Satellite Atmospheres

Most satellites have no atmospheres (why?)

A few have extremely tenuous, transient atmospheres.

Io:

Very low density atmosphere of sulfur (S), sodium (Na), and some sulfur dioxide (SO2). Where would this come from?
SO2 frost in Jovian shadow --> SO2 gas when back in sunlight: Io brightens briefly sometimes just after eclipse.
Ionized gases interact w/ Jovian magnetic field: Io plasma torus around Io's orbit.

Io Plasma Torus

Ganymede:

very thin hydrogen atmosphere, plus some heavier elements. Where would this come from?

Other satellites have even weaker atmospheres -- except for Titan (of Saturn) and Triton (of Neptune).