Caverns of Mercury by Jeff Tanyard

Dale Kinmont spends his days helping the new government of Venus adjust to independence. He is comfortable there, and certainly doesn't want to go to some horrible place like the Crater prison colony on Mercury. But when Uncle Zack goes there and winds up missing, Dale knows he's the only one who can find him.

Crater is too big for him to search for Zack by himself, so he must enlist the aid of the local partisans. Their assistance comes at a price, though, and paying isn't easy. Dale's presence there stirs up trouble for everyone. He hopes the civil unrest on Mercury, like that of Hesperus, will have a happy ending. But when a prison planet has a revolution, no one leaves unscathed.