NASA’s Jet Propulsion Laboratory just announced some early findings from the rover’s first three attempts at drilling rock cores. After an unsuccessful first attempt due to crumbly rock, Perseverance drilled its first successful core sample, dubbed “Montdenier,” on Sept. 6, and took its second, “Montagnac,” from the same rock called “Rochette” on Sept 8. The results from these samplings support what NASA already hoped and suspected, but needed more hard evidence to prove: the idea that Jezero Crater rocks had been exposed to water for a long time.
‘Little atomic clocks’
The samples were of crystalline igneous rock, meaning rock that solidified into crystals from hot lava or magma. The rocks also show signs that they were chemically changed: weathered and laced with added salt minerals, likely left when nearby water evaporated. When Perseverance ground away the surface of the rock, the team could tell that it had been oxidized, judging by the difference in color between the outer surface and the freshly exposed patch. “That rock … in so many ways, is absolutely ideal and for it to be the first sample that we collected is really quite incredible,” says David Shuster, a geochemist at the University of California at Berkeley and a participating scientist with NASA’s sample return team. He says the team has been hoping to sample this region—in the floor of Jezero Crater—for over a year. The fact that these first samples appear to be igneous rock is helpful for dating the region, says Jack Mustard, a planetary geologist at Brown University and chair of the science definition team for Perseverance, which outlined what goals the mission should prioritize. That’s because as rock cools from liquid lava or magma, it locks in “little atomic clocks” as Mustard calls them—or radioactive isotopes trapped inside Martian minerals. “If you can then open the mineral up and read the clock, you can tell how old it is,” he says. “Crystalline, igneous minerals are particularly good at that.”
Calibrating for Martian craters
It’s also possible that the salts in Perseverance’s cores will contain tiny bubbles of ancient Mars water, which would hint at Mars’ ancient climate, according to the announcement. Studying the details of such “water bearing minerals” in the lab will give scientists a better understanding of how long water was present in Jezero Crater and reveal “whether it was rainfall, a standing body of water, [or] groundwater,” Mustard says. Using core samples to date Martian features is critical, Shuster says, because currently, researchers don’t have a solid timeline. Scientists estimate the age of the Martian surface by gauging the number of impact craters—the older the surface, the more craters pile up. But the estimate for how many craters appear in a given time is based on samples returned from the moon, which is much less massive and has a different orbit. Getting that “calibration” done for Mars could drastically change all our current estimates for ages. Ideally, the rover will sample a wide range of rock ages to get the best baseline.
Delta deliberations
With this end in mind, choosing Perseverance’s landing site was a tough decision. It was a close choice between Jezero Crater and a highland, “upriver” region nearby called Nili Planum, Mustard says. On the one hand, if you’re looking for life, you should look where “it’s clearest that there was liquid water,” Shuster says. And for that, Jezero Crater’s delta seemed like the best pick around. A delta is formed “when a river carrying lots of sediment comes into a big lake,” Mustard says, and “the sediment drops out because the current loses velocity and it can’t hold the sediment anymore.” River water on Earth is full of living organisms, and when they die, they too sink to the bottom. But the signs of these once-living things “don’t last long unless you find some way to put them in a coffin,” Mustard says. If Martian organisms lived and died in rivers, delta sediments could land on top of their corpses and bury them—protecting them from decay, and perhaps letting them be found millennia later by a wandering rover. But Jezero Crater does have a similar environment to other sites that have been explored, like Gale Crater, where NASA’s Curiosity rover landed, Mustard says. And outside of looking for life, the area is less scientifically interesting.
Upriver opportunities
But Mustard favored Nili Planum, which could give a glimpse at a “totally unexplored type of terrain”: a window to the subsurface of Mars. While Jezero may be the best bet for finding life that lived on Mars, Nili Planum is a better bet for finding life if it ever lived under the Martian surface, which Mustard thinks is a more likely place for life to survive given the brutal radiation, temperature swings, and other hassles that come with living on the exposed surface of a planet. The upriver Nili Planum has large chunks of rock called breccia blocks, some as big as houses, which were formed when powerful meteorite or asteroid impacts ejected chunks of the Martian subsurface. Even if there aren’t ancient remains of life, Nili Planum could tell scientists more about the subsurface in general, Mustard says. The committee debated and debated, but eventually decided to land in Jezero Crater, because it made for a more straightforward and unique target in the mission’s limited time.
Red planet reconciliation
Even though Jezero won, there’s hope yet for peace between the Jezero junkies and Nili nuts. It’s possible that when Perseverance completes its mission in Jezero Crater, the rover will be able to explore a region closer to Nili Planum in an extended mission, Mustard says. But that’s still a ways out, and for now the rover will next scout out the South Séítah region, a rough area of terrain covered in rocky sand dunes about 200 meters away. Perseverance will keep collecting samples and drop them in caches, to be picked up and returned by a future European Space Agency mission, hopefully in the early 2030s. Shuster is excited to see the mission’s astrobiology focus, but not quite for the reasons most would expect. While some may presume that the mission is looking for signs of life, “I don’t actually think of it that way,” Shuster says. “I think we’re really looking for signs of conditions that could have supported life.” The difference may seem subtle. But if scientists find habitable conditions, Shuster says, and even after looking thoroughly, find no signs of life, it will beg the question: Why not? Why didn’t life appear somewhere that could have supported it? Which raises bigger questions—like, how rare is life in the universe? To the question of whether or not there was life on a habitable Mars, Shuster says, “all the answers are interesting.”
