Shocked Soil Discovered: Rare On Earth, But Abundant On Mars?

When a meteoroid strikes, it generates a wave of energy that moves faster than the speed of sound. When all that energy propagates through material in seconds or less before being quickly cooled and resolidified by a secondary wave, it produces glass.

Planetary Science Institute Senior Scientist Shawn Wright was looking for such glassy material while doing field work among the basaltic volcanic rock of Lonar crater in the Deccan region of India, when he found something unexpected.

“Some glassy samples were fluffy and light, like popcorn,” he said. “It had a really low density, it was airy, and it crumbled in my fingers. It looked different than all the other samples I’d seen and collected, so I aimed to find out what it was by trying to figure out what it used to be.”

Wright collected three different samples of the material in the region. He and his co-author, PSI alum Joseph Michalski of Hong Kong University, compared the physical structure, chemical makeup and microscopic texture of the strange samples to both shocked and unshocked basalt, as well as local soil, to ensure that what they found was something new and different.

It was. Analyses showed that the material was what Wright would dub ‘shocked soil,’ rather than the more common shocked basalt.

“We compared the geochemical abundances of many of the eight major elements – silicon, iron, magnesium, oxygen, sodium, potassium, calcium, aluminum – of the shocked soils to Deccan basalt and to the present-day local soil,” Wright said. “We compared their minerals as well. Whereas the Deccan volcanic basalt in the Lonar region is very well-constrained to a narrow range of these elements, soil has undergone more than 60 million years of chemical alteration.”

The new samples were not in that narrow range, and were more like present day soils.

“Mineralogically, while Deccan basalt has what we call primary minerals formed by volcanism, while soils have what are known as secondary minerals such as clay, zeolite, hematite, and a different texture than bedrock basalt,” he said.

Also key to their finding was the discovery of unmelted, unshocked soil remnants encased within the glassy shocked soil, he said, which suggested that unshocked soil was the precursor to the shocked soil found after the impact event.

This shocked soil qualifies as an impactite, which is a material modified by an impact. Wright and Michalski published their shocked soil discovery in a paper in the Journal of Geophysical Research: Planets.

Wright has since found just four other samples of the same material, and added these to his large sample collection of hundreds of the other types of impactites.

“We geologists have a biased sampling of whatever time leaves behind, such as old sedimentary rocks and old granites. These are the toughest stuff that can withstand weathering,” he said. “Greater than 99% of the 180 impact sites on Earth are emplaced into this long-lasting material, but their ejecta and impactites with glass do not stick around for long. Impactites usually disappear quickly due to weathering, erosion and active geology. Because of this, shocked impactites are rare on Earth.”

On the desolate Moon and Mars, on the other hand, such impactites might be more prevalent, as basalt is the target rock for several million impact sites on the surfaces of these two bodies.

“Soil might have more interesting data for astrobiologists and atmospheric scientists,” Wright said. “It can reveal information about the presence of past microbes, organics, life and also the results of water flowing or ponding, and the atmospheric conditions over time, rather than just the present day.”

What’s more, Wright and Michalski suggest that if the original soil was physically weathered or chemically altered away, the shocked soil impactites – which are little pieces of glass encasing soil, might preserve the only information left from the time before the impact.

Shocked Soil—A Rare Terrestrial Impactite, Journal of Geophysical Research: Planets

Astrobiology, Astrogeology,