NASA's Mars Exploration Rover Spirit has been analyzing sulfur-rich rocks and surface materials in the "Columbia Hills" in Gusev Crater on Mars. This image shows rocky debris and dust, which planetary scientists call "regolith" or "soil," that has been churned up by the rover wheels. This 40-centimeter-wide (16-inch-wide) patch of churned-up dirt, nicknamed "Paso Robles," contains brighter patches measured to be high in sulfur by Spirit's alpha particle X-ray Spectrometer. Spirit's panoramic camera took this false-color image on martian day, or sol, 400 (Feb. 16, 2005), using filters at wavelengths of 750, 530, and 430 nanometers. Darker red hues in the image correspond to greater concentrations of oxidized soil and dust. Whiter and bluer hues correspond to sulfur-rich deposits that are not as heavily coated with soils or are not as highly oxidized.