An international team of planetary researchers led by University of Oxford scientists has performed an analysis of visible/near-infrared observations of Uranus and Neptune made by the NASA/ESA Hubble Space Telescope, NASA’s Infrared Telescope Facility (ITF), and the Gemini North telescope.
This picture of Neptune was produced from the last whole planet images taken through the green and orange filters on the narrow angle camera of NASA’s Voyager 2 spacecraft. The images were taken at a range of 7.1 million km (4.4 million miles) from the planet, 4 days and 20 hours before closest approach. The picture shows the Great Dark Spot and its companion bright smudge; on the west limb the fast moving bright feature called Scooter and the little dark spot are visible. These clouds were seen to persist for as long as Voyager’s cameras could resolve them. North of these, a bright cloud band similar to the south polar streak may be seen. Image credit: NASA / JPL.
Neptune and neighboring Uranus are classified as ice giants, as opposed to the gas giants Jupiter and Saturn.
They have much in common, yet Neptune looks distinctly bluer than its planetary neighbor.
In the new research, University of Oxford’s Professor Patrick Irwin and colleagues used Hubble, ITF and Gemini North data to develop a model describing aerosol layers in the atmospheres of both ice giants.
“This is the first model to simultaneously fit observations of reflected sunlight from ultraviolet to near-infrared wavelengths,” Professor Irwin said.
“It’s also the first to explain the difference in visible color between Uranus and Neptune.”
Uranus in natural colors. Image credit: NASA / ESA / Hubble Team / Erich Karkoschka, University of Arizona.
The team’s model involves three haze layers at different heights in the atmospheres of each planet.
The middle layer of haze particles, just above the methane condensation level, is found to be thicker on Uranus than on Neptune, which affects the visible color of the two planets.
On both planets, methane ice condenses on the particles in the middle layer forming a shower of methane snow that pulls the haze particles deeper into the atmosphere, where they can then promote the condensation of hydrogen sulfide ice, forming a separate, deeper layer of cloud/haze.
Neptune has a more active, turbulent atmosphere than Uranus, suggesting Neptune’s atmosphere is more efficient at churning up gaseous methane into the haze layer where it can condense on the haze particles and produce this snow.
This action removes more of the haze and keeps Neptune’s haze layer thinner than it is on Uranus, making Neptune appear bluer than Uranus.
In contrast, excess haze on Uranus builds up in the planet’s stagnant, sluggish atmosphere giving it a lighter tone than Neptune.
This diagram shows three layers of aerosols in the atmospheres of Uranus and Neptune. The height scale on the diagram represents the pressure above 10 bar. The deepest layer (Aerosol-1 layer) is thick and composed of a mixture of hydrogen sulfide ice and particles produced by the interaction of the planets’ atmospheres with sunlight. The key layer that affects the colors is the middle layer, which is a layer of haze particles (Aerosol-2 layer) that is thicker on Uranus than on Neptune. Irwin et al. suspect that, on both planets, methane ice condenses onto the particles in this layer, pulling the particles deeper into the atmosphere in a shower of methane snow. Because Neptune has a more active, turbulent atmosphere than Uranus does, the team believes Neptune’s atmosphere is more efficient at churning up methane particles into the haze layer and producing this snow. This removes more of the haze and keeps Neptune’s haze layer thinner than it is on Uranus, meaning the blue color of Neptune looks stronger. Above both of these layers is an extended layer of haze (Aerosol-3 layer) similar to the layer below it but more tenuous. On Neptune, large methane ice particles also form above this layer. Image credit: Gemini Observatory / NOIRLab / NSF / AURA / J. da Silva / NASA / JPL-Caltech / B. Jónsson.
The authors also showed that the presence of a second, deeper layer in the model that, when darkened, could account for dark spots occasionally visible on Neptune and more sporadically on Uranus, such as the famous Great Dark Spot on Neptune observed by NASA’s Voyager 2 spacecraft in 1989.
While astronomers were already aware of the presence of dark spots in the atmospheres of both planets, they didn’t know which haze layer was causing these dark spots or whether they were caused by a thinning or darkening of this layer.
“We hoped that developing this model would help us understand clouds and hazes in the ice giant atmospheres,” said Dr. Mike Wong, an astronomer at the University of California, Berkeley.
“Explaining the difference in color between Uranus and Neptune was an unexpected bonus!”
The findings were published in the Journal of Geophysical Research: Planets.
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P.G.J. Irwin et al. Hazy blue worlds: A holistic aerosol model for Uranus and Neptune, including Dark Spots. Journal of Geophysical Research: Planets, published online May 23, 2022; doi: 10.1029/2022JE007189
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