Cosmic rays are known to degrade the organic molecules that may be telltale fossils of ancient life. So while the mudstone indicates the existence of an ancient lake—and a habitable environment some time in the planet's distant past—neither crater counting nor potassium-argon dating can directly determine exactly when this was. Reliability in the dating of a geological feature is increased by sampling disparate areas which have been subjected to slightly different thermal histories. These results were attributed to incomplete Ar extraction arising from the coarse grainsize and Ar-retentive characteristics of sanidine, the major K-bearing mineral in the sample. 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. 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. In March, Curiosity drilled holes into the mudstone and collected powdered rock samples from two locations about three meters apart. Previously, such clays—evidence of a habitable environment—were thought to have washed in from older deposits. Jarosite is precipitated from acidic, sulfate-rich waters and is a suitable mineral for KAr dating. Findings from the first such experiment on the Red Planet—published by Farley and coworkers this week in a collection of Curiosity papers in the journal Science Express—provide the first age determinations performed on another planet. Ar—Ar dating is a similar technique which compares isotopic ratios from the same portion of the sample to avoid this problem.
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. Over time, atoms of the radioactive form of potassium—an isotope called potassium—will decay within a rock to spontaneously form stable atoms of argon To provide an answer for how the geology of Yellowknife Bay has changed over time, Farley and his colleagues also designed an experiment using a method called surface exposure dating. 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. Departures from this assumption are quite common, particularly in areas of complex geological history, but such departures can provide useful information that is of value in elucidating thermal histories. The small amount of jarosite in this sample prohibits generalization to a larger Martian context. Farley's work is published in a paper titled "In-situ radiometric and exposure age dating of the Martian surface. 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. Geologists have developed a relatively well-understood model, called the scarp retreat model, to explain how this type of environment evolves. At 80 million years ago, wind would have caused this scarp to migrate across the surface and the rock below the scarp would have gone from being buried—and safe from cosmic rays—to exposed," Farley explains. The paper is one of six appearing in the journal that reports results from the analysis of data and observations obtained during Curiosity's exploration at Yellowknife Bay—an expanse of bare bedrock in Gale Crater about meters from the rover's landing site. Now, for the first time, researchers have successfully determined the age of a Martian rock—with experiments performed on Mars. One technique, potassium-argon dating, determines the age of a rock sample by measuring how much argon gas it contains. For shorter timescales, it is unlikely that enough 40 Ar will have had time to accumulate in order to be accurately measurable. 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. 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. Although this method is simple, it has large uncertainties. By extension, it could also indicate the time of the very final stages of evaporation from the lake in Gale Crater. This also helps researchers looking for evidence of past life on Mars. However, shortly before the rover left Earth in , NASA's participating scientist program asked researchers from all over the world to submit new ideas for experiments that could be performed with the MSL's already-designed instruments. Previously, such clays—evidence of a habitable environment—were thought to have washed in from older deposits. 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. Miller, and Edward Stolper. In another paper in the same issue of Science Express, Grotzinger—who studies the history of Mars as a habitable environment—and colleagues examined the physical characteristics of the rock layers in and near Yellowknife Bay. Ar—Ar dating is a similar technique which compares isotopic ratios from the same portion of the sample to avoid this problem. Recently, a drilled bedrock sample from the Murray mudstone formation termed Mojave 2 was found by the CheMin instrument to contain approximately 3. K—Ar dating was instrumental in the development of the geomagnetic polarity time scale.
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