Whats on or underneath Vall's surface?

[Dockillar]

Vall, the one moon of Jool that you don't hear much about, but I was taking a look a Vall and I wondered, what's on or underneath Vall's surface??? Besides crystal (which I highly doubt) no rock is colored like what's on Vall's surface. The next possibility is frozen water, which means that much like Europa, a liquid ocean could exist. This I also have trouble believing simply because It doesn't reflect Kerbol's (Sun's) light like other bodies that have a liquid or frozen surface, for example Eeloo. Or is it some unknown element in the Kerbal universe that hasn't been documented or studied by the players...

I don't can't decide, can you guys?

[DisarmingBaton5]

I like to think of it as a much larger version of Minmus. Much, much larger.

[Themohawkninja]

It's terrain scattering looks like granite if you ask me, so that might give you a clue.

[dharak1]

It could be oxidized copper, I also have this greenish rock that looks just like Vall's surface.