Why Uranus and Neptune are different colors

Astronomers may now understand why Uranus and Neptune, two identical planets, have distinct hues. Researchers built a single atmospheric model that fits observations of both planets using data from the Gemini North telescope, the NASA Infrared Telescope Facility, and the Hubble Space Telescope. Excess haze on Uranus builds up in the planet's stagnant, sluggish atmosphere, giving it a lighter tone than Neptune, according to the model.

Neptune and Uranus have many similarities, including comparable masses, diameters, and atmospheric compositions, yet their looks are strikingly different. Neptune has a noticeably bluer hue at visible wavelengths than Uranus, which is a faint shade of cyan. The color differences between the two planets have finally been explained by astronomers.

According to new study, a concentrated haze layer that appears on both planets is thicker on Uranus than on Neptune and 'whitens' Uranus' look more than Neptune's [1]. If the atmospheres of Neptune and Uranus were devoid of haze, they would seem nearly identically blue [2].

This is based on a model [3] created by an international team led by Patrick Irwin, Professor of Planetary Physics at Oxford University, to characterize aerosol layers in Neptune and Uranus' atmospheres [4].

Previous research into the higher atmospheres of these planets has concentrated on the appearance of the atmosphere at only a few wavelengths. However, throughout a wide range of wavelengths, this new model, which consists of many atmospheric layers, matches evidence from both worlds. The new model contains haze particles in deeper levels that were previously considered to solely include methane clouds and hydrogen sulfide ices.

"This is the first model to simultaneously fit observations of reflected sunlight from ultraviolet to near-infrared wavelengths,"said Irwin, who is the main author of a report published in the Journal of Geophysical Research: Planets. "It's also the first to explain the difference in visible color between Uranus and Neptune." 

Three layers of aerosols at various heights make up the team's model [5]. The middle layer, which is a layer of haze particles (referred to as the Aerosol-2 layer in the research) that is thicker on Uranus than on Neptune, is the critical layer that impacts the colors. Methane ice condenses onto the particles in this layer on both worlds, dragging them further into the atmosphere in a shower of methane snow, according to the study.  The team believes Neptune's atmosphere is more efficient at churning up methane particles into the haze layer and creating this snow because it has a more active, turbulent atmosphere than Uranus'. This eliminates more haze and maintains Neptune's haze layer thinner than Uranus', making Neptune's blue hue appear brighter.

"We hoped that developing this model would help us understand clouds and hazes in the ice giant atmospheres," said Mike Wong, an astronomer at the University of California, Berkeley, who was part of the team that came up with the discovery. "Explaining the difference in color between Uranus and Neptune was an unexpected bonus!"

Irwin's team used archival data from the NASA Infrared Telescope Facility, as well as observations of the planets 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 — to create this model.

The NIFS instrument on Gemini North played a critical role in this discovery since it can produce spectra (measurements of how luminous an object is at various wavelengths) for every location in its field of view. This allowed the scientists to make precise measurements of how reflective both planets' atmospheres are throughout their whole disks as well as a spectrum of near-infrared wavelengths.

"The Gemini observatories continue to deliver new insights into the nature of our planetary neighbors," stated Martin Still, the National Science Foundation's Gemini Program Officer."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 concept also explains why black patches appear on Neptune and Uranus less frequently. While astronomers were aware of black areas in both planets' atmospheres, they had no idea which aerosol layer was creating them or why the particles in those levels were less reflective. The team's findings answer these problems by demonstrating that darkening the deepest layer of their model will result in dark areas similar to those found on Neptune and maybe Uranus.


[1] This whitening effect is analogous to how exoplanet atmospheric clouds dull or 'flatten' characteristics in their spectra.

[2] Methane molecules in the planets' atmospheres absorb more of the red hues of sunlight dispersed from haze and air molecules. This phenomenon, known as Rayleigh scattering, is what causes blue sky on Earth (albeit sunlight is typically dispersed by nitrogen molecules rather than hydrogen molecules in Earth's atmosphere). At shorter, bluer wavelengths, Rayleigh scattering is more common.

[3] An aerosol is a gaseous suspension of small droplets or particles. Mist, soot, smoke, and fog are all common instances on Earth. Aerosol hazes in the atmospheres of Neptune and Uranus are caused by particles created by sunlight reacting with substances in the atmosphere (photochemical reactions).

[4] A scientific model is a computer program that scientists use to evaluate predictions about phenomena that are hard to verify in real life.

[5] The lowest layer (dubbed the Aerosol-1 layer in the report) is made up of a mixture of hydrogen sulfide ice and particles created by the planets' atmospheres interacting with sunlight. The top layer (the Aerosol-3 layer) is an extended layer of haze, comparable to the intermediate layer but more tenuous. Large methane ice particles grow above this layer on Neptune.