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. Johnson
Observations from the Gemini Observatory and other telescopes reveal that the excess fog of Uranus makes it paler than Neptune.
Now astronomers can understand why such planets Uranus and Neptune have distinctive nuances. Researchers have built a unified atmospheric model that matches the observations of the two planets, using observations from the Gemini North telescope, NASA’s infrared telescope and the Hubble Space Telescope. The model reveals that excess fog on Uranus accumulates in the stagnant, slow atmosphere of the planet, giving it a lighter hue than Neptune.
The planets Neptune and Uranus have much in common – they have similar masses, sizes and atmospheric composition – but their appearance is significantly different. At visible wavelengths, Neptune has a clear blue color, while Uranus is pale cyan. Astronomers already have an explanation for why the two planets are different colors.
New research suggests that a layer of concentrated fog that exists on both planets is thicker than Uranus than a similar layer on Neptune and “whitens” Uranus’ appearance more than Neptune’s.[1] If there was no fog in the atmospheres of Neptune and Uranus, they would both look almost equally blue.[2]
This conclusion comes from a model[3] that an international team led by Patrick Irwin, a professor of planetary physics at Oxford University, developed to describe aerosol layers in the atmospheres of Neptune and Uranus.[4] Previous research on the upper atmospheres of these planets has focused on the appearance of the atmosphere only at certain wavelengths. However, this new model, consisting of multiple atmospheric layers, coincides with observations from 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 clouds of methane and hydrogen sulfide ice.
This diagram shows three layers of aerosols in the atmospheres of Uranus and Neptune, modeled by a team of scientists led by Patrick Irwin. The scale height of the diagram represents a pressure above 10 bar. The deepest layer (aerosol layer-1) is thick and consists of a mixture of hydrogen sulfide ice and particles obtained from the interaction of the planets’ atmospheres with sunlight. The layer that affects colors is the middle layer, which is a layer of mist particles (referred to in paper as the aerosol-2 layer) that is thicker than Uranus than Neptune. The team suspects that on both planets, methane ice is condensing on the particles in this layer, pulling the particles deeper into the atmosphere under a rain of methane snow. Because Neptune has a more active, turbulent atmosphere than Uranus, the team believes that Neptune’s atmosphere is more efficient at breaking up methane particles in the fog layer and producing that snow. This removes more of the fog and keeps Neptune’s fog layer thinner than Uranus, which means that Neptune’s blue color looks stronger. Above the two layers there is an elongated layer of fog (aerosol-3 layer), similar to the layer below it is weaker. On Neptune, large methane ice particles also form above this layer. Credit: International Gemini Observatory / NOIRLab / NSF / AURA, J. da Silva / NASA / JPL-Caltech / B. Johnson
“This is the first model to combine reflections of reflected sunlight from ultraviolet to near-infrared wavelengths,” said Irwin, lead author of an article in the Journal of Geophysical Research: Planets. “This is the first to explain the difference in visible color between Uranus and Neptune.”
The model of the team 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 mist particles (referred to in paper as the aerosol-2 layer), which is thicker than Uranus than Neptune. The team suspects that on both planets, methane ice is condensing on the particles in this layer, pulling the particles deeper into the atmosphere under a rain of methane snow. Because Neptune has a more active, turbulent atmosphere than Uranus, the team believes that Neptune’s atmosphere is more efficient at breaking up methane particles in the fog layer and producing that snow. This removes more of the fog and keeps Neptune’s fog layer thinner than Uranus, which means that Neptune’s blue color looks stronger.
“We hoped that developing this model would help us understand the clouds and fog in the atmosphere of the ice giants,” said Mike Wong, an astronomer at the University of California, Berkeley and a member of the team behind the 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 planetary observations covering ultraviolet, visible, and near-infrared wavelengths (0.3 to 2.5 micrometers) taken with the near-infrared integrated field spectrometer (NIFS). the Gemini North Telescope near the Maunakea Summit in Hawaii – part of the Gemini International Observatory, NSF’s NOIRLab program – as well as archival data from NASA’s infrared telescope facility, also located in Hawaii, and NASA’s Hubble Space. ESA telescope.
Gemini North’s NIFS instrument was particularly important for this result, as it was able to provide spectra – measurements of how bright it is subject to different wavelengths – for each point in its field of view. This provided the team with detailed measurements of how reflective the atmospheres of the two planets are, both across the planet’s disk and in the range of near to infrared wavelengths.
“Gemini Observatories continue to provide new insights into the nature of our planetary neighbors,” said Martin Steele, Gemini’s program director at the National Science Foundation. “In this experiment, Gemini North provided a component within a range of terrestrial and space facilities that are critical to the detection and characterization of atmospheric fog.
The model also helps explain the dark spots that are sometimes seen on Neptune and less commonly found on Uranus. While astronomers were already aware of the presence of dark spots in the atmospheres of the two planets, they did not know which aerosol layer caused these dark spots or why the aerosols in these layers were less reflective. The team’s research sheds light on these issues, showing that darkening the deepest layer of their model will lead to dark spots similar to those observed on Neptune and perhaps Uranus.
Notes
- This whitening effect is similar to how clouds in the atmosphere of exoplanets dull or “smooth” the characteristics in the spectra of exoplanets.
- The red colors of sunlight scattered by fog and air molecules are absorbed more than methane molecules in the planet’s atmosphere. This process – called Rayleigh scattering – is what makes the sky blue here on Earth (although in the Earth’s atmosphere sunlight is scattered mainly by nitrogen molecules, not hydrogen molecules). Rayleigh scattering occurs mainly at shorter, bluer wavelengths.
- An aerosol is a suspension of fine droplets or particles in a gas. Common examples on Earth include fog, soot, smoke and fog. On Neptune and Uranus, particles produced by sunlight interacting with elements in the atmosphere (photochemical reactions) are responsible for the aerosol mist in the atmospheres of these planets.
- The scientific model is a computational tool used by scientists to test predictions of phenomena that would be impossible to make in the real world.
- The deepest layer (referred to in the article as the aerosol-1 layer) is thick and consists of a mixture of hydrogen sulfide ice and particles derived from the interaction of the planets’ atmospheres with sunlight. The top layer is an elongated fog layer (aerosol-3 layer), similar to the middle layer, but weaker. Large methane ice particles also form on Neptune above this layer.
More info
This study is presented in the article “Turbid Blue Worlds: A Holistic Aerosol Model for Uranus and Neptune, Including Dark Spots,” which will appear in the Journal of Geophysical Research: Planets.
The team consists of PGJ Irwin (Department of Physics, University of Oxford, United Kingdom), NA Teanby (School of Earth Sciences, University of Bristol, United Kingdom), LN Fletcher (School of Physics and Astronomy, University of Leices) ), 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 and 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, Oxford University, United Kingdom).
NSIR’s NOIRLab (National Research Laboratory for Optical-Infrared Astronomy), the American Center for Terrestrial Optical-Infrared Astronomy, operates the Gemini International Observatory (NSF facility, NRC – Canada, ANID – Chile, MCTIC – Brazil, MINCyT – Argentina Republic of Korea), Kitt Peak National Observatory (KPNO), Cerro Tololo Inter-American Observatory (CTIO), Community Science and Data Center (CSDC) and Vera C. Rubin Observatory (operated in collaboration with the National Accelerator Laboratory SLAC of Energy) . It is administered by the Association of Universities for Astronomical Research (AURA) under a cooperation agreement with the NSF and is based in Tucson, Arizona. It is an honor for the astronomical community to have the opportunity to conduct astronomical …
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