Life After Impact: What a Finnish Crater Can Tell Us

When a meteorite slammed into what is now western Finland some 77 million years ago, it left behind more than just a scar. The Lappajärvi crater, a 23-kilometre-wide basin, became a geological time capsule, and, as it turns out, a biological one too.

We often think of impact events as extinction-level catastrophes, such as what happened with the extinction of all non-avian dinosaurs 66 million years ago. And they certainly can be. But they also create something else as an impact event delivers considerable energy onto a planetary crust, causing extensive fracturing and heating of rock in proportion to the size of the crater they leave behind. The heat, fractures, and fluid pathways often lead to the development of hydrothermal systems, which manifest as vents similar to those found at the bottom of Earth's oceans. 

These hydrothermal vents stand out as hotspots where rich ecosystems flourish despite the absence of sunlight; numerous organisms, including archaea and extremophiles, derive energy through chemosynthesis, utilising the heat, methane, and sulfur compounds emitted by hydrothermal vents known as black smokers. Higher-order species, such as clams and tubeworms, sustain themselves by consuming these primary producers. These environments demonstrate how life can not only endure but thrive under extreme conditions.

Indeed, while impacts eradicate all life nearby, the resulting spaces created by the fractures and the geochemical and thermal conditions could allow microbes to gain a foothold. The questions here are: how long do hydrothermal systems keep the impact site habitable after the cooling-down period (below 122 °C), and is this window sufficient for extremophiles to establish themselves? Understanding these can help us better determine if a hypothetical lifeform could do the same on other planets, such as Mars, where deep biosphere habitats in impact structures are considered favourable targets, and if so, could traces of that life still be preserved in the mineral record, waiting to be discovered?

Back in Finland, the Lappajärvi crater drew scientific interest as researchers sought to better understand these questions. Studies on similar-sized craters suggest that hydrothermal systems may have remained active for periods lasting above 50,000 years (at the Haughton impact structure) up until 250,000 years (at the Ries Impact structure). High-precision dating of Lappajärvi demonstrated that its hydrothermal systems persisted significantly longer, enduring for over a million years following the impact. This extended duration of the hydrothermal system would have provided ample time for microorganisms to colonise, making it a prime target for the investigation of ancient microbial biosignatures. Nevertheless, no evidence of microbial colonisation at Lappajärvi has previously been reported. Actually, among the more than 200 confirmed impact structures on Earth, evidence of microbial colonisation has been found in only a few.

As presented in the illuminating scientific paper here, researchers found evidence for past microbial colonisation at the Lappajärvi impact crater using micro-scale analytical techniques. The team focused on analysing isotopes found in calcite and pyrite crystals that lined fractures and small cavities within the melt rock and breccia (a rock made up of broken fragments), from 3 to 145 meters below the surface. Sulfur isotopes in pyrite and carbon and oxygen isotopes in calcite can provide an indication as to their origin and at what temperature they were formed.

Based on the study, the sulfur isotopes were consistent with microbial sulfate reduction, pointing to methanogenesis and methanotrophy, both processes found in hydrothermal vents. The researchers also estimated that the calcite formed at temperatures ranging from 8°C to 47°C, well within the threshold for microbial life, and was indicative of both anaerobic microbial consumption and production of methane. Dating of the samples suggests that microbes re-established themselves within approximately 4 million years after the impact, and remained active for over 10 million years as the crater gradually cooled.

Ultimately, this paper suggests that hydrothermal systems created by impact events may have sustained habitable conditions for millions of years, offering a potential pathway for life to establish itself on planetary bodies like Mars or Ceres. I don’t know about you, but I’ll never look at impact craters the same way again.

As always, onwards and upwards.