Several hundred million years after Earth formed, when life was emerging, our young planet had an atmosphere, oceans and primordial continents. But it did not yet have an ozone layer to shield the surface from the sun’s harshest ultraviolet rays. Because UV radiation can damage DNA, that would have made it difficult for any but the most extreme forms of life to survive.
In 2002, a team led by astrobiologist Charles Cockell at the University of Edinburgh, UK, discovered a unique group of cyanobacteria in Haughton crater in northern Canada. The bacteria live in tiny pores and cracks of near-translucent rock, formed during the intense heat and pressure of the asteroid or comet impact that made the crater, about 23 million years ago.
Cockell’s team found that the altered crystal structure of the rocks absorbed and reflected UV rays. This suggests the rock could shield the bacteria while letting enough sunlight through to allow them to photosynthesise.
Complex life evolved long before the crater formed, but there have been countless space rock strikes in Earth’s history. “That raised a whole bunch of questions about whether the unique geology of impact craters could have been a good UV shield on the early Earth,” says Casey Bryce, a member of Cockell’s lab.
Bryce and her colleagues got an unusual chance to test the notion in 2008. As part of the European Space Agency’s EXPOSE mission, the team sent some of the crater rocks to the International Space Station (ISS). Before lift-off, they grew samples of the cyanobacteria either in plain glass discs or in discs of the impact-altered rock. Once in space, these discs were mounted on the outside of the ISS, where they were left exposed for nearly two years.
The bacteria received radiation doses far more intense than conditions on early Earth. When the samples were returned to the lab, the microbes in the glass discs were dead.
“However, when we cracked open the impact-shocked rocks we were able to detect chemical signals of life and rejuvenate the dormant cyanobacteria,” says Bryce. The team’s findings provide the first direct evidence that crystal cocoons formed by impacts might have been radiation-proof cradles for early life.
Asteroid and comet impacts are ubiquitous in the solar system, so Pontefract thinks impacts could have helped kick-start life on rocky planets and then shielded whatever emerged. Crater rocks could provide refuges even now for life on other planets, such as Mars, she says.
I just wanna rant about directed panspermia and ancient aliens and start an Asteroid Cult and… maybe I’ll finally set up the basic podcast kit I got for Xmas tomorrow.