Interestingly, the upper atmospheres of both Uranus and Neptune are warmer than what theory would predict given their composition and the amount of sunlight they receive. This is an open mystery that has yet to be adequately explained. Furthermore, compared to Neptune, Uranus appears to have much less heat escaping from its deep interior and fewer trace gas molecules (that absorb sunlight) in its upper atmosphere. This leads to Uranus being somewhat colder, but the reasons for this are also unknown.