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In the northern reaches of Russia, enigmatic craters have begun appearing in broad expanses of windswept tundra. These craters can reach 230 feet across and plunge more than 100 feet deep into dark frozen soils known as permafrost.
The first crater was discovered by a helicopter pilot in 2014 on the Yamal Peninsula, a finger of frozen land extending into the Arctic Ocean. Since then, reindeer herders have stumbled upon another 16 craters. As the number of craters identified grew, so did the hypotheses about how the craters came to be. Some speculated that missiles, meteorites, or even UFOs were responsible. But when researchers dug into the mystery, they found high levels of methane in the atmosphere around the craters, suggesting that the forces responsible were not objects falling from above but rather explosive belches of gas from deep below the surface.
It would swell up like a water balloon and the ground would crack to release this pressure.
Still, many questions remain—namely, what forces are causing the methane release, or why they might happen in some places and not others. Now, one team of engineers and physicists from the University of Cambridge and the University of Granada in Spain have developed models that suggest permafrost thaw in a warming climate is causing the methane explosions and that osmosis plays a critical role. They recently published their findings in Geophysical Research Letters.
Through osmosis, fresh water naturally moves toward salty water, explains Ana Morgado, a chemical engineer at University of Cambridge and lead author of the study. According to the researchers’ modeling, osmosis could cause water from recently thawed permafrost near the surface to trickle down to cryopegs, layers of salt water that lie deep in the permafrost. The water in these cryopegs is so salty and weighted under so much pressure that it doesn’t freeze. If freshwater were to flow into a cryopeg, it would swell up like a water balloon and the ground would crack to release this pressure, say the researchers.
“Our idea was that the pressure would build up and push the soil, the ice up toward the surface. This would create a mound on the surface,” says Morgado. The mounds on the permafrost would be a little bit like pimples on a teenager’s face, only in this case the pressure would be so great that the pimple would explode.
A swelling cryopeg wouldn’t, by itself, cause an explosion. Morgado and her colleagues believe that the explosions and resulting craters happen where they do because these particular swelling cryopegs are close to methane hydrates—methane frozen in ice—which are easily disrupted by changes in pressure. In the Yamal Peninsula, where the permafrost is 600 to 1000 feet deep, cryopegs are typically found more than 160 feet down, just above a layer of methane hydrates.
“It was thought that methane release from methane hydrates in this region should be a slow process. Now, more studies are showing the different ways in which methane release can be sudden,” says Lauren Schurmeier, a geophysicist at University of Hawai’i at Manoa who has also researched the craters and was not a part of this study.
The mounds on the permafrost would be a little bit like pimples on a teenager’s face.
A growing cryopeg that is just above a layer of methane hydrate is rare, according to Morgado, which is why explosions are unusual even though permafrost is thawing and methane is escaping across the Arctic as climate warms. “The geological conditions that allowed it to happen are very, very niche, very precise,” she says.
Researcher Evgeny Chuvilin, who has also studied the craters, calls the hypothesis “novel”; however, he thinks it does not entirely describe the complexity underground, including how deep layers of ice could prevent melt water from moving downward. “The cryopeg is located far enough from the surface and water cannot migrate into it, since the permeability of the icy environment is very small,” he says. “Many questions arise regarding the proposed hypothesis of crater formation.”
Chuvilin and his colleagues suggest the methane causing the explosions that create the craters may also come from rocks deep below the permafrost, which formed more than 65 million years ago and are associated with the area’s oil and gas reserves. This methane would travel upward via faults to reach the permafrost. But Morgado believes that this idea is compatible with her group’s findings. Methane trapped within permafrost would be released first since it’s shallower, she says. “Existence of methane at deeper depths does not contradict our model.”
Whether the methane that explodes to the surface comes from ancient rocks or thawing permafrost, what’s clear is that more of this potent greenhouse gas enters the atmosphere with each blast. It’s difficult to estimate where future explosions could occur, but the researchers predict the number of explosions and craters will increase as warming continues. Having an estimate of how much methane is released with each blast is the most important thing to understand next, says Morgado.
Despite the unknowns, what is happening in these remote Arctic craters could have a global impact on climate change, given methane’s powerful ability to trap heat in the atmosphere, the researchers say. Even a small amount of methane could make a difference. According to the model, it’s a vicious cycle: Warming causes more methane release in explosions, which causes more warming. Says Schurmeier, “It is a terrifying, positive feedback loop.” 
Lead image: Aleksandr Lutcenko / Shutterstock
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