Barnacle Bill is a 40-centimetre (16 in) rock on Mars in Ares Vallis.

It was the first rock on Mars analyzed by the Sojourner rover using its Alpha Proton X-ray Spectrometer.

Early analysis of data sent from Sojourner led scientists to speculate that the rock was andesite.

The name was inspired in mission scientists by barnacle-like structures on the rock that appeared in transmitted photos.

This article about an impact crater on Mars or its moons is a stub.