Excess haze on Uranus renders it paler than Neptune, according to observations from the Hubble Space Telescope, the NASA Infrared Telescope, and the Gemini Observatory, and dark patches are caused by a darkening of a second deeper cloud/haze layer.
Astronomers may now understand why Uranus and Neptune, two identical planets, have different hues. Researchers built a single atmospheric model that matches observations of both planets using data from the Hubble Space Telescope, the NASA Infrared Telescope Facility, and the Gemini North telescope. Excess haze on Uranus builds up in the planet's stagnant, sluggish atmosphere, giving it a lighter tone than Neptune, according to the model. The model also suggests the presence of a second, deeper layer that, when darkened, can explain dark areas in these atmospheres, such as the well-known Great Dark Spot (GDS) discovered by Voyager 2 in 1989.
Neptune and Uranus share many similarities, including similar masses, diameters, and atmospheric compositions, but their appearances are strikingly different. At visible wavelengths, Neptune is noticeably bluer than Uranus, and astronomers have discovered why.
According to new research, a concentrated haze layer that appears on both planets is thicker on Uranus than on Neptune, and "whitens" Uranus' appearance more than Neptune's. Both Neptune and Uranus would appear nearly identically blue if their atmospheres were free of haze.
This result is based on a model built by an international team led by Patrick Irwin, Professor of Planetary Physics at Oxford University, to describe the aerosol layers in Neptune and Uranus' atmospheres. Previous research into the higher atmospheres of these planets had focused on the appearance of the atmosphere at only a few wavelengths. This new model, which consists of many atmospheric layers, matches observations from both planets concurrently over a wide range of wavelengths. The new model contains haze particles in deeper strata previously assumed to solely include methane clouds and hydrogen sulphide ices.
Professor Irwin, who is the primary author of an article published in the Journal of Geophysical Research: Planets, said, "This is the first model to concurrently fit measurements of reflected sunlight from ultraviolet to near-infrared wavelengths." "It's also the first to explain why Uranus and Neptune have different visible colours."
Three layers of aerosols at various heights make up the team's model. The middle layer, which is a layer of haze particles (referred to as the Aerosol-2 layer in the research) that is thicker on Uranus than on Neptune, is the critical layer that impacts the colours. Methane ice condenses onto the particles in this layer on both worlds, dragging them deeper into the atmosphere in a shower of methane snow, according to the study. The team believes Neptune's atmosphere is more efficient at churning up methane particles into the haze layer and producing this snow because it has a more active, turbulent atmosphere than Uranus'. This clears off more haze and keeps Neptune's haze layer thinner than Uranus', making Neptune appear bluer.
Mike Wong, an astronomer at the University of California, Berkeley, and a part of the team behind this study, says, "We thought that constructing this model would help us comprehend clouds and hazes in the ice giant atmospheres." "Explaining the colour difference between Uranus and Neptune was a pleasant surprise!"
Professor Irwin's team used the Hubble Space Telescope, the NASA Infrared Telescope Facility near the summit of Maunakea in Hawai'i, and the Gemini North Telescope, all of which are located in Hawai'i, to develop this model.
The model also explains the black areas that can be seen on Neptune and Uranus on a more sporadic basis. While astronomers were already aware of black spots in both planets' atmospheres, they had no idea which aerosol layer was creating the spots or why the aerosols at those layers were less reflecting. The team's findings answer these problems by demonstrating that darkening the particles in the deepest layer of their model produces black areas that resemble those seen on Neptune and, on rare occasions, Uranus.
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