As the Mars rover Spirit recently trudged up a rocky edifice known as Husband Hill, the vehicle’s wheels churned up material that drew the immediate attention of scientists. The soil appeared unusually bright, and researchers directed Spirit to take a closer look.

The rover’s survey camera and three spectrometers, which study the composition of materials at wavelengths ranging from gamma rays to the near infrared, revealed that the bright soil patch is laden with a greater amount of chemical salts than any other sample examined on Mars. Salts typically suggest the presence of water.

The patch’s main ingredient appears to be iron sulfate with water molecules bound inside, says Steve Squyres of Cornell University, principal scientist for the rover missions. The soil also contains an abundance of phosphorus, his team announced March 2.

Researchers are still puzzling over the sample, but Squyres says its high salt content indicates that water has flowed abundantly through the region, leaving behind the salt when it evaporated.

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