Traces of dust particles in the Antarctic ice are 2.3 to 2.7 million years old, analysis shows. This would make them the oldest legacy of an airburst: an asteroid that exploded in the atmosphere, rather than hitting the ground while large enough to leave a trace. The discovery could be the first step on a path that allows us to assess the danger of these events in the future.
Asteroids or comets that leave behind giant impact craters when they hit Earth can change the course of history, but aerial eruptions are more common. As the Chelyabinsk explosion revealed, air bursts can cause quite a bit of damage to people in the area – and the Tunguska event would have been much more destructive if it had hit a populated area.
By some calculations, the cumulative threat to life from the many air eruptions that Earth experiences exceeds that from the much larger, but also much rarer, crater-forming events.
“All the energy is released into the atmosphere in the form of shock waves and thermal radiation,” study author Dr. Matthias van Ginneken from the University of Kent told ScienceNews.
Air eruptions must have occurred since Earth first gained an atmosphere, but their legacy is being erased much faster than craters, some of which last billions of years. In most conditions, rain, other sources of dust, and biological activity quickly take away our ability to identify ancient airbursts.
Ice can act as a protector, but alpine glaciers usually carry away the remains even if they don’t melt. This makes Antarctica – especially the parts where snow builds up slowly – the best and possibly only chance to find such evidence.
Two types of debris have been found in Antarctica and are believed to come from air eruptions 430 and 480 thousand years ago. Now a team led by Van Ginneken has presented evidence particles that were more than five times as old as those from a similar event.
The dust field known as BIT-58 was first found 30 years ago in Antarctica’s Allan Hills, the site of the famous Martian meteorite once thought to contain evidence of life. About 90 percent of the particles are chondritic (from unmodified stony meteorites), so it wasn’t hard to tell that this was the remnant of a visitor from space, and not a volcanic eruption. This led to the extraction of approximately 100 kilograms (220 pounds) of dust-laden ice and its transfer to McMurdo Station for analysis.
What wasn’t clear when this was initially done was whether the balls of meteorite material came from an impact whose crater we hadn’t yet found, or whether they were the product of an air burst.
Hundreds of dust particles were found in the ice. After removing the terrestrial contamination, the team studied 116 of them with an electron probe microanalyzer and an ion beam. About 30 percent turned out to be perfectly spherical, as is often the case for particles created by atmospheric impacts.
Meteoritic spheres found in Allan Hills in Antarctica. Chemical analysis suggests they are consistent with a type of asteroid known as a common chondrite that disintegrated in the atmosphere
Image courtesy of Matthias van Ginneken
The authors note the absence of microtektites that form when the heat of the impact melts terrestrial material, or of the translucent microcrystites that condense from the impact plume. Instead, the composition is similar to that of ‘touchdown scenarios’, where the jet of superheated gas produced by evaporating parts of the asteroid maintains its momentum until it reaches the ground. Van Ginneken told ScienceNews that touchdowns are like; “A huge torch that hits the ground and everything evaporates.” One of the two younger events in Antarctica also appears to have been a landing scenario.
“I think my work is the first essential step in understanding what the remains of large eruptions look like in the geological record,” Van Ginneken told IFLScience. “The next step will be to find more examples of such events, especially in other environments (e.g. lower latitudes). This would allow us to establish a protocol to identify explosion residues with a high degree of certainty and ultimately help us determine the frequency of such events in the past.”
Van Ginneken added; “We could try to incorporate the spheroids we already have into numerical models of airbursts, which could potentially help understand their formation and geographic distribution. This could help us discover a link between specific properties of spheres (e.g. size) and the size of air blasts and thus their destructive potential.”
The research has been published open access in the journal Earth and Planetary Science Letters