Light Rock, Dark Rock?

a)
b)

With only four sols left and mission success nearly achieved, the choices of what to do and where to go become harder and harder as real-time events play the role of the "wild card". Scientist John Grant explains, "On one hand, there is a strong desire to approach a 'light rock' which may be a piece of the resistant rock that lies throughout the region. On the other hand, the team is curious about the mysterious 'dark rocks' that have been scattered in the landing region. These rocks are thought to be fragments of cooled lava, but remain unexplored and very interesting. With time running out, one of the major decisions that looms larger and larger is which of the two types of rocks to target. It is simply not possible to do both."

John Grant

The good news is that approaching either kind of rock and using the Rock Abrasion Tool (RAT) to help characterize its composition is an important unfulfilled exercise and would be a valuable learning experience. It is also likely that either type of rock will yield its own particular surprises. These exercises and uncertainties form important parts of the excitement and process of exploration and discovery.

But why bother to go to "light rocks" and "dark rocks," and why all the discussion? "It all relates to the different stories that they may tell regarding the geology of the region." Based on current information, some of the team members advocate for taking a better look at the "light rocks", suspecting they may be limestone, a type of rock that is typically created in warm, shallow seas. Others suggest going to the "dark rocks" in order to understand whether they are volcanic, heavily weathered, or represent a rock layer that is somewhere in between that of modern surfaces and ancient rock formations on the local cliffs.

It may sound like a simple decision, but there's a catch. FIDO's most recent traverse took it a little off the expected track and its Navcam image coverage only reveals the view directly in front of the rover, not the wider view up Long Valley that would have helped the team make a decision. Additionally, the rover did not find the expected plethora of large easily accessible "light" and "dark" rocks at the end of the traverse. Some "light rocks" may lie on the ground in the distance or in place, but are probably too far or difficult to reach and sample. The "dark rocks" are widely scattered, but are all too small to approach and scour with the RAT.

Now what? Work is underway to use FIDO's remote sensing instruments to target the "light rocks" along the cliffs near Camelback and should help resolve the origin of the regional cap rock and whether a sea once was present. It is hoped that some of the questions about the origin of the "light rock" can be resolved without actually touching it. Hopes are also high that new the Navcam data expected in the upcoming sol's downlink of data will reveal some sufficiently large "dark rocks" nearby enough to be approached in time to use the RAT and take measurements. In the meantime, the mission clock ticks ever louder as time runs on.

As you can see in these images taken after Sol 22, the RAT is a powerful grinder, able to create a hole 45 millimeters (about 2 inches) in diameter and 5 millimeters (0.2 inches) deep into a rock on the martian surface.

The RAT is located on the arm of the rover and weighs less than 720 grams (about 1.6 lbs). It uses three electric motors to drive rotating grinding teeth into the surface of a rock. Two grinding wheels rotate at high speeds. These wheels also rotate around each other at a much slower speed so that the two wheels sweep the entire cutting area. The RAT is able to grind through hard volcanic rock in about two hours.

Once a fresh surface is exposed, scientists can examine the abraded area in detail using the rover's other science instruments. This means that the interior of a rock may be very different from its exterior. That difference is important to scientists as it may reveal how the rock was formed and the environmental conditions in which it was altered. A rock sitting on the surface of Mars may become covered with dust and will weather, or change in chemical composition from contact with the atmosphere.