Uranus and Neptune have almost the same size and mass. They also have similar compositions and structures, even turnover rates. However, their color is different. If these two planets are so similar, how can this difference be explained?
Uranus and Neptune, the two most distant (and forgotten) planets in the Solar System, have a very similar structure. Imagine a small rocky core surrounded by a mantle of water ice, ammonia and methane. Cover everything with a gaseous atmosphere composed mainly of hydrogen, helium and methane. Finally, coat the whole thing in a layered high atmosphere. On paper, these two "ice giants", as they are called, therefore look a lot alike, but not quite.
Indeed, their main difference lies in their hue . When Neptune displays an attractive azure blue where several storms loom, Uranus shows us a pale, featureless face. In a study, a team from the University of Oxford led by planetary physicist Patrick Irwin tried to explain this difference. Note that this work is still pending peer review.
For this study, researchers analyzed observations in the visible and near-infrared regions of the two planets to generate new models of their atmospheric layers.
These two planets offer a layer of photochemical haze (dubbed Aerosol-2). This very common type of structure forms when ultraviolet radiation from the Sun breaks down aerosol particles in the atmosphere. On both planets, this hazy layer appears to be the source of clouds that condense into methane ice at the lower boundary and snow deeper in the atmosphere. The difference is that on Uranus this layer appears to be twice as opaque as on Neptune . This is why the two planets do not show the same face.
"Since these particles absorb UV, this explains the lower observed UV reflectivity of Uranus which therefore appears paler to the human eye “, note the authors. "The lower opacity of Neptune's Aerosol-2 layer also explains why dark spots are easier to observe in the atmosphere than in Uranus. "
Beneath the Aerosol-2 layer is a layer deeper haze called Aerosol-1 where the methane again evaporates to form haze particles. These then condense into submicron crystals of hydrogen sulfide. The reason why the Aerosol-2 layer of Neptune is not as dense as that of Uranus is not yet very clear . Researchers believe its atmosphere may be better at shedding haze by snowing methane more efficiently than Uranus.
Study details are published on the arXiv preprint server.
