For decades, Mars scientists have been clamoring for a chance to bring back a few choice samples from the red planet. But on such a diverse planet, where would one go? And with any return capsule capped at about half a kilogram of precious cargo, which scientific objectives should be prioritized?
Advocates now have a short list of seven nominal targets and, more importantly, a scientific framework for evaluating their worth. “We know what samples we want to get, we know how to get them, and we know why they’re important,” says Scott McLennan of Stony Brook University in New York, who presented the result of a large international effort to come to such a consensus on Wednesday at a conference of the American Geophysical Union in San Francisco, California.
The group created a clear hierarchy of four types of samples to gather: first, rocks either deposited or altered in water; second, unaltered igneous rocks; third, samples from the soil; and last, atmospheric samples. The group also said a scientific ‘sweet spot’ would be reached if 30–35 samples, of roughly 15 grams apiece, were gathered. After the samples are returned to Earth, 40% of them should be saved for future scientific analysis, in the hopes that instrument technologies will be continuously improved, the group recommended.
These recommendations stemmed from the group’s dutiful attempt to list and rank scientific objectives. Tops on the list was a desire to “critically assess any evidence for past life or its chemical precursors”. Although the team recognized that a search for life on Mars today was important — all samples would certainly be tested for life — the group did not include that as a factor in targeting a sample-return mission. There would be no way of knowing for sure where to go to test for life on Mars, says McLennan. “That’s a huge science risk,” he says.
The group took a brief look at 85 sites that have been proposed for past missions, and whittled them down to seven. Among the sites on that list were Gusev crater, where the rover Spirit landed; Mawarth Vallis, one of the runners-up in the site selection process for the Curiosity rover; and an intriguing site, Nili Fossae, where not only carbonates have been spotted, but also minerals that produce methane.
McLennan notes that this is just the opening salvo in a process that will be open to other site proposals. Moreover, it’s unclear whether a sample-return mission will get started. A 2018 joint mission between NASA and the European Space Agency, which is supposed to send a caching rover as the first stage in a three-pronged mission, has yet to get budgetary approval.
Image credit (artist’s impression): NASA/JPL-Caltech