After collecting a dozen finger-sized rock samples during its 18 months on Mars, the Perseverance rover has a message for planetary scientists: your order is ready to be picked up.
Next week, at a community workshop on Mars, mission leaders will unveil a plan to lay 10 or 11 of the titanium sample tubes on the floor of Jezero Crater, which housed a lake billions of years ago. years. If NASA officials approve the plan, the rover could begin depositing 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 ferry rocks to an orbiting spacecraft that would deliver them to a special facility on Earth by 2033. There, lab researchers could track the rover’s tantalizing discovery that many samples contain organic molecules – the building blocks of life – and find out if 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 rocket back to around 2030. But if the rover gets stuck or fails in 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 this cache on the ground, we know that we always have the possibility of recovering it.”
For the rover team, establishing the backup cache is a milestone that shows how MSR, a dream of Mars scientists for a generation, is beginning to materialize. “The fact that we’ve reached this point is pretty amazing,” says Ken Farley, rover mission project scientist and geologist at the California Institute of Technology. “It’s getting really real.” The cache is also a rock inventory of the rover’s 13 kilometers of exploration, stretching from the floor of the crater where it landed to the edge of a fossilized river delta.
Some are from lava flows, a surprising and welcome finding for rover scientists who expected to find mostly lakebed sediments at the bottom of the crater. These igneous rocks contain radioactive elements like uranium. Their disintegration provides a clock that terrestrial laboratories can use to date when rocks crystallized. Some of the volcanic rocks are thought to have been deposited before the delta, and others may have come after, so they could provide temporal boundaries for the aquatic episode that created it.
The researchers also want to use laboratory tools to detect ancient magnetic fields frozen in certain volcanic minerals. Mars has no magnetic field today, but the planet’s meteorites show traces of an ancient field. Its loss could have allowed water molecules to escape into space, explaining why Mars is so dry today. Dating the disappearance of the magnetic field could strengthen this theory, says Tanja Bosak, geobiologist on the rover team at the Massachusetts Institute of Technology.
The volcanic rocks could even contain signs of ancient life. Perseverance has already discovered that some contain carbonates and sulfates, a sign that hot water once seeped through rocks, causing reactions favorable to early biochemistry. “There are water-rock interactions that would produce hydrogen and methane that could form a habitable environment,” says Katherine French, organic geochemist at the US 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 the sediments could hold telltale signs. These could be chemical: organic molecules adsorbed on clay minerals in the sludge. They could even be physical: microbial fossils buried as silt particles cemented together over time. “The cell is effectively isolated from the processes that would degrade it,” Bosak explains.
In April, the rover arrived at the 40-meter-high cliff at the edge of the delta. Last week, the rover team revealed that one of the drill targets there, a fine-grained mudstone, contained the highest concentration of organic molecules the rover had ever seen – a class of molecules shaped rings called aromatics.
Further examination on Earth could show if living things made these molecules. Researchers will want to see if they contain more light carbon isotopes than life prefers, says Chris Herd, a planetary geologist with the rover team at the University of Alberta, Edmonton. “We are 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’re hoping for a glimpse of a cell,” she said. “You will never find peptides and proteins, but lipids may persist.”
Rover managers want to add a few more samples to their collection before deleting the backup cache. Next week they plan to drill at a site called Enchanted Lake, which has the potential to provide the finest delta rock of all. Shortly after, the rover will collect a wind-deposited soil sample, which “integrates” information from across Mars, says Katie Stack Morgan, the mission’s assistant 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 include a tube containing only air, an important resource for those studying the Martian atmosphere.
Once the rover team completes its cache and NASA approves the plan, a small arm under the rover will begin unloading the sample tubes. He’s not going to drop them in a heap. 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 pool table,” says Meenakshi Wadhwa, MSR senior scientist at Arizona State University, Tempe. “It’s as good as it gets in terms of where to land a sample retrieval mission.”
Current plans call for a pair of autonomous helicopters, like the one deployed by Perseverance last year, to collect individual samples and transport them to the 3-meter-tall rocket that will launch them into orbit. Farley says he’s not worried about finding the tubes. “We’ll know to within a centimeter where they are.”
If the rover stays healthy, of course, the backup cache may never make it to Earth. But psychologically, the cache will be an incentive to continue with the rest of the expensive and risky MSR program and an incentive to make sure it works perfectly. “When we place this cache, it sends a message,” Bosak says, “that this is a set of logged samples.”
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