We think the reason for this is that its interior is still undergoing slow gravitational collapse, and that as the constituents sort themselves out, this produces a warm core. The temperature at the top of the atmosphere is about 57 Kelvins, and would only be 50 Kelvins if sunlight were the only source of heat. Actually, there is no real mystery about this. If a planet's core is still hot, like Earth's, Jupiter's, Saturns' etc, There will be a specific distance in the solar system where the internal heat from the core is to be slightly beyond that distance and still has a hot core.