The rock at Kimberley, a science stop for the Mars rover Curiosity enroute to Mount Sharp, is sandstone. Sandstone is usually formed in a two-step process.
First, one or more layers of sand-grain-size rock particles accumulate as a result of sedimentation, a process where the grains settle out of a liquid and onto the bottom of a stream or lake. Then, these layers are compacted by the pressure of overlaying deposits, and the grains are cemented together by precipitation of minerals in the pore spaces between the grains.
Over time, wind or water erosion can remove the upper layers and expose the sandstone at the surface. This is what we find at Kimberley. The Curiosity science team hopes to analyze the cement that holds the grains together at this location.
“We want to learn more about the wet process that turned sand deposits into sandstone here,” said Curiosity Project scientist John Grotzinger. “What was the composition of the fluids that bound the grains together? That aqueous chemistry is part of the habitability story we’re investigating.”
The hardness of the sandstone at Kimberley varies. The harder sandstone is more erosion-resistant, and its chemical history is most likely different from that of other less-resistant sandstone found in the area. The capping layer of the mesas and buttes is made of the harder sandstone.
These differences in hardness could help to explain the major shapes of the landscape. Understanding these differences may also help explain why Gale Crater has a large layered mountain at its center.
Curiosity has a set of tools and instruments that will allow project scientists to unlock these secrets from the rocks. Let’s look at the general steps they have used before to analyze rock samples.
First, images using several cameras on Curiosity are taken of the rock and the area around it. The next step is to remove any dust in the sample area on top of the rock with a wire-bristle brush. Once the dust is removed, more images are taken with the Mars Hand Lens Imager, a close-up camera mounted on the end of the robotic arm.
Curiosity can then use two other instruments to determine some of the chemical properties of the rock’s surface: the Alpha Particle X-Ray Spectrometer and the Chemistry and Camera (ChemCam).
The APXS exposes the rock to alpha particles and X-ray radiation to determine what rock-forming elements are in the surface material.
ChemCam fires a laser to vaporize a small area on the surface of the rock, about one twenty-fifth of an inch square. This vapor is then analyzed by an onboard spectrograph that provides details about the minerals and microstructure of the rock.
But to examine the inside of the rock, Curiosity has to use a rock drill, which can drill a hole in the rock about a half-inch in diameter and about two-and-a-half inches deep.
During the drilling process, the rock powder from the hole is collected in a storage area near the drill on the end of the robotic arm. The robotic arm can then sieve and deliver a small part of this material to two onboard laboratories: the Chemistry and Mineralogy instrument and the Sample Analysis at Mars instrument.
These instruments can complete a more detailed analysis of the sample as the rover drives on to a new destination. The rock sample can also be kept in the storage area for future analysis, until a new sample is taken.
In preparation for drilling the deep hole, a test mini-drill is done. The hammer drill is used to bore a hole of the same diameter, but only about three-quarters of an inch deep. This hole tests the properties of the rock, and this information is used to determine if a deeper hole is possible.
At Kimberley, a drill site was selected near the base of Mount Remarkable at the southern end of the Kimberley area. Selection of the drill site to completion of the sampling process took about three weeks, from April 23 to May 14.
A link showing the whole three-week process, captured with time-compressed images, is at bit.ly/1n8eF63. Note the cascading of loose sand during the two drilling activities.
Samples have now been given to most of the instruments and Curiosity has left Kimberley. It will be several weeks, or even months, before we know the results of the analysis of these samples. But at least for now, you have a better idea of how rock samples are collected on Mars.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.