It’s only a conference paper, so it needs some more work and refereeing before the case is more secure, but I was pointed to this paper today which reports the possibility that a natural thorium-based nuclear reactor once functioned in the northern regions of Mars:
Evidence exists that a large natural nuclear reac- tor formed and operated on Mars in the northern Mare Acidalium region of Mars. However, unlike its terre- strial analogs this natural nuclear reactor was apparent- ly much larger, bred 233U off of thorium, and appar- ently underwent explosive disassembly, ejecting large amounts of radioactive material over Mars surface.
Natural fission reactors are known on Earth, in the Oklo region of Africa, but this is the first extraterrestrial one to be claimed, and the first based on thorium.
There are clearly science-fictional implications in this – Stephen Baxter used Oklo in his novel Origin – but this has other implications for me, since I’m giving a lecture on terrestrial planets tomorrow, including Mars.
I wonder if any of my students will ask me about this?