Although there is no well-defined solid surface within Uranus's

Uranus's internal heat appears markedly lower than that of the other giant planets; in astronomical terms, it has a low thermal flux.[17][63] Why Uranus's internal temperature is so low is still not understood. Neptune, which is Uranus's near twin in size and composition, radiates 2.61 times as much energy into space as it receives from the Sun.[17] Uranus, by contrast, radiates hardly any excess heat at all. The total power radiated by Uranus in the far infrared (i.e. heat) part of the spectrum is 1.06 ± 0.08 times the solar energy absorbed in its atmosphere.[12][64] In fact, Uranus's heat flux is only 0.042 ± 0.047 W/m2, which is lower than the internal heat flux of Earth of about 0.075 W/m2.[64] The lowest temperature recorded in Uranus's tropopause is 49 K (-224 °C), making Uranus the coldest planet in the Solar System.[12][64]

One of the hypotheses for this discrepancy suggests that when Uranus was hit by a supermassive impactor, which caused it to expel most of its primordial heat, it was left with a depleted core temperature.[65] Another hypothesis is that some form of barrier exists in Uranus's upper layers which prevents the core's heat from reaching the surface.[11] For example, convection may take place in a set of compositionally different layers, which may inhibit the upward heat transport;[12][64] it is possible that double diffusive convection is a limiting factor.[11]

Atmosphere

Main article: Atmosphere of Uranus
Although there is no well-defined solid surface within Uranus's interior, the outermost part of Uranus's gaseous envelope that is accessible to remote sensing is called its atmosphere.[12] Remote sensing capability extends down to roughly 300 km below the 1 bar (100 kPa) level, with a corresponding pressure around 100 bar (10 MPa) and temperature of 320 K.[66] The tenuous corona of the atmosphere extends remarkably over two planetary radii from the nominal surface, which is defined to lie at a pressure of 1 bar.[67] The Uranian atmosphere can be divided into three layers: the troposphere, between altitudes of -300 and 50 km and pressures from 100 to 0.1 bar; (10 MPa to 10 kPa), the stratosphere, spanning altitudes between 50 and 4000 km and pressures of between 0.1 and 10-10 bar (10 kPa to 10 µPa), and the thermosphere/corona extending from 4,000 km to as high as 50,000 km from the surface.[12] There is no mesosphere.

Composition