The Mars Opportunity rover
San Francisco—Their aging robot has an arthritic arm and some myopic sensors, minor infirmities that only heightened the boyish enthusiasm of two veteran scientists as they announced on Wednesday that the Mars Opportunity rover was still going strong—and had discovered a rock on the Martian surface apparently created by liquid, moving water.
“I think this is the single most bulletproof observation we’ve made with this rover about liquid water,” said Steven W. Squyres, a professor of astronomy at Cornell University and lead investigator of the mission, at the American Geophysical Union meeting here. “Both the chemistry and the mineralogy both scream water.”
The word they are hollering is “gypsum,” which appears to be what the rock is made from, added his colleague Raymond E. Arvidson. “As groundwater comes up, the first thing to come out of would be gypsum,” said Arvidson, Opportunity’s deputy chief scientist and a professor of planetary science at Washington University in St. Louis.
Water on Mars, or evidence there was once water on Mars, is an ongoing quest for researchers because water means life, at least as far as we understand life. An article on a NASA Mars exploration web site begins “After Earth, Mars is the planet with the most hospitable climate in the solar system. So hospitable that it may once have harbored primitive, bacteria-like life.” But that theory depends on a warm, wet past with lakes and rivers. There’s nothing like that on cold Mars today. There’s some evidence of frozen ice. And the Mars Phoenix Lander kicked up a fuss in 2008 when it appeared to kick up water droplets on its landing struts, though that interpretation is still strongly disputed. So this thin finger of gypsum, a mineral on Earth that’s used for drywall and caps the blinding dunes of White Sands National Monument in New Mexico, beckons aluringly in a watery, life-sustaining direction.
The "Homestake" vein of rock found by Mars Opportunity
Opportunity, the robot that landed on Mars in 2004, has been inching along a crater called Endeavor. It found this unusual vein, 16 to 20 inches long and a fingers-width wide, and set about figuring out what it was made of. Spectrometers and multi-filter cameras, instruments that analyze light and x-rays reflecting off an object, were deployed. Different minerals have different reflection patterns. This one indicated calcium and sulfur, in a ratio typically seen in gypsum, a form that’s heavily saturated with water molecules.
How did it get there? “Long ago,” speculated Squyres, “an impact hit this basalt crater. The energy from that impact may have produced hydrothermal activity” deep in the planet. “Later—a long time later—groundwater percolated up through the surface, and calcium and sulfur precipitated out.” Carried by the water up through these surface cracks, those minerals came together to form gypsum.
Gypsum signatures have been seen elsewhere on the planet, but never this pure and always at sites that lent themselves to non-watery explanations for its creation, such as particles blown together and compacted by wind. This vein, which the scientists have named Homestake, seems to be a narrow fracture where water once bubbled up.
The scientists plan to move Opportunity to the northern end of the crater to angle its solar panels towards the sun and maintain power as the rover prepares to weather the Martian winter. In the Martian spring, more vein prospecting is in order, but exactly what the robot will do depends on what it sees. “We’re going to do discovery-based science,” Squyres said, with a rover that has crawled 20 miles since it landed and is still prepared to be what the scientist called “a field geologist on Mars.”
(Images courtesy NASA – JPL)
