Mars Rover Snaps Fine-Grained Materials

The first close-up photos of the rock outcropping discovered by NASA’s Opportunity rover rule out the possibility that the layered Martian rock was created by volcanic lava flows. The photos also suggest that it is unlikely there was once a large body of water at the Meridiani Planum site.

But the photos revealed a new mystery: small, spherical grains of an apparently different material embedded within the layers of stone.

“The deeper we get into this, the more it is reminding me of a mystery novel,” said principal investigator Steve Squyres of Cornell University. “We start getting clues, one at a time. Some of them mean something and some are probably red herrings, and we don’t know which is which yet.”

The photographs of the outcropping -- which the rover team named Opportunity Ledge -- show layers of very fine-grained materials, about one- to two-tenths of an inch thick. That’s much too thin for lava flows.

The team at NASA’s Jet Propulsion Laboratory in Pasadena has narrowed the possibilities for the rock’s formation down to two, Squyres said.

“One is that it is volcanic ash, and the other is that it is windblown dust compacted into sedimentary rock,” he said. “Because it is so fine-grained, it is not sandstone,” which is formed by minerals in standing water.

“It’s been sitting there for very long periods of time being sandblasted,” he said. The photographs even reveal some of those grains trapped in cracks and depressions.

What is really intriguing, however, are the spherical grains, “embedded in it like blueberries in a muffin,” he said. “These are pretty tough. As the rock is eroded away [by sandblasting], these spherical grains are dropping out.”

The grains are very dark gray, much different from the material that forms the layers. The team has two hypotheses for their origin: They were formed when molten rock -- from volcanoes or meteor impacts -- was sprayed into the air and froze into spherical shapes, or they were formed when mineral-laden water, possibly from underground springs, diffused through the rock. The mineral could precipitate out to form the spheres.

Geologists should be able to choose between the two possibilities after they examine the spheres with the rover’s instruments; but first the rover is going to perform a maneuver called “scoot and shoot.”

Opportunity, which landed Jan. 24, will move a short distance parallel to the outcropping, stop and take more photographs, and then continue the process until it has traversed the entire 50-foot length. The team will then choose the most promising sites to study at length.

As Opportunity climbed the slight grade to reach the outcropping, its added elevation enabled it to see over the rim of the small crater in which it landed.

The first photos gave engineers another surprise. About 450 yards away on the flat plain of Meridiani Planum, the lander’s parachute and heat shield are clearly visible.

“There is the hardware that we littered the surface with,” said imaging specialist Michael Malin of Malin Space Science Systems.

Malin said that his team had been able to use the camera on the Mars Odyssey orbiter to take a photo of the landing site that also showed the parachute and heat shield, as well as the lander sitting inside the crater. “I think we can even see the rover, but we’ll have to wait until it is moved to be sure,” he said.

Halfway around Mars, the Spirit rover, which landed Jan. 3, completed its work on the rock called Adirondack and began its trek across Gusev Crater toward a large crater nicknamed Bonneville.

Over the weekend, the rover used its rock abrasion tool to drill a small hole about a tenth of an inch deep into the surface of Adirondack, then used its Mossbauer and X-ray spectrometers to examine the freshly exposed surface. “It opened up a window into the interior that we can use to understand this rock very well,” Squyres said.

“There is compelling compositional evidence that it is volcanic in origin,” he said. “We know what it is. It is time to move on.”