Why Does Neptune Appear Bluer Than Uranus? Hubble May Have Helped Find The Answer

The excess haze on Uranus builds up in the planet's stagnant, sluggish atmosphere, and makes it appear a lighter tone than Neptune, a new model has revealed.

New study explains why Uranus (left) is paler than Neptune (right)

Source : NASA

New Delhi: The ice giant Uranus has an excess haze on it, and dark spots due to the darkening of a second deeper haze layer. The excess haze on Uranus is what makes it paler than Neptune, observations from NASA's Hubble Space Telescope, the NASA Infrared Telescope, and the Gemini Observatory have revealed. 

These observations will now help astronomers understand why the similar planets Uranus and Neptune are of different colours. The study describing the findings was recently published in the Journal of Geophysical Research: Planets.

A team of researchers have developed a single atmospheric model that matches observations of both planets, using data collected by the Hubble Space Telescope, the NASA Infrared Telescope Facility, and the Gemini North Telescope. 

Excess Haze On Uranus Makes It Appear Paler Than Neptune

The excess haze on Uranus builds up in the planet's stagnant, sluggish atmosphere, and makes it appear a lighter tone than Neptune, the model has revealed. The researchers also observed the presence of a second, deeper layer which, when darkened, can account for dark spots in these atmospheres. In 1989, the Voyager 2 spacecraft had discovered the famous Great Dark Spot (GDS), which is thought to be a hole in the methane cloud deck of Neptune. When the Hubble Space Telescope observed Neptune in 1994, the Great Dark Spot was gone, and a different dark spot had appeared in the northern atmosphere of Neptune.

Why Are Appearances Of Uranus And Neptune Different Despite Similar Masses & Sizes? 

Neptune and Uranus have similar masses, sizes, and atmospheric compositions. Despite this, their appearances are notably different. Neptune has a distinctly bluer colour than Uranus at visible wavelengths. Now, astronomers may have an explanation to this. 

According to new research, a layer of concentrated haze that exists on both planets is thicker on Uranus than a similar layer on Neptune. It is this layer of concentrated haze that "whitens" Uranus's appearance more than that of Neptune, a statement released by University of Oxford, England said. Both Neptune and Uranus would appear almost equally blue if there were no haze in the atmospheres of the ice giants. 

New Model Describes Aerosol Layers In Ice Giants’ Atmospheres

The model described in the new study was developed by an international team of researchers led by scientists from the University of Oxford. Using the model, the scientists were able to describe aerosol layers in the atmospheres of Neptune and Uranus. 

Previous research on the ice giants' upper atmospheres had focused on the appearance of the atmosphere at only specific wavelengths. The new model, however, matches observations from both the ice giants across a wide range of wavelengths simultaneously. The model consists of multiple atmospheric layers, and includes haze particles at deeper layers that had previously been thought to contain only clouds of methane and hydrogen sulphide ices. 

First Study To Explain Difference In Visible Colour Between Uranus And Neptune

According to the statement, Professor Patrick Irwin, the lead author on the paper, said this is the first model to simultaneously fit observations of reflected sunlight from ultraviolet to near-infrared wavelengths. He stated that this study is also the first to explain the difference in visible colour between Uranus and Neptune.

Three Layers Of Aerosols In The Model

According to the study, the model consists of three layers of aerosols at different heights. The middle layer is the key layer which affects the colours. It is a layer of haze particles, and is referred to as the Aerosol-2 layer in the paper. 

The layer is thicker on Uranus than in Neptune. The researchers believe that on both planets, methane ice condenses onto the particles in the Aerosol-2 layer. This pulls the particles deeper into the atmosphere in a shower of methane snow. 

Why Neptune Is Bluer Than Uranus

Since Neptune has a more active and turbulent atmosphere than that of Uranus, Neptune's atmosphere might be more efficient at churning up methane particles into the haze layer and producing the snow. The researchers concluded that this removes more of the haze and keeps Neptune's haze layer thinner than it is on Uranus. This is what makes Neptune bluer than Uranus.

Mike Wong, one of the researchers involved in the study, said the team hoped that developing the model would help the scientists understand clouds and hazes in the ice giant atmospheres. He added that explaining the difference in colour between Uranus and Neptune was an "unexpected bonus". 

According to the study, the model also helps explain the dark spots that are occasionally visible on Neptune, and more sporadically on Uranus. Astronomers were already aware of the presence of dark spots in the atmospheres of both planets, but did not know which aerosol layer was causing these dark spots or why the aerosols at those layers were less reflective. 

The new study sheds light on these questions by showing that a darkening of the particles in the deepest layer of the model would produce dark spots very similar to those seen on Neptune and occasionally on Uranus.