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Three moons are juicy targets for Jupiter mission

Paul Sutherland, Feature writer
May 12, 2012, 7:00 UTC

Sen—Planetary scientists are busy preparing for Europe's next big space adventure - the unmanned JUICE probe's exploration of Jupiter's neighbourhood.

Prime targets on the mission, due to launch in 2022 following its selection by the European Space Agency (ESA), will be the giant planet's largest moons, Callisto, Ganymede and Europa, which were, with Io, first spotted by the astronomer Galileo Galilei back in 1610.

All three moons have internal oceans and could be possible habitats for life. JUICE will not arrive at Jupiter until 2030. But we already know a fair deal about the Galilean moons, largely thanks to an earlier NASA probe Galileo that explored Jupiter's family from 1995 to 2003.

Callisto is the outermost of the four Galilean moons, lying 1.88 million km from Jupiter and orbiting it in 16.69 days. It is also Jupiter's second biggest satellite and the third-largest in the Solar System with a diameter of more than 4,800km, making it nearly as big as Mercury.

Callisto's ancient surface is thought to have changed little over four billion years but its interior appears to be mostly a jumble mix of ice and rock with a possible liquid ocean around 200km down. Signs of weathering were detected on the ancient surface which may have been caused by ice turning to vapour on the peaks in an extremely thin atmosphere and leaving dust that drifts into the craters.

Ganymede is the biggest moon in the Solar System, being even larger than Mercury with a diameter of 5,262 km. The Galileo probe suggested that Ganymede has an interior of metal and rock surrounded by a thick icy crust. Grooves and ridges that break up its heavily cratered surface indicate tectonic activity.

Ganymede has its own magnetic field which could be explained either by a liquid iron outer core around its solid heart or an ocean beneath the surface.

Europa is a mysterious moon because it has no craters or mountains, just a strange pattern of cracks over its surface of of ice. They resemble ridges of ice found in the Arctic and are thought to be due to the tidal pull of Jupiter and the other main moons. Measurements of a magnetic field add to evidence that a salty subterranean sea is wrapped around the moon which could be more than 100km deep and a prime place to look for alien life.

Europa is 3,122 km in diameter and lies 671,000 km from Jupiter orbiting once every 3.55 days. Like our own Moon, it is tidally locked, keeping the same face towards the planet. There is a thin atmosphere, mainly of oxygen.

JUICE will not study Jupiter's fourth Galilean moon, Io, partly because it is a volcanic world thought unsuitable for life but also because it lies in a region of heavy radiation that would shorten the probe's working life.

Professor Michele Dougherty from Imperial College London led the proposal for JUICE. She told Sen that four conditions needed to be present for life to form - water, a heat source, the presence of complex organic compounds and a stable environment.

She said: "Studying these three moons will give us an understanding about whether there are habitable environments elsewhere in our Solar System and beyond other than on Earth."

Professor Dougherty, who also works on the Cassini mission at Saturn, said a team of 16 had defined a model payload when planning JUICE for their proposal to ESA. An announcement of opportunity would now go out to the science community for ideas for other instruments to fly aboard the probe.

She said: "We will have to wait a long time for JUICE to arrive at Jupiter but if you want to explore the outer solar system that is what you have to do. From my perspective, if someone had not done this for me on the Cassini spacecraft I wouldn't be doing the great science I'm able to do at Saturn now.

"So I see this as a way for us to ensure that the planetary community and people interested in Solar System exploration are able to continue doing what we do. What Cassini is essentially doing now is training the young scientists who are going to be able to do great science at Jupiter in 2030."