Ritchey Crater, located near the Martian equator, has impact melt deposits containing clay minerals. Impact melt forms when rock melted during an impact cools and hardens. The clay minerals found within these deposits are very likely to have formed after the impact event. Most clay minerals on Mars are thought to have formed during the earliest Martian epoch, known as the Noachian. However, evidence from Ritchey crater and other post-Noachian craters, suggests that clay formation after the Noachian was not uncommon. Credit: NASA/JPL/University of Arizona/Brown University
Clays and other minerals formed when rocks are altered by water have been found in multiple locations on Mars. It’s been assumed that these minerals probably formed in the earliest Martian epoch, >3.7B yrs ago. But a new study finds that later clay formation (last 2B yrs) might have been more common than many scientists thought.
The lion’s share of the clay deposits found on Mars thus far have turned up in terrains that date back to the earliest Martian epoch, ie Noachian period. Clays also tend to be found in and around large impact craters, where material from deep below the surface has been excavated. Scientists have generally assumed that the clays found at impact sites probably formed in the ancient Noachian, became buried over time, and then were brought back to the surface by the impact.
That assumption is true of clay deposits found in crater central peaks which are formed when, post-impact, rocks from within the crust rebound upward, bringing layers to the surface that had been buried many kilometers deep. “Because central peaks contain rocks uplifted from depth, some previous studies have assumed the clays found within central peak regions are uplifted too,” said Milliken, assistant professor of Earth, environmental and planetary sciences.
Milliken and Sun performed a survey of 633 crater central peaks. They looked at detailed mineralogy data collected by NASA’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), combined with high-resolution stereo images taken by NASA’s HiRISE camera. Both instruments fly aboard NASA’s Mars Reconnaissance Orbiter. 265 peaks have evidence of hydrated minerals, the majority of which were consistent with clays. They then used HiRISE images to establish a detailed geologic context to help determine if the clays were in rocks that had indeed been excavated from depth. In ~65% of cases the clay minerals were indeed associated with uplifted bedrock. “but it still leaves a substantial number of craters – 35% – where these minerals are present and not clearly associated with uplift,” Milliken said.
Within those 35%, they 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 of rock that had been melted by the heat of the impact and then re-solidified as it cooled. Both of these scenarios suggest that the clay minerals at these sites are likely “authigenic,”ie 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, formed in the last 2 billion years or so.
One mechanism for forming these clays could be related to the impact process itself, which generate heat, which could melt any ice or pre-existing hydrated minerals that may have been present within the nearby crust. Any liberated water could then percolate through surrounding rock to form clays. Some impact simulations suggest that these hydrothermal conditions could persist for perhaps thousands of years, making for potentially habitable conditions, with implications for the search for past life on Mars.
“So far, much of our surface exploration by rovers has focused on ancient terrains and whether or not the environments they record were habitable,” said Sun, lead author on the study and a graduate student working with Milliken. “But if we wanted to look at an environment that was more recent, we’ve identified craters that might be possible candidates.” https://news.brown.edu/articles/2015/12/mars
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