Life Under Bombardment
No-one knows when life first established a firm foothold on Earth. Ask around in the scientific community, though, and you’ll probably hear that the surface of early Earth, before about 3.8 billion years ago, was too hostile an environment for even a lowly microbe to set up shop.
The main problem, as the conventional argument goes, was that between around 4.1 and 3.8 billion years ago, Earth was constantly being bombarded by comets and asteroids. The disastrous effects of these impacts would have rendered the Earth’s surface uninhabitable.
Not necessarily so, say a team of astrobiologists who are studying the oldest known sedimentary rocks on Earth. They have come to the conclusion that conventional wisdom on the subject may need some revision. Their conclusions will be published in a forthcoming issue of the Journal of Geophysical Research.
The rocks in question appear to be a banded iron formation, or BIF, from Akilia Island in West Greenland. BIFs were deposited on Earth’s ocean floors during the first 2 billion years of the planet’s history. Iron and oxygen present in the oceans combined to form rust, which settled to the sea floor in layered sediments. Movements of the Earth’s crust later pushed some of these sediments to the surface, where they can now be studied.
It’s not possible to date the Akilia BIF sediments directly because they have undergone metamorphism – pressure cooking – so that traditional radioactive dating techniques cannot be used to determine their age. But jutting into these sediments are younger igneous rocks that can be accurately dated with these techniques.
An earlier analysis of this igneous rock, performed by a group headed by Dr. Allen Nutman of the Australian National University, put its age at 3.85 billion years. And because the igneous rock intrudes into the banded iron formation, it must have formed later than the sediments did. So the Akilia sediments must have been formed at least 3.85 billion years ago. Exactly how old they are, there is no way to know. But they are more ancient than any other sedimentary rocks found so far on Earth.
Rocks this old are rare on Earth because tectonic recycling action has crushed, buried and melted all of the material that formed the Earth’s crust during its first half-billion years of existence. Finding sedimentary rocks this old is important to geologists because they provide invaluable clues about what Earth was like in its early years.
Few people dispute the notion that between 4.1 and 3.8 billion years ago, our planet was heavily bombarded by debris from space, a period known as the Late Heavy Bombardment. If you look up at a full moon on a clear night, you see that its surface is riddled with impact craters. Scientists who study the size and distribution of those craters see clear evidence that the Moon underwent an intense period of impacts between 4.1 and 3.8 billion years ago. Although no craters from this time remain on Earth, because the Earth and Moon are so near each other, the assumption is that Earth suffered a similar fate.
Ariel D. Anbar, an Assistant Professor in the Department of Earth and Environmental Sciences at the University of Rochester, working with Gail L. Arnold, a graduate student, decided to look for traces of this bombardment in the Akilia sediments. Comets and asteroids contain greater quantities of the chemical element iridium than does the Earth’s crust. So Anbar and Arnold, members of the NASA Astrobiology Institute (NAI), probed the Akilia sediments for abnormally high traces of iridium.
They didn’t find them. "Our naive expectation going in," explained Anbar, was that "these sediments date from this bombardment period, so we should see evidence of the bombardment in them, right? So we looked for iridium in these rocks and didn’t find any. They were clean as a whistle."
But earlier study of the Akilia sediments by one of Anbar’s collaborators, Steve Mojzsis, had turned up a very different type of signature in the Akilia formation – a signature of biological activity. Mojzsis, also a member of the NAI, performed his analysis of the Akilia sediments while at the University of California San Diego.
Carbon atoms come in two distinct forms, or isotopes. Carbon-12 atoms, the lighter of the two, contain 6 neutrons; carbon-13 atoms contain 7 neutrons. Microorganisms that take in carbon dioxide prefer to use the lighter carbon-12 atoms to construct the organic building blocks of which they are made.
When ancient ocean-dwelling organisms died, the carbon that was formerly part of their living tissue settled to the ocean floor, becoming part of the sedimentary material deposited there. When Mojzsis found that the Akilia sedimentary rock samples contained higher-than-normal quantities of carbon-12, he concluded that biological activity must have been taking place at the time the sediments were formed – at least 3.85 billion years ago.
So Anbar, Arnold and Mojzsis were faced with seemingly contradictory evidence. Life appeared to have been flourishing during the period of the Late Heavy Bombardment, at a time when Earth’s surface was thought to be uninhabitable. And traces of the bombardment were nowhere to be found in the Akilia rocks.
The solution lay in quantifying more carefully the effects of bombardment, using models developed by NAI member Kevin Zahnle at the NASA Ames Research Center. The essence of these models is that they treat the bombardment as a series of impact episodes, rather than assuming continuous pummeling of the Earth. They also take into account that smaller impact events are far more common than larger ones.
The Akilia sediments would not be expected to contain telltale traces of extraterrestrial iridium unless a massive asteroid had slammed into the Earth, spewing iridium into the global environment, precisely during the period when the Akilia formation was being deposited. Zahnle’s models indicate, however, that even during the Late Heavy Bombardment, such massive impacts were rare – too rare for there to be much chance of seeing their signs in sediments like those found on Akilia Island. So it made perfect sense that the sediments didn’t contain elevated levels of iridium.
Anbar and his colleagues reason that if the bombardment had a smaller-than-expected effect on the composition of sediments, it may also have had a smaller-than-expected effect on early life. Although small impacts were more common during the Late Heavy Bombardment than at any time since then, each such impact would destroy life at the surface only in one small area, not globally. Only the rarest, most massive impacts had the potential to wipe out all life on the planet’s surface.
Zahnle’s models indicate such impacts occurred only once every ten to one hundred million years. Moreover, the worst of their effects lasted for only about ten thousand years, after which time conditions on the Earth’s surface returned to normal. "So during most of this violent period of Earth’s history," says Anbar, "the Earth’s surface – if you’re a microbe, anyway – was a perfectly balmy place to be. Which runs contrary to this picture that is out there that this was a very inhospitable period of time for life."
What Next? Getting Above the Surface
That still leaves open one important question: Where could life hang out in safety during those rare, massive impact events that caused the surface literally to boil away? One suggestion is that hydrothermal vents might have filled that role. If life migrated down to these vents – or perhaps even began there – it could have continued on during major impact events, oblivious to what was going on the surface. And when the environment topside returned to habitability, life could have moved back up and recolonized the surface.
"So as long as microorganisms had places on the Earth where they’d be sheltered from really massive impact events," concluded Anbar, "there’s no reason that they couldn’t have repopulated the surface multiple times. And therefore there’s no reason not to expect to find evidence of life if you find sediments from the earth’s surface during the period of heavy bombardment."