As they flew over the sweeping Siberian tundra, a Russian TV crew recently spotted an intriguing feature: a crater more than half a football field deep gouged from the frozen ground. Blocks of ice and dirt lay hundreds of feet away from the crater, flung from the deep scar on the surface.
This is just the latest in a series of such curious craters discovered in the Siberian Arctic, after the first was identified in 2014. Scientists believe they form from blasts of methane and carbon dioxide gas trapped within mounds of dirt and ice—a phenomenon that may be increasingly common as the climate warms. But much remains uncertain.
“We still don’t know what’s going on,” says Sue Natali, a permafrost expert at the Woodwell Climate Research Center in Falmouth, Massachusetts. “And is it going to happen somewhere else?”
Recent studies of other craters point to one likely mechanism: cryovolcanism, in which eruptions take the form of frosty mud or slush rather than fiery molten rocks. Such phenomena are well known elsewhere in our solar system, such as on Saturn’s watery moon Enceladus. But cryovolcanism is thought to be uncommon on our planet. Studying these Siberian features could provide clues to what’s happening on those far-flung worlds.
What’s more, their discovery highlights just how much there still is to learn about our blue marble, especially as scientists continue to untangle the consequences of a warmer future. “There’s processes that may occur that we haven’t even thought about,” Natali says. “There may be more out there; you only know what you know.”
Outsiders in the Arctic
The first Siberian crater was discovered in July 2014—and the public’s theories behind how it formed rapidly proliferated. Meteorite strike! Missile blast! Aliens!
In the years since, researchers have identified 15 more suspected craters from natural blasts. The newfound hole, number 17, may be the largest yet, says Evgeny Chuvilin, a permafrost expert at Russia’s Skoltech Center for Hydrocarbon Recovery. The Arctic craters are not easy to study—they fill with water in the months to years post blast, disguising them as one of the many lakes that dot the region.
Soon after this latest discovery, Chuvilin and his colleagues rushed to sample the icy crater, which is located in the Yamal Peninsula of northwest Siberia. Set against the gray, yellow, and green backdrop of the tundra, the crater stands out as “something of an outsider,” Chuvilin says. “When you are near a new crater, first of all, you are struck by its size.” Noises emanate as the slowly melting soils of its near-vertical walls crumble into the depths—“giving the impression of being alive,” he says.
The team is now “urgently processing” the samples for a scientific journal article, he explains via email.
Researchers hope to not only better understand the process behind the blasts but also predict where they may strike in the future. The explosions could pose a risk to locals, who have reported hearing booms or seeing flames near where new craters were found, says Andrey Bychkov, a geochemist at Lomonosov Moscow State University who has studied other craters but not yet visited the new one. In 2017, a crater was reported to have exploded near a camp of Nenets reindeer herders. The threat also potentially extends to the region’s abundant oil and gas infrastructure.
Ingredients for an icy explosion
Analyses of other craters, including sampling their icy walls, have provided some clues to what’s going on. In 2018, Bychkov and his colleagues proposed the blasts were a form of cryovolcanism that centers around an explosive combination of gas, ice, water, and mud.
The craters form within permafrost, ground that typically remains frozen through the summer, which blankets nine million square miles of the Northern Hemisphere. They seem to start in deep pockets of unfrozen ground, known as taliks. One common place taliks form is under lakes, where the overlying water heats and insulates the land below. Yet the lakes are ever-shifting features as the surrounding permafrost freezes and thaws, so they frequently fill or can drain entirely. And if a lake drains, the ground starts to ice over.
“It can refreeze from the bottom, and the sides, and the top, so it’s freezing from all directions,” says Katey Walter Anthony, an ecologist at the University of Alaska Fairbanks. Since ice takes up more space than water, the growth of ice squeezes the unfrozen slurry, concentrating and pressurizing the gas and water, eventually bulging at the surface in a wart-like hill, called a pingo.
Not all of the craters are related to lakes, Natali notes. Taliks can form in other situations, such as within a subterranean zone high in salt, which reduces the temperature at which water freezes. Some pingos are continually fed from below by rising groundwater.
Pingos are common throughout the Arctic, with more than 11,000 dotting the Northern Hemisphere. But crater-forming blasts, it seems, are much rarer. They’ve been observed only in Siberia’s Yamal and Gydan peninsulas. And those explosions require an excess of one particular ingredient: gas.
Natural gas abounds in western Siberia, and some of it likely percolates up along cracks and porous zones in the ground into the mushy talik. But there are other possible sources of gas. Microbes munch on organic matter and exude methane or carbon dioxide. Some of the gas may also come from the breakdown of what’s known as methane hydrates, a crystalline form.
“It may not be one thing,” Natali says. Different mounds might have slightly different gaseous contributors, but the gases all likely serve a similar purpose: pressurizing. Eventually, either because of mounting gaseous pressure or destabilization of the overlying ice cap, the system will let loose in a powerful blast that can spew the slurry across the surface and leave a steep-sided crater behind.
“It’s like champagne,” Bychkov says.
Connections to climate and beyond
Studying the explosions could help untangle some of the icy blasts on other bodies in the solar system. In particular, the Siberian craters may prove to be an intriguing analog for ice volcanism on the dwarf planet Ceres, which—unlike many icy worlds that have cryovolcanism—has some of the same ingredients found in the Arctic, says Lynnae Quick, a planetary geophysicist who specializes in cryovolcanism at NASA’s Goddard Space Flight Center.
“Ceres is really interesting because it has this rocky soil component that takes part in these processes that we don’t see when we look at icy moons,” Quick says. “We’re still trying to figure out what we’re looking at there.”
Likewise, lingering questions remain about the Siberian craters. One is their connection with climate change. The Arctic has seen a number of extraordinarily warm temperatures in recent years. Just this summer, on June 20, the small town of Verkhoyansk, Russia, reached a scorching 100.4 degrees Fahrenheit, the highest temperature the region has seen since record keeping began in 1885.
While the craters seem to have proliferated since their discovery in 2014, the phenomenon could stretch back thousands of years and we’ve only just recently noticed, Walter Anthony says. Flights over the region have become more common, and Yamal’s population in particular has grown tremendously. “Now there’s a railway and huge towns,” Bychkov says.
Warmer climate, however, may contribute to more frequent blasts, since melting can destabilize the icy cap on the gaseous pockets, sparking an explosion. Melting could also increase the connections between the ground and surface, creating “chimneys” through which the deep gases can more readily seep upward into taliks, Walter Anthony adds.
In the grand scheme of greenhouse gas emissions, the puffs of methane and carbon dioxide released from each explosion are likely insignificant. But the explosions may provide a “short-term glimpse of a longer-term phenomenon,” Walter Anthony says.
Climate change has already taken a toll on the Arctic, which is warming at least twice as fast as the rest of the planet. An increasingly thick layer of the carbon-rich permafrost thaws each year—and in some places, the ground isn’t re-freezing, even in the winter months. Such thawing allows microbes to munch on the once frozen organic material and burp out carbon dioxide or methane. But it also poses deeper concerns. Permafrost acts as a lid on stores of geologic methane gas deep underground, slowing down their trek to the skies, Walter Anthony explains. As the permafrost melts, that lid may grow increasingly rife with holes that allow the methane to escape at the surface.
Walter Anthony studies this phenomenon in Arctic lakes, and notes that recent studies on crater formation may be further evidence of deep gas already burbling up to the surface. “As we turn permafrost from a block of cheddar cheese into a block of Swiss, we should see more of it,” she says.
“It’s a wild card in the climate change story.”