The Mars Science Laboratory Curiosity is on its way to the red planet, scheduled for a landing in August. In the meantime, the Opportunity rover, which has been operating for nearly eight years, is still sending back scientific results. Its latest, announced at the annual meeting of the American Geophysical Union, provides a clear indication that water once flowed through underground fissures, giving us a better picture of Mars' geological history.

The findings are based on a vein of material that's picked up the name "Homestake," found near the rim of the Endeavour Crater. The material is only a couple centimeters across, but about 50cm long. Readings with a spectrometer indicated it was a form of calcium sulfate, and that it contained significant amounts of water. Gypsum crystals are formed when calcium sulfate associates with water; they tend to dissolver readily, but Mars' dry climate allows them to be stable. The material has been spotted elsewhere on Mars, in the form of sand dunes in the northern polar region.

Gypsum is formed when calcium-containing water comes in contact with sulfates, either from rocks or via volcanic activity. The calcium sulfate precipitates, capturing water molecules within the crystal structure in the process. The gypsum vein clearly indicates that water was flowing through through cracks below the surface of Mars at some point in its past; the rover's scientific lead, Cornell's Steve Squyres, calls it a "slam-dunk story."

By now, the evidence for liquid water on Mars' surface in its past is so extensive that we can probably treat it as a fact. But, so far at least, we haven't found a way to determine whether that water ever supported living creatures. Still, the Homestake find is another indication that Mars' waters were present in a variety of environments, both on the surface and percolating beneath it.

(Incidentally, many of you may be familiar with a common form of gypsum: drywall. If we ever want to put a housing development on Mars, we maybe able to locally source some of the material.)