Neptune and its Internal heat

A size comparison of Neptune and Earth.

Neptune's more varied weather when compared to Uranus is believed to be due in part to its higher internal heat. Although Neptune lies half again as far from the Sun as Uranus, and receives only 40% its amount of sunlight, the two planets' surface temperatures are roughly equal.The upper regions of Neptune's troposphere reach a low temperature of −221.4 °C (51.7 K). At a depth where the atmospheric pressure equals 1 bar, the temperature is −201.15 °C (72.0 K).Deeper inside the layers of gas, however, the temperature rises steadily. As with Uranus, the source of this heating is unknown, but the discrepancy is larger: Uranus only radiates 1.1 times as much energy as it receives from the Sun;Neptune radiates about 2.61 times as much, which means the internal heat source generates 161% of the solar input.Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System. Several possible explanations have been suggested, including radiogenic heating from the planet's core,dissociation of methane into hydrocarbon chains under atmospheric pressure,and convection in the lower atmosphere that causes gravity waves to break above the tropopause.