The speculation is that Galileo gradually sunk into thicker layers of Jupiter body, and eventually reached level where pressure squeezed its RTG battery so much the plutonium went supercritical.
Galileo was powered by RTGs. That is a heat source powered by the decay of Plutonium-238.
Nuclear decay rate does not change. It isn't affected by pressure, density, or literally any chemical process, because the decay is a nuclear process. Since the decay is constant, the heat production rate is constant. That shouldn't cause anything like a bubble detectable from space, although admittedly, it could cause bubbling.
Nuclear criticality is a process of a fission chain reaction. So the question becomes:
- Does Pu-238 have a fission cross section sufficient to go critical?
- Are there any other radioisotopes that could go critical? (other Pu isotopes probably)
The answer to both of these is pretty handily "no". The only fissile Pu isotopes are Pu-239 and Pu-241. While I don't doubt that someone might be able to detect some Pu-239 atoms in an RTG, its fission cross section would have to dominate over the other cross sections (neutron dead-ends) within the material. Looking up the data, the neutron absorption cross section of Pu-238 looks sufficiently large to shut down any reaction. Clearly this mechanism would not lead to runaway fission regardless of size.
