Xenon-127 is a rare and stable isotope of xenon, a noble gas that is present in trace amounts in the Martian atmosphere. Xenon-127 has an atomic mass of 127 and a half-life of more than 10 billion years. It is produced by the decay of iodine-127 or by the fission of uranium or plutonium. Xenon-127 has a very low natural abundance on Earth, but it is found at much higher levels on Mars. The reason for this discrepancy is not fully understood.

One possible explanation for the abnormally high levels of xenon-127 on Mars is that it is a product of a large, anomalous nuclear explosion that occurred in the past. According to a paper by Brandenburg (PDF), the isotopic ratios of xenon and krypton in the Martian atmosphere suggest that there was a thermonuclear reaction that involved both fast and slow neutrons, unlike a natural nuclear reactor. The paper also claims that the explosion was caused by an artificial device, possibly of alien origin, that was detonated near the Cydonia region.

Another possible explanation is that xenon-127 is a result of neutron capture by barium or bromine atoms in the Martian regolith. This process can be triggered by cosmic rays or solar wind particles that bombard the surface material. A study by Conrad et al. found evidence that this mechanism can account for some of the variations in xenon and krypton isotopes in the Martian atmosphere. However, this process may not be sufficient to explain the high levels of xenon-127 alone.

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