Stretching along "Low Ridge" in front of the winter haven for NASA's Mars Exploration Rover Spirit are several continuous rock layers that make up the ridge. Some of these layers form fins that stick out from the other rocks in a way that suggests that they are resistant to erosion. Spirit is currently straddling one of these fin-like layers and can reach a small bit of light-toned material that might be a broken bit of it. Informally named "Halley," this rock was broken by Spirit's wheels when the rover drove over it.

Spirit's microscopic imager took this picture during the rover's 861st sol, or Martian day, of exploring Mars (June 5, 2006). The field of view is about 31 millimeters square (a square with sides of 1.2 inches). The light-toned soils in the bottom center and the top center of the image correspond to small, bright, bluish-white deposits just to the right of the rover's tracks in the lower left corner of an image from the panoramic camera (see PIA08567).

The first analyses of Halley showed it to be unusual in composition, containing a lot of the minor element zinc relative to the soil around it and having much of its iron tied up in the mineral hematite. When scientists again placed the scientific instruments on Spirit's robotic arm on a particularly bright-looking part of Halley, they found that the chemical composition of the bright spots was suggestive of a calcium sulfate mineral. Bright soils that Spirit has examined earlier in the mission contain iron sulfate.

This discovery raises new questions for the science team: Why is the sulfate mineralogy here different? Did Halley and the fin material form by water percolating through the layered rocks of Low Ridge? When did the chemical alteration of this rock occur? Spirit will continue to work on Halley and other light-toned materials along Low Ridge in the coming months to try to answer these questions.