While at Yellowknife Bay, Curiosity collected a drilled rock sample that was later age-dated by using its science instruments in two different ways. The first one has to do with the age of the rock itself: When did this rock first form?
I explained last month that Curiosity used its Sample Analysis at Mars instrument to determine the amount of potassium-40 that had decayed into argon-40 in the rock sample.
This radioactive process has a half-life of 1.25 billion years, so by measuring the amount of argon-40 and assuming that the rock had none at the beginning, we can determine that the age of the rock is the amount of time it would take to create that much argon — in this case, about 4.2 billion years.
The second measurement is a time interval: How long has the rock been exposed at the surface?
Mars lacks a magnetic field to protect the surface from high-energy galactic cosmic rays, which means that all material within six to 10 feet of the surface is bombarded by these cosmic rays.
This bombardment creates isotopes of noble gases such as argon-36 and helium-3, which were not present when the rock formed.
If we know the amount of each isotope in the rocks and how fast they form on Mars, we can determine how long the rock has been at or near the surface.
From the drilled-rock samples taken at Yellowknife Bay, we learned that this 4-billion-year-old rock had only been at or near the surface for about 80 million years. This is a shorter time than scientists had expected.
Looking at close-up images taken by the Mars Hand Lens Imager on the end of Curiosity’s robotic arm, nodules can be seen protruding from the otherwise-flat surface of the mudstone rock, with wind tails formed downwind.
This is evidence that the mechanism that has exposed the rock is wind erosion; the rocks have been sandblasted. The erosion is happening quickly in geological time, and it is likely an ongoing process.
Sequential layers of rocks form the Yellowknife Bay formation. The oldest is called Sheepbed, followed by Gillespie Lake, and finally Point Lake.
The Sheepbed layer is the softest and is more easily eroded than the other two layers. This means that the harder upper layers are being undermined as the Sheepbed layer is being eroded. This creates scarps, and the erosion process is called “scarp retreat” because the process is ongoing.
As the surface erosion continues, rocks of harder material at different depths will be exposed. The shallow rocks will be exposed first, then the deeper rocks. This means that the deeper rocks will be exposed for a shorter period of time.
From the differences in the exposure times of the rocks, we can determine the rate of erosion. The limited data obtained so far indicate that the erosion rate at Yellowknife is about 3 feet per million years.
If our understanding of this erosion process is correct, it may be of great help in finding organic compounds on Mars. Those same galactic cosmic rays that are necessary for creating the noble gases needed to determine surface exposure time can also destroy organic material in a relatively short time.
Curiosity has not found any large organic compounds in any sample it has tested. This may be because the samples never contained any organic material, but that is unlikely.
Because we find organic material in meteorites that hit the Earth, it is likely that organic material has also been delivered to Mars, even if life processes never created it there.
A more likely explanation for the absence of organic material in our samples is that whatever was once there has been destroyed.
Exposure to galactic cosmic rays for 80 million years could have destroyed any organic material in the samples Curiosity tested.
The erosion process tells us where to find a sample that has had minimal surface exposure time and therefore the least likelihood of the organic material having been destroyed. If we follow the direction of the wind until we get to the scarp, the eroded surface at the scarp should be the most freshly exposed material and the best place to search for organics.
Today Curiosity is a long way from Yellowknife Bay, and it is not going to drive back there anytime soon. But the rover team has spotted several other potential sampling sites in images taken from orbit. One of these is at Waypoint 4, and Curiosity should be there in a few weeks.
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.