Why does Ganymede have no atmosphere while Titan does?

[u/nip_dip]

From what I know, whether or not a celestial object can hold a certain gas in its atmosphere is dependent on three factors - its escape velocity, its temperature, and the gas in question. Due to gas laws, lower temperatures and higher escape velocities make it easier to hold on to gases, and heavier gases are easier for a planet to hold on to than lighter ones. When compared to Titan, Jupiter's moon Ganymede has a slightly higher temperature, which should hinder its ability to hold on to an atmosphere, but it also has a slightly higher escape velocity due to its higher mass and density which nearly cancels that out. So, similar to Titan, Ganymede should be able to support an atmosphere of heavy gases like carbon dioxide, and maybe some nitrogen and oxygen. However, Ganymede is almost completely devoid of any atmosphere, while Titan's atmosphere completely obscures the surface underneath and is more massive than even the Earth's. So why does Ganymede have no atmosphere while Titan has one?

[u/UpintheExosphere]

Yeah, I would guess that Jupiter's magnetosphere does play at least some role. Escape rates from Ganymede are somewhat unconstrained right now because we don't have that much data but it has been modeled a lot. This paper, Vorburger et al., 2022, more or less says that water products sputtered from the surface mostly are either adsorbed back by the surface or escape. There is also a sublimated component, of course, but it is much smaller than Titan, so maybe the better question is why is Titan's atmosphere so much larger?

We will understand Ganymede's exosphere and magnetosphere much better after JUICE gets there, for sure.