What was the first rock analyzed by NASA's Sojourner rover on Mars?

What was the first rock analyzed by NASA’s Sojourner rover on Mars? Barnacle Bill. Sojourner examined the rock with its Alpha Proton X-ray Spectrometer during the Mars Pathfinder mission.

Barnacle Bill was a roughly 40-centimeter rock in Ares Vallis, near Pathfinder’s landing site. The encounter occurred on Sol 3, and the rover’s instrument took about 10 hours to complete its analysis. The spectrometer measured the abundance of elements in the rock, helping scientists assess its composition. Early results suggested that Barnacle Bill might be andesite, although interpreting Martian rocks from limited data is challenging.

The informal name came from the rock’s knobby, barnacle-like appearance in images. A common mix-up is Adirondack, which was the first rock examined by NASA’s Spirit rover in 2004, not Sojourner. Yogi was another Pathfinder-era rock that Sojourner studied later. Sojourner was only about the size of a microwave oven, but its successful deployment in 1997 demonstrated that a small wheeled rover could investigate the Martian surface.