Scientists discover a possible source of shock-darkened meteorites

The RAPTORS system atop the Kuiper Space Sciences Building at the University of Arizona was used to collect data for this study. Credit: Vishnu Reddy/University of Arizona

When the Chelyabinsk fireball burst over the Russian sky in 2013, it left a rather rare sort of meteorite on Earth. The black veins formed by a process known as shock darkening distinguish the Chelyabinsk meteorites and others like them. Yet, until today, planetary scientists have been unable to establish a close asteroid source of these types of meteorites. University of Arizona scientists discovered an asteroid designated 1998 OR2 as one probable source of shock-darkened meteorites in recent research published in The Planetary Science Journal. The 1.5-mile-wide near-Earth asteroid made a close pass to Earth in April 2020. Meteorites are asteroid fragments that break off into space and crash land on Earth.

According to lead study author Adam Battle, a UArizona graduate student studying planetary science, shock darkening is a process that occurs when something impacts a planetary body hard enough that the temperatures partially or completely melt those rocks, altering their appearance both to the human eye and in our data. This process has been observed in meteorites several times, but it has only been observed on asteroids on one or two occasions far out in the main asteroid belt, which lies between Mars and Jupiter.

Vishnu Reddy, a planetary sciences professor, and Battle's mentor, identified shock darkening on main belt asteroids in 2013 and 2014. Reddy co-directs the Lunar and Planetary Laboratory's Space Domain Awareness lab with engineering professor Roberto Furfaro. Battle has been employed in the lab since 2019.

Reddy stated. Impacts are exceedingly prevalent in asteroids and any solid body in the solar system, as evidenced by impact craters seen in spacecraft photos. Impact melt and shock-darkening effects on meteorites generated from these bodies, however, are uncommon. The discovery of a near-Earth asteroid dominated by this mechanism has implications for assessing impact danger. Adam's research has revealed that shock darkening may cause typical chondrite asteroids to seem carbonaceous in our classification tools. The physical strengths of these two materials differ, which is essential when attempting to deflect a dangerous asteroid.

Battle, Reddy, and their colleagues collected data on the surface composition of 1998 OR2 using the RAPTORS system, a telescope atop the Kuiper Space Sciences building on campus, and discovered that it appeared like a typical chondrite asteroid. Chondrite asteroids are lighter in color and include the minerals olivine and pyroxene. When the researchers processed the data through a classification algorithm, it revealed that the asteroid was a carbonaceous asteroid, a kind of asteroid that is black and featureless.

According to Battle, the inconsistency was one of the first things that sparked the project's investigation into probable sources of the gap. Based on its minerology, the asteroid is undoubtedly an ordinary chondrite that has been transformed to seem like a carbonaceous asteroid to the classification tool, rather than a combination of ordinary chondrite and carbonaceous asteroids.

Shock darkening was proposed in the late 1980s but did not gain momentum or attention until 2013 when a fireball over Russia generated meteorites with shock-darkened properties.

Scientists, including Reddy, were increasingly interested in shock darkening, according to Battle, and Reddy soon identified shock-darkened asteroids in the main asteroid belt. On Earth, 2%, or around 1,400, of the approximately 60,000 typical chondrite meteorites have been subjected to some form of shock or impact activity.

Researchers were able to rule out several other possible explanations for why 1998 OR2 seemed to be a carbonaceous asteroid rather than a regular chondrite. One possible explanation for the mismatch is space weathering, which occurs when an asteroid's surface changes due to exposure to the space environment; nevertheless, if this were the case, the asteroid would seem somewhat redder in color than it is. Shock darkening is a mechanism that can reduce the appearance of olivine and pyroxene while also darkening the surface of the asteroid to make it seem carbonaceous.

Journal Information: Adam Battle et al, Physical Characterization of Near-Earth Asteroid (52768) 1998 OR2: Evidence of Shock Darkening/Impact Melt, The Planetary Science Journal (2022). DOI: 10.3847/PSJ/ac7223