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In June, 2024, Perseverance, a NASA rover sent to collect samples on the surface of Mars, came upon a cluster of rocks in what is thought to be a former riverbed. Most of the rocks were identified as mudstones—they likely formed from the sediment in slow-moving water—meaning they would be perfect vessels for any traces of aquatic life in the area. After a monthlong, systematic geological survey, scientists took a special interest in an arrowhead-shaped stone slab dubbed Cheyava Falls. The rover drilled a sample of it, which researchers called Sapphire Canyon, for an eventual return to Earth. (Confusingly, the names are borrowed from Grand Canyon National Park and do not reflect the geography or the scale of the Martian specimens; the red planet’s Cheyava Falls is two feet across, and its Sapphire Canyon could fit in a tube of lipstick.)
The discovery may go down in history. Perseverance determined that the whole area around Cheyava Falls is rich in oxidized iron, phosphorus, sulfur, and organic carbon—a combination that microbes could potentially feed on. Colorful spots on Cheyava Falls contain the mineral greigite, which some microbes on Earth excrete, and vivianite, which is often found around decaying organic matter. Producing such minerals in a lifeless place would probably require acidic conditions or high temperatures—and the area showed signs of neither. Together, these findings are a “potential biosignature,” Katie Stack Morgan, Perseverance’s project scientist, said at a press conference on Wednesday. This means that they are more likely to be the result of biology than the result of something else. Scientists published their findings this week in the journal Nature. Sean Duffy, the interim administrator of NASA, called them “the clearest sign of life that we’ve ever found on Mars.”
Billions of years ago, as life was emerging on Earth, Mars is thought to have fostered a wide, shimmering ocean, as well as rivers and deltas that might have flooded when it rained. If biology was possible on Earth, then it was possible on the ancient surface of Mars. The red planet eventually lost most of its atmosphere, presumably wiping out whatever might have flourished on it, but there could still be traces, even fossils.
Life on other planets has been “discovered” before. At the turn of the twentieth century, Percival Lowell, an American astronomer, spent years mapping artificial canals that he believed had been built on Mars. Other astronomers spent decades challenging his interpretation. The issue wasn’t settled until 1965, when NASA’s Mariner 4 spacecraft flew past Mars and saw no canals. Even after that, many scientists thought that Mars harbored life. The Martian surface darkened during certain parts of the year, giving rise to theories that plants grew there. Carl Sagan, who said that extraordinary claims required extraordinary evidence, hypothesized that the dark patches were caused by windstorms, not flora, but even he hadn’t abandoned the possibility that Martian life forms existed. In the planning stages of the two-part Viking mission, which landed spacecraft on Mars in the mid-seventies, Sagan argued that the probes should include lights and cameras, in case creatures scurried past.
In the end, the Viking landers found no creatures, and biological experiments proved inconclusive. Then, in the nineties, NASA scientists studied a Martian meteorite discovered in the Allan Hills of Antarctica. It contained strange blobs and wormlike structures, which the scientists interpreted as evidence of fossilized bacteria. President Bill Clinton gave a speech to mark what was potentially “one of the most stunning insights into our universe that science has ever uncovered.” But when other scientists reëvaluated the meteorite, they came up with several explanations that did not require the existence of aliens. Inorganic crystals could have caused the wormy features; the types of chemical reactions that produce limestone could have caused the blobs.
There was potential evidence for extraterrestrial life in our solar system, but it didn’t reach the threshold of proof. “We have a bunch of bridges built halfway, from various lines of evidence,” Kirby Runyon, a research scientist at the Planetary Science Institute, headquartered in Tucson, told me. In 2020, astronomers asserted that they’d found phosphine gas in Venus’s atmosphere, and that life could have produced it. (On Earth, bacteria produce phosphine, but so do chemical reactions involving phosphorus.) Some scholars countered that Venusian volcanoes could have produced phosphine; others said that the measurements were dubious, and the mystery gas wasn’t phosphine.
Runyon described the authors of the recent Nature paper as appropriately cautious: they offered many caveats and didn’t jump to conclusions. If an identical rock were found on Earth, he said, we would assume it had a biological origin. “The geochemistry is very reminiscent of life,” he told me. But claims of life on Mars are extraordinary, and verifying them requires extraordinary evidence. “The skeptical posture says we’re just running up against how far rocks and geochemistry can go to look like life—but not be life. And that reveals the extent to which we must be cautious in interpreting our scientific results.”
There could be a way to prove that the Cheyava Falls rock contains signs of life: by studying it more closely than Perseverance is able to do. “If this is the most compelling potential biosignature on Mars, and it seems to be, logic dictates that NASA should go back with more missions, or bring that sample home for analysis,” Runyon said. Unfortunately, NASA is currently facing its own extinction-level event: the Trump Administration has recommended a budget that cuts the agency’s over-all federal funding by nearly a quarter, and essentially halves its spending on its science program. The proposal would also cancel the mission to return the samples to Earth. Duffy, a Trump appointee, seemed pleased during Wednesday’s announcement, but he is part of an Administration that would leave the bridge half built.
The authors of the Cheyava Falls paper spent a year in the peer-review process, and during that time their discovery was publicly known. Did NASA headquarters seize this moment to publicize its findings in hopes of resurrecting the sample-return mission? “The announcement was more earnest than calculated, I believe,” Casey Dreier, the chief of space policy at the Planetary Society, a space-exploration-advocacy group that is headed by Bill Nye, told me. “But it only raises the issues of the President’s self-contradictory and self-sabotaging budget.” Trump’s proposed budget would cancel forty-one science missions and slash Perseverance funding by twenty-three per cent. There’s no money to be saved on the building, launching, and landing of Perseverance—these things have already happened—so “the only dial you can turn to achieve that is by doing less science,” Dreier said. The budget, which could take effect on October 1st if Congress does not pass an appropriations bill, would also effectively disable two healthy spacecraft that are orbiting Mars: MAVEN and Mars Odyssey, both of which Perseverance uses to send communications back to Earth. (Early this year, Trump vowed to land humans on Mars, but his proposed budget invests very little in that effort.)
Methodical science could perhaps be accused of constraining our collective imagination. We no longer dream of discovering moon bats, Venusian dinosaurs, and Martian beavers, as scientists and sci-fi writers of old once did. Yet NASA is arguably within reach of something even more wondrous: the truth about life on another planet. In Dreier’s view, that would seem to call for more science, not less. “NASA just found potential signatures of life, and the official plan is to walk away from it,” Dreier said. Still, he seemed hopeful that Duffy and the rest of the Trump Administration might change course. “This is the exciting part of NASA,” he told me. “Discoveries like this are why we do this, and highlight what we could be giving up. I hope some people get inspired.” ♦
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David W. Brown
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