The "clearest evidence" yet that life once thrived on Mars has been discovered, Nasa has said.
Colourful spots on rocks were discovered last year by the Perseverance rover, the agency said.
The Nasa robot found the markings on an ancient dry riverbed within the Jezero Crater, just north of the Martian equator.
After a peer review in the journal Nature, researchers now believe it is "the closest we have come to discovering ancient life on Mars".
According to Nasa, the marks could have been 'excreted' by ancient microbes
|NASA
Having originally suspected that the spots were left behind by microbial life, scientists have spent the last year researching other ways they may have come to be.
Distinct patterns of minerals could be seen in images sent back by the rover.
Scientists call the clusters, which form from points of contact where chemical and physical reactions occur, "reaction fronts".
Two iron-rich minerals were detected in the spots, sparking significant interest from scientists.
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The Perseverance rover discovered the spots last year
|NASA
Vivianite is one of the minerals discovered, and is frequently found in sediments, peat bogs, and near to decaying organic matter.
The other was greigite, which on Earth is produced by microbial life.
According to Nasa, the marks, which resemble leopard spots, could have been "excreted" by ancient microbes which used the organic carbon, sulphur and phosphorus in the rock as an energy source.
Nasa's acting administrator Sean Duffy said: "A year ago, we thought we found signs of microbial life on the Mars surface and we put it out to our scientific friends to pressure test it. Did we get this right?
"After a year of review, they have come back and said: 'Listen, we can't find another explanation,' so this could be the clearest sign of life that we have ever found on Mars which is incredibly exciting."
Nasa describes the 45km-wide Jezero Crater, where the Perseverance is currently exploring and where the spots were found, as "a location on Mars that shows promising signs of a place that was likely friendly to life in the distant past".
The sample, which contains "potential biosignatures", was taken from a rock named "Cheyava Falls".
Scientists were particularly surprised by the discovery because it was found on some of the youngest sedimentary rocks the mission has investigated.
Previously, it had been believed that signs of ancient life on the Red Planet would be confined to older rock formations.