SAN FRANCISCO — While many obsessed over speculation that NASA’s newest Mars rover, Curiosity, had dug up signs of life — it had not — it is the agency’s older, smaller jalopy, Opportunity, that has been exploring a more intriguing plot of Martian real estate.

“This is our first glimpse ever at conditions on ancient Mars that clearly show us a chemistry that would have been suitable for life,” Steven W. Squyres, the principal investigator for Opportunity, said at a news conference last week at a meeting of the American Geophysical Union here.

Opportunity could be sitting on rocks chock-full of organic molecules — but the rover and the scientists back on Earth would never know. Unlike Curiosity, Opportunity is not carrying instruments that can detect those kinds of molecules.

But the scientists are not complaining. Everything from Opportunity over the past eight years has been a bonus for a mission that was to have ended long ago.