Photo: NASA/JPL-Caltech/B. Johnson
Astronomers may now understand why the similar planets Uranus and Neptune have different colors. Using observations from the Gemini North Telescope, NASA’s Infrared Telescope Facility, and the Hubble Space Telescope, the researchers developed a single model of the atmosphere that matches observations of both planets. The model reveals that excess nebula on Uranus is accumulating in the planet’s slow, stagnant atmosphere, making it appear a shade lighter than Neptune.
Neptune and Uranus have much in common – they have similar masses, sizes, and atmospheric compositions – yet their appearances are markedly different. At visible wavelengths, Neptune has a distinctly bluer color, while Uranus has a paler cyan color. Astronomers now have an explanation for why the two planets’ colors are different. New research suggests that the concentric nebula layer on both planets is thicker on Uranus than a similar layer on Neptune and makes Uranus’ appearance more ‘white’ than Neptune’s. † If there were no nebulae in the atmospheres of Neptune and Uranus, they would both appear roughly the same blue †
This conclusion comes from a model  that an international team led by Patrick Irwin, Professor of Planetary Physics at the University of Oxford, has developed to describe the aerosol layers in the atmospheres of Neptune and Uranus † Previous investigations of the upper atmosphere of these planets focused on the appearance of the atmosphere at specific wavelengths only. However, this new model, consisting of several atmospheric layers, is consistent with observations of both planets over a wide wavelength range. The new model also includes hazy particles in deeper layers that were previously thought to contain only clouds of methane and hydrogen sulfide ice.
“This is the first model that synchronously fits observations of reflected sunlight from ultraviolet to near infrared,” explains Irwin, lead author of a research paper presenting the finding in the Journal of Geophysical Research: Planets. He is also the first to explain the difference in visible color between Uranus and Neptune.
The team’s model consists of three layers of aerosols at different altitudes † The main layer affecting colors is the middle layer, which is a layer of fog particles (referred to in the article as the aerosol layer-2) which is thicker on Uranus than on Neptune. The team suspects that on both planets, methane ice condenses on the particles in this layer, pulling the particles deeper into the atmosphere as the methane snows fall. Because Neptune’s atmosphere is more active and turbulent than that of Uranus, the team believes that Neptune’s atmosphere is more efficient at hunting down methane particles in the nebula’s layer and producing this snow. This removes more of the nebula and keeps the layer of the Neptune nebula thinner than that of Uranus, making Neptune’s blue color appear stronger.
“We were hoping that developing this model would help us understand clouds and nebulae in the atmospheres of the ice giants,” said Mike Wong, an astronomer at the University of California, Berkeley, and a member of the team responsible for this finding. “Explaining the difference in color between Uranus and Neptune was an unexpected bonus!” To create this model, Irwin’s team analyzed a series of observations of the planets spanning at ultraviolet, visible, and near-infrared wavelengths (from 0.3 to 2.5 micrometers). 5 micrometers), captured with the Near Infrared Field Spectrometer (NIFS) on the Gemini North Telescope near Maunakea Peak in Hawaii—part of the Gemini International Observatory, a program of NSF NOIRLab—plus archival data from NASA. The Infrared Telescope Facility, also in Hawaii, and the NASA/ESA Hubble Space Telescope.
The NIFS instrument at Gemini North was particularly important to this finding because it can provide spectra — measurements of how bright an object at different wavelengths is — for any point in its field of view. This provided the team with detailed measurements of atmospheric reflectance for both planets, across the entire planet’s disk and across a range of near-infrared wavelengths.
“The Gemini observatories continue to provide new insights into the nature of our neighboring planets,” said Martin Steele, Gemini Program Officer at the National Science Foundation. “In this experiment, Gemini North provided a component within a suite of terrestrial and space facilities critical to the discovery and characterization of atmospheric nebulae.”
The model also helps explain dark spots that sometimes appear on Neptune and less frequently on Uranus. Although astronomers were already aware of the presence of dark spots in the atmospheres of both planets, they did not know which layer of aerosols caused these dark spots to appear or why the aerosols in those layers were less reflective. The team’s research sheds light on these questions by showing that an eclipse of the deepest layer in their model would create dark patches similar to those seen on Neptune and possibly Uranus.
Nuts: This whiter effect is similar to how clouds in the atmospheres of exoplanets are made faint or ‘flatten’ in the spectra of exoplanets.  Methane particles in the atmospheres of planets absorb the red colors of sunlight scattered in nebulae and air particles. This process – called Rayleigh scattering – is what makes the sky blue here on Earth (although sunlight in Earth’s atmosphere is usually 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 are fog, soot, smoke, and mist. On Neptune and Uranus, particles from the interaction between sunlight and elements in the atmosphere (photochemical reactions) are responsible for the aerosol nebulae in the atmosphere of these planets.  A scientific model is a computational tool that scientists use to test predictions about a phenomenon that would be impossible to achieve in the real world.  The deepest layer (called aerosol layer 1 in the article) is thick and consists of a mixture of hydrogen sulfide ice and particles from the interaction of planetary atmospheres with sunlight. The upper layer is an extended nebula layer (aerosol layer 3) similar to the middle layer but thinner. On Neptune, large methane ice particles also form above this layer.
source: NSF’s NOIRLab
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