The Deep Impact mission, which last year fired a massive projectile at the nucleus of comet Tempel 1, has inspired a proposal for a similar mission to Jupiter鈥檚 moon Europa.
Evidence that Europa has a subsurface ocean, which would make it the only known 鈥渨ater world鈥� besides Earth, emerged between 1996 and 2003 from NASA鈥檚 Galileo mission. While this makes Europa a prime target in the search for alien life, any plan to reach its ocean faces the daunting challenge of how to drill or melt down through the 20 kilometres or more of ice lying above it.
To begin to tackle the problem, a team led by researchers at Johns Hopkins University in Baltimore, Maryland, is proposing to smash a spacecraft into the ice and analyse the plume of ejected material. While the impact would come nowhere near breaking through to the ocean below, the energy released would throw up a tonne of material for every gram that the craft itself weighed.
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This would provide the best chance of measuring the true composition of Europa鈥檚 ice, potentially revealing the residue of life or its precursor molecules. The probe would 鈥渄ig into pristine dirt and analyse that stuff for organics鈥�, says Karl Hibbits of the Applied Physics Laboratory at Johns Hopkins.
Getting below Europa鈥檚 surface is necessary for the task, as the top layer of ice has been chemically altered by radiation caused by Jupiter鈥檚 powerful magnetic field, and by constant sputtering from volcanic eruptions on the neighbouring moon Io. Because of these changes, 鈥渁strobiology can鈥檛 do anything useful at the surface鈥�, says Louise Prockter, also at Johns Hopkins. Using an 鈥渋mpactor鈥� craft would also avoid the problem of radiation destroying a lander and its instruments during an extended operation on the surface.
The proposal, outlined last week at a meeting of the American Astronomical Society鈥檚 Division for Planetary Sciences in Pasadena, California, envisages that the impactor would have a camera and transmitter to get close-up pictures during the descent. As with the Deep Impact mission, a separate fly-by spacecraft would do a spectroscopic analysis of the impact plume, go into orbit around Jupiter and loop back to take pictures of the crater. 鈥淚t鈥檚 quite an elegant way to look at a very difficult problem,鈥� says Prockter. The same concept could be used for exploring other icy bodies such as Neptune鈥檚 huge moon Triton, the team says.
Funds even just to develop the proposal will be a problem. As a stand-alone mission, it would cost about a billion dollars, and NASA has no development funding for such missions. 鈥淭he axe hit鈥� when NASA cut back on science funding this year, says team member Peter Schultz of Brown University in Rhode Island. An initial proposal to NASA was turned down this year, but the group hopes to find other sources of funding to pursue the research and then resubmit the plan. One possibility is to make it a component of NASA鈥檚 planned Juno mission, now set for launch around 2011.
