The hard part is not "warm the ice enough to melt it into liquid water" but "don't let all the water turn into gas an escape". The phase diagram is a harsh mist

The hard part is not "warm the ice enough to melt it into liquid water" but "don't let all the water turn into gas an escape". The phase diagram is a harsh mistress at low pressures. https://commons.wikimedia.org/wiki/File:Phase_diagram_of_water.svg https://commons.wikimedia.org/wiki/File:Phase_diagram_of_wat... For reference, the surface temperature on Ceres varies from 110K to 235K, which is -163°C to -38°C in chart units, but "effectively zero pressure" (I can't find exact numbers for Ceres, but the 100×-heavier Moon is measured in nanopascals) is off the bottom of the chart.