The temperature on the surface of Venus, Earth, or Mars is determined by two properties of these planets: the amount of energy they receive from the Sun and their atmosphere . The energy received depends mainly on its distance from the star , with Venus receiving considerably more energy than Mars. The composition and size of its atmosphere are the result of a complex and long evolution, but basically they accentuate the tendency, being that of Venus one hundred times more massive than the terrestrial one and this one hundred times more massive than the Martian one.
Well, the temperature at which said atmosphere should be can be established knowing this information. The energy that reaches the planet will heat the surface and its atmosphere will retain part of that heat and emit the rest. Well, when this calculation is made for Jupiter , it is found that the temperature of the last layers of its atmosphere should be around 100 K (about -170 ºC). In other words, according to calculations, at this temperature Jupiter would receive as much energy from the Sun as it would emit energy , due to being in thermal equilibrium.
However, the temperature obtained by measuring the emission spectrum in radio and infrared waves is somewhat higher, between 20 and 40 K higher , depending on the area of the atmosphere studied. This difference may seem small, perhaps insignificant, but the reality is that it implies an even greater difference in emitted energy . The energy emitted by a body at a certain temperature depends on the fourth power of that temperature , therefore a body that doubles its temperature starts to emit sixteen times more energy. This means that a Jupiter at a temperature of 120 K emits approximately twice as much energy into space as a Jupiter at 100 K. This would in turn mean that Jupiter emits twice as much energy into space as it receives from the Sun , but from where? does that energy come from?
It cannot be the disintegration of radioactive elements , such as uranium, thorium or potassium, because although these elements must be present inside the gas giant, they certainly do not appear in sufficient quantity to significantly heat the interior of the planet. This is in stark contrast to what happens in the Earth’s core, where we think these disintegrations could be the main source of heat that keeps the Earth’s core molten. Nor can it be the processes of nuclear fusion that make the Sun shine, because although the temperature inside Jupiter is high, it remains a thousand times below the millions of degrees necessary for these nuclear reactions to begin. As far as we know, the main source of heat for the interior of Jupiter would be the residual heat accumulated during the contraction of the corresponding bit of the gigantic cloud of gas and dust that formed the solar system .
When this bit contracted to give rise to Jupiter, it would have heated up to temperatures somewhat higher than almost 40,000 K (and pressures 50 million times the atmospheric pressure on the Earth’s surface) that we think exist in the nucleus of Jupiter today. . This core would be made up of material heavier than the atmosphere , with large amounts of iron, silicon and magnesium, although not in a solid state, as conditions would not allow it. The heat from the core has been slowly transferred to the outer layers over the last few billion years, by convection. That is, due to the movement of the hot material from the interior towards shallower areas.
Although it may seem that this process could cool the gas giant rapidly, since Jupiter emits more than a trillion watts of energy in excess of what it receives from the Sun, this is completely insignificant compared to the energy stored in the interior of the planet. It is estimated that this emission of energy causes the temperature of Jupiter’s interior to decrease by one millionth of a degree per year . This would mean that it still has energy left to continue with the current trend until after the Sun dies and becomes a white dwarf, five billion years from now.
The complex system of zones, belts, storms, and vortices that we see on Jupiter with our telescopes is probably caused by the last 200 kilometers of atmosphere . Beneath this outermost cloud layer the temperature and pressure increase rapidly , until we reach the interior of the planet proper. This interior escapes our observation so, as in the case of the Sun, we resort to theoretical models and observations from the outside to understand it in detail.
Beyond the occasional glimpse of some structure in the interior of the planet, as we were able to take after the Shoemaker-Levy 9 collision in 1994 , you must infer the relevant information. These models to date coincide with the observations made. Also, since Jupiter is mainly made up of hydrogen and helium , two gases that we understand quite well, modeling its interior is easier .
Reference:
FH Busse, 1976, A simple model of convection in the Jovian atmosphere, Icarus. 29 (2), doi:10.1016/0019-1035(76)90053-1
JC Pearl et al, 1990, The albedo, effective temperature, and energy balance of Uranus, as determined from Voyager IRIS data, Icarus. 84 (1), doi:10.1016/0019-1035(90)90155-3
