[meteorite-list] Study Finds Evidence for More Recent Clay Formation on Mars

[Ron Baalke]

https://news.brown.edu/articles/2015/12/mars

Study finds evidence for more recent clay formation on Mars
Brown University
December 14, 2015   

Contact: Kevin Stacey   401-863-3766

PROVIDENCE, R.I. - Recent orbital and rover missions to Mars have turned 
up ample evidence of clays and other hydrated minerals formed when rocks 
are altered by the presence of water. Most of that alteration is thought 
to have happened during the earliest part of Martian history, more than 
3.7 billion years ago. But a new study shows that later alteration - within 
the last 2 billion years or so - may be more common than many scientists 
had thought.

The research, by Brown University geologists Ralph Milliken and Vivian 
Sun, is in press in the Journal of Geophysical Research: Planets.

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, known as 
the 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 particularly true of clay deposits found in crater 
central peaks. Central peaks are formed when, in the aftermath of an 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. "What we wanted to do was look at lots of these craters in detail 
to see if that's actually correct."

Milliken and Sun performed a survey of 633 crater central peaks distributed 
across the Martian surface. 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.

Of those 633 peaks, Milliken and Sun found 265 that have evidence of hydrated 
minerals, the majority of which were consistent with clays. The researchers 
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," Milliken said, "but it still leaves a substantial 
number of craters - 35 percent - where these minerals are present and 
not clearly associated with uplift."

Within those 35 percent, Milliken and Sun 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," 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.

"What this tells us is that the formation of clays isn't restricted to 
the most ancient time period on Mars," Milliken said. "You do apparently 
have a lot of local environments in these crater settings where you can 
still form clays, and it may have occurred more often than many people 
had thought."

One mechanism for forming these clays could be related to the impact process 
itself, the researchers say. Impacts 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.

And that could have implications for the search for evidence of 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."

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