Around 375 million years ago, a vast rock came crashing down to Earth, leaving an immense crater in modern-day Sweden. Now, scientists have uncovered evidence to suggest ancient microbes dwelled in the cavity, which could have implications for the search for life beyond Earth.
Sweden’s Siljan crater, aka the Siljan Ring, is the biggest impact structure in Europe, measuring about 52 kilometers (32 miles) across. The crater is being drilled for natural gas and researchers at Linnaeus University have managed to get their hands on some of the resulting rock cores.
The researchers examined fractured rock found deep within the crater and spotted signs of ancient life. The rock fractures contained teeny crystals of calcium carbonate and sulfide, which appear to be the result of microbial activity.
“Specifically, the relative abundance of different isotopes of carbon and sulfur within these minerals tells us that microorganisms that produce and consume the greenhouse gas methane have been present, and also microbes that reduce sulfate into sulfide,” said lead author Henrik Drake in a statement. “These are isotopic fingerprints for ancient life.”
To work out when the microbes might have been active, the researchers used radioisotope dating techniques and concluded that the crystals formed between 80 and 22 million years ago. While this suggests that microbes were active in the crater for a very long time, it also suggests they lived there as long as 300 million years after the initial impact. The findings are reported in Nature Communications.
Life doesn’t just exist on Earth’s surface, much thrives deep beneath our feet in what is known as the deep biosphere. Critters that survive at these depths are sometimes referred to as intraterrestrials, and it’s thought that their homes are often created by meteorite impacts.
So what do Siljan’s intraterrestrials tell us about the potential existence of extraterrestrials? Well, if life were to exist on other planets, it may well have been triggered by meteorite impacts. These impacts allow life to colonize the area by creating pores for microbes to live in, and by driving hydrothermal convection – the circulation of fluids deep in the Earth – which benefits deep ecosystems.
“Detailed understanding of microbial colonization of impact craters has wide-ranging astrobiological implications,” explained study co-author Magnus Ivarsson. “The methodology that we present should be optimal to provide spatiotemporal constraints for ancient microbial methane formation and utilization in other impact crater systems, such as the methane emitting craters on Mars.”
“Our findings indeed confirm that impact craters are favorable microbial habitats on Earth and perhaps beyond,” added Drake.
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