I've noticed that the Angel Moon (the one orbiting Hiigara) has been going through various states of... geological activity, or something like that throughout the Homeworld series. For example, in HW1, it looks like a fairly ordinary moon, similar to ours. In HW:C, it has this weird dark grey stuff on it. Then, in HW2 it looks like there are firestorms all around it.
Here's a pic of the moon in HW1.
Smallish pic from HW:C
The moon in HW2, behind the bombers
So, is there any "official" explanation or anything like that?