Dating planetary surfaces
One technique, potassium-argon dating, determines the age of a rock sample by measuring how much argon gas it contains.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.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 500 meters from the rover's landing site.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.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.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.
Believing they knew how old the earth-moon system was, and something about its geological history, scientists had plotted crater density on the moon against surface age.
Farley had the idea of performing the experiment on Mars using the SAM instrument.
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.
They applied this to Mars and other planets and moons, such that any surface could be dated by reference to the lunar standard. Now, awareness of the potential for single impacts to generate vast numbers of secondary craters has yanked the guy on the bottom, bringing the scheme crashing down.
(May 26, 2006) reported that at a conference last March, "125 planetary scientists deadlocked" over how to apply the method, with many doubting that crater counts have anything to do with telling time.
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.