NASA’s Voyager 2 spacecraft captured these views of Uranus (left) and Neptune (right) as they flew over the planets in the 1980s. Credit: NASA / JPL-Caltech / B. Jónsson
Observations from the Gemini Observatory and other telescopes reveal that the excess fog on Uranus makes it paler than Neptune.
Astronomers can now understand why the similar planets Uranus and Neptune have distinctive nuances. The researchers built a single atmospheric model that matches the observations of the two planets using observations from the Gemini North telescope, the NASA infrared telescope installation, and the Hubble Space Telescope. The model reveals that excess fog in 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 compositions, but their appearances are markedly different. At visible wavelengths, Neptune is a slightly bluer color, while Uranus is pale cyan. Astronomers now have an explanation for why the two planets have different colors.
New research suggests that a layer of concentrated fog that exists on both planets is thicker in Uranus than a layer similar to 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 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 the layers of aerosols in the atmospheres of Neptune and Uranus.[4] Previous research into the upper atmospheres of these planets had focused on the appearance of the atmosphere only at specific wavelengths. However, this new model, made up of multiple atmospheric layers, coincides with the observations of the two planets over a wide range of wavelengths. The new model also includes particles of fog inside deeper layers that were previously thought to contain only clouds of methane and hydrogen sulfide gels.
This diagram shows three layers of aerosols in the atmospheres of Uranus and Neptune, modeled on a team of scientists led by Patrick Irwin. The height scale of the diagram represents the pressure above 10 bar. The deepest layer (the Aerosol-1 layer) is thick and consists of a mixture of hydrogen sulfide ice and particles produced by the interaction of the planets’ atmospheres with sunlight. The key The layer that affects the colors is the middle layer, which is a layer of fog particles (referred to in the document as the Aerosol-2 layer) that is thicker in Uranus than in Neptune. The team suspects that on both planets, methane ice condenses into particles in this layer, dragging the particles deeper into the atmosphere in a methane snowfall. Because Neptune has a more active and turbulent atmosphere than Uranus, the team believes that Neptune’s atmosphere is more efficient at shaking methane particles into the fog layer and producing this snow. This removes more of the fog and keeps Neptune’s fog layer thinner than Uranus, which means Neptune’s blue color looks stronger. Above these two layers is an extended layer of fog (the Aerosol-3 layer) similar to the layer below. but more tenuous. In Neptune, large particles of methane ice also form above this layer. Credit: Gemini International Observatory / NOIRLab / NSF / AURA, J. da Silva / NASA / JPL-Caltech / B. Jónsson
“This is the first model that simultaneously adjusts to observations of reflected sunlight from ultraviolet wavelengths to near infrared,” said Irwin, who is the lead author of an article that presents 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 equipment 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 (referred to in the document as the Aerosol-2 layer) that is thicker in Uranus than in Neptune. The team suspects that on both planets, methane ice condenses into particles in this layer, dragging the particles deeper into the atmosphere in a methane snowfall. Because Neptune has a more active and turbulent atmosphere than Uranus, the team believes that Neptune’s atmosphere is more efficient at shaking methane particles into the fog layer and producing this snow. This removes more of the fog and keeps Neptune’s fog layer thinner than Uranus, which means Neptune’s blue color looks stronger.
“We hoped that the development of this model would help us understand the clouds and mists in the atmospheres of the ice giants,” said Mike Wong, an astronomer at the University of California, Berkeley and a member of the team. behind this result. “Explaining the color difference between Uranus and Neptune was an unexpected advantage!”
To create this model, Irwin’s team analyzed a set of observations of the planets that include near-ultraviolet, visible, and near-infrared wavelengths (0.3 to 2.5 micrometers) taken with the field spectrometer. near infrared (NIFS) in the nearby Gemini North telescope. the summit of Maunakea in Hawaii, which is part of the Gemini International Observatory, a program of NSF’s NOIRLab, as well as archival data from the installation of NASA’s infrared telescope, also located in Hawaii, and the NASA / ESA Hubble Space. Telescopes.
The NIFS instrument in Gemini North was especially important for this result, as it is able to provide spectra (measurements of the brightness of an object at different wavelengths) for each point in its field of view. This provided the team with detailed measurements of the reflection of the atmospheres of the two planets on both the planet’s entire disk and a range of near-infrared wavelengths.
“Gemini observatories continue to offer new insights into the nature of our planetary neighbors,” said Martin Still, head of the National Science Foundation’s Gemini program. “In this experiment, Gemini North provided a component within a set of critical ground and space installations for the detection and characterization of atmospheric fog.”
The model also helps to explain dark spots that are occasionally visible in Neptune and are less frequently detected in Uranus. Although astronomers were already aware of the presence of dark spots in the atmospheres of both planets, they did not know what layer of aerosols was causing these dark spots or why the aerosols in these 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 in Neptune and perhaps Uranus.
Notes
- This bleaching effect is similar to how clouds in exoplanet atmospheres go out or “flatten” the characteristics of the spectra of exoplanets.
- The red colors of sunlight scattered by fog and air molecules are more absorbed by the methane molecules in the planets’ atmosphere. This process, known as Rayleigh scattering, is what causes blue skies here on Earth (although in the Earth’s atmosphere sunlight is mostly dispersed by nitrogen molecules instead of 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. In Neptune and Uranus, the particles produced by sunlight that interact with the elements of the atmosphere (photochemical reactions) are responsible for the aerosol mists in the atmosphere 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 deepest layer (known in the document as the Aerosol-1 layer) is thick and consists 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 fog (the Aerosol-3 layer) similar to the middle layer but thinner. In Neptune, large particles of methane ice also form above this layer.
More information
This research was presented in the paper “Nebula Blue Worlds: A Holistic Spray Model for Uranus and Neptune, including Dark Spots” in the Journal of Geophysical Research: Planets.
The team consists of PGJ Irwin (Department of Physics, Oxford University, UK), NA Teanby (School of Earth Sciences, University of Bristol, UK), LN Fletcher (School of Physics and Astronomy, University of Leicester, United Kingdom), D. Toledo (National Institute of Aerospace Technology, 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, Oxford University, UK) , J. Dobinson (Department of Physics, Oxford University, UK).
The NOIRLab (National Optical-Infrared Astronomy Research Laboratory) of NSF, the American center for terrestrial optical-infrared astronomy, operates the Gemini International Observatory (a facility of NSF, NRC-Canada, ANID-Chile, MCTIC -Brazil, MINCyT-Argentina). , and KASI-Republic of Korea), the Kitt Peak National Observatory (KPNO), the Cerro Tololo Inter-American Observatory (CTIO), the Community Data and Science Center (CSDC), and the Vera C. Rubin Observatory (operated in cooperation with the Department). of the National Accelerator Laboratory SLAC Energy). It is managed by the Association of Universities for Astronomy Research (AURA) under a cooperation agreement with NSF and is headquartered in Tucson, Arizona. The astronomical community has the honor of having the opportunity to carry out astronomical activities …