thumb|left|Views of eroding (top) and mostly eroded (bottom) ice knobs (~100 m high), possibly formed from the ejecta of an ancient impact The further evolution of Callisto after accretion was determined by the balance of the radioactive heating, cooling through thermal conduction near the surface, and solid state or subsolidus convection in the interior. Details of the subsolidus convection in the ice is the main source of uncertainty in the models of all icy moons. It is known to develop when the temperature is sufficiently close to the melting point, due to the temperature dependence of ice viscosity. Subsolidus convection in icy bodies is a slow process with ice motions of the order of 1 centimeter per year, but is, in fact, a very effective cooling mechanism on long timescales. It is thought to proceed in the so-called stagnant lid regime, where a stiff, cold outer layer of Callisto conducts heat without convection, whereas the ice beneath it convects in the subsolidus regime. For Callisto, the outer conductive layer corresponds to the cold and rigid lithosphere with a thickness of about 100 km. Its presence would explain the lack of any signs of the endogenic activity on the Callistoan surface. The convection in the interior parts of Callisto may be layered, because under the high pressures found there, water ice exists in different crystalline phases beginning from the ice I on the surface to ice VII in the center. The early onset of subsolidus convection in the Callistoan interior could have prevented large-scale ice melting and any resulting differentiation that would have otherwise formed a large rocky core and icy mantle. Due to the convection process, however, very slow and partial separation and differentiation of rocks and ices inside Callisto has been proceeding on timescales of billions of years and may be continuing to this day.
thumb|left|views of eroding (top) and mostly eroded (bottom) ice knobs (~100 m high) possibly formed from the ejecta of an ancient impact the further evolution of callisto after accretion was determined by the balance of the radioactive heating cooling through thermal conduction near the surface and solid state or subsolidus convection in the interior details of the subsolidus convection in the ice is the main source of uncertainty in the models of all icy moons it is known to develop when the temperature is sufficiently close to the melting point due to the temperature dependence of ice viscosity subsolidus convection in icy bodies is a slow process with ice motions of the order of 1 centimeter per year but is in fact a very effective cooling mechanism on long timescales it is thought to proceed in the so-called stagnant lid regime where a stiff cold outer layer of callisto conducts heat without convection whereas the ice beneath it convects in the subsolidus regime for callisto the outer conductive layer corresponds to the cold and rigid lithosphere with a thickness of about 100 km its presence would explain the lack of any signs of the endogenic activity on the callistoan surface the convection in the interior parts of callisto may be layered because under the high pressures found there water ice exists in different crystalline phases beginning from the ice i on the surface to ice vii in the center the early onset of subsolidus convection in the callistoan interior could have prevented large-scale ice melting and any resulting differentiation that would have otherwise formed a large rocky core and icy mantle due to the convection process however very slow and partial separation and differentiation of rocks and ices inside callisto has been proceeding on timescales of billions of years and may be continuing to this day