This image taken by the panoramic camera on the Mars Exploration Rover Spirit shows a trench dug by the rover on its way toward the "Columbia Hills." Measurements taken of the soil contained in the trench by Spirit's alpha particle X-ray spectrometer showed the presence of sulfur and magnesium. Concentrations of those two elements varied in parallel at different locations in the trench, suggesting that they may be paired as a magnesium-sulfate salt. One possible explanation for these findings is that water percolated through underground material and dissolved out minerals, then as the water evaporated near the surface, it left concentrated salts behind.