The last, dramatic moments of some microbes' lives may tell us more about how serious space rock impacts on Earth were in the ancient past.
The charred bodies of microorganisms killed by even a moderate asteroid impact can show the amount of damage produced by a cosmic crash, a new study suggests.
A research team examined four craters in Estonia, Poland and Canada that were created thousands of years apart. Despite their geographical distance and the amount of time between these various impacts, the team found millimeter-to-centimeter sized pieces of charcoal mixed in with the material that was formed during each of them, the authors said.
The charcoal "was formed from organisms killed, grilled and buried by the asteroid," lead author Anna Losiak, who is with the Institute of Geological Sciences at the Polish Academy of Sciences, told Space.com. That discovery of ancient asteroid-battered organisms differed from charcoal associated with normal wildfires, which was the team's leading hypothesis for a while.
Charcoal formed by impact instead of wildfire, she added, is "much more homogeneous and points to a lower temperature of formation."
She said the impact charcoal found in the craters was similar, but not identical, to charcoal that is formed when wood is intermixed with pyroclastic flows. (Pyroclastic flows form from erupting volcanoes.)
The smaller impact craters Losiak studies — those that are only up to 656 feet (200 meters) in diameter — form every 200 years or so and thus present numerous opportunities to study formation conditions, she said.
But her focus is distinct: "Most people are interested in gigantic collisions because those are capable of causing planet-scale damage — the diminishment of dinosaurs is the best, and so far the only, example of this kind of event," she said, referring to the asteroid event that led to the extinction of the non-avian dinosaurs 66 million years ago.
In photos: Asteroids in deep space
Losiak first came across the mysterious charcoal near a small impact crater in Estonia. She began work during a summer school opportunity as a newly minted Ph.D. and then returned a year later to lead a project to uncover and study the "paleosoil." Paleosoil, she said, is an ancient soil covered by the material removed from the crater during its formation.
As it turned out, the team never found the paleosoil. But after three days of digging by hand, a time-consuming necessity due to environmental protection, her team found charcoal.
"At first, we thought this charcoal was formed by wildfires that occurred shortly before the impact, and charcoal just got tangled in this extraterrestrial situation," she said. "But later, I found similar charcoal in other impact craters, and started to think that something was not right with this hypothesis."
What seemed strange to the team, she said, was why there would be so many large wildfires shortly before the formation of four different impact craters created geographically far away from each other, and across a timespan of thousands of years.
'It made no sense, so we decided to investigate further and analyze properties of charcoal pieces found intermixed within material ejected from craters, and compare it with wildfire charcoal," she said. That's when the team uncovered that wildfires were not involved at all.
NASA and other entities are continually on the hunt for extraterrestrial bodies, like comets or asteroids, that may cause a crater on Earth's surface. So far, scientists have found no impending hazards to worry about. But Losiak said proper disaster preparedness will benefit from studies such as hers.
"This study improves our understanding of environmental effects of small impact crater formation," she said. For incoming impactors, she added, "we will be able to more precisely determine the size and type of evacuation zone necessary."
Relatively large impact events do crop up recently in recorded history. One of the most famous examples is the Tunguska event, which flattened roughly 770 square miles (2,000 square kilometers) of Siberian forest in 1908.
More recently, in 2014, a small body exploded over the Russian city of Chelyabinsk. Thousands of people were injured due to glass and other debris, but otherwise, the damage was minimal.
Losiak and her team plan to go to another set of small impact craters in Argentina, in a region called Campo del Cielo, in late September to follow up on the research.
"We will collect more data, and samples, and hopefully we will be able to find more organisms killed by the asteroids," Losiak said. "Campo del Cielo is particularly interesting because there are not only true impact craters — sites where an asteroid literally exploded when it touched the ground — but also penetration funnels."
A penetration funnel occurs when an asteroid slows down in the atmosphere during its entry to Earth. This formation happens when it is hitting the ground with velocity similar to that of a sniper rifle bullet, the researchers say.
"In this case, most of the asteroid survives, and the temperatures and pressures experienced by the ground are much less extreme," Losiak said. The goal is to perform "a perfect natural experiment" by comparing the craters and the funnels in the same area, she added.
A study based on the research was published Aug. 31 in the journal Geology (opens in new tab).