Life, especially the microbes too small to see, has left an indelible mark on rocks on Earth, creating minerals that would otherwise not be there.
If some of those minerals turn up in rocks on Mars, is that not good evidence that life once existed there, too?
Maybe.
NASA’s Perseverance robotic rover has discovered such minerals in one particular rock on Mars. It came across the specimen in an outcrop of hardened mud along what was once a flowing river.
“I think it’s very intriguing,” said Joel Hurowitz, a professor of geosciences at Stony Brook University on Long Island and a member of the Perseverance science team.
He said that he had worked on NASA Mars missions for more than two decades and that this was the most confident he had been of potential biosignatures in a Martian rock.
“These observations, to me, are really compelling,” he said.
Dr. Hurowitz and his colleagues describe the findings in a paper published on Wednesday in the journal Nature. NASA is also holding a news conference on Wednesday to discuss the research.
A year ago, NASA scientists were already giddy about this rock, because it possessed features that were reminiscent of what microbes could have left behind when this area was warm and wet several billion years ago, part of an ancient river delta.
Jezero Crater, the landing site for Perseverance, was once a lake, and the sediments along the river that flowed into the crater were a promising place to look for signs of past life on Mars. However, Perseverance found few indications of it until this part of the riverbed, one of the rover’s last stops before it exited the crater.
The Nature paper describes in much greater detail the mineralogy of the rock, which the scientists named Cheyava Falls, and of the nearby terrain.
“Last summer was kind of a, ‘Hey, we think we found something really interesting and cool,’” Dr. Hurowitz said. “Now we’re at the point where we’re actually saying in detail, ‘Here is what we have found.’”
What attracted the most attention were what the scientists called poppy seeds — dark specks a few thousandths of an inch wide within the mudstone. The specks contain vivianite, an iron phosphate mineral.
The scientists also observed larger features nicknamed leopard spots — dark rings containing vivianite surrounding off-white centers that included greigite, an iron sulfide mineral.
“The leopard spots kind of look like they maybe started out as poppy seeds but were able to continue growing to a larger size,” Dr. Hurowitz said.
On Earth, vivianite and greigite often form in sedimentary deposits in freshwater lakes, river estuaries and marshlands.
“These would be examples of microbially influenced environments where the microbes are consuming the organic matter and making these minerals as a byproduct,” Dr. Hurowitz said.
The minerals can also be produced by chemical reactions not involving living organisms. But in laboratory experiments, the greigite reactions require heating to more than 250 degrees Fahrenheit, and, so far, it appears that the minerals on Mars formed at cooler temperatures.
“Within the sort of limitations of our rover payload capabilities, they don’t look like they’ve been cooked,” Dr. Hurowitz said.
The rock also contains organic compounds — molecules with carbon and hydrogen — which adds more intrigue. Organic compounds are the building blocks for life, but those, too, can be produced by geological processes that have nothing to do with life.
Janice Bishop, a senior research scientist at the SETI Institute in Mountain View, Calif., said the results were “super exciting” in showing that Mars once possessed habitable conditions.
But with the lack of evidence of life on Mars, past or present, nonbiological explanations were more likely, said Dr. Bishop, who was an author of an accompanying commentary on the analysis in Nature.
Scientists looking for alien life often cite a Carl Sagan quote, “Extraordinary claims require extraordinary evidence.” So far, the evidence in the Cheyava Falls rock is intriguing, not extraordinary.
Perseverance drilled a sample of the mudstone, part of a collection of rocks and dirt, to be brought back on Earth for closer study. More sophisticated instruments in labs here that could not be packed into a rover would be able to study, with more precision, the various elements in the rocks. Those tools may help scientists determine if the characteristics, like the mix of isotopes, match what would be expected if they were the remnants of Martian microbes.
Dr. Hurowitz said he would be willing to make a small bet that there had been life on Mars and that Cheyava Falls contained convincing evidence of that.
“It’s better than a coin flip,” he said. “If you say, ‘Let’s put 20 bucks on whether these rocks turn out to have biology in them, and we’ll figure that out when the samples come back,’ I’ll take that bet.”
However, the rocks and the answers they hold may remain stranded on Mars indefinitely. Last year, the Mars Sample Return mission, which is to bring back the samples that Perseverance collected, was put on hold as cost estimates spiraled upward to $11 billion. NASA then solicited proposals for alternative possibilities.
In its budget request this year, the Trump administration sought to cancel the Mars Sample Return mission entirely.
Kenneth Chang, a science reporter at The Times, covers NASA and the solar system, and research closer to Earth.
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