A new discovery reveals the reason for the different colors of Uranus and Neptune

Observations from the Gemini Observatory and other telescopes show excessive haze[{“ 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 share a lot in common – they have similar masses, sizes, and atmospheric compositions – yet their appearances are markedly different. At visible wavelengths, Neptune is conspicuously bluer in color, while Uranus is a paler cyan. Astronomers now have an explanation for why the two planets are so different in color.

New research suggests that the layer of Uranus-focused haze on both planets is thicker than a similar layer on Neptune and “illuminates” the appearance of Uranus more than Neptune.^[1] When there is no fog in it the atmosphere Neptune and Uranus appear roughly the same blue.^[2]

This conclusion comes from a model^[3] It was developed by an international team led by Patrick Irwin, Professor of Planetary Physics at the University of Oxford, to describe the aerosol layers in the atmospheres of Neptune and Uranus.^[4] Previous studies of the upper atmosphere of these planets focused only on the appearance of the atmosphere at specific wavelengths. However, this new model, made up of several atmospheric layers, agrees with observations of both planets over a wide range of wavelengths. The new model also includes hazy particles in deeper layers, which were previously thought to contain only clouds of methane and hydrogen sulfide ice.

This diagram shows three layers of aerosols in the atmospheres of Uranus and Neptune as designed by a team of scientists led by Patrick Irwin. The altimeter in the graph represents the pressure above 10 bar.
The deepest layer (aerosol layer-1) is thick and consists of a mixture of hydrogen sulfide ice and particles from the interaction of planetary atmospheres with sunlight.
The main layer that affects the colors is the middle layer, which is a layer of nebula particles (called Aerosol Layer-2 in the prism), which is thicker on Uranus than on Neptune. The team suspects that on both planets, methane ice condenses on the particles in this layer and pulls the particles deeper into the atmosphere as the methane snows fall. Because Neptune’s atmosphere is more active and turbulent than that of Uranus, the team believes that Neptune’s atmosphere is more efficient at pushing methane particles into the layer of fog and producing that snow. This removes more haze and keeps Neptune’s haze layer thinner than that on Uranus, which means that Neptune’s blue color appears to be stronger.
Above both layers is a broad mist layer (aerosol layer 3) which is similar to the substrate but more fragile. On Neptune, large particles of methane ice also form above this layer.
Image Credit: Gemini International Observatory / NOIRLab / NSF / AURA, J. da Silva / NASA / JPL-Caltech / B. Johnson

“This is the first model to synchronously tune observations of reflected sunlight from ultraviolet to near infrared,” said Irwin, lead author of a research paper presenting the findings in the Journal of Geophysical Research: Planets. He is also the first to explain the difference in visible color between Uranus and Neptune.

The team’s model consists of three layers of aerosols at different altitudes.^[5] The main layer affecting the colors is the middle layer, which is a layer of fog particles (referred to in the publication as Aerosol Layer-2) that is thicker on top of the layer Uranus to Neptune. The team suspects that on both planets, methane ice condenses on the particles in this layer and pulls the particles deeper into the atmosphere as the methane snows fall. Because Neptune’s atmosphere is more active and turbulent than that of Uranus, the team believes that Neptune’s atmosphere is more efficient at pushing methane particles into the layer of fog and producing that snow. This removes more haze and keeps Neptune’s haze layer thinner than that on Uranus, which means that Neptune’s blue color appears to be stronger.

Mike Wong, 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.

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.