Why Uranus and Neptune Are Different Colors – NASA Solar System Exploration
Observations from the Gemini Observatory, a program of NSF’s NOIRLab, and other telescopes reveal that excess haze on Uranus makes it paler than Neptune
Astronomers can now understand why similar planets Uranus and Neptune are different colors. Using observations from the Gemini North Telescope, NASA’s Infrared Telescope Facility, and the Hubble Space Telescope, researchers have developed a unique atmospheric model that matches observations of both planets. The model reveals that excess haze on Uranus is building up in the planet’s stagnant, slow-moving atmosphere and giving it a lighter tone than Neptune.
Neptune and Uranus have a lot in common – they have similar masses, sizes and atmospheric compositions – but their appearances are noticeably different. At visible wavelengths, Neptune has a distinctly bluer color while Uranus is a pale shade of cyan. Astronomers now have an explanation for why the two planets are different colors.
New research suggests that a layer of concentrated haze that exists on both planets is thicker on Uranus than a similar layer on Neptune and “whitens” Uranus’ appearance more than Neptune’s. . If there were no haze in the atmospheres of Neptune and Uranus, both would appear almost equally blue. .
This conclusion comes from a model  that an international team led by Patrick Irwin, professor of planetary physics at the University of Oxford, developed to describe the layers of aerosols in the atmospheres of Neptune and Uranus . Previous investigations of the upper atmospheres of these planets have focused on the appearance of the atmosphere at specific wavelengths only. However, this new model, composed of several atmospheric layers, corresponds to observations of the two planets over a wide range of wavelengths. The new model also includes haze particles in deeper layers that were previously thought to contain only methane and hydrogen sulfide ice clouds.
“This is the first model to simultaneously scale observations of reflected sunlight from ultraviolet to near-infrared wavelengths,” explained Irwin, who is the lead author of a paper presenting this result in the Journal of Geophysical Research: Planets. “It is also the first to explain the visible color difference between Uranus and Neptune.”
The team’s model consists of three layers of aerosols at different heights . The key layer that affects colors is the middle layer, which is a layer of haze particles (referred to in the article as the Aerosol-2 layer) that is thicker on Uranus than on Neptune. The team suspects that on both planets, methane ice is condensing on particles in this layer, dragging the particles deeper into the atmosphere in a shower of methane snow. Because Neptune has a more active and turbulent atmosphere than Uranus, the team thinks Neptune’s atmosphere is more efficient at stirring up methane particles in the haze layer and producing this snow. This removes more haze and keeps Neptune’s haze layer thinner than it is on Uranus, which means Neptune’s blue color looks stronger.
“We hoped that developing this model would help us understand clouds and haze in the atmospheres of ice giants,” commented Mike Wong, an astronomer at the University of California at Berkeley and member of the team behind this result. “Explaining the color difference 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 spectrometer. near-infrared integral field (NIFS) data from the Gemini North Telescope near the summit of Maunakea in Hawai’i – part of the Gemini International Observatory, a program of NSF’s NOIRLab – as well as data from archives of 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 because it is able to provide spectra – measurements of an object’s brightness at different wavelengths – for every point in its field of view. This provided the team with detailed measurements of the reflection of the two planets’ atmospheres both across the planet’s full disk and over a range of near-infrared wavelengths.
“The Gemini observatories continue to provide new insights into the nature of our planetary neighbors,” said Martin Still, Gemini program manager at the National Science Foundation. “In this experiment, Gemini North provided one component in a suite of ground-based and space-based facilities critical to the detection and characterization of atmospheric haze.”
The model also helps explain the dark spots that are sometimes visible on Neptune and less frequently detected on Uranus. While astronomers were already aware of dark spots in the atmospheres of both planets, they didn’t know which aerosol layer was causing the dark spots or why aerosols in 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 possibly Uranus.
 This whitening effect is similar to how clouds in exoplanet atmospheres attenuate or “flatten” the spectra features of exoplanets.
 The red colors of sunlight scattered by haze and air molecules are further absorbed by methane molecules in the atmospheres of planets. This process – called Rayleigh scattering – is what makes the sky blue here on Earth (although in Earth’s atmosphere, sunlight is mostly scattered by nitrogen molecules rather than hydrogen molecules). Rayleigh scattering mainly occurs at shorter, bluer wavelengths.
 An aerosol is a suspension of fine droplets or particles in a gas. Common examples on Earth include haze, soot, smoke, and fog. On Neptune and Uranus, particles produced by sunlight interacting with atmospheric elements (photochemical reactions) are responsible for aerosol haze in the atmospheres of these planets.
 A scientific model is a computational tool used by scientists to test predictions about a phenomenon that would be impossible to make in the real world.
 The innermost layer (referred to in the article as the Aerosol-1 layer) is thick and is composed of a mixture of hydrogen sulfide ice and particles produced by the interaction of planetary atmospheres with sunlight. . The top layer is an extensive 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.
This research was presented in the article “Misty Blue Worlds: A holistic aerosol model for Uranus and Neptune, including dark spotsto appear in the Journal of Geophysical Research: Planets.
The team is composed of PGJ Irwin (Department of Physics, University of Oxford, UK), NA Teanby (School of Earth Sciences, University of Bristol, UK), LN Fletcher (School of Physics & Astronomy, University of Leicester, UK) , D. Toledo (Instituto Nacional de Tecnica Aeroespacial, Spain), GS Orton (Jet Propulsion Laboratory, California Institute of Technology, USA), MH Wong (Center for Integrative Planetary Science, University of California, Berkeley, USA) , MT 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 National Optical-Infrared Astronomy Research Laboratory (NOIRLab), 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 SLAC National Accelerator Laboratory Department of Energy). It is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the 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, Maunakea in Hawai’i, and Cerro Tololo and Cerro Pachón in Chile. We recognize and recognize the very important cultural role and respect these sites have for the Tohono O’odham Nation, the Native Hawaiian community and the local communities of Chile, respectively.
University of Oxford Physics Department
Email: [email protected]
Tel: +1 520 318 8591
Email: [email protected]