Uranus and Neptune are the two-outer planets of our solar system that share different hues of blue. In a recent discovery, scientists have discovered the reason why the two planets reflect different shades of blue despite having much in common. With the help of data from the NASA/ESA Hubble Space Telescope, Gemini North telescope and the NASA Infrared Telescope Facility, researchers developed a single atmospheric model that matches observations of both planets.
Before we delve into the differences in the colours of the planets, let us take a look at factors that make them similar: Neptune and Uranus have similar masses, sizes, and atmospheric compositions, yet their appearances are notably different. According to NASA, Neptune is a rich, deep azure hue whereas Uranus is a distinctly pale shade of cyan at visible wavelengths.
The research was led by an international team led by Patrick Irwin, Professor of Planetary Physics at Oxford University. The team developed a model to describe aerosol layers in the atmospheres of Neptune and Uranus. According to a statement shared by Hubble, previous investigations of these planets’ upper atmospheres had focused on the appearance of the atmosphere at only specific wavelengths. With the latest model, multiple atmospheric layers match observations from both planets across a wide range of wavelengths. The new model also included haze particles within deeper layers that had previously been thought to contain only clouds of methane and hydrogen sulphide ices.
Their analysis revealed that there are three aerosol layers in the atmospheres of Neptune and Uranus. Among these three layers, the one that affects the colours of the two planets is the middle layer. The middle layer of haze particles is thicker on Uranus than on Neptune. Researchers suggest that, on both planets, methane ice condenses onto the particles in this layer, pulling the particles deeper into the atmosphere.
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.
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— HUBBLE (@HUBBLE_space) May 31, 2022
The study also mentions that since Neptune has a more active, turbulent atmosphere than Uranus does, the latter’s atmosphere is more efficient at churning up methane particles into the haze layer. This process removes more of the haze and keeps Neptune’s middle layer thinner than it is on Uranus, with the result that the blue colour of Neptune looks stronger.
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