"Was There a Natural Nuclear Blast on Mars?"

"A massive nuclear explosion on Mars would have created huge craters on the surface, visible from orbiting telescopes like Hubble and from the Mars rovers. Brandenburg said such craters could have filled in with sand over the past 180 million years, leaving no visual cues to prove the theory.

Another possibility is that the reaction occurred in mid-air and did not leave a crater -- which is exactly what happened at the Tunguska event in Russia in 1909, presumably by a large comet."