The discovery of what appears to be a clay-rich rock on Mars adds to the portrait of the planet as one that once — in its youth, more than three and a half billion years ago — was a water-rich world with conditions amenable for life, NASA scientists said Friday.

The rock in question, about the size of a person’s forearm, was examined by the Mars rover Opportunity, the older of the spacecraft still in operation on the planet. The newer rover, the Curosity, landed last August and has been hogging the headlines ever since; in a news conference on Friday, NASA officials proudly called attention to the fact that the Opportunity, launched nearly a decade ago, was still soldiering on with valuable field work.

The newly discovered rock, which scientists named Esperance, is one of the oldest rocks that the Opportunity has looked at during its nine and a half years on Mars. From the abundances of elements like aluminum, calcium and magnesium, the mission’s scientists concluded that the rock is very rich in clay minerals, which could have formed from copious water running over volcanic rocks.

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