The Mars rovers go ditch digging and find sparkling spheres

NASA/JPL

Above: A view from the front hazard avoidance camera of NASA's Spirit rover on its 47th Martian day, or sol, shows a trench excavated by the rover's left front wheel within the "Laguna Hollow" area. The trench, dubbed "Road Cut," is 7 centimeters (3 inches) deep. Below: An image, taken by the microscopic imager on the rover Opportunity's robotic instrument arm reveals shiny, spherical objects embedded within the trench wall at the rover's landing site on Meridiani Planum. The area in the image measures approximately 3 centimeters (1.2 inches) across.

NASA/JPL/USGS

By David Brand

PASADENA, Calif. -- It was time for ditch digging by the two rover vehicles on Mars last week, using technology developed by engineers at Cornell and NASA's Jet Propulsion Laboratory (JPL). First Opportunity then Spirit spun their wheels back and forth into the Martian soil and then used the high-tech instruments on their robotic arms for the remote study of the soil's stratigraphy and an analysis of whether water once existed.

The strangest discovery was made by Opportunity on Feb. 19 in a crater in a vast plain, the Meridiani Planum, where it landed Jan. 25. Perched on a 9 degree downward slope in the crater, the rover used its right front wheel to dig a trench 20 inches long and found, embedded in its walls, spherical, sparkling pebbles, the size of BBs. The rover's instruments previously had found the shiny spheres, dubbed "blueberries" by mission scientists, scattered over the surface.

"We don't know what their pedigree is," said rover science team leader Steven Squyres, Cornell professor of astronomy. But, he said, he expects the spherical granules to be common over most of the Martian surface.

The next day, Feb. 20, it was the rover Spirit's turn in the Gusev crater on the other side of Mars, where it landed Jan. 3. The rover had traveled into a depression dubbed Laguna Hollow, where it carved out a somewhat smaller trench. Because the ground at this location is harder than on Meridiani Planum, Spirit had to dig twice as long as Opportunity, going back and forth over the surface 11 times instead of 6.

Any one of the rovers' set of six wheels can be spun backward or forward -- while the other five are locked in place -- to dig a trench to examine the geology of the rocks and soils beneath the surface. For controllers at JPL the process involved complicated maneuvers -- a "rover ballet," according to Rob Sullivan, the Cornell researcher and member of the Mars mission science team who helped develop the trenching mechanism with Harry Stewart, Cornell associate professor of civil engineering (aided by Cornell undergraduates Lindsey Brock and Craig Weinstein) and JPL engineers. Indeed, controllers had to send Opportunity 66 "mobility commands" for its trenching.

At a press conference at JPL Feb. 17, Sullivan described the eight black-and-white images taken by Opportunity's hazard identification cameras, showing "clotty" soil along the upper walls of the trench and "contrasts in brightness between brighter floor material and material outside of the trench on the plain." Said Sullivan: "We're not sure of the cause of that. It could be the material itself is intrinsically brighter than the stuff seen on the surface." Or, he said, "it could be that we have discovered something that is intrinsically different down on the floor of this trench than we have seen so far on the surface."

To find out more about the glassy pebbles, Opportunity embarked last weekend on a 15-meter (50-foot) trip -- its longest journey yet -- to what Squyres called "a blueberry patch," a "wonderful finely layered laminated bedrock" dubbed El Capitan, where the rover had seen the pebbles embedded on a recent cruise past. The exposed bedrock has a gently sloping lower portion and a much steeper upper portion, overhanging in places. The aim is, Squyres said, to "zero in on a place where there are lots of those spherical granules and look in detail with the microscopic imager."

On Monday, Feb. 23, Opportunity took measurements of El Capitan, with its varying textures and layers of dirt and rock, using the front hazard avoidance camera, miniature thermal emission spectrometer (Mini-TES) and microscopic imager. Science team members then found the perfect spot to use the rover's rock abrasion tool (RAT) to grind a small hole into an area of the outcrop, dubbed Opportunity Ledge. It was Opportunity's first use of the RAT, which was designed at Cornell and at Honeybee Robotics in New York City by a team that included alumnus Paul Bartlett.

Other plans called for placing the rover's alpha-particle X-ray spectrometer up against a rock to see if it is sulfur-rich all the way through. The rover's Mössbauer spectrometer also is being used to identify any minerals present.

There are, Squyres told a JPL press briefing on Feb. 19, basically three questions that Opportunity is trying to answer. First, how were these materials deposited -- by air or by water? Second, where did the spherical pebbles embedded in the outcrop come from -- were they dropped in from above, or did they grow in place? Third, he said, what is the composition of the outcrop and "what does it tell us or not tell us about the possibility that water was involved in forming the minerals that are in this outcrop?"

It took Spirit two hours to dig a trench -- dubbed "Road Cut" by mission engineers -- in Laguna Hollow with its left front wheel, uncovering fresh soil and possibly ancient information. After the trench was completed, Spirit backed up about a yard and then analyzed the area with its Mini-TES before driving forward and imaging the excavation site with a panoramic camera.

Spirit is on its way to a crater nicknamed "Bonneville." The first driving session on Feb. 16 covered 19 meters (62.3 feet). After a rest, Spirit continued another 8.5 meters (27.9 feet) in the afternoon, resulting in a total drive of 27.5 meters (90.2 feet), a new one-day Martian driving record. The remaining distance to Bonneville is about 245 meters (about 800 feet) from Laguna Hollow.

Both rovers, it seems, have an abundance of time to carry out their investigations. Squyres noted that both vehicles' power and thermals systems "are holding up spectacularly well." And although the mission originally was expected to last about 90 Martian days, "I will say that I have talked to people at my apartment complex about staying through the summer," he said.