The image on the left shows the difference between the red-specified iron-altered shocked basalt, where the rocky basalt is the precursor, and the light gray shocked soil at the bottom right of the large sample. On the right, individual samples of impact soil can be seen. It is light gray in color with white caliche – cemented sedimentary deposits – soil horizons within just like the rammed earth clinging to the rock on the left. Credit: Shaun Wright/Michaela Kelly
When a meteorite hits, it generates a wave of energy that moves faster than the speed of sound. When all this energy spreads through the material in seconds or less before it is quickly cooled and refrozen by a secondary wave, it produces glass.
Sean Wright, a senior scientist at the Planetary Science Institute, was searching for such a glassy substance while doing field work among basalt volcanic rocks in Lonar Crater in the Deccan region of India, when he found something unexpected.
“Some of the glass samples were thin and light, like popcorn,” he said. “Its density was very low, it was airy, and it crumbled between my fingers. It looked different from all the other specimens I had seen and collected, so I aimed to find out what it was by trying to find out what it had been like before.”
Wright collected three different samples of the material in the area. He and his co-author, Joseph Michalski of the University of Hong Kong, compared the physical structure, chemical composition and microscopic texture of the exotic samples with both shocked and unshocked basalt, as well as local soil, to ensure that what they found was something new and different.
It was. Analyzes showed that the material was what Wright called “shock soil,” not the more common shock basalt.
“We compared the geochemical abundances of several of the eight major elements — silicon, iron, magnesium, oxygen, sodium, potassium, calcium, and aluminum — in the shock soils with Deccan basalt and present-day local soils,” Wright said. “We also compared their mineralogy. While the Deccan volcanic basalts in the Lonar region are very well restricted to a narrow range of these elements, the soil has undergone more than 60 million years of chemical changes.”
The new samples were not in this narrow range, and were more similar to current soil.
He said: “Mineralogically, while Deccan basalt contains what we call primary minerals that were formed as a result of volcanic activity, while the soil has what are known as secondary minerals such as clay, zeolite and hematite, and a different texture from rocky basalt.”
Also key to their discovery, he said, was the discovery of remnants of unmelted, unshocked soil encased within the shocked glassy soil, suggesting that the unshocked soil was a precursor to the shocked soil found after the impact event.
This shocked soil is an impact material, which is a material that has been modified as a result of the impact. Wright and Michalski published their shocking soil discovery in a paper in the journal Geophysical Research: Planets.
Since then, Wright has found only four other samples of the same material, adding them to his large collection of hundreds of other types of impacts.
“We geologists have a bias for sampling anything that time leaves behind, like ancient sedimentary rocks and ancient granite,” he said. “These are the hardest materials to weather.” “More than 99% of the 180 impact sites on Earth are encased in this long-lived material, but their ejecta and glass collisions do not stay for long. Impacts usually disappear quickly due to weathering, erosion and active geology. For this reason, impact impacts are rare on Earth.”
On the other hand, such impacts may be more common on the desolate Moon and Mars, as basalt is the target rock for several million impact sites on the surfaces of these bodies.
“The soil may contain more interesting data for astrobiologists and atmospheric scientists,” Wright said. “It can reveal information about the presence of microbes, organic matter, and past life as well as the results of water flow or ponds, and weather conditions over time, not just the present.”
Furthermore, Wright and Michalski suggest that if the original soil was physically weathered or chemically altered, rammed soil collisions—small pieces of glass encasing the soil—may preserve the only remaining information from the time before the impact.
Impacted soil — a rare earth impactJournal of Geophysical Research: Planets
Astrobiology, Astrogeology,
