Listen to this article - Play Now

Researchers at the Association of Universities for Research in Astronomy (AURA) have constructed a single atmospheric model that matches observations of both planets, according to their findings.

Excess haze on Uranus builds up in the planet’s stagnant, sluggish atmosphere, giving it a lighter tone than Neptune, according to the model. Observations from the Gemini North telescope, NASA’s Infrared Telescope Facility, and the Hubble Space Telescope were used in this study.
Neptune and Uranus share many similarities, including similar masses, diameters, and atmospheric compositions, but their appearances are strikingly different. Neptune has a noticeably bluer colour at visible wavelengths than Uranus, which is a faint shade of cyan. The difference in colour between the two planets has finally been explained by astronomers.

According to new research, a concentrated haze layer that appears on both planets is thicker on Uranus than on Neptune, and ‘whitens’ Uranus’ appearance more than Neptune’s. Both Neptune and Uranus would appear nearly identically blue if their atmospheres were free of haze.
This result is based on a model built by an international team led by Patrick Irwin, Professor of Planetary Physics at Oxford University, to describe the aerosol layers in Neptune and Uranus’ atmospheres. Previous research into the higher atmospheres of these planets had focused 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 layers that were previously considered to solely include methane clouds and hydrogen sulphide ices.
“This is the first model to fit measurements of reflected sunlight from ultraviolet to near-infrared wavelengths at the same time,” said Irwin, who is the lead author of a report published in the Journal of Geophysical Research: Planets that details the findings. “It’s also the first to explain why Uranus and Neptune have different visible colours.”

Three layers of aerosols at various heights make up the team’s model. 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 colours. Methane ice condenses onto the particles in this layer on both worlds, dragging them deeper 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 producing this snow because it has a more active, turbulent atmosphere than Uranus’.

This removes more haze and keeps Neptune’s haze layer thinner than Uranus’, making Neptune’s blue colour appear brighter.
“We thought that constructing this model would help us comprehend 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 colour difference between Uranus and Neptune was a pleasant surprise!”

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 across their whole discs as well as a spectrum of near-infrared wavelengths.
“The Gemini observatories continue to provide fresh insights into the nature of our planetary neighbours,” stated Martin Still, the National Science Foundation’s Gemini Program Officer. “Gemini North contributed to a range of ground- and space-based facilities important to the detection and characterisation of atmospheric hazes in this experiment.”

The concept also explains the black spots that can be seen on Neptune but are less frequently seen on Uranus. While astronomers were already aware of black spots in both planets’ atmospheres, they had no idea which aerosol layer was creating the spots or why the aerosols at those layers were less reflecting. The team’s findings answer these questions by demonstrating that darkening the lowest layer of their model produces dark spots comparable to those seen on Neptune and maybe Uranus.