Not every impact event is a massive extinction-level event. More often, the asteroid or comet fragments that make it through our atmosphere are small and usually burn up before they hit the ground. However, there is a lot of in-between when it comes to meteorite sizes: between ‘extinction event’ and ‘burns up in the atmosphere’ is a range of impactors. One such middle-of-the-road event was the Chelyabinsk meteor in 2013 – not large enough to leave a crater but large enough to cause a sonic boom that destroyed windows.

However, along with a bunch of fragments, the Chelyabinsk meteor also left behind an important mystery. The meteorites were an uncommon type, containing dark veins in the rocks left by a process called ‘shock darkening’.

Until recently, scientists didn’t know of any asteroid from which these types of meteorites could originate. Now, in a new paper published in the Planetary Science Journal and led by Adam Battle, a possible asteroid source has been discovered – asteroid 1988 OR2. Battle explains: 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.

This particular near-Earth asteroid is 2.4 kilometers wide and last made a close approach to Earth back in April 2020, and while it’s not the first asteroid with evidence of shock darkening to be found, it is the closest. The others were all in the main asteroid belt, closer to Mars and Jupiter. Co-author Vishnu Reddy notes: Impacts are very common in asteroids and any solid body in the solar system because we see impact craters on these objects from spacecraft images. But impact melt and shock-darkening effects on meteorites derived from these bodies are rare. Finding a near-Earth asteroid dominated by this process has implications for impact hazard assessment.

At first glance, asteroid 1998 OR2 seems to be an ordinary chondrite asteroid, containing olivine and pyroxene and appearing lighter relative to other asteroids. However, a classification tool used by the research team suggested that the asteroid was carbonaceous, and that discrepancy was the crux of the question. Carbonaceous asteroids are usually dark and featureless. But as Battle further explains: The asteroid is not a mixture of ordinary chondrite and carbonaceous asteroids, but rather it is definitely an ordinary chondrite, based on its mineralogy, which has been altered – likely through the shock darkening process – to look like a carbonaceous asteroid to the classification tool.

Mineralogy is important to planetary defense. Reddy notes: …ordinary chondrite asteroids can appear as carbonaceous in our classification tools if they are affected by shock darkening. These two materials have different physical strengths, which is important when trying to deflect a hazardous asteroid.

We would not enjoy expecting a rubble pile and getting something far more solid and more difficult to move if we were trying to deflect a potentially hazardous asteroid., so this research is vital to planetary defense.

Just ask the dinosaurs… oh, wait.

More Information

The University of Arizona press release

“Physical Characterization of Near-Earth Asteroid (52768) 1998 OR2: Evidence of Shock Darkening/Impact Melt,” Adam Battle et al., 2022 October 4, The Planetary Science Journal