Scientists have explained why, despite having much in common, Uranus and Neptune have very different appearances.
The furthest two planets in the Solar System have similar masses, sizes and atmospheric compositions – yet Neptune looks distinctly bluer.
New research, led by Professor Patrick Irwin at the University of Oxford, suggests a layer of haze on both planets is behind the different hues.
Both would appear almost equally blue if there were no haze in their atmospheres, the study suggests.
Using observations from the Hubble Space Telescope, the Nasa Infrared Telescope Facility and the Gemini North telescope, an international team of researchers developed a model to describe aerosol layers in the atmospheres of both planets.
The model involves three haze layers at different heights in the two planets’ atmospheres.
The middle layer of haze particles is thicker on Uranus than on Neptune, which affects the visible colour of the two planets, the scientists say.
On both planets, methane ice condenses on the particles in the middle layer, forming a shower of methane snow that pulls the haze particles deeper into the atmosphere.
Neptune has a more active, turbulent atmosphere than Uranus, suggesting its atmosphere is more efficient at churning up the gas into the haze layer where it can condense on the haze particles and produce this snow.
This removes more of the haze and keeps Neptune’s haze layer thinner. So, Neptune appears bluer, while excess haze on Uranus builds up in the planet’s stagnant atmosphere, giving it a lighter tone.