Atmosphere of Uranus


May 25, 2022

The atmosphere of Uranus, like that of Neptune, is different from the atmospheres of the major gas giants, Jupiter and Saturn. Although it is also composed mainly of hydrogen and helium, it contains a greater proportion of volatile elements (nicknamed "ice") such as water, ammonia and methane. Scholars believe that Uranus, unlike Jupiter and Saturn, does not possess a mantle of metallic hydrogen beneath the upper atmosphere, but that the interior of the planet consists of an "ocean" of ammonia, water and methane that gradually transforms into a gaseous atmosphere dominated by hydrogen and helium. Because of these differences, some astronomers group Uranus and Neptune in the category of "frozen giants" to distinguish them from Jupiter and Saturn. Although there is no well-defined solid surface, the outermost gaseous layer, accessible to detections from the outside, is called the atmosphere. The capabilities of the detection instruments allow to reach a depth of about 300 km below the layer at a pressure of 1 bar assumed as zero altimetric, corresponding to a pressure of 100 bar and a temperature of 320 K. The tenuous crown extends for more than two planetary rays with an ideal surface at 1 bar of pressure. The atmosphere of Uranus can be divided into three layers: the troposphere, between −300 and 50 km of altitude and between 100 and 0.1 bar of pressure; the stratosphere, between 50 and 4000 km of altitude and between 0.1 and 10-10 bar of pressure and the thermosphere / corona, which extends from 4000 to 50,000 km in height from the ideal surface. There is no mesosphere present. At the top of the clouds the temperature is about −220 ° C with small differences between the different areas of the planet, probably it has a particular system of redistribution of solar energy. The various measurements revealed that the amount of heat it receives from the sun is almost the same as that which it radiates.


The composition of Uranus' atmosphere is different from that of the planet as a whole. The atmosphere is in fact made up of 83% molecular hydrogen, 15% helium and 2.3% methane. The mole fraction of helium, i.e. the number of helium atoms per hydrogen / helium molecule, was determined by analyzing the occultation measurements of the Voyager 2 probe in the radio and infrared frequencies. The commonly accepted value is 0.15 ± 0.03 in the upper atmosphere, which corresponds to a mass fraction of 0.26 ± 0.05, which is close to the mass fraction for the helium of the nebula from which the Solar System originated, 0.275 ± 0.01. This indicates that during the accretion of Uranus, helium did not concentrate in the center of the planet, as it did in the gas giants. The abundance of deuterium in relation to hydrogen, 5.5 + 3.5−1.5 × 10−5, was measured in the 1990s thanks to the Infrared Space Observatory (ISO) and appears to be higher than the value for the proto-solar nebula measured in Jupiter and equal to 2.25 × 10−5. Deuterium is present almost exclusively in the molecular form, coupled with other hydrogen atoms (HD). The fourth abundant constituent of Uranus' atmosphere is methane (CH4), the presence of which had already been detected by the Earth through spectroscopic analyzes. The main absorption bands of methane are found in the visible and near infrared, resulting in the characteristic coloration of Uranus, which appears aquamarine or cyan. Methane molecules make up 2.3% of the planet's atmosphere by molar fraction, below the methane cloud layer at 1.3 bar; a percentage equal to 20-30 times that measured in the Sun. The quantity of methane in the upper atmosphere is much lower in percentage due to the extremely low temperatures, which lower the degree of saturation and freeze the excess methane. On the other hand, the