Oklo genesis?

  • No natural reactors exist today, as the relative density of fissile uranium has now decayed below the level needed for a sustainable reaction. Pictured here is Fossil Reactor 15, located in Oklo, Gabon. Uranium oxide remains are visible as the yellowish rock. Image credit: Robert D Loss/CEMS
  • Alexander Meshik (foreground), Charles Hohenberg and Olga Pravdivtseva combined a selective laser with ion-counting mass spectrometry to find out how the world’s only natural nuclear reactor worked. They determined that the Oklo reactor, which comprises several separate sites, ran for 30 minutes, then shut off for 2,5 hours before starting over. And it did this for two million years. Image credit: US Department of Energy
  • (1) Nuclear reactor zones; (2) Sandstone;(3) Ore layer; (4) Granite. Image credit: David Kilper/WUSTL
Date:25 February 2012 Tags:, , , ,

It’s a startling concept by anyone’s measure, but more than one scientist is putting two and two together… and coming up with the same question: could natural nuclear reactors have triggered the start of life on Earth? By Clara Moskowitz

If you thought all nuclear reactors were made-made, or buried in the hearts of stars, you’d be wrong. Evidence for a cluster of natural nuclear reactors has been found on Earth, and some scientists say our planet may have had many more in its ancient past.

There’s also reason to think other planets might have had their own naturally occurring nuclear reactors, although evidence for this is hazy. If they did exist, say experts, the large amounts of radiation and energy released by such reactors would have had complicated effects on any developing life.

Natural nuclear reactors occur when deposits of the radioactive element uranium build up in one spot, and eventually ignite a self-sustaining nuclear chain reaction where uranium divides in a process called fission, producing other elements. The reaction releases a powerful punch of energy – energy that could prove beneficial or highly detrimental to developing life, depending on the circumstances. (The jury is still out on this one.)

Only known example
The only known examples of natural nuclear reactors were found in the Oklo region of Gabon, Africa, in 1972. French miners discovered that the uranium samples they extracted were depleted in the rare isotope uranium 235, the only naturally occurring material on Earth capable of sustaining fission reactions. It was as if the material had already gone through a nuclear reaction and been used up.

In fact, that’s the scenario most supported by studies. Scientists think a concentration of uranium 235 in that location went critical about 2 billion years ago and underwent fission, just as it does inside man-made nuclear reactors. “As far as we know, we have evidence of natural reactors forming and operating only at the one site in Gabon, but that demonstrates that it’s possible, and our calculations suggest it was much more probable earlier in Earth’s history,” said Jay Cullen of the University of Victoria in Canada.

Cullen and Laurence A Coogan, a colleague at the University of Victoria, researched how likely these reactions were when Earth was much younger, based on how much uranium in a given area is necessary for the material to go critical and start a self-sustaining fission reaction. They found that during the Archaean epoch, between about 2,5 billion and 4 billion years ago, natural nuclear reactors may have been relatively common.

“It certainly seems more than likely that these sorts of reactors would have been much more common in Earth’s early history because the amount (of uranium) you need is actually quite small,” Cullen told Astrobiology Magazine. However, because the time scale is so vast and David Kilper/WUSTL the geological record so poor, scientists have very little chance of confirming this idea.

Spark of life
If natural nuclear reactors were indeed present on early Earth, they could have had interesting effects on any nascent life. The ionising radiation released by a nuclear reaction can damage DNA, the precious instruction code built into every living cell. If organisms were living too close to the site of a reactor, they could have been wiped out completely. However, life hanging out on the outskirts of a nuclear reactor might have received a smaller dose of radiation – not enough to kill, but enough to introduce mutations in the genetic code that could have boosted diversity in the local population.

“The ionising radiation would actually provide some genetic variation,” Cullen explains. “That’s the quantity that natural selection is going to act upon, and with time, it might help to promote change in organisms. I think that most people view ionising radiation as a bad thing, but that’s not necessarily so.”

Furthermore, some scientists think the nuclear reactors themselves could have provided an even greater boon to life by giving it the spark it needed to originate in the first place. Zachary Adam, now a graduate student at Montana State University in Bozeman, suggested the possibility in a 2007 paper in the journal Astrobiology, which he wrote as a graduate student at the University of Washington.

Scientists don’t know for sure how life got started on Earth, but some think it required a burst of energy. This energy would have been required to break the bonds of simple elements such as carbon, nitrogen, hydrogen and oxygen, so that they could recombine to form the first complex organic molecules. Other researchers have suggested that a lightning strike might have provided the requisite energy, but Adam thinks the energy released by a natural nuclear reactor is more likely to have provided the catalyst.

“I think it is at least as possible as other ideas, if not more plausible, but I realise everyone is partial to their own ideas.”

Source: Astrobiology Magazine

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