Io, moon of Jupiter

(Color enhanced by NASA. But here are some views with natural color on the top to compare with the enhanced color on the bottom.)

1st Galilean moon of Jupiter.
6 R_J from Jupiter.
2 day orbital period.
Radius ~ 0.3 R_E.
Density ~ 3500 kg/m³.

The density suggests that it is rock.

A special effect on Io:

Since it is near to Jupiter, there are big tidal forces.
Since the other moons pull its orbit in and out, the tidal forces are continually changing.
This constant bending puts a lot of energy into Io from friction.
Prediction: heating of Io should produce volcanos.
One might also call them geysers [see below].

Look at the many black spots (the volcanos). Note that there are no craters, favoring the theory that the volcanos continually renew the surface.

What is going on? There appear to be two kinds. Either kind is a little similar to a geyser is on Earth.

The important fact is that water vapor (steam) has a much higher volume than liquid water.

Water is heated by hot rock.
The water is under high pressure because of the water above it.
Its temperature gets to more than 373 K.
It doesn't boil because of the high pressure.
The temperture goes up a little more.
The water begins to boil, pushing some out at the surface.
This releases some pressure, causing more boiling.
This pushes more water out, making the pressure even less.
Now all the water turns to steam.
It is pushed out at the surface with high velocity.

For Io, substitute sulfur dioxide, SO₂, for water. Another possibility is sulfur, S. Both seem to occur.

SO₂ is a gas on earth.
After discharge in a plume from an Io volcano, it would crystalize in the cold and make a white snow [cf white deposits].
S is a solid on earth with a low melting point. At high temperatures, it could also be turned into a vapor.
So the stuff in a volcano plume could be gaseous S.
It is usually yellow, but can be black or red [cf. yellow and red on Io].

There also appears to be flows of dark rock lava, as in volcanos on Earth.

Volcano plume at the horizon.

Better view of the volcano plume.

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Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA soper@bovine.uoregon.edu