With a gravitational pull of about 0.04% of Earth’s, the difference between Deimos’ gravity and zero gravity is academic. As far as molten cores go, there are three main sources of heat: formation heat, tidal forces, and radioactive decay. Tidal forces only really apply to inner moons around gas giants. The tidal forces have to be huge in order to melt the core just by “massaging” the moon in question. So age, size, tidal forces, and density are just some of the many variables that go into whether or not a core will be molten.