Traces of life found in ancient impact crater

Artist's impression of meteorite striking the Earth

8 October 2010 by Adrian Boyce and John Parnell

In remotest Arctic Canada, scientists are discovering that life can exploit the harshest of conditions on our planet - not the Arctic winter, but the aftermath of a massive meteorite collision. Could traces of life be found in this sort of area on Mars too? Adrian Boyce and John Parnell tell us more.

Disaster movies like Deep Impact with comets colliding catastrophically with Earth inevitably involve the extinction of 'life as we know it'. And just ask the dinosaurs how big an influence meteorite impacts have on survival prospects on our planet! But, that doesn't mean that all life is destroyed by impacts. Far from it - our recent research is providing evidence that some bacteria may actually thrive in the thermal spring systems these events leave behind.

These bugs leave behind distinctive chemical traces, and we may be able to find similar traces in the impact craters of Mars. Discussions are under way to develop instruments for future Mars landers to do just that.

The Haughton impact crater lies in the wilderness of the Canadian High Arctic on Devon Island - the largest uninhabited island on Earth. Nearly 40 million years ago, a meteorite two kilometres across crashed into Earth, leaving behind a 23km-wide crater in the bedrock and causing serious damage over an area of 50km2. It melted stone and formed what are known as impact 'breccias' - a tell-tale pattern of smashed rocks.

In fact the movies exaggerate only slightly. These asteroids do strike with enormous speed (more than 10km a second). On impact, much of this energy dissipates into the rocks around as heat, generating temperatures of thousands of degrees centigrade. The rocks the meteorite encountered were mainly ancient carbonates, around 470 million years old, but they also contained thick beds of sulfate salts, called gypsum. These are the remnants of ancient seas and lakes that dried up, of which there are many examples through geological time.

Researchers sampling rocks

Researchers carry out sampling in the Haughton impact structure breccias

The sulfates around the Haughton crater were broken up and even melted by the impact. In some areas they were dissolved by the scalding water circulating around the newly formed underground fractures and voids - a natural mechanism called a hydrothermal system that cools the Earth after such events. This system lasted for around 10,000 years - this sounds a long time to us, but in geological time is just the blink of an eye.

The occurrence of sulfate also sparks an intriguing possibility. Sulfate is at the heart of one of the oldest and most important biological metabolic functions on Earth - bacterial sulfate reduction. Just as we metabolise oxygen and organic matter to produce carbon dioxide, so sulfate-reducing bacteria (SRB) metabolise sulfate and organic matter and produce hydrogen sulfide, a chemical with a characteristic rotten-egg smell that makes it a favourite ingredient in stink bombs.

Of microbes and meteorites

SRB can live only where there is no oxygen, so they are generally found in soils, mud on the seabed, or even deep in the Earth in what scientists have called the deep biosphere. Wherever there's sulfate, organic matter and no oxygen you're likely to find SRB activity - even at extreme temperatures.

Much of the hydrogen sulfide they produce escapes into the atmosphere, but some of it combines with iron in the surrounding rocks and mud to produce iron sulfide minerals. Most commonly these are pyrite - fool's gold - but also another compound called marcasite. Both minerals are abundant in cracks and fissures in the Haughton impact breccia, deposited by the flowing hydrothermal waters.

However, there are other natural processes that can make iron sulfides with no need for living things. So, how could we tell that SRB were responsible if all this happened many millions of years ago?

We looked at the precise chemical make up of 25 samples of iron sulfide from all over the crater, and found a distinctive chemical signature, very different from that which can arise without the presence of life.

Fragments of rock

Fragments of rock in the soil zone where iron sulfides are weathered to rusty-coloured sulfate minerals

Atoms of the same chemical element come in different varieties, called isotopes. All atoms of an element have the same number of protons - that's why they're the same element. But the number of neutrons in the atom varies. Some kinds of sulfur have more neutrons than others, and we found that the split between different sulfur isotopes in the Haughton crater sulfides could have arisen only through the activity of microbes.

SRB much prefer the slightly lighter sulfur-32 isotope to the heavier sulfur-34 variety, so the sulfides they produce contain lots more sulfur-32 than sulfur-34. This isn't the case with sulfides that form naturally. So, there's little chance this isotopic signature could have been produced by a non-biological process - the difference between the starting sulfates and the eventual sulfides is just too great.

Furthermore, we have found that when this 'bacteriogenic' sulfide is oxidised back to sulfate by exposure to the weather at the surface, there is very little change from the original sulfide isotopic value. This means that even these sulfate minerals retain the tell-tale sulfur isotopic signature after weathering.

Among those planetary bodies nearby which are thought most likely to harbour life are Mars and Europa, one of Jupiter's moons. It also seems that their surfaces are rich in sulfates, left behind from the gases given off by ancient volcanoes. This abundance has fuelled speculation that simple life on Mars could get energy from the transformation of sulfur compounds - sulfur metabolisms are thus a credible component of life on Mars.

Areas of Mars that are thought to be rich in sulfate have already been identified as priority targets in the search for life. Our new observations of widespread sulfide precipitation, mediated by bacteria, in impact breccias in a sulfate-rich terrain, indicate that Martian sulfur minerals in impact crater settings should be strong candidates for sulfur isotopic analysis, and that the next missions to return to Mars should aim to gather such samples.

A programme has also started to develop a mass spectrometer system to do the analysis via laser-based instruments on a lander. It may be that the answer to the question of whether there is life out there could be just a laser zap away.

Dr Adrian Boyce is manager of the NERC Isotope Community Support Facility at the Scottish Universities Environmental Research Centre. Professor John Parnell is chair in geology & petroleum geology at the Department of Geology & Petroleum Geology at the University of Aberdeen.

'Sulfur isotope signatures for rapid colonization of an impact crater by thermophilic microbes' - J Parnell, A Boyce et al (2010). Geology, 38, 271-74.