WASHINGTON, D.C.--The Opportunity rover has found strong evidence that eons ago water flowed across Mars, pooled on the surface, and then returned to the atmosphere, leaving behind a thick layer of minerals leached from the land. "We've found our ancient water on Mars," says rover science team member Harry McSween of the University of Tennessee, Knoxville.
At a press conference at NASA headquarters here today, rover principal investigator Steven Squyres of Cornell University ticked off the evidence for a soaking wet Mars he and his team had developed from Opportunity observations of the previous weeks. The new findings don't prove that Mars ever sustained life or even that Mars was ever as warm and wet as Earth has been. However, they do mean that at least in one patch the size of Oklahoma, water soaked the rock for age upon age, which is all that life could ask for.
Part of the evidence involves the structure of the outcrop. The round balls 2 or 3 millimeters in diameter seen weathering out of the rock, they decided, resemble "concretions" that grow as mineral-laden water lays down salts in the rock pores. Also indicative are vugs, "very weird-looking, tabular holes" that riddle the rock, in Squyres's words. They are probably the voids left by now vanished crystals of gypsum--to judge by their shape--that grew from mineral-laden water.
The mineralogy is telling, too. The rover's spectrometer found high concentrations of jarosite, an iron-rich mineral "you have to have water around to make," said Squyres. Another sign of water was chemical. The rover's alpha particle x-ray spectrometer found "an enormous quantity of sulfur in this rock," said Squyres. "It must be full of sulfate, a telltale sign of liquid water." The rover's minithermal emission spectrometer also saw abundant signs of sulfate in the infrared colors of the rock, much of it probably magnesium sulfate, the familiar epsom salts. "This is an astonishing amount of salt," said Benton Clark, a science team member at Lockheed Martin in Denver. On Earth, such an abundance of salts most often forms when water carrying dissolved material pools and then evaporates.
All this salty water on early Mars tells team members that water soaked the subsurface or, more likely, the bottom of a lake or ocean. "We may never know which," said Squyres, "but we're going to give it our best shot."
Background and news about the Mars rovers