Brandenburg is a physicist who submitted a paper to the 42nd Lunar and Planetary Science Conference a few years ago. It’s way outside my area of expertise, but it postulated an interesting scenario from the ratios of rare isotopes in the atmosphere of Mars: that there was evidence of a natural nuclear reactor, like Oklo on Earth, that had exploded over 180 million years ago. He makes a good case, at least to this biologist’s eyes, and it seems reasonable.
Natural Nuclear Reactors formed and operated on Earth, there is no reason this could not have happened on Mars. Conditions on Mars: lack of plate tectonics, and nearness to the asteroid belt, may have favored such occurrences in larger size and duration than on Earth. Changes in groundwater distribution, due to either climate change of loss of geothermal heat, may have triggered this event. The occurrence of such a large natural reactor may explain some puzzling aspects of Mars data, such as the superabundance of K and Th on the surface and the large inventory of radiogenic isotopes in the Mars atmosphere.