The accepted view of Mars is red rocks and craters as far as the eye can see. That’s much what scientists expected when they landed the rover Perseverance in the Jezero Crater, a spot chosen partly for the crater’s history as a lake and as part of a rich river system, back when Mars had liquid water, air and a magnetic field. What the rover found once on the ground was startling: Rather than the expected sedimentary rocks — washed in by rivers and accumulated on the lake bottom — many of the rocks are volcanic in nature. Specifically, they are composed of large grains of olivine, the muddier less-gemlike version of peridot that tints so many of Hawaii’s beaches dark green.