NASA’s Voyager 2 spacecraft captured these views of Uranus (on the left) and Neptune (on the fitting) throughout its flybys of the planets within the 1980s. Credit: NASA/JPL-Caltech/B. Jónsson
Observations from Gemini Observatory and different telescopes reveal that extra haze on 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 a lot in frequent — they’ve comparable lots, sizes, and atmospheric compositions — but their appearances are notably totally different. At seen wavelengths Neptune has a distinctly bluer shade whereas Uranus is a pale shade of cyan. Astronomers now have a proof for why the 2 planets are totally different colours.
New analysis suggests {that a} layer of concentrated haze that exists on each planets is thicker on Uranus than the same layer on Neptune and ‘whitens’ Uranus’s look greater than Neptune’s.[1] If there have been no haze within the atmospheres of Neptune and Uranus, each would seem virtually equally blue.[2]
This conclusion comes from a mannequin[3] that a global group led by Patrick Irwin, Professor of Planetary Physics at Oxford University, developed to explain aerosol layers within the atmospheres of Neptune and Uranus.[4] Previous investigations of those planets’ higher atmospheres had centered on the looks of the ambiance at solely particular wavelengths. However, this new mannequin, consisting of a number of atmospheric layers, matches observations from each planets throughout a variety of wavelengths. The new mannequin additionally consists of haze particles inside deeper layers that had beforehand been thought to comprise solely clouds of methane and hydrogen sulfide ices.
This diagram reveals three layers of aerosols within the atmospheres of Uranus and Neptune, as modeled by a group of scientists led by Patrick Irwin. The peak scale on the diagram represents the strain above 10 bar.
The deepest layer (the Aerosol-1 layer) is thick and composed of a mix of hydrogen sulfide ice and particles produced by the interplay of the planets’ atmospheres with daylight.
The key layer that impacts the colours is the center layer, which is a layer of haze particles (referred to within the paper because the Aerosol-2 layer) that’s thicker on Uranus than on Neptune. The group suspects that, on each planets, methane ice condenses onto the particles on this layer, pulling the particles deeper into the ambiance in a bathe of methane snow. Because Neptune has a extra energetic, turbulent ambiance than Uranus does, the group believes Neptune’s ambiance is extra environment friendly at churning up methane particles into the haze layer and producing this snow. This removes extra of the haze and retains Neptune’s haze layer thinner than it’s on Uranus, which means the blue shade of Neptune seems stronger.
Above each of those layers is an prolonged layer of haze (the Aerosol-Three layer) much like the layer beneath it however extra tenuous. On Neptune, massive methane ice particles additionally kind above this layer.
Credit: International Gemini Observatory/NOIRLab/NSF/AURA, J. da Silva/NASA /JPL-Caltech /B. Jónsson
“This is the first model to simultaneously fit observations of reflected sunlight from ultraviolet to near-infrared wavelengths,” defined Irwin, who’s the lead creator of a paper presenting this outcome within the Journal of Geophysical Research: Planets. “It’s also the first to explain the difference in visible color between Uranus and Neptune.”
The group’s mannequin consists of three layers of aerosols at totally different heights.[5] The key layer that impacts the colours is the center layer, which is a layer of haze particles (referred to within the paper because the Aerosol-2 layer) that’s thicker on Uranus than on Neptune. The group suspects that, on each planets, methane ice condenses onto the particles on this layer, pulling the particles deeper into the ambiance in a bathe of methane snow. Because Neptune has a extra energetic, turbulent ambiance than Uranus does, the group believes Neptune’s ambiance is extra environment friendly at churning up methane particles into the haze layer and producing this snow. This removes extra of the haze and retains Neptune’s haze layer thinner than it’s on Uranus, which means the blue shade of Neptune seems stronger.
“We hoped that developing this model would help us understand clouds and hazes in the ice giant atmospheres,” commented Mike Wong, an astronomer on the 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
- This whitening effect is similar to how clouds in exoplanet atmospheres dull or ‘flatten’ features in the spectra of exoplanets.
- 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.
- 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.
- 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.
- 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.
