Ten years ago, NASA’s Opportunity rover bounded to the surface of Mars for what was planned to be a three-month exploration.

Opportunity is still going today — and still making discoveries.

The latest, scientists said on Thursday at a news conference celebrating an anniversary none had expected 10 years ago, is a small rock that seemingly appeared out of nowhere.

The rock, whose chemical composition was also unexpected, appears in an image taken Jan. 8. There was no rock in a picture taken of the same spot less than two weeks earlier.

“This is strange,” said Steven W. Squyres, the principal investigator for Opportunity, during the NASA news conference. But he added, “We don’t think anything particularly exotic happened here.”