Africa

A Nuclear Explosion on Mars

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.

African Lake Study Leaves Some Questions

The headline from the University of Arizona News, and many other news outlets said, “Twentieth-Century Warming in Lake Tanganyika is Unprecedented.” The headline from Brown University press release (home of the lead author) said, “Brown Geologists Show Unprecedented Warming in Lake Tanganyika.”

Well, not exactly. The title of the study referred to is “Late-twentieth-century warming in Lake Tanganyika unprecedented since AD 500,” published in Nature Geoscience (16 May 2010). Even that more modest claim doesn’t tell the whole story.

First some background. Lake Tanganyika occurs within the East African Rift, which is a divergent tectonic plate boundary that is gradually separating East African countries from the main continent. The rift contains both active and dormant volcanoes. The lake is 418 miles long and 45 miles wide. Its average depth is 1,870 feet with a maximum depth of 4,820 feet. Portions of the lake are claimed by Burundi, Democratic Republic of the Congo, Tanzania, and Zambia. Fishing the lake provides a major food source for people in the surrounding lands. There is concern that lake warming will disrupt the fish supply.

The abstract of the paper concludes, “Our records indicate that changes in the temperature of Lake Tanganyika in the past few decades exceed previous natural variability. We conclude that these unprecedented temperatures and a corresponding decrease in productivity can be attributed to anthropogenic global warming, with potentially important implications for the Lake Tanganyika fishery.”

The questions I had upon reading this were: 1) Are the temperatures really unprecedented? 2) Do they exceed natural variability? 3) What is the evidence that the warming was caused by anthropogenic global warming? 4) Could there be some other cause of fish decline?

The researchers studied lake sediment cores going back 60,000 years and by using proxies deduced a temperature record for the lake surface temperature. In the current study, the researchers said that during the last 1,500 years, temperature varied between 22.5º C and 25.7º C, and that in the last 50 years the temperature rose by 1.6º C.

However, in 2008, these same researchers published a paper in Science (Vol. 322. no. 5899, pp. 252 – 255) which said the lake surface temperature fluctuated between 27° and 29°C over the last 60,000 years according to their interpretation of lake sediment cores.

I emailed a co-author of the paper, a UofA professor, asking for an explanation of this apparent discrepancy. He replied by referring me to the website of the lead author at Brown University. There, she explained that there was a problem in calibration of the temperature proxies. She presents a graph showing the records after recalibration. It is reproduced below. It should be noted that there are two separate core sample locations. The more recent core was taken closer to shore than the older, longer record. The more recent record initially shows cooler temperatures where the two records overlap. The researchers attribute this discrepancy to upwelling cold water from deeper in the lake. So which record is closer to the real surface temperature?

TanganyikaTemp

According to the lead author’s own data as shown on the graph, it is obvious that the current temperatures are not unprecedented, nor do they exceed natural variability. The title of their paper is technically correct only if one accepts cherry-picking start dates.

That leaves the question about the cause of the warming. The UofA scientist replied to my email, “our record only demonstrates a lake surface temperature history, not the cause of that history.” The allegation of an anthropogenic cause, a major conclusion of the paper, was made without any supporting evidence, just speculation.

I am wondering why the paper abstract contains the conclusions it does. Is it time for some scary scenarios to promote more study and more funding?

This whole study purports to be about lake surface temperatures, but it contains very few such measurements from the lake surface. From my reading, the researchers deduce surface temperatures from only two core sample locations. As the NOAA satellite graphic below shows, on any given day, at any given time, the variation in lake surface temperature can be as much as 4º C in different parts of the lake, and that equals or exceeds the entire range of temperatures found in the studies. It would seem, therefore, that any temperature record derived from sediment cores could vary greatly depending on location. Since this study had just two sample locations, it makes one wonder if it gives a true representation of actual conditions.

satellite_tanganyika

And about the fish. The current paper says that warming is causing a decline in fish abundance. Yet an earlier study, of which the UofA scientist was a co-author, says the fish decline is caused by land disturbance. “Watershed deforestation, road building, and other anthropogenic activities result in sediment inundation of lacustrine habitats.” “Our faunal analyses suggest that all three taxonomic groups are negatively affected by sediment inundation but may have varying response thresholds to disturbance.” (Citation: Conservation Biology, vol. 13, no. 5, Oct. 1999).