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A new discovery reveals why Uranus and Neptune are different colors

Voyager 2 Uranus and Neptune

NASA’s Voyager 2 spacecraft captured these views of Uranus (left) and Neptune (right) as it flew over the planets in the 1980s. Credit: NASA / JPL-Caltech / B. Jonsson

Observations from the Gemini Observatory and other telescopes reveal that the fog is in excess[{” attribute=””>Uranus makes it paler than Neptune.

Astronomers may now understand why the similar planets Uranus and Neptune have distinctive hues. Researchers constructed a single atmospheric model that matches observations of both planets using observations from the Gemini North telescope, the NASA Infrared Telescope Facility, and the Hubble Space Telescope. The model reveals that excess haze on Uranus accumulates in the planet’s stagnant, sluggish atmosphere, giving it a lighter hue than Neptune.

The planets Neptune and Uranus have much in common - they have similar masses, sizes and atmospheric compositions - yet their appearance is significantly different. At visible wavelengths, Neptune is a slightly bluer color, while Uranus is a pale shade of cyan. Astronomers now have an explanation for why the two planets have different colors.

New research suggests that a layer of concentrated fog on both planets is thicker on Uranus than a similar layer on Neptune and “whitens” Uranus’ appearance more than Neptune’s.[1] If there were no fog in the atmospheres of Neptune and Uranus, both would look almost as blue.[2]

This conclusion comes from a model[3] which an international team led by Patrick Irwin, a professor of planetary physics at Oxford University, has developed to describe the aerosol layers in the atmospheres of Neptune and Uranus.[4] Previous investigations of the upper atmospheres of these planets have focused on the appearance of the atmosphere only at certain wavelengths. However, this new model, consisting of several atmospheric layers, matches the observations on both planets over a wide range of wavelengths. The new model also includes fog particles in deeper layers that were previously thought to contain only methane ice clouds and hydrogen sulfide.

The atmospheres of Uranus and Neptune

This diagram shows three layers of aerosols in the atmospheres of Uranus and Neptune, as modeled by a team of scientists led by Patrick Irwin. The height scale on the diagram represents the pressure over 10 bar.
The deepest layer (the Aerosol-1 layer) is thick and consists of a mixture of ice with hydrogen sulfide and particles produced by the interaction of the planet’s atmosphere with sunlight.
The key layer that affects colors is the middle layer, which is a layer of fog particles (called the Aerosol-2 layer in paper) that is thicker on Uranus than on Neptune. The team suspects that on both planets, methane ice condenses on the particles in this layer, pulling the particles deeper into the atmosphere in a rain of methane snow. Because Neptune has a more active, more turbulent atmosphere than Uranus, the team believes that Neptune’s atmosphere is more efficient at shaking methane particles in the fog and producing this snow. This removes more fog and keeps Neptune’s fog layer thinner than it is on Uranus, which means that Neptune’s blue color looks stronger.
Above both layers is an extended layer of fog (Aerosol-3 layer) similar to the layer below it, but thinner. On Neptune, large particles of methane ice form above this layer.
Credit: Gemini / NOIRLab / NSF / AURA International Observatory, J. da Silva / NASA / JPL-Caltech / B. Jonsson

“This is the first model to match simultaneous observations of reflected sunlight from ultraviolet to near-infrared wavelengths,” said Irwin, who is the lead author of this paper in the Journal of Geophysical Research: Planets. “He is also the first to explain the visible color difference between Uranus and Neptune.”

The team model consists of three layers of aerosols at different heights.[5] The key layer that affects colors is the middle layer, which is a layer of fog particles (called the Aerosol-2 layer in paper) that is thicker on Uranus than on Neptune. The team suspects that on both planets, methane ice condenses on the particles in this layer, pulling the particles deeper into the atmosphere in a rain of methane snow. Because Neptune has a more active, more turbulent atmosphere than Uranus, the team believes that Neptune’s atmosphere is more efficient at shaking methane particles in the fog and producing this snow. This removes more fog and keeps Neptune’s fog layer thinner than it is on Uranus, which means that Neptune’s blue color looks stronger.

“We hope that the development of this model will help us understand the clouds and fog in the atmosphere of the ice giant,” said Mike Wong, an astronomer at[{” attribute=””>University of California, Berkeley, and a member of the team behind this result. “Explaining the difference in color between Uranus and Neptune was an unexpected bonus!”

To create this model, Irwin’s team analyzed a set of observations of the planets encompassing ultraviolet, visible, and near-infrared wavelengths (from 0.3 to 2.5 micrometers) taken with the Near-Infrared Integral Field Spectrometer (NIFS) on the Gemini North telescope near the summit of Maunakea in Hawai‘i — which is part of the international Gemini Observatory, a Program of NSF’s NOIRLab — as well as archival data from the NASA Infrared Telescope Facility, also located in Hawai‘i, and the NASA/ESA Hubble Space Telescope.

The NIFS instrument on Gemini North was particularly important to this result as it is able to provide spectra — measurements of how bright an object is at different wavelengths — for every point in its field of view. This provided the team with detailed measurements of how reflective both planets’ atmospheres are across both the full disk of the planet and across a range of near-infrared wavelengths.

“The Gemini observatories continue to deliver new insights into the nature of our planetary neighbors,” said Martin Still, Gemini Program Officer at the National Science Foundation. “In this experiment, Gemini North provided a component within a suite of ground- and space-based facilities critical to the detection and characterization of atmospheric hazes.”

The model also helps explain the dark spots that are occasionally visible on Neptune and less commonly detected on Uranus. While astronomers were already aware of the presence of dark spots in the atmospheres of both planets, they didn’t know which aerosol layer was causing these dark spots or why the aerosols at those layers were less reflective. The team’s research sheds light on these questions by showing that a darkening of the deepest layer of their model would produce dark spots similar to those seen on Neptune and perhaps Uranus.

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Notes

  1. This whitening effect is similar to how clouds in exoplanet atmospheres dull or ‘flatten’ features in the spectra of exoplanets.
  2. The red colors of the sunlight scattered from the haze and air molecules are more absorbed by methane molecules in the atmosphere of the planets. This process — referred to as Rayleigh scattering — is what makes skies blue here on Earth (though in Earth’s atmosphere sunlight is mostly scattered by nitrogen molecules rather than hydrogen molecules). Rayleigh scattering occurs predominantly at shorter, bluer wavelengths.
  3. An aerosol is a suspension of fine droplets or particles in a gas. Common examples on Earth include mist, soot, smoke, and fog. On Neptune and Uranus, particles produced by sunlight interacting with elements in the atmosphere (photochemical reactions) are responsible for aerosol hazes in these planets’ atmospheres.
  4. A scientific model is a computational tool used by scientists to test predictions about a phenomena that would be impossible to do in the real world.
  5. The deepest layer (referred to in the paper as the Aerosol-1 layer) is thick and is composed of a mixture of hydrogen sulfide ice and particles produced by the interaction of the planets’ atmospheres with sunlight. The top layer is an extended layer of haze (the Aerosol-3 layer) similar to the middle layer but more tenuous. On Neptune, large methane ice particles also form above this layer.

More information

This research was presented in the paper “Hazy blue worlds: A holistic aerosol model for Uranus and Neptune, including Dark Spots” to appear in the Journal of Geophysical Research: Planets.

The team is composed of P.G.J. Irwin (Department of Physics, University of Oxford, UK), N.A. Teanby (School of Earth Sciences, University of Bristol, UK), L.N. Fletcher (School of Physics & Astronomy, University of Leicester, UK), D. Toledo (Instituto Nacional de Tecnica Aeroespacial, Spain), G.S. Orton (Jet Propulsion Laboratory, California Institute of Technology, USA), M.H. Wong (Center for Integrative Planetary Science, University of California, Berkeley, USA), M.T. Roman (School of Physics & Astronomy, University of Leicester, UK), S. Perez-Hoyos (University of the Basque Country, Spain), A. James (Department of Physics, University of Oxford, UK), J. Dobinson (Department of Physics, University of Oxford, UK).

NSF’s NOIRLab (National Optical-Infrared Astronomy Research Laboratory), the US center for ground-based optical-infrared astronomy, operates the international Gemini Observatory (a facility of NSF, NRC–Canada, ANID–Chile, MCTIC–Brazil, MINCyT–Argentina, and KASI–Republic of Korea), Kitt Peak National Observatory (KPNO), Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and Vera C. Rubin Observatory (operated in cooperation with the Department of Energy’s SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona. The astronomical community is honored to have the opportunity to conduct astronomical research on Iolkam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawai‘i, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence that these sites have for the Tohono O’odham Nation, the Native Hawaiian community, and the local communities in Chile, respectively.

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