A Learning Opportunity

Mars Rover Examines Odd Material at Small, Young Crater

This image from the Mars Reconnaissance Orbiter shows NASA’s Mars Exploration Rover, Opportunity, perched on the edge of "Concepción" crater in Meridiani Planum, Mars. Note rover tracks in ripples to the north and northwest of the rover.
Credit: NASA/JPL-Caltech/University of Arizona

Weird coatings on rocks beside a young Martian crater examined by NASA’s Opportunity rover are puzzling scientists.

The rover spent six weeks investigating the crater called “Concepcion”. The crater is about 10 meters (33 feet) in diameter. Dark rays extending from it, as seen from orbit, flagged it in advance as a target of interest because the rays suggest the crater is the result of a recent impact.

The rocks ejected outward from the impact that dug Concepcion are chunks of the same type of bedrock Opportunity has seen at hundreds of locations since landing on Mars in January 2004: soft, sulfate-rich sandstone holding harder peppercorn-size dark spheres like berries in a muffin. The little spheres, rich in iron, gained the nickname “blueberries.”

“It was clear from the images that Opportunity took on the approach to Concepcion that there was strange stuff on lots of the rocks near the crater,” said Steve Squyres of Cornell University, Ithaca, N.Y., principal investigator for Opportunity and its twin rover, Spirit. “There’s dark, grayish material coating faces of the rocks and filling fractures in them. At least part of it is composed of blueberries jammed together as close as you could pack them. We’ve never seen anything like this before.”

Opportunity used tools on its robotic arm to examine this unusual material on a rock called “Chocolate Hills.” In some places, the layer of closely packed spheres lies between thinner, smoother layers.

“It looks like a blueberry sandwich,” said Matt Golombek, a rover science-team member at NASA’s Jet Propulsion Laboratory, Pasadena, Calif.

Initial analysis of the coating’s composition does not show any obvious component from whatever space rock hit Mars to dig the crater, but that is not a surprise, Golombek said. “The impact is so fast, most of the impactor vaporizes,” he said. “Thin films of melt get thrown out, but typically the composition of the melt is the stuff that the impactor hit, rather than the impactor material.”

The composition Opportunity found for the dark coating material fits at least two hypotheses being evaluated, and possibly others. One is that the material resulted from partial melting of blueberry-containing sandstone from the energy of the impact. Another is that it formed from filling of fractures in this type of rock before the impact occurred.

This image from the microscopic imager on NASA’s Mars Exploration Rover Opportunity shows details of the coating on a rock called "Chocolate Hills," which the rover found and examined at the edge of a young crater called "Concepción." The view covers an area about 3 centimeters (1.2 inches) across.
Credit: NASA/JPL-Caltech/Cornell University

“It’s possible that when you melt this rock, the sandstone melts before the blueberries do, leaving intact blueberries as part of a melt layer,” Squyres said. “As an alternative, we know that this type of rock has fractures and that the sandstone can dissolve. Long ago, water flowing through fractures could have dissolved the sandstone and liberated blueberries that fell down into the fracture and packed together. In this hypothesis, the impact that excavated the crater did not play a role in forming this material, but split rocks along fractures so the material is exposed on the exterior like a coating.”

Golombek said, “One consideration that jumps out is that we’ve been driving around this part of Mars for six years and never seen this stuff before, then we get to this young crater and it’s coating rocks all around the crater. Sure looks like there’s a connection, but it could just be a coincidence.”

The observation that the rocks thrown from the crater have not yet eroded away much is evidence that the crater is young, confirming the suggestion from the dark rays. Squyres said, “We’re not ready to attach a number to it, but this is really young. It is the youngest crater we’ve ever seen with Opportunity and probably the youngest either rover has seen.”

One question Opportunity’s visit did answer was about the dark rays: “We wondered before getting to Concepcion why the rays are dark,” Golombek said. “We found out that the rays are areas with blocks of light-toned sandstone ejected from the crater. They look dark from orbit because of the shadows that the blocks are casting when the orbital images are taken in mid-afternoon.”

Since departing Concepcion on March 9, Opportunity has driven 614 meters (2,014 feet) farther along the route to its long-term destination at Endeavour Crater, which is about 19 kilometers (12 miles) in diameter.

Endeavour crater is at a drive distance of more than 12 kilometers (7 miles) away. Opportunity has driven more than 20 kilometers (12.5 miles) since first landing on Mars.

Squyres said, “We’re on the road again. We have a healthy rover and we have enough power for substantial drives. We want to get to Endeavour with a healthy rover. It takes a compelling target for us to stop and study. And Concepcion was a compelling target.”

Rover Getting Smarter as it Gets Older

NASA’s Mars Exploration Rover Opportunity took this image in preparation for the first autonomous selection of an observation target by a spacecraft on Mars.
Credit: NASA/JPL-Caltech

While scientists continue to keep an eye out for interesting targets, Opportunity now has the capability to make its own choices about where to look on arrival at a new location.

Software uploaded this winter allow Opportunity’s computer to examine images that the rover takes with its wide-angle navigation camera, and recognize rocks that meet specified criteria, such as rounded shape or light color. It can then center its narrower-angle panoramic camera on the chosen target and take multiple images through color filters.

“It’s a way to get some bonus science,” said Tara Estlin of JPL. She is a rover driver, a senior member of JPL’s Artificial Intelligence Group and leader of development for this new software system.

The new system is called Autonomous Exploration for Gathering Increased Science, or AEGIS. Without it, follow-up observations depend on first transmitting the post-drive navigation camera images to Earth for ground operators to check for targets of interest to examine on a later day. Because of time and data-volume constraints, the rover team may opt to drive the rover again before potential targets are identified or before examining targets that aren’t highest priority.

The first images taken by a Mars rover choosing its own target show a rock about the size of a football, tan in color and layered in texture. It appears to be one of the rocks tossed outward onto the surface when an impact dug a nearby crater. Opportunity pointed its panoramic camera at this unnamed rock after analyzing a wider-angle photo taken by the rover’s navigation camera at the end of a drive on March 4. Opportunity decided that this particular rock, out of more than 50 in the navigation camera photo, best met the criteria that researchers had set for a target of interest: large and dark.

NASA’s twin robot geologists, the Mars Exploration Rovers, launched toward Mars on June 10 and July 7, 2003, in search of answers about the history of water on Mars. They landed on Mars January 3 and January 24 of 2004, and continue to make important scientific discoveries.
Credit: NASA

“It found exactly the target we would want it to find,” Estlin said. “This checkout went just as we had planned, thanks to many people’s work, but it’s still amazing to see Opportunity performing a new autonomous activity after more than six years on Mars.”

Opportunity can use the new software at stopping points along a single day’s drive or at the end of the day’s drive. This enables it to identify and examine targets of interest that might otherwise be missed.

“We spent years developing this capability on research rovers in the Mars Yard here at JPL,” said Estlin. “Six years ago, we never expected that we would get a chance to use it on Opportunity.”

The developers anticipate that the software will be useful for narrower field-of-view instruments on future rovers.

Other upgrades to software on Opportunity and its twin, Spirit, since the rovers’ first year on Mars have improved other capabilities. These include choosing a route around obstacles and calculating how far to reach out a rover’s arm to touch a rock. In 2007, both rovers gained the know-how to examine sets of sky images to determine which ones show clouds or dust devils, and then to transmit only the selected images. The newest software upload takes that a step further, enabling Opportunity to make decisions about acquiring new observations.