The eight planets in the solar system, Uranus and Neptune, are the most like twins, with similar size, mass, composition and structure, and even rotational speed. However, Neptune has a fascinating and dreamy sky blue, and there are visible rotating storms, while Uranus is a monotonous light cyan color. The planets are so similar, but the colors are obviously different, why?
The team, led by planetary physicist Patrick Irwin at the University of Oxford, found that a layer of haze has diluted Uranus’ blue hue. Uranus and Neptune are very similar in structure, with small rocky cores covered by water, ammonia and methane ices; gaseous atmospheres are composed of hydrogen, helium and methane, but like other planets in the solar system atmospheres are not evenly distributed but layered . The research team analyzed the visible light and near-infrared observations of the two planets to construct a new atmosphere model that fully replicates the observations.
In the model, both planets have a layer of photochemical haze, haze particles created by ultraviolet radiation from the sun breaking down atmospheric aerosol particles, which the researchers call the Aerosol-2 layer. On Uranus, the Aerosol-2 layer is almost twice as opaque as Neptune, because these particles absorb ultraviolet light and have a nearly white visible reflection spectrum, so Uranus observes lower ultraviolet reflectance, which explains why Uranus is more visible to the human eye than Neptune. The blue is lighter, and why the black dots are easier to observe on Neptune.
The Aerosol-1 layer below the Aerosol-2 layer is a deeper haze layer, which is haze particles from the re-evaporation and deposition of methane, which condense into submicron crystals of hydrogen sulfide, with spectral characteristics consistent with ice and black fog, the team believes, Aerosol- Region 1 is where the dark features such as blobs and bands observed on Neptune originate, and these features would be more apparent if the Neptune Aerosol-2 layer was thinner and more transparent.
It’s unclear why Neptune’s Aerosol-2 layer is less dense than Uranus, but researchers believe that Neptune’s atmosphere may be more effective at releasing methane than Uranus to clear the haze.