After collecting dozens of samples of pinky-sized rocks over the course of 18 months on the surface of Mars, the Perseverance rover carries a message for planetary scientists: Your order is ready for pickup.
Next week, at a community workshop on Mars, mission managers will unveil a plan to deposit 10 or 11 titanium sample tubes on the floor of Jezero crater, which has housed a lake for billions of years. If NASA officials approve the plan, the craft could begin dropping samples as soon as November, to accumulate a cache that will play a key role in an ambitious plan to recover the first rocks from another planet. The Mars Sample Return (MSR) mission will use a small rocket to transport the rocks to an Earth-orbiting spacecraft that will transport them to a special facility on Earth by 2033. There, laboratory researchers can follow the rover’s baffling discovery that many of the samples contain organic matter. Molecules – the building blocks of life – and learn if they are made of living things.
The cache model is actually an MSR backup plan. Plan A is for the rover to store a collection of more than 30 samples in its belly while it continues to hunt the scientific treasure and deliver it to the return rocket around 2030. But if the rover malfunctions or fails along the way, the researchers don’t want to be left empty-handed. “I call it an insurance policy,” says Susan Schwenzer, a planetary mineralogist at the Open University and a member of the MSR Campaign’s science group. “Once we have the cache on Earth, we always know we have the option to receive it.”
For the rover team, the creation of the backup cache is a milestone that illustrates how MSR – the dream of Mars scientists for a generation – began to emerge together. “The fact that we got to this point is pretty amazing,” says Ken Farley, a rover mission scientist and geologist at Caltech. “It’s really real.” The bunker is also an inventory of rocks from the exploration of the 13-kilometer rover, extending from the bottom of the crater where it descended to the edge of a petrified river delta.
Some of it comes from lava flows, a surprising and welcome discovery for rover scientists who had been expecting to find mostly lake-bottom sediments on the crater floor. These igneous rocks contain radioactive elements such as uranium. Its decay provides a clock that laboratories on Earth can use to date the moment the rock crystallized. Some igneous rocks are thought to have been laid before the delta, and some may have come after, so they can provide time limits for the aquatic loop that created it.
The researchers also want to use lab instruments to detect ancient, frozen magnetic fields in certain volcanic minerals. Mars lacks a magnetic field today, but meteorites from the planet show traces of an ancient field. His loss would have allowed water molecules to escape into space, which would explain why Mars is so dry today. Determining the date of the magnetic field’s disappearance can support this theory, says Tanya Bosak, a geologist on the rover team at MIT.
Igneous rocks may even bear signs of ancient life. Perseverance has already found that some of them contain carbonates and sulfates – a sign that hot water was seeping through the rocks, leading to favorable reactions for early biochemistry. “There are water-rock interactions that would produce hydrogen and methane that could constitute a habitable environment,” says Catherine French, a USGS organic geochemist and member of the science group for the MSR campaign.
However, in the search for past life, petrified river deltas have always been the main attraction because of how the sediments retain telltale signs. Those could be chemical: organic molecules adsorbed onto the clay minerals in the mud. It could be physical: buried microbial fossils where silt particles stick together over time. “The cell is effectively sequestered away from processes that might degrade it,” Boussac says.
In April, the rover reached a 40-meter cliff at the edge of the delta. Last week, the rover team revealed that one of the drilling targets there, a fine-grained mudstone, contains the highest concentration of organic molecules the rover has ever seen — a class of ring-shaped molecules called aromatics.
Further scrutiny on Earth can show whether these molecules were made by living organisms. Researchers will want to see if it contains more of the light carbon isotopes favored by life, says Chris Heard, 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 clearer signs of ancient life: the solid fat particles that can form cell walls. “You’re hoping for a blueprint for a cell,” she says. “You’ll never find peptides and proteins, but fats can go on.”
Rover managers want to add a few more samples to their collection before dropping the backup cache. Next week, they plan to drill into a site called Enchanted Lake, which has the potential to provide the best-grained delta rocks ever. Shortly thereafter, the rover will collect a sample of wind deposited soil, which “incorporates” information from across Mars, says Katie Stack Morgan, deputy mission project scientist at NASA’s Jet Propulsion Laboratory. “We could get a really global sample of the fine dust circulating on Mars.” The team also wants the cache to include a tube that contains nothing but air, an important resource for those studying Mars’ atmosphere.
Once the rover team has completed its cache and NASA has approved the plan, a small arm under the rover will begin emptying the sample tubes. You won’t drop them in a pile. Instead, the rover will spend about two months depositing them one by one, several meters apart, in a flat area of the crater. “It’s like a pool table,” says Meenakshi Wadua, principal MSR 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 using a pair of autonomous helicopters, such as the one deployed last year, to collect individual samples and transfer them to the 3-meter-high rocket that will launch them into orbit. Farley says he’s not worried about finding the tubes. “We’ll know to a centimeter or so where they are.”
If the rover remains in good shape, of course, the backup cache may never hit the ground running. But psychologically, the cache will be an incentive to move forward with the rest of the expensive and risky MSR scheme and an incentive to ensure it works flawlessly. “When we put this cache in, it sends a message, that this is a set of samples that can be returned,” Bussac says.
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