Clay deposits in impact craters on Mars have often been assumed to have been formed in the earliest Martian epoch – the Noachian period – over 3.7 billion years ago. New research published in the Journal of Geophysical Research: Planets finds clay minerals that appear to have formed after an impact event within the last 2 billion years.
Ritchey Crater on Mars has impact melt deposits containing clay minerals. The clay minerals found within these deposits are very likely to have formed after the impact event. Image credit: NASA / JPL / University of Arizona / Brown University.
Two geologists at Brown University carried out a global survey of 633 crater central peaks distributed across the surface of Mars.
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% 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%, where these minerals are present and not clearly associated with uplift,” said Dr Ralph Milliken of Brown University’s Department of Earth, Environmental, and Planetary Sciences.
Within those 35%, 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 Dr Milliken and his colleague, Dr Vivian Sun, also of Brown University’s Department of Earth, Environmental, and Planetary Sciences. “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.”
_____
Vivian Z. Sun & Ralph E. Milliken. Ancient and recent clay formation on Mars as revealed from a global survey of hydrous minerals in crater central peaks. Journal of Geophysical Research: Planets, published online November 20, 2015; doi: 10.1002/2015JE004918
