Yogi and Soujourner - Sol 15

Caption NASA originale:"The Soujourner Rover is taking its Alpha Proton X-Ray Spectrometer measurement.
It clearly shows the "two-toned" surface of this large rock.
The nature of this color difference is not known, however.
It might consist of wind-blown dust accumulated on the surface (the rock is leaning into the prevailing wind) or might be evidence of a break from a larger boulder as it was deposited in the ancient flood that scoured this area".