The Glint of Martian Glass

Scientists have detected glass in Martian craters, created by the fierce heat of impacts that melted the Red Planet’s surface.

Like someone who suffered an acne-marred adolescence, Mars has a face pockmarked with craters. Its ravaged surface bears scars from countless impacts. When a high-speed impactor hits, it melts the rock it slams into. If this material cools fast enough — say, when exposed to the rock and cold air of ancient Mars — it’ll form glass.

But thus far, planetary scientists haven’t definitively found impact glass on the Red Planet. They have seen glass in general; in 2012, for example, Briony Horgan and Jim Bell (both then at Arizona State University) detected a huge deposit in the north polar sand sea and across the northern plains, covering some 10 million square kilometers (4 million square miles). They still don’t know its origin, but it’s hard to explain that much glass with impacts, so they’re leaning toward explosive volcanism, Horgan says. (And Mars had plenty of volcanism in its early days.)

glass on Mars

Scientists have found deposits of impact glass (green) preserved in several Martian craters, including Alga (crater's central peak shown above). Also detected are the minerals pyroxene (blue) and olivine (red). The color-coded composition information from NASA's Mars Reconnaissance Orbiter is shown over a terrain model based on observations, but with the vertical dimension exaggerated by a factor of two.Credit: NASA / JPL-Caltech / JHUAPL / University of Arizona 

Now it looks like we have finally detected impact-created glass on Mars. Kevin Cannon and his advisor John Mustard (both Brown University) took an inventive tack to do it, combining lab work and spacecraft observations in a search for glass’s spectral pattern. Researchers can use spectroscopy to differentiate between glass and minerals based on how the material absorbs certain wavelengths of light. So Cannon whipped up a rock-powder recipe similar to what’s found on Mars, then fired it to create glass. With the pseudo-Martian glass in hand, he determined the material’s spectral pattern.

Then the team picked several well-preserved impact craters on Mars for which we have excellent spectroscopic data from NASA’s Mars Reconnaissance Orbiter. They used a homemade computer algorithm to tease glass’s signal out of the spectroscopic mess of these craters’ reflected light, detecting glass in many of the craters they explored.

The glass matches up well with other features interpreted as impact-spurred, sometimes even following the sharp margins of impact melts (such as in Alga crater) or staying confined to a melt’s thin draping across the surface (as in Ritchey crater), the team reports June 5th in the journal Geology.

“This is pretty cool,” says Horgan (now at Purdue University). Detecting glass’s spectral signal is a major challenge, she explains. Glass is partially translucent, so it doesn’t absorb light well. On the other hand, iron-bearing minerals like olivine and pyroxene — which are common in basalt and also appear in the craters — absorb a lot more light, producing absorption bands that are much deeper and easier to recognize. “Because they absorb so much more light than glass, their signature tends to swamp out the glass signature,” she says.

A Window on Ancient Life

Glass is an intriguing find because it’s an excellent preserver of past signs of life, kind of like the combination of amber and ancient bugs. Now, I’m as skeptical as anyone when it comes to waving the “implications for the search for life” flag in a discussion of Martian results, but this time I’ll play ball. We’ve previously talked about the clays that Curiosity is exploring as a good place to look for life’s vestiges, because clays preserve fossils well. But glasses are good for entombing biosignatures, too, because they form by very rapid cooling, Cannon explains.

“If the impact melt cooled slowly, it would cook any kind of organic matter trapped inside,” he says. Plus, glass is friendlier to microbes. “In terms of colonization, the chemical bonds in a glass tend to be weaker, so it's easier for microbes to tunnel inside.”

If we found glass in places that were also wet around the same time the glass formed, that would be exciting. Ancient Mars had its fair share of water, and most candidates for bygone lakes on the planet are in craters, with some overlap between those craters and the ones Cannon and Mustard studied. But although the glass might have reacted with water after it formed, the team doesn’t see any evidence that water altered any of the glass they detected. So it’s unclear how promising these places would be for a dead microbe hunt.

Still, one of the glassy craters, Hargraves, lies near a site under consideration for the Mars 2020 rover’s landing. That mission aims to squirrel away soil and rock samples, with the hope of being ready for a future pickup mission. But for the moment, a sample-return spacecraft is just a twinkle in planetary scientists’ eyes.

Less provocatively, scientists could use this type of spectral analysis to find many other kinds of minerals on Mars, or on other bodies. Because we have so few well-preserved craters on Earth, doing these studies of Mars or elsewhere could also help us better understand the variety of stuff impacts produce — how glassy impact deposits are, where they end up with respect to the crater — and how they affect the material dug up from beneath the surface, Horgan says.

 

Reference: K. M. Cannon and J. F. Mustard. “Preserved glass-rich impactites on Mars.” Geology. Published online June 5, 2015.


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