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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.Over time, atoms of the radioactive form of potassium—an isotope called potassium-40—will decay within a rock to spontaneously form stable atoms of argon-40.This decay occurs at a known rate, so by determining the amount of argon-40 in a sample, researchers can calculate the sample's age.Cosmic rays are known to degrade the organic molecules that may be telltale fossils of ancient life.However, because the rock at Yellowknife Bay has only been exposed to cosmic rays for 80 million years—a relatively small sliver of geologic time—"the potential for organic preservation at the site where we drilled is better than many people had guessed," Farley says.

"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."In one sense, this is an utterly surprising result—it's the number that everybody expected," Farley says.Indeed, prior to Curiosity's geochronology experiment, researchers using the "crater counting" method had estimated the age of Gale Crater and its surroundings to be between 3.6 and 4.1 billion years old.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.Now, for the first time, researchers have successfully determined the age of a Martian rock—with experiments performed on Mars.

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