SAN FRANCISCO – While attention has been focused on the Mars rover Curiosity, NASA's other active Mars rover, Opportunity, has quietly been going about its business and may have stumbled across an intriguing new geologic puzzle. Opportunity has begun examining ancient clays on Mars that would have formed in the presence of water with neutral acidity, a condition favorable for life as we know it.

"This is our first glimpse ever at an ancient Mars where conditions would be suitable for life," said astronomer Steve Squyres of Cornell University, the lead scientist for Opportunity's mission, here at the American Geophysical Union conference on Dec 4.

Previous minerals that Opportunity and its defunct twin Spirit had found thus far on Mars would have required extremely harsh conditions to form, something akin to battery acid, and not very conducive to life. The clays that the rover is now exploring would have been created billions of years earlier in water with a neutral pH. "These clays point to water you could drink," said Squyres.

Since it landed on Mars in 2004, Opportunity has roved more than 22 miles. After nine years on Mars, the rover is getting a bit old, with a squeaky right front wheel and a creaky arm joint, though is overall in good condition. Last year, the probe arrived at Endeavor crater, where it discovered the most unambiguous evidence for water on ancient Mars ever found.

Scientists directed Opportunity to Endeavor crater to study the clays, which had been spotted earlier from orbit. Roving around one of the half-buried rims of the crater, Opportunity found a small hill that Squyres described as the "sweet spot" where clays are known to be present. The rover took photos showing two types of rocks.

The dominant material is light-toned, flat-lying, and easily eroded by Martian wind. This is the rock that contains the clays and shows a chemistry typical of Martian material. But Opportunity also found a fin sticking up through the dominant rocks made of another material that is dark, gray, and more resistant to erosion.

"What we have stumbled upon is what is turning out to be one of the most delightful geologic puzzles we have ever found on Mars," said Squyres.

When they drove up to the fin, the science team found it covered in a dense concentration of spherical bubbles quite similar looking to the iron-containing Martian "blueberries" that Opportunity has seen before. But when they looked at the chemical composition of these spheres, scientists found they contained no iron.

"It's something totally different, and I've started calling them 'newberries'," said Squyres.

The team doesn't yet know what these newberries are made of and will spend the next few months investigating them. They could be mineral concretions, impact ejecta spherules, or volcanic hailstones. Squyres said that researchers will also examine the clays to determine what conditions on early Mars would have been like. He said many questions remained and Opportunity had a lot of work ahead of it.

"It's like we've been exploring Mars for nine years, and now Mars has given us a final exam," said Squyres.