It has long been a mystery of why there is a super-abundance of uranium, thorium, and potassium on the Martian surface concentrated near Mare Acidalium in the region of the large, shallow depression. Also, the Martian atmosphere has an unusual amount of radiogenic isotopes.
An explanation for this Martian mystery was presented by Space Physicist John Brandenburg at the 42nd Lunar and Planetary Science Conference in Houston, TX this month. According to the press release, Brandenburg suggests, “evidence shows that approximately 180 million years ago the planet Mars was devastated by a massive natural nuclear explosion. This natural event filled its atmosphere with radio-isotopes, irradiated its soil and atmosphere with neutrons, and spread a layer of radioactive material on the surface of Mars. His analysis estimates the force of the explosion to have been in excess of 1 million one megaton hydrogen bombs. This explosion created a region of enhanced radioactivity centered in the northern Mare Acidalium region at approximately 55N and 15 W.” You can read Brandenburg’s paper here.
If you think Brandenburg’s proposal is far-fetched, consider that something similar almost happened in Africa about two billion years ago.
In 1956, uranium was discovered in Oklo, Gabon, then a French colony. The uranium was mined and used in French nuclear reactors. Most uranium is the stable heavy isotope U-238 and has to be refined to recover the fissionable U-235. It was found that the uranium from Gabon was unusually depleted in U-235. Geologists investigated and in 1972 proposed that the U-235 was depleted because the Gabon deposits were the remains of a natural spontaneous nuclear reactor.
As explained by Andrew Alden, writing in about.com, geology:
What made such a thing possible was that in the distant past uranium was naturally enriched in U-235, that is, less of it had decayed away by nuclear fission. About 1.7 billion years ago, to be more precise, a natural deposit of uranium ore was radioactive enough to generate about 100 kilowatts of heat, off and on, for more than a million years.
Geologic forces gathered the uranium together. First a layer of sandstone was infiltrated by uranium-bearing groundwater, leaving a relatively thin sheet of uranium-oxide ore. Then the rocks were tilted, and as they eroded downward the groundwater concentrated the uranium minerals, sweeping them downward within the sandstone until a thick stripe of ore was built up. That’s when things heated up.
To understand what happened next, you need to know a little about nuclear reactors. The nuclei of uranium atoms normally decay with the release of energetic neutrons—so energetic that they fly away without interacting with other uranium nuclei. The neutrons need to be slowed down before they can start splitting other uranium nuclei, which release more neutrons and start a feedback cycle. Something needs to moderate the neutrons. The first artificial reactor, built in 1942, used balls of enriched uranium spread out inside a large pile of graphite blocks, which served as a moderator.
But water acts as a moderator, too. At Oklo there was a lot of water, probably a river flowing above the buried orebody. The water allowed the nuclear interactions to reach the critical point, and the reactor began to work. But as it heated up, the water turned to steam and flowed away. With the moderator gone, the chain reaction stopped and did not start again until the orebody cooled and the water returned. This simple feedback cycle kept the Oklo reactors (there were at least a dozen of them) active until the U-235 was depleted. That took about a million years. When the Oklo mine was producing ore in the 1970s it was that telltale depletion of U-235, unheard-of in nature, that tipped scientists off.
A remarkable thing about the Oklo reactors is that the highly radioactive waste products stayed put without the elaborate containment we use today on nuclear power plant waste. More than a billion years later, everything is contained within a few meters of its source.
Recently a team of scientists took advantage of this excellent preservation and studied the isotopes of xenon gas—a product of uranium decay—trapped in phosphate minerals at Oklo. Led by Alex Meshik of Washington University of St. Louis, they reported in 2004 that the reactor went through eight cycles a day, running for 30 minutes then shutting down for two and a half hours. The whole thing is reminiscent of geysers.
Why was uranium so much more radioactive then? With a half-life of 700 million years, U-235 started out making up nearly half of all uranium when the solar system began some 4560 million years ago. Many shorter-lived radioisotopes that existed in the beginning, like aluminum-26, have become extinct. We know of their former existence by the presence of their decay products in ancient meteorites—nuclear fossils.
The abundance of water in Gabon prevented a nuclear explosion. On Mars, however, it is hypothesized that the uranium deposit was much larger than that at Oklo, large enough to contain the errant neutrons after being triggered by a deep intrusion of groundwater.