Mercury probably has one from insolation and tidal forces from the sun. Venus might still have one thanks to its thermal insulation blanket. Earth and Moon flex each other with tidal forces, and Earth also has a much thinner insulation blanket. Mars might still have one, but its moons don't have much tidal influence, and its blanket is even thinner than Earth's.
So if Mars still has one, chances are good that all the inner planets also have one. They would all be warmed some by radioactive decay, too.
Io (vulcanism) and Ganymede (magnetic field) probably have layers of molten rock/metal in their cores, from tidal forces with Jupiter. Icy moons Europa and Callisto probably have a liquid water layer for the same reason.
I may be wrong, but insolation probably isn't a substantial factor in keeping cores molten because of how small the energy input is. Venus does have extremely active volcanoes. There's suspicion that it might have a stronger magnetic field if it were spinning faster (it's rotational period is currently longer than its year).
The metal part is in fact partially liquid and partially solid depending on how deep in you are. The increasing pressure makes the liquid go to solid state.
It is the rotation of this solid part inside of the liquid part that generates magnetic field
How does pressure affect melting points? I thought the core was like ~6000 C and the melting point of iron is like ~1538 C.
My brain is seg-faulting trying to figure out why pressure would make something need to be hotter to melt. I know melted things generally take more space than solid things (except ice, right?). But why does that have to be? Doesn't pressure affect volume already?
