How did life arise on Earth? How did it survive the Hadean eon, a time when repeated massive impacts excavated craters thousands of kilometres in diameter into the Earth's surface? Those impacts turned the Earth into a hellish place, where the oceans turned to steam, and the atmosphere was filled with rock vapour. How could any living thing have survived?
Ironically, those same devastating impacts may have created a vast subterranean haven for Earth's early life. Down amongst all those chambers and pathways, pumped full of mineral-rich water, primitive life found the shelter and the energy needed to keep life on Earth going. And the evidence comes from the most well-known extinction event on Earth: the Chicxulub impact event.
A new study presents evidence that the Chicxulub crater was host to an enormous subterranean network of hydrothermal vents that could have provided a sanctuary for microbial life.
By extension, much earlier impact craters likely provided the same sanctuary. The study is titled 'Microbial Sulfur Isotope Fractionation in the Chicxulub Hydrothermal System'. The lead author is David Kring from the Lunar and Planetary Institute. It's published in the journal Astrobiology.
The idea that life could have arisen and persisted in the network under impact craters is called the impact origin of life hypothesis. David Kring is a leading scientific voice supporting that hypothesis.
While massive repeated impacts made Earth's surface uninhabitable during the Hadean eon, the same wasn't likely true of the region under the impact craters.
According to Kring, those same impact events "…were producing vast subsurface hydrothermal systems that were perfect crucibles for pre-biotic chemistry and habitats for the early evolution of life."
In this new study, Kring and his colleagues present evidence from the International Ocean Discovery Program and the International Continental Scientific Drilling Program.
Those programs provided rock cores from the Chicxulub crater ring. Specifically, this study is based on about 15,000 kilograms (33,000 pounds) of rock retrieved from a bore-hole 1.3 km (0.8 miles) deep.
Above: Section of the Chicxulub core with the hydrothermal minerals dachiardite (bright orange) and analcime (colourless and transparent). The minerals partially fill cavities in the rock that were niches for microbial ecosystems.
Since pyrite is an iron sulphide, it contains isotopes of sulphur. Those isotopes showed that the framboids were formed by microbes, and those microbes were part of an ecosystem that was adapted to the heated mineral-laden fluid that flowed through the underground network. That network was present under the shattered peak ring of the Chicxulub impact crater.
Life needs energy to survive, and this microbial life got its energy from chemical reactions in the system of rock and fluids.
They converted sulphate in the fluid into sulphide, which was then preserved as pyrite. These ancient thermophilic microbes would have been quite similar to the thermophilic microbes that populate extreme environments on present-day Earth, like deep ocean hydrothermal vents and the hot springs in Yellowstone Park.
In their paper the authors write:
"Sulfur isotope analyses of pyrite framboids in impact breccia from the Chicxulub crater indicate thermophilic colonies of sulfate-reducing organisms inhabited the porous, permeable rock beneath the floor of the crater and fed on sulfate transported through the rock via an impact-generated hydrothermal system."
They add that the same sulfur-reducing organisms persisted for 2.5 million years after the impact, and that organisms found under Chicxulub now are probably the direct descendants of those earlier organisms.
This study comes at the tail end of 20 years of research into the impact origin of life hypothesis. Dr Kring's involvement began before that when he co-authored a 1992 study that linked the Chicxulub crater to the K-T boundary mass extinction. Dr Kring was also involved in research showing that the region underneath the Chicxulub crater was porous.
Subsequent research showed that the region was riven with a vast system of hydrothermal vents. Now, this study shows that the system of vents hosted life.
It's been a long arduous path to this latest discovery. The timing and duration of bombardment during the Hadean was difficult to establish, since none of Earth's Hadean craters are still around. Study of lunar craters and lunar samples from the Apollo missions showed that the Moon was subjected to intense bombardment for approximately 400 million years during the Hadean. The Earth would've suffered through the same episode.
The Chicxulub crater is the only reasonable proxy for a Hadean crater. So the crater is one way to develop and test the impact origin of life hypothesis.
As the authors write in their paper, "Chicxulub is the only large peak-ring basin that is still intact and provides an opportunity to study the remnants of an impact-generated hydrothermal system, from depth up to and including the venting surface environment, similar to those that may have existed earlier in Earth's history."
It also took time and effort to establish how long the vast hydrothermal systems under these craters existed. The question was: Did they last long enough for evolutionary processes to occur, and for biological material to migrate to adjacent impact craters and their systems?
Extensive modelling showed that the systems of vents were long-lived enough for both things to happen.
The authors write: "Thermal evolution models of that and other hydrothermal systems indicated they were long-lived and produced significant volumes of porous, permeable rock suitable for thermophilic organisms."
There's still more work to be done to confirm the impact origin of life hypothesis. In a two-page brief, Dr Kring outlines some of that work.
First of all, core samples from the Chicxulub crater need more study. Kring wants to retrieve more information from those samples about the evolution - both thermal and chemical - of the system of hydrothermal vents.
There's also a need for a better understanding of the energy available to microorganisms in the granitic crust that was present during the Hadean.
And to clarify the chronology of impacts on Earth, researchers need more samples from lunar impact basins like the South-Pole Aitken basin and others.
But the cherry on top might be finding samples of Hadean Earth on the Moon itself.
"If sedimentary and fossiliferous samples can be located, they would provide a direct record of the early evolution of life on Earth," Kring writes.