Several Athena instruments will measure or photograph the surfaces of Martian rocks to determine different aspects of their compositions. All of these tools closely approach the surface of a rock and measure only the surface; they cannot "see" inside the rock.
|Microscopic Imager with lens at left and frame for MI electronics at right.
Credit: NASA/JPL/Cornell/ USGS
But Martian rocks lie under a blanket of fine dust, and beneath the dust the rocks may include a weathered layer, like the layer of rust on an abandoned car. This weathered surface can differ from the rock underneath just as rust differs from the steel under it.
To reveal the real Martian rock beneath the dross, the Athena rovers will carry an instrument called the Rock Abrasion Tool, or RAT. The RAT scrapes away a few millimeters of a rock surface. The freshly exposed circle of surface is then ready for instruments such as the Microscopic Imager, which takes close-up photographs and the three spectrometers, which measure the chemical composition of the rock surface.
"Cutting into the rock beyond the weathered surface rind will reveal a flat cross section of fresh rock," says Stephen Gorevan, Co-Investigator for the Athena Science Payload. Gorevan is chairman of Honeybee Robotics, in New York, which designed and built the RAT.
The RAT works somewhat like a surface grinder, the sort a machinist might use to clean or shape a metal surface. But, Gorevan says, "the RAT must cut strong rock with much less force than a typical person would exert with a hand held grinder." "The RAT needed to be specially designed to execute low-force cutting."
The RAT rasps rock with two spinning diamond-studded epoxy plastic cutting tips mounted on a wheel, which is turned by a motor. (A second wheel holds a set of spinning brushes to wipe away dust and grit.) As the wheel holding the cutting tips rotates, the tips trace out a pair of spirals, eventually sweeping out a 45-millimeter (1.8-inch) circle. A separate motor moves the tips toward the rock surface. Because of its low-speed cutting action it takes two to four hours to cut the 45-by-5-millimeter (1.8-by-0.2-inch) depression the RAT will not heat the rock enough to cause any change in its chemical composition.
|The Instrument Deployment Device.
Once the rover's robotic arm, the Instrument Deployment Device, presses the RAT against a rock, the RAT takes over.
"The RAT is truly a robotic device," Gorevan says. "Encoders attached to each of the three RAT motors provide us with precise position information. The encoders are discs attached to the motor shaft; a particular shaft position gives a particular encoder signature that is readable by the sensors accompanying the encoder." Data from the encoders feeds directly to the Rover's central computer, which contains software to interpret the data.
"We can count revolutions and know exactly where the motor shaft rotations have taken us. The position feedback primarily tells us how deeply we have cut into the rock. The torque feedback (provided by monitoring how much current the grinder motor is drawing) is used by the grinding algorithm to cut through the rock according to how strong the rock is. One of the great challenges we face is not having any information at all as to how strong any Martian rock is. No previous mission provided any rock penetration."
"Testing, testing and more testing," Gorevan says. "This is what consumes us now and will be part of what we do even after we land; indeed even after the surface mission is over."
|A prototype of the Rock Abrasion Tool (the RAT) is put to the test.
Credit: NASA/JPL/Cornell/Honeybee Robotics
The team plans to test the RAT in wind tunnels at the NASA Ames Research Center. "Here we will simulate high Martian winds at Mars [atmospheric] pressure to see if the ground-up material transported by the thin Martian atmosphere poses any hazard to the Hazcams, which are located close to the RAT."
But the most important tests will challenge the RAT with rocks of varying types on Earth.
Each rock type has different hardness and brittleness. And the IDD might press the RAT against rocks in different ways in different situations, pressing harder in some situations and less firmly in others. The RAT readouts will provide a numeric signature of how the RAT behaved while cutting each type of rock. The team can then match known rock types with these signatures.
"One can imagine searching our rock library [on Earth] for rocks that match the feedback signature we saw on the mission," Gorevan says. This process takes the RAT to a new level. It becomes more than a service robot for other MER instruments; it can return its own data, data that the RAT team can use to analyze the material of the rock.