They stressed that in-depth analysis is needed of the sample gathered there by Perseverance — ideally in labs on Earth — before reaching any conclusions.

Roaming Mars since 2021, the rover cannot directly detect life.

Instead, it carries a drill to penetrate rocks and tubes to hold the samples gathered from places judged most suitable for hosting life billions of years ago.

The samples are awaiting retrieval to Earth — an ambitious plan that’s on hold as NASA seeks cheaper, quicker options.

Calling it an “exciting discovery,” a pair of scientists who were not involved in the study — SETI Institute’s Janice Bishop and the University of Massachusetts Amherst’s Mario Parente — were quick to point out that non-biological processes could be responsible.

“That’s part of the reason why we can’t go so far as to say, ‘A-ha, this is proof positive of life,’’’ lead researcher Joel Hurowitz of Stony Brook University told The Associated Press.

“All we can say is one of the possible explanations is microbial life, but there could be other ways to make this set of features that we see.”

Either way, Hurowitz said it’s the best, most compelling candidate yet in the rover’s search for potential signs of long-ago life.

It was the 25th sample gathered; the tally is now up to 30, with six more to go.

“It would be amazing to be able to demonstrate conclusively that these features were formed by something that was alive on another planet billions of years ago, right?” Hurowitz said.

But even if that’s not the case, it’s “a valuable lesson in all of the ways that nature can conspire to fool us.”

Collected last summer, the sample is from reddish, clay-rich mudstones in Neretva Vallis, a river channel that once carried water into Jezero Crater.

This outcrop of sedimentary rock, known as the Bright Angel formation, was surveyed by Perseverance’s science instruments before the drill came out.

Along with organic carbon, a building block of life, Hurowitz and his team found minuscule specks, dubbed poppy seeds and leopard spots, that were enriched with iron phosphate and iron sulfide.

On Earth, these chemical compounds are the byproducts when microorganisms chomp down on organic matter.

The findings appeared in the journal Nature.

Ten of the titanium sample tubes were placed on the Martian surface a few years ago as a backup to the rest aboard the rover, the main target in NASA’s still fuzzy return mission.

When Perseverance launched in 2020, NASA expected the samples back on Earth by the early 2030s.

But that date slipped into the 2040s as costs swelled to $US11 billion ($16 billion) stalling the retrieval effort.

Until the samples are transported off of Mars by robotic spacecraft or astronauts, scientists will have to rely on Earthly stand-ins and lab experiments to evaluate the feasibility of ancient Martian life, according to Hurowitz.

On Earth, microorganisms commonly interact with minerals in Antarctic lakes.

“There is no evidence of microbes on Mars today, but if any had been present on ancient Mars, they too might have reduced sulfate minerals to form sulfides in such a lake at Jezero Crater,” Bishop and Parente wrote in an accompanying editorial.