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The question looms large, not only for Mars, but for other worlds in the Solar System: As we look for evidence of extraterrestrial life, is it better to do the science investigations in situ— onsite—or to bring samples back to Earth for study?

Enceladus is a relatively small moon of Saturn, about the size of Great Britain, which has geyser-like jets in the south-polar region that eject a plume of gases and solid particles into space. These are thought to come from a liquid subsurface water reservoir, which may contain life. Thus, Enceladus has become one of the most important objects in the Solar System for astrobiological study. Since the organic and inorganic components of the subsurface water are ejected into space, a lander is not needed. Samples could be taken from orbit, allowing a much cheaper mission.

There are currently two competing mission designs for Enceladus. Jonathan Lunine of Cornell University made the case at this week’s meeting of the American Geophysical Union in San Francisco for an orbiter that would fly through the plume and analyze its constituents. Called ELF (Enceladus Life Finder), the spacecraft would include more sophisticated instrumentation than the Cassini orbiter that is currently investigating Saturn’s moons. ELF would be much better suited to detecting the building blocks of life and to assessing environmental conditions in the liquid water reservoir underneath the icy surface of Enceladus.

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