Washington: Clay deposits on Mars have often been assumed to have been formed in the earliest Martian epoch, the Noachian period - over 3.7 billion years ago.
Now, a Brown University research finds clay minerals that appear to have formed after an impact event within the last 2 billion years.
They looked at detailed mineralogy data collected by two instruments on board NASA's Mars Reconnaissance Orbiter: the High Resolution Imaging Science Experiment (HiRISE) and the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM).
Of those 633 peaks, the scientists found 265 that have evidence of clay minerals. They then used HiRISE images to establish a detailed geologic context for each of those craters to help determine if the clays were in rocks that had indeed been excavated from depth.
They found that in about 65 percent of cases the clay minerals were indeed associated with uplifted bedrock.
"That's a majority, but it still leaves a substantial number of craters, 35 percent, where these minerals are present and not clearly associated with uplift," said Ralph Milliken. Within those 35 percent, the team found examples where clays exist in dunes, unconsolidated soil, or other formations not associated with bedrock. In other cases, clays were found in impact melt deposits.
Both of these scenarios suggest that the clay minerals at these sites are likely 'authigenic,' meaning they formed in place sometime after impact occurred, rather than being excavated from underground.
In a number of cases, these authigenic clays were found in fairly young craters, ones formed in the last 2 billion years or so.
"Over 35% of central peak clays are not associated with uplifted rocks, thus caution must be used when inferring deeper crustal compositions from surface mineralogy of central peaks," said Milliken and his colleague Vivian Sun. "Uplifted clay-bearing rocks suggest the Martian crust hosts clays to depths of at least 7 km."
"Our observations are consistent with widespread Noachian/Early Hesperian clay formation, but a number of central peak clays are also suggestive of clay formation during the Amazonian."
The study is in press in the Journal of Geophysical Research: Planets.