After collecting a dozen rock samples the size of a pinkie during its 18 months on Mars, the Perseverance rover has a message for planetary scientists: your order is ready for pickup.
At a Mars community workshop next week, mission managers will unveil a plan to deposit 10 or 11 of the titanium sample tubes at the bottom of Jezero Crater, which contained a lake billions of years ago. If NASA officials endorse the plan, the rover could begin dropping the samples as early as November, assembling a cache that will play a key role in an ambitious plan to retrieve the first rocks from another planet. The Mars Sample Return (MSR) mission would use a small rocket to deliver rocks to an orbiting spacecraft that would take them to a special facility on Earth by 2033. There, lab researchers could follow up on the rover’s tempting finding that many samples contain organic compounds. molecules – the building blocks of life – and learn whether they are made by living things.
The sample cache is actually MSR’s backup plan. Plan A is for the rover to store a larger set of 30 monsters in its belly as it continues its scientific treasure hunt and deliver it to the return rocket around 2030. But if the rover gets stuck or fails along the way, researchers don’t want to be left empty-handed. “Call it an insurance policy,” says Susanne Schwenzer, a planetary mineralogist at the Open University and member of the MSR campaign science group. “Once we have that cache on the ground, we know we’ll always have the opportunity to retrieve it.”
For the rover team, setting up the backup cache is a milestone that shows how MSR — a dream of Mars scientists for a generation — is beginning to come together. “The fact that we’ve reached this point is pretty amazing,” said Ken Farley, the rover mission’s project scientist and a geologist at the California Institute of Technology. “It really gets real.” The cache is also an inventory of rocks from the rover’s eight-mile exploration, stretching from the crater floor where it landed to the edge of a fossilized river delta.
Some come from lava flows, a surprising and welcome discovery for rover scientists who expected to find mostly sediments from the bottom of the crater floor. These igneous rocks contain radioactive elements such as uranium. Their decay provides a clock that Earth-based labs can use to date when the rocks crystallized. Some of the volcanic rocks would have been deposited before the delta, and some may have come after, so they can provide time limits on the watery episode it caused.
Researchers also want to use lab instruments to detect ancient magnetic fields frozen in certain volcanic minerals. Mars today has no magnetic field, but meteorites from the planet show traces of an ancient field. Its loss could have allowed water molecules to escape into space, which explains why Mars is so dry today. Dating when the magnetic field disappeared could bolster that theory, says Tanja Bosak, a geobiologist on the rover team at the Massachusetts Institute of Technology.
The volcanic rocks may even contain signs of ancient life. Perseverance has already found that some contain carbonates and sulfates — a sign that hot water once seeped through the rocks, triggering reactions beneficial for early biochemistry. “There are water-rock interactions that would produce hydrogen and methane that could form a habitable environment,” said Katherine French, an organic geochemist with the US Geological Survey and member of the MSR campaign science group.
However, in the search for past life, the fossilized river delta has always been the main attraction because of the way sediments can preserve telltale signs. These can be chemical: organic molecules adsorbed to clay minerals in the mud. They can even be physical: Microbial fossils buried as silt particles were cemented together over time. “The cell is effectively shut off from the processes that would affect it,” Bosak says.
In April, the rover arrived at the 40-meter-high cliff on the edge of the delta. Last week, the rover team revealed that one of the drilling targets there, a fine-grained mud rock, contains the highest concentration of organic molecules the rover has ever seen — a class of ring-shaped molecules called aromatics.
Further research on Earth could reveal whether living things made those molecules. Researchers will want to see if they contain more of the light isotopes of carbon that life prefers, said Chris Herd, a planetary geologist on the rover team at the University of Alberta, Edmonton. “We’re really looking for evidence of metabolism.” Bosak wants to find even clearer signs of ancient life: the tough lipid molecules that can form cell walls. “You hope for a sketch of a cell,” she says. “You will never find peptides and proteins, but lipids can persist.”
Rover executives want to add a few more samples to their collection before dropping the backup cache. Next week they plan to drill at a location called Enchanted Lake, which has the potential to deliver the finest granular delta rock of all. Soon after, the rover will collect a sample of wind-deposited soil, which “integrates” information from all over Mars, said Katie Stack Morgan, the mission’s deputy project scientist at NASA’s Jet Propulsion Laboratory. “We could get a truly global sample of the fine-grained dust circulating on Mars.” The team also wants the cache to contain a tube containing only air, an important resource for those studying the Martian atmosphere.
Once the rover team has completed its cache and NASA has approved the plan, a small arm under the rover will begin to discharge the sample tubes. It won’t make them fall in a heap. Instead, the rover will spend about 2 months depositing them one at a time, several meters apart, in a flat part of the crater. “It’s like a billiard table,” said Meenakshi Wadhwa, MSR’s chief scientist at Arizona State University, Tempe. “It’s as good as it gets when it comes to a place to land a monster retrieval mission.”
Current plans call for a pair of autonomous helicopters, like the one from Perseverance last year, to collect individual samples and deliver them to the 3-foot-tall rocket that will launch them into orbit. Farley says he’s not worried about finding the tubes. “We’ll know to within an inch or so where they are.”
Of course, if the rover stays healthy, the backup cache will never reach Earth. But psychologically, the cache will be an incentive to continue with the rest of the expensive, risky MSR scheme and an incentive to make sure it runs flawlessly. “When we put that cache, we’ll send a message,” Bosak says, “that this is a set of samples that can be returned.”