Scientists finally have an answer to why Neptune and Uranus, despite being very similar in many ways, are very different shades of blue.
Neptune has a distinctly bluer colour whereas Uranus is a pale shade of cyan. The two farthest planets from the Sun are similar in mass, size and atmospheric composition but Neptune is distinctly bluer than its neighbour.
New research led by Professor Patric Irwin at the Department of Physics, University Of Oxford, suggests that a hazy layer that exists on both planets is behind the different shades of blue. Their appearances would be identical if it wasn't for this haze, the lead author Mr Irwin explained.
Astronomers may now know why Uranus and Neptune are different colours. If you are curious to understand this yourself and find out how observations from @HUBBLE_space contributed to explaining this difference, watch our latest #SpaceSparks episode.— HUBBLE (@HUBBLE_space) May 31, 2022
Using observations from the Hubble Space Telescope, the NASA infrared telescope facility and the Gemini North Telescope, researchers developed a model to describe aerosol layers in the atmosphere of both planets. As per a press release, the model revealed that excess haze on Uranus builds up in the planet's stagnant, sluggish atmosphere, which makes it appear a lighter tone than Neptune.
"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 colour between Uranus and Neptune," explained Mr Irwin.
Further, according to the press note, the researchers explained that the model involves three haze layers at different heights. On Uranus, the middle layer of haze is thicker than on Neptune, which affects the visible colour.
Scientists explained that on Uranus, methane ice condenses on the particles in the middle layer of haze, which forms a shower of methane snow that pulls haze particles deeper into the atmosphere. Once there, the haze particles then can promote the condensation of hydrogen sulphide ice, forming a separate, deeper layer of haze.
Neptune's atmosphere, on the other hand, is more active and turbulent than that of Uranus, suggesting that it is more efficient at producing the snow which removes more of the haze and keeps Neptune's middle layer thinner.
Therefore, scientists stated that Neptune appears bluer, while excess haze on Uranus builds up in the sluggish atmosphere and causes a lighter shade.