Radiometric dating of surface rocks


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.

The exposure of rock in Yellowknife Bay has been caused by wind erosion.

Over time, as wind blows sand against the small cliffs, or scarps, that bound the Yellowknife outcrop, the scarps erode back, revealing new rock that previously was not exposed to cosmic rays."Imagine that you are in this site a hundred million years ago; the area that we drilled in was covered by at least a few meters of rock.

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.

That is probably the most remarkable thing I've ever seen as a scientist, given the difficulty of the analyses," Farley says.

This also helps researchers looking for evidence of past life on Mars.

One technique, potassium-argon dating, determines the age of a rock sample by measuring how much argon gas it contains.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.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.Furthermore, the "young" surface exposure offers insight into the erosion history of the site.


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