Io

In the outer solar system, the equilibrium temperatures are low that small bodies should be rocky/icy, made up of rocky cores covered with ice.

Then Voyager 1 (1979) took the first detailed images of Io. Wow!

And here is the latest image from Galileo
This ain't no ice ball!

Voyager found a very young surface, with active, ongoing volcanic eruptions:

Note: Io volcanos are more like Earth's geysers than like Earth's volcanos.

Earth's geysers: steam vents upward through cracks, blowing out steam and water
Io's volcanos: sulfur dioxide (SO₂) outgasses, throwing up SO₂ and sulfur.

Tvashat Catena (from Galileo)

Questions:

Where are the ices (the volatiles)?

Io too hot for volatiles; they escaped, or perhaps never condensed.

Why is the surface orange/yellow?

When sulfur cools, it turns a variety of colors.
Why so much sulfur? Actually sulfur is common in solar system, but its locked in the interiors of the terrestrial planets...

Why the heck is there so much volcanic activity?

Io is hot. Why? Tidal effects

Remember how tides work:

Now, compare the tidal force Jupiter exerts on Io to the tidal force the Earth exerts on the Moon:

Big tidal force! But the presence of a tidal force by itself won't act to heat Io. Why not?

We need another factor: Orbital resonances.

Io, Europa, and Ganymede live in a 1:2:4 orbital resonance:

So Io is constantly being pulled out of its locked orbit into an eccentric orbit. Jupiter, Europa, and Ganymede are playing tug-of-war with Io. As a result, Io lives on a slightly eccentric orbit, sometimes a bit closer to Jupiter, sometimes a bit farther.

The tidal force from Jupiter changes with distance, so Io is constantly being ``stretched and squeezed'' by Jupiter -- the tidal bulges change elevation due to the changing tidal force. Result of this flexing: heat!

This was actually predicted by Peale etal (1979) just a few days before Voyager flyby:

"...one might speculate that widespread and recurrent surface volcanism would occur... Consequences of a largely molten interior may be evident in pictures of Io's surface returned by Voyager 1."