Future mission concepts
Enceladus has captivated us and given us more than enough reasons to go back. Many possible missions would do the job, and a few have been proposed in the post-Cassini era, although not yet selected by NASA to proceed.
Some would do as Cassini did — fly through the plume and analyze the gas and grains — but with upgraded instruments capable of much more sensitive and effective tests for life. Others would land on Enceladus’ south polar terrain, sampling fresh snow deposited onto the surface from the plume.
Even more ambitious concepts include a sample return mission (although with a round-trip time of 14 years, we would have to wait awhile to get that sample) or various climbing or melting robots to descend the 1.2 to 6.2 miles (2 to 10 km) through the ice shell and reach the ocean itself.
Whatever we send, the next mission to Enceladus — if indeed astrobiology is its main objective — will need a well-designed suite of instruments capable of searching for multiple, independent lines of evidence for life. Our understanding of life’s characteristics has advanced greatly since the Viking era, the last time NASA openly stated the search for life as the primary goal.
Back when the two Viking landers touched down on Mars in 1976, for example, we knew only two of the three branches of life. (Archaea, the third and most primitive branch of the tree of life, was discovered in 1977.) The Viking landers had three biological experiments designed to search for life in the martian regolith. One test result was positive, one was negative, and one was ambiguous. Since then, we have learned a great deal about how to design experiments such that an ambiguous result is much less likely.
We are also getting better at searching for biosignatures that are as agnostic to Earth life as possible. For example, a future mission to Enceladus might not target DNA, which is Earth-life-specific, but it might look for a molecule that could serve the same function for alien life: a large molecule with repeating subunits (akin to an alphabet) capable of storing information, such as the blueprints to build an alien cell. If such a molecule is detected, along with positive identification of multiple other biosignatures, a strong case could be made for the first detection in human history of life on another world.
Active, accessible, and relevant
Enceladus is not the only place that could host life. Europa has an even larger liquid water reservoir, and Titan’s ocean may entertain an unimaginably rich organic chemistry.
But Enceladus is the one place where researchers know for certain that they can access material from the ocean without the need to dig or drill (or even land). We can use technology available right now to test the hypothesis of whether life may be present somewhere else in the solar system.
Enceladus may be a tiny moon, but good things often come in small packages. The time is now to answer the key question that has driven us since we first looked up: Are we alone?