Scientists crack 228-year-old mystery about the colour of Uranus and Neptune | Science | News

Top researchers have cracked the 228-year-old mystery about the colours of the two planets which are furthest from the Sun and planet Earth.

New research by top professors at the University of Oxford show that the distinct bluish appearance of Neptune, compared to its neighbour Uranus, is due to a layer of haze present in their atmospheres. It was previously thought that Uranus was a “darker green” compared to Neptune – but latest findings suggest they are more similar in colour than previously thought.

Patrick Irwin, who is leading the research, said: “Although the familiar Voyager 2 images of Uranus were published in a form closer to ‘true’ colour, those of Neptune were, in fact, stretched and enhanced, and therefore made artificially too blue.”

He added: “Even though the artificially saturated colour was known at the time amongst planetary scientists – and the images were released with captions explaining it – that distinction had become lost over time.”

Uranus was first discovered in 1781, while Neptune was located 14 years later in 1995. Since then, no scientists have been able to explain why the colours of planets are as they are – until now, 228 years later.

Despite their shared mass, size, and atmospheric composition characteristics, the distinct difference in their visible colours has long intrigued astronomers.

Scientists used data from various telescopes such as, the Hubble Space Telescope’s Imaging Spectrograph (STIS) and the Multi Unit Spectroscopic Explorer (MUSE) on the European Southern Observatory’s Very Large Telescope, and found out that a layer of haze within the atmospheres of these ice giants plays a crucial role in determining their perceived colours.

This groundbreaking research, detailed in the Journal of Geophysical Research: Planets, marks a significant leap forward in understanding the complexities of these distant worlds.

The team’s model revolves around multiple layers of haze situated at different elevations within the atmospheres of Neptune and Uranus. Interestingly, Uranus possesses a thicker layer of haze than Neptune, primarily due to its relatively stagnant atmospheric conditions.

This haze thickness disparity influences the two planets’ visual appearance, with Neptune exhibiting a more intense blue tone than Uranus. The research also suggests the presence of an additional, deeper layer within the model that could account for the sporadic dark spots occasionally observed on both Neptune and Uranus.

These dark features, such as Neptune’s Great Dark Spot (GDS-89) documented by Voyager 2 in 1989, have remained a mystery to astronomers until now.

The study explains the colour disparity between the two ice giants and delves into these distant worlds’ atmospheric dynamics and peculiarities. Dr Mike Wong, an astronomer from the University of California, Berkeley, and a research team member, was surprised by the unexpected insight he uncovered regarding the planets’ appearance while aiming to comprehend their cloud and haze formations.

He said: “We hoped that developing this model would help us understand clouds and hazes in the ice giant atmospheres.”

The research also resolved the mystery behind Uranus’s varying colour during its 84-year orbit around the sun, attributing it to the thickness of certain gases at the planet’s poles as they interact with sunlight.

The full study, ‘Hazy blue worlds: A holistic aerosol model for Uranus and Neptune, including Dark spots‘, is available in the Journal of Geophysical Research: Planets.


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