Cold as ice, but less dense.

BA tells us about Hyperion, a small moon of Saturn. 360km across and with half the density of water!

So why is it such a puffball? Probably it’s suffered multiple low speed impacts with other bodies. This ruptured the moon, creating cracks and fissures all through it. If it got whacked by something of just the right size and speed, it could have actually broken apart and recoalesced; forming what astronomers call a "rubble pile". It would have so many holes in it that this would account for the extremely low density. Note– I’m still speculating, but it’s hard to imagine what else could have caused this moon to be so lightweight.

Check out the article.