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The smooth floor of Yellowknife Bay is made up of a fine-grained sedimentary rock, or mudstone, that researchers think was deposited on the bed of an ancient Martian lake.In March, Curiosity drilled holes into the mudstone and collected powdered rock samples from two locations about three meters apart.Although the potassium-argon method has been used to date rocks on Earth for many decades, these types of measurements require sophisticated lab equipment that could not easily be transported and used on another planet.Farley had the idea of performing the experiment on Mars using the SAM instrument.Once the rock samples were drilled, Curiosity's robotic arm delivered the rock powder to the Sample Analysis on Mars (SAM) instrument, where it was used for a variety of chemical analyses, including the geochronology—or rock dating—techniques.One technique, potassium-argon dating, determines the age of a rock sample by measuring how much argon gas it contains.Crater counting relies on the simple fact that planetary surfaces are repeatedly bombarded with objects that scar their surface with impact craters; a surface with many impact craters is presumed to be older than one with fewer craters.Although this method is simple, it has large uncertainties."What surprising was that our result—from a technique that was implemented on Mars with little planning on Earth—got a number that is exactly what crater counting predicted," Farley says.
We can use the leaky nature of rocks and minerals to isotopic diffusion to estimate the cooling history of rocks - which is very important in tracking the passage of rocks to the surface as their overburden is eroded.A key assumption is that a sample has remain a closed system so that the number of parent and daughter atoms can be fully audited.To examine these problems of diffusion, click here.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.