In a larger view, the Opportunity mission is about understanding the mineral hematite and how that fits into the martian water history. Gray hematite covers an estimated 15 to 20 percent of the surface in the vicinity of the total Meridiani landing site. It appears as a dark cap layer atop a brighter layer that is exposed at many places within the large ellipse-shaped landing target.

The site is within an even larger region that has been known as Meridiani since the earliest days of telescopic study of Mars because it lies near the planet's arbitrarily designated prime meridian, or line of zero longitude.

But the microscopic view is not about mineralogy yet; it is about shapes and sizes. So as scientists consider how to identify their elephant, another proverb comes to mind because the tools at hand to investigate shapes include a rodent-like instrument not tried before on another planet. The proverb for Sol 11 might refer instead to the tiny mouse that scared the elephant.

The analogy might go something like this: When scientists imagined what kinds of dusty rocks they might encounter, they decided to include an instrument which became known as the RAT. The RAT works on medium-sized rocks--using an instrument on the robotic turret that combines a diamond drill (the Rock Abrasion Tool, or RAT) usually with a microscopic imager.

Eventually it may be the RAT that scares the Meridiani elephant.

In combinations of RAT experiments with the microscope, one could use this fresh surface and imager together to take pictures of the interior of martian rocks in great detail, enough detail at least to determine the interior rock grain shape. If that interior grain was round, the landform might be sedimentary and if shard-like or glassy, the landform might be volcanic or geothermal.

Driving up to a rock, scraping it clean if dusty, and extending an arm-like geology tools are the major roving activities for science opportunities. Credit: NASA/JPL/Cornell University

But on Sol 11, at Meridiani, the first microscope pictures of the crater floor showed round pebbles which Squyres referred to as 'the little round guys'.

Potentially thousands of little round guys were strewn all over the soil, even without exposing any grains interior to a rock face.

It might have required some speculative mission luck to plan for a pebble too small to RAT but showing so much detail under the microscopic imager. The science team won't try to RAT these spheres, but they will use their instrument turret and robotic arm to squish a few of them.

How to Squish a Little Round Guy? Squyres said that if they find a large concentration of the 'round guys' in one soil picture, they are equipped to use a face plate especially designed to 'go nose down' into the soil and test these pebbles' fragility. When they tried this move on the Spirit site, the soil barely moved.

But judging by how easily the Opportunity site changed to a much brighter red color when the rover's airbags squished some of these tiny pebbles at Meridiani, the answer of 'how to squish a round guy' likely is addressable using the robotic arm.

"This is a 5-degrees-of-freedom [rotating] robotic arm," said Squyres. "It is a very difficult [mechanical] maneuver to pick one of these pebbles out, move 2 mm to one side, another 2-3 mm to another, and center the microscope on one. But there are thousands of these, so an alternative strategy will be to find another place with alot of them, and then press down on a few."

"If there are broken ones, we can image those," continued Squyres, noting the scientific interest in discovering what might be interior to one pebble.

"There are [high-temperature] processes that form spheres like glass in air," referring to what volcanic or geothermal debris might have contributed to the Meridiani landscape. "There are sedimentary processes that would build up a sphere layer by layer."

The science team, with some positioning expertise and luck, hopes to find a large vein of pebbles, squish some, and take close-up views of the resulting broken examples. Alternatively they may see scores of ones already broken. "Dust particles that are very fine can blow into the crater, but round pebbles can roll. These little round guys might have weathered off the bedrock, or something, and rolled."

The Microscopic Imager is not really a classic laboratory model, but resembles a geologist's field lens or a gem-cutter's eyepiece. It is a combination of a microscope and a camera. Compared to any instrument ever previously deployed on another planet, it produces extreme closeup black-and-white views (at a scale of hundreds of microns) of rocks and soils examined by other instruments on the rover arm, providing context for the interpretation of data about minerals and elements.

The imager helps to characterize any sedimentary rocks that formed in water, and thus helps scientists understand past watery environments on Mars. This instrument also yields information on the small-scale features of rocks formed by volcanic and impact activity as well as tiny veins of minerals. The shape and size of particles in the martian soil can also be determined by the instrument, which provides valuable clues about how the soil formed.

To evaluate soil properties and expose fresh layers, the wheels of the rover may be used to dig shallow trenches. Such a trenching maneuver is a highlight of plans for the next several Sols while inside the Meridiani crater. The rover will drive about 3 meters, dig a hole about 20 centimeters (one half-wheel diameter), and then photograph the layers below the surface. Any exposed layers will be uncovered by this 'trenching'.

The actual maneuver itself takes only a few hours to complete. Usually the front right or left wheel of the six-wheeled rovers can be turned independently while the others remained locked and keep the rover from backing up. Eventually when one wheel has traction and finishes digging in, the rover will indeed pull back and photograph its fresh sand trench.

After this trenching, the rover will drive another 3 meters and begin to examine the bedrock outcrop, the science team's real elephant. After taking several hundred high-resolution pictures of that outcrop, the team will likely take a pause to replot their next high-value targets on the outcrop or look out beyond the crater for later driving destinations.

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