Several times while rolling across Mars, the treads of the >robotic rover Spirit have serendipitously uncovered unusually bright soil.
Spirit uncovered another batch unexpectedly last month while >rolling toward its winter hibernation location on McCool Hill.
The physics and chemistry >instruments on Spirit have determined the soil, >shown above, contains a high content of salts including iron-bearing sulfates.
A leading hypothesis holds that these salts record the presence of past water, with the salts becoming concentrated as the water >evaporated.
