Shock-darkened Meteorites – Asteroid 1998 OR2 A Possible Source

By Michael Horton •  Updated: 10/25/22 •  4 min read

During the Chelyabinsk fireball explosion in 2013, a type of meteorite that is rare fell to Earth. The dark veins, which are a result of a process known as shock darkening, are what distinguish the Chelyabinsk meteorites from others of a similar type.

But until now, scientists haven’t been able to figure out which nearby asteroid these meteorites may have come from.

The asteroid 1998 OR2 has now been identified by scientists as one potential source of shock-darkened meteorites. The 1.5-mile-wide near-Earth asteroid passed by the planet in close proximity in April 2020.

What Is Shock Darkening

Asteroids that fragment into space and then crash to Earth are referred to as meteorites.

In ordinary chondrites, the shock-darkening or shock-blackening process involves melting iron sulphides and metals. They create a network of tiny melt veins by melting solid silicate grain boundaries and cracks.

“Shock darkening is an alteration process caused when something impacts a planetary body hard enough that the temperatures partially or fully melt those rocks and alter their appearance both to the human eye and in our data,”

said Adam Battle. A graduate student studying planetary science at the University of Arizona, Battle is the study’s lead author.

Meteorites often go through this process, but asteroids outside of the main asteroid belt, which is between Mars and Jupiter, don’t do it very often.

Impact Melted Or Shock-darkened Meteorites Rare

Asteroids and other solid bodies in the solar system get hit by things frequently. Scientists can see where these things hit by looking at images taken by spacecraft. However, these bodies rarely produce impact melt or shock-darkening effects on meteorites that come from them.

Professor of planetary sciences Vishnu Reddy, Battle’s advisor and co-author of the study, first noticed shock darkening on main belt asteroids in 2013 and 2014. Reddy works with engineering professor Roberto Furfaro to run the Space Domain Awareness lab at the Lunar and Planetary Laboratory. Since 2019, Battle has been employed by the lab.

“Finding a near-Earth asteroid dominated by this process has implications for impact hazard assessment,”

Reddy said.

Adam’s research has shown that shock darkening can make common chondrite asteroids show up in classification tools as being made of carbon. When attempting to deflect a dangerous asteroid, it is important to consider the physical strengths of these two materials.

Asteroid 1998 OR2 Carbonaceous Or Chondrite?

Battle, Reddy, and their team used the RAPTORS system, a telescope atop the campus Kuiper Space Sciences building, to gather information for this study on the surface composition of 1998 OR2. They found that it resembled a typical chondrite asteroid.

Known as a potentially dangerous asteroid, 1998 OR2 is an asteroid of the Apollo class in the JPL Small Body Database. Its diameter is estimated to be about 2.5 kilometers.

Asteroids known as chondrites are lighter in colour and contain the minerals olivine and pyroxene.

The asteroid was instead suspected to be a carbonaceous asteroid, a type of asteroid that is typically dark and featureless, when the team ran the data through a classification tool.

“The mismatch was one of the early things that got the project going to investigate potential causes for the discrepancy. The asteroid is not a mixture of ordinary chondrite and carbonaceous asteroids, but rather it is definitely an ordinary chondrite, based on its minerology, which has been altered—likely through the shock darkening process—to look like a carbonaceous asteroid to the classification tool,”

Battle said.

Shock Darkening

After being proposed in the late 1980s, shock darkening remained an unexplored phenomenon until 2013, when a fireball over Russia produced meteorites that had shock-darkened characteristics.

Reddy soon found shock-darkened asteroids in the main asteroid belt as interest in shock-darkening among scientists increased. According to Battle, 1,400 out of 60,000 common chondrite meteorites on Earth, or about 2% of them, have been subjected to some sort of shock or impact process.

Numerous other potential causes were successfully ruled out by researchers. Instead of being a typical chondrite, 1998 OR2 seemed to be a carbonaceous asteroid. Space weathering, which occurs when an asteroid’s surface is exposed to the space environment and changes as a result, could be one explanation for the discrepancy.

However, if that were the case, the asteroid would appear to be a little bit redder than it actually is. Shock darkening is a procedure that can darken the asteroid’s surface and suppress the appearance of olivine and pyroxene, giving it the appearance of a carbonaceous asteroid.

References:

Adam Battle, Vishnu Reddy, Juan A. Sanchez, Benjamin Sharkey, Neil Pearson, and Bryn Bowen. Physical Characterization of Near-Earth Asteroid (52768) 1998 OR2: Evidence of Shock Darkening/Impact Melt. 2022 Planet. Sci. J. 3 226

Moreau, J., Kohout, T. and Wünnemann, K. (2017), Shock-darkening in ordinary chondrites: Determination of the pressure-temperature conditions by shock physics mesoscale modeling. Meteorit Planet Sci, 52: 2375-2390.