After collecting a dozen pinkie-size rock samples over its 18 months on Mars, the Perseverance rover has a message for planetary scientists: Your order is ready for pickup.
Next week, at a Mars community workshop, mission managers will reveal a plan to deposit 10 or 11 of the titanium sample tubes on the floor of Jezero crater, which held a lake billions of years ago. If NASA officials endorse the plan, the rover could begin to drop the samples as soon 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 ferry rocks to an orbiting spacecraft that would deliver them to a special facility on Earth by 2033. There, laboratory researchers could follow up on the rover’s tantalizing finding that many samples contain organic molecules—the building blocks of life—and learn whether they were made by living things.
The sample cache is actually MSR’s backup plan. Plan A is for the rover to stow a larger set of 30 samples in its belly as it continues its scientific treasure hunt and deliver them 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 a member of the MSR campaign science group. “Once we have that cache on the ground we know we always have the option to pick it up.”
For the rover team, establishing the backup cache is a milestone that shows how MSR—a dream of Mars scientists for a generation—is starting to come together. “The fact that we have reached this point is pretty amazing,” says Ken Farley, the rover mission’s project scientist and a geologist at the California Institute of Technology. “It’s really getting real.” The cache is also an inventory of rocks from the rover’s 13 kilometers of exploration, extending 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 were expecting to find mostly lakebed sediments on 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 the moment when the rocks crystallized. Some of the volcanic rocks are thought to have been laid down before the delta, and some may have come after, so they could provide time bounds on the watery episode that created it.
Researchers also want to use lab tools to detect ancient magnetic fields frozen into certain volcanic minerals. Mars lacks a magnetic field today, but meteorites from the planet show traces of an ancient field. Its loss could have allowed water molecules to escape to space, explaining 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 might even hold signs of ancient life. Perseverance has already found that some contain carbonates and sulfates—a sign that hot water once percolated through the rocks, driving reactions favorable for early biochemistry. “There are water-rock interactions that would produce hydrogen and methane that could form a habitable environment,” says Katherine French, an organic geochemist at the U.S. Geological Survey and member of the MSR campaign science group.
In the quest for past life, however, the fossilized river delta has always been the main attraction because of how sediments might preserve telltale signs. Those could be chemical: organic molecules adsorbed on clay minerals in the muds. They could even be physical: microbial fossils entombed as silt particles got cemented together over time. “The cell effectively gets sealed away from the processes that would degrade it,” Bosak says.
In April, the rover arrived at the 40-meter-tall cliff at the delta’s edge. Last week, the rover team revealed that one of the drilling targets there, a fine-grained mudstone, contained the highest concentration of organic molecules the rover has ever seen—a class of ring-shaped molecules called aromatics.
Further scrutiny on Earth could show whether living things made those molecules. Researchers will want to see whether they contain more of the light isotopes of carbon that life prefers, says 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 an outline of a cell,” she says. “You will never find peptides and proteins, but lipids can persist.”
Rover managers want to add a few more samples to their collection before they drop the backup cache. Next week, they plan to drill at a site called Enchanted Lake, which has the potential to provide the finest grained delta rock of all. Soon after that, the rover will collect a sample of wind-deposited soil, which “integrates” information from across all of Mars, says Katie Stack Morgan, the mission’s deputy project scientist at NASA’s Jet Propulsion Laboratory. “We could be getting a truly global sample of the fine-grained dust that circulates on Mars.” The team also wants the cache to include a tube containing nothing but air, an important resource for those who study 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’s not going to drop them in a pile. Instead, the rover will spend about 2 months depositing them one by one, several meters apart, in a flat area of the crater. “It’s like a billiards table,” says Meenakshi Wadhwa, MSR principal scientist at Arizona State University, Tempe. “It’s as good as it gets in terms of a place to land a sample retrieval mission.”
Current plans call for a pair of autonomous helicopters, like the one Perseverance deployed last year, to collect individual samples and carry them to the 3-meter-tall rocket that will launch them into orbit. Farley says he’s not worried about finding the tubes. “We will know to within a centimeter or so where they are.”
If the rover remains healthy, of course, the backup cache may never make it to Earth. But psychologically, the cache will be a spur to proceed with the rest of the expensive, risky MSR scheme and an incentive to ensure it works flawlessly. “When we place that cache, that’s sending a message,” Bosak says, “that this is a returnable set of samples.”
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