This graph compares amounts of magnesium and sulfur in the soil lining the trenches dug by the Mars Exploration Rover Spirit on sols 114 and 140 (April 28 and May 25, 2004) at Gusev Crater. Measurements were taken of the soil at the surface, floor and walls of the trench dug on sol 140 (squares), and at the surface and floor of the trench dug on sol 114 (diamonds). Non-trenched soil samples from Gusev Crater are represented as dots. The more recently made trench is located near the base of the "Columbia Hills."
Because concentrations of magnesium and sulfur occur in the same ratio throughout the trench dug on sol 140, scientists believe the soil there contains the salt magnesium sulfate. The walls of this trench appear to contain the highest concentrations of the salt. The trench from sol 114 may also possess magnesium sulfate, but the data is less clear. These data were taken by Spirit's alpha particle X-ray spectrometer.
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.