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"The surface of Mars, the surface of Earth, and basically all surfaces in the solar system are being bombarded by cosmic rays," explains Farley, and when these rays—very high-energy protons—blast into an atom, the atom's nucleus shatters, creating isotopes of other elements.Cosmic rays can only penetrate about two to three meters below the surface, so the abundance of cosmic-ray-debris isotopes in rock indicates how long that rock has been on the surface."That gives us some idea about why the environment looks like it does and it gives us an idea of where to look for rocks that are even less exposed to cosmic rays," and thus are more likely to have preserved organic molecules, Farley says.Curiosity is now long gone from Yellowknife Bay, off to new drilling sites on the route to Mount Sharp where more dating can be done."MSL instruments weren't designed for this purpose, and we weren't sure if the experiment was going to work, but the fact that our number is consistent with previous estimates suggests that the technique works, and it works quite well."The researchers do, however, acknowledge that there is some uncertainty in their measurement.One reason is that mudstone is a sedimentary rock—formed in layers over a span of millions of years from material that eroded off of the crater walls—and thus the age of the sample drilled by Curiosity really represents the combined age of those bits and pieces.Although researchers have determined the ages of rocks from other planetary bodies, the actual experiments—like analyzing meteorites and moon rocks—have always been done on Earth.
"When we first came up with this number, the geologists said, 'Yes, now we get it, now we understand why this rock surface is so clean and there is no sand or rubble,'" Farley says.
There, the sample was heated to temperatures high enough that the gasses within the rock were released and could be analyzed by an onboard mass spectrometer.
Farley and his colleagues determined the age of the mudstone to be about 3.86 to 4.56 billion years old.
Using the SAM mass spectrometer to measure the abundance of three isotopes that result from cosmic-ray bombardment—helium-3, neon-21, and argon-36—Farley and his colleagues calculated that the mudstone at Yellowknife Bay has been exposed at the surface for about 80 million years.
"All three of the isotopes give exactly the same answer; they all have their independent sources of uncertainty and complications, but they all give exactly the same answer.