Glasses formed by asteroid impacts are an important target to search for signs of ancient life on Mars, but until now they have not been detected on the Red Planet’s surface.
Deposits of impact glass have been preserved in Martian craters, including Alga Crater, shown here. In color coding based on analysis of data from the CRISM instrument on NASA’s Mars Reconnaissance Orbiter, green indicates the presence of glass; blues are pyroxene; reds are olivine. This view shows Alga Crater’s central peak, which is about 3 miles (5 km) wide within the 12-mile (19-km) diameter of this southern-hemisphere crater. The information from CRISM is shown over a terrain model and image based on observations by the High Resolution Imaging Science Experiment (HiRISE) camera. The vertical dimension is exaggerated by a factor of two. Image credit: NASA / JPL-Caltech / JHUAPL / University of Arizona.
Over the last few years, a number of studies have shown that, here on our planet, ancient biosignatures can be preserved in impact glass. One of those studies found organic molecules and even plant matter entombed in glass formed by an impact that occurred millions of years ago in Argentina.
Scientists suggested that similar processes might preserve signs of life on Mars, if indeed they were present at the time of an impact.
In a new study, Prof Jack Mustard and PhD student Kevin Cannon, both from Brown University, used orbital remotely sensed data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), which flies aboard NASA’s Mars Reconnaissance Orbiter, to investigate spectral signatures of Martian impactites – geologic units that were formed during impacts.
Using spectral mixture modeling, the researchers found that glass is in fact present in these units, mixed with other minerals like olivine and pyroxene.
This modeling approach allows for the identification glass signatures that are not otherwise obvious when glass is present in a mixture.
“Glasses tend to be spectrally bland or weakly expressive, so signatures from the glass tend to be overwhelmed by the chunks of rock mixed in with it,” Prof Mustard noted.
The glass-rich impactites the team has identified have been preserved on billion-year timescales, old enough to date back to more clement surface conditions on Mars.
Their preservation is likely due to the current cold and dry surface environment; therefore, fossilization in glass, as proposed previously, seems to be a promising target to search for possible ancient Martian biological activity.
“The metastable glass has been preserved by the cold and dry Martian climate during the Amazonian period (2.9-3.3 billion years ago to present), and this preservation – as confirmed here across the planet – provides a means to trap signs of ancient life on the accessible Martian surface.”
“We think these could be interesting targets for future exploration,” said Prof Mustard, who is the senior author of the paper published in the journal Geology. In fact, the team has a particular spot in mind.
One of the craters found to contain glass is called Hargraves, and it’s located near the Nili Fossae trough, a 400 mile (640 km) long depression that stretches across the Martian surface.
The region is one of the leading landing site contenders for the Mars 2020 rover, a mission that aims to cache soil and rock samples for possible future return to Earth. Nili Fossae trough is already of scientific interest because the crust in the region is thought to date from when Mars was a much wetter place. The region is also rife with what appear to be ancient hydrothermal fractures, warm vents that could have provided energy for life to thrive just beneath the surface.
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Kevin M. Cannon & John F. Mustard. 2015. Preserved glass-rich impactites on Mars. Geology, published online June 05, 2015; doi: 10.1130/G36953.1
