Satellite masses

Not very familiar with the masses of several of the solar system moons, I decided to find them out.

Io -> 1.216 lunar masses
Europa -> 0.653 lunar masses
Ganymede -> 2.017 lunar masses.
Callisto -> 1.464 lunar masses.

Mimas -> 0.0005103 lunar masses.
Enceladus -> 0.00147 lunar masses.
Tethys -> 0.0084033 lunar masses.
Dione -> 0.0149 lunar masses.
Rhea -> 0.0313910 lunar masses.
Titan -> 1.8308 lunar masses.
Iapetus -> 0.02457 lunar masses.

Miranda -> 0.000897 lunar masses.
Ariel -> 0.01837 lunar masses.
Umbriel -> 0.0163 lunar masses.
Titania -> 0.048 lunar masses.
Oberon -> 0.0410196

Triton -> 0.291 lunar masses.


I was quite surprised about the low masses of the moons, especially Triton, which I expected to be nearly that of Luna.
Why are some moons active, but not others? Tides seems to be the obvious solution, especially for Io and Enceladus. There is evidence that Titan is active. If it is, why would it be?

And why is Triton active? What provides the energy for that?

It can't just be tides alone for triton could it be?

Hmm.....I know that since it was most likely captured that it's orbit was probably eccentric at first......meaning it was definitely heated alot then......according to solar views at least it was possibly liquid for up to a billion years after it's capture

However it should have cooled down by now right? Then again this is nitrogen thats being erupted meaning that the heating it's receiving isn't to much...... Maybe tritons been captured alot more recently than we think?


As for titan......darnit I remeber reading something on this......lemme look for it again.

Sirius_Alpha wrote:I was quite surprised about the low masses of the moons...

Further from the sun than earth, i believe ice is a significant ingredient in a lot of the moons. Since ice has about 1/3rd the density of rock, the total mass can be much less than you would expect a luna-like body of the same size to have.