As NASA’s Curiosity rover treks up a three-mile-high mountain on Mars, the rocks are changing. That says something about how the planet’s climate and environment changed more than three billion years ago — but scientists are not sure what.

Since it landed more than three years ago in a 96-mile-wide depression known as Gale Crater, Curiosity has made a number of discoveries, notably that the crater once held lakes of fresh water. For most of that time, the rocks it encountered were generally basaltic, a volcanic composition typical on Mars.

“Now in the recent few months, that has changed,” Ashwin R. Vasavada, the project scientist for the mission, said at a news conference on Thursday at a meeting of the American Geophysical Union in San Francisco, where researchers were presenting some of their newest results.

They have surprising clues but no definitive story, yet.

The attraction of Gale Crater to planetary scientists is the mountain at the center. A space rock slamming into Mars created the crater about 3.6 billion years ago. It then filled with sediment, which was subsequently carved away by the wind, leaving behind the formation known as Mount Sharp.