Figure 1

A rock dubbed "Palenque" in the "Columbia Hills" of Mars has contrasting textures in upper and lower portions. This view of the rock combines two frames taken by the panoramic camera on NASA's Mars Exploration Rover Spirit during the rover's 278th martian day (Oct. 14, 2004). The layers meet each other at an angular unconformity that may mark a change in environmental conditions between the formation of the two portions of the rock. Scientists would have liked the rover to take a closer look, but Palenque is not on a north-tilted slope, which is the type of terrain needed to keep the rover's solar panels tilted toward the winter sun. The exposed portion of the rock is about 100 centimeters (39 inches) long.