NASA’s Voyager 2 spacecraft captured these views of Uranus (left) and Neptune (right) during its flybys of the planets in the 1980s. Credit: NASA/JPL-Caltech/B. Jonsson
Observations from the Gemini Observatory and other telescopes reveal that excess haze over[{” attribute=””>Uranus makes it paler than 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
The planets 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.[1] If there were no haze in the atmospheres of Neptune and Uranus, both would appear almost equally blue.[2]
This conclusion comes from a model[3] that an international team led by Patrick Irwin, professor of planetary physics at the University of Oxford, has developed to describe the layers of aerosols in the atmospheres of Neptune and Uranus.[4] 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 diagram shows three aerosol layers in the atmospheres of Uranus and Neptune, as modeled by a team of scientists led by Patrick Irwin. The height scale on the diagram represents the pressure above 10 bar.
The innermost layer (the Aerosol-1 layer) is thick and composed of a mixture of hydrogen sulfide ice and particles produced by the interaction of planetary atmospheres with sunlight.
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.
Above these two layers is an extended layer of haze (the Aerosol-3 layer) similar to the layer below but more tenuous. On Neptune, large methane ice particles also form above this layer.
Credit: Gemini International Observatory/NOIRLab/NSF/AURA, J. da Silva/NASA /JPL-Caltech /B. Jonsson
“This is the first model to simultaneously fit observations of reflected sunlight from ultraviolet to near-infrared wavelengths,” explained Irwin, who is the lead author of a paper reporting this result in the Journal of Geophysical Research: Planets. “It’s 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.[5] 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, astronomer at the[{” 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
- This whitening effect is similar to how clouds in
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,