls due to tidal pulls. This effect is expected to be in the order of between a few centimetres and a few metres and so detectable by the mission.

Studies for the mission have been carried out by the Royal Observatory of Belgium, supported by ESA. They say that the movements in Ganymede’s crust will help to reveal the satellite’s interior properties, including the depth of the liquid water ocean and the thickness, rigidity and makeup of its ice shell.

Ganymede is 5,262 km across (3,280 miles) making it the biggest moon in the Solar System and larger than planet Mercury.