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.
Another proposed mission, called LIFE (Life Investigation for Enceladus), is led by Peter Tsou, and uses the same technology he developed for NASA’s Stardust comet flyby mission. The probe would use aerogel, a synthetic ultralight material, to trap particles from the plume and bring it back to Earth for detailed analysis.
Both mission designs have their advantages and disadvantages. An in-situ mission can analyze samples right away, before they become altered during storage or a years-long journey back to Earth. A sample return mission has the advantage of allowing scientists to analyze samples in much more detail, using a wider variety of methods. But a sample return mission also raises planetary protection concerns if alien life is actually found in these plumes.
It would be great to see one of these proposals selected for NASA’s Discovery program of planetary missions. And maybe we could even find a way to investigate the enigmatic moon Titan if we’re already undertaking the long journey to Saturn.
This animation shows Cassini’s flyby through the plume of Enceladus last October.