From what I recall, one of the hazards of long term space travel is that nearly any material will start sublimating atoms in the hard vacuum of space, with things like cosmic rays adding to the woes. Some over time it will start deteriorating.

Not sure about “melting” into an amorphous mass, I guess in theory the probes gravity could do that, but I would imagine even the tiniest force would disturb that and dissipate it.