To the untrained eye, it may be difficult to distinguish between Uranus and Neptune. Yes, Uranus’ color is frequently described as cyan, while Neptune’s color is frequently described as azure. But, regardless of your classification, it’s difficult to deny that the ice giants look strikingly similar at first glance. And it’s not surprising given that both worlds are made up of similar components.
Researchers have been working for years to develop a model that accurately explains the slight color difference between Uranus and Neptune. Scientists believe they have cracked the code as to why our solar system’s two most distant planets are slightly different shades of blue.
Astronomers may have figured out why the planets Uranus and Neptune are different colors. Researchers developed a single atmospheric model that matches observations of both planets using data from the Gemini North telescope, the NASA Infrared Telescope Facility, and the Hubble Space Telescope. According to the model, excess haze on Uranus accumulates in the planet’s stagnant, sluggish atmosphere, giving it a lighter tone than Neptune.
Neptune and Uranus share many characteristics, including similar masses, sizes, and atmospheric compositions, but their appearances are markedly different. Neptune is clearly bluer at visible wavelengths than Uranus, which is a pale shade of cyan. Astronomers now know why the two planets have different colors.
This is the first model to simultaneously fit observations of reflected sunlight from ultraviolet to near-infrared wavelengths. It’s also the first to explain the difference in visible color between Uranus and Neptune.
Patrick Irwin
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’s appearance more than Neptune’s. If there were no haze in the atmospheres of Neptune and Uranus, both would appear almost equally blue.
This conclusion is based on a model developed by an international team led by Patrick Irwin, Professor of Planetary Physics at Oxford University, to describe aerosol layers in Neptune and Uranus’ atmospheres. Previous studies of these planets’ upper atmospheres concentrated on how the atmosphere appeared at specific wavelengths. This new model, which consists of multiple atmospheric layers, however, matches observations from both planets over a wide range of wavelengths. The new model includes haze particles within deeper layers that were previously thought to only contain clouds of methane and hydrogen sulfide ices.
“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 presenting this result in the Journal of Geophysical Research: Planets. “It’s also the first to explain the difference in visible color between Uranus and Neptune.”
The team’s model is made up of three layers of different height aerosols. The middle layer, which is a layer of haze particles (referred to in the paper as the Aerosol-2 layer) that is thicker on Uranus than on Neptune, is the key layer that affects the colors. The team believes that methane ice condenses onto the particles in this layer on both planets, pulling the particles deeper into the atmosphere in a shower of methane snow. The team believes Neptune’s atmosphere is more efficient at churning up methane particles into the haze layer and producing this snow because it has a more active, turbulent atmosphere than Uranus. This removes more of the haze and keeps Neptune’s haze layer thinner than it is on Uranus, meaning the blue color of Neptune looks stronger.
“We hoped that developing this model would help us understand clouds and hazes in the ice giant atmospheres,” commented Mike Wong, an astronomer at 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 critical to this result because it can 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 the atmospheres of both planets are across the entire disk of the planet as well as across a range of near-infrared wavelengths.
“The Gemini observatories continue to provide new insights into the nature of our planetary neighbors,” said Martin Still, National Science Foundation Gemini Program Officer. “Gemini North provided a component within a suite of ground- and space-based facilities critical to the detection and characterization of atmospheric hazes in this experiment.”
The model also helps to explain the dark spots that are occasionally seen on Neptune and less frequently seen 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 demonstrating that darkening the deepest layer of their model would result in dark spots similar to those seen on Neptune and possibly Uranus.
