In the outer solar system, the equilibrium temperatures
low that small bodies should be rocky/icy, made up of rocky cores
Then Voyager 1 (1979) took the first detailed images
And here is the latest image
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
than like Earth's volcanos.
Tvashat Catena (from Galileo)
vents upward through cracks, blowing out steam and water
dioxide (SO2) outgasses, throwing up SO2 and
Where are the ices (the volatiles)?
Io too hot for volatiles; they escaped, or
Why is the surface orange/yellow?
When sulfur cools, it turns a variety of colors.
Why so much sulfur? Actually sulfur is common in
but its locked in the interiors of the terrestrial planets...
Why the heck is there so much volcanic
Io is hot. Why? Tidal effects
Remember how tides work:
Now, compare the tidal force Jupiter exerts on Io to
tidal force the Earth exerts on the Moon:
Big tidal force!
the presence of a tidal force by itself won't act to heat Io. Why
We need another factor: Orbital
Io, Europa, and Ganymede live
a 1:2:4 orbital resonance:
So Io is constantly being pulled out of its locked
into an eccentric orbit. Jupiter, Europa, and Ganymede are playing
with Io. As a result, Io lives on a slightly eccentric
orbit, sometimes a bit closer to Jupiter, sometimes a bit
The tidal force from Jupiter changes with distance,
is constantly being ``stretched and squeezed'' by Jupiter --
the tidal bulges change elevation due to the changing tidal force.
of this flexing: heat!
This was actually predicted by
etal (1979) just a few days before
that widespread and recurrent surface volcanism would occur...
of a largely molten interior may be evident in pictures of Io's surface
returned by Voyager 1."