This may not apply, but I'd like to point out that as far as we can tell, the Alternian sun doesn't actually burn trolls' skin, as shown in [s]: make her pay, when Terezi walks to a point where she can stare into the sun with no ill effects (y'know, other than being rendered blind).

Here’s the thing with the sun, it doesn’t cause your skin to spontaneously combust upon stepping into it. It sort of takes time to cause damage to your skin.

-Mod L