As we go away from the Sun and into the outer solar system, two planets hold our breath for their beautiful shades of colour — blue. Both Uranus and Neptune, while they appear to be painted in the same colour, they can not be more different from each other. Scientists have now figured out why the hues of these two planets are very different.
It’s due to the excess haze on Uranus that builds up in the planet’s stagnant, sluggish atmosphere and makes it appear a lighter tone than Neptune. Researchers used the Gemini North telescope, the NASA Infrared Telescope Facility, and the Hubble Space Telescope to gather the data around the two planets.
The study published in the journal Advancing Earth and Space Science states that methane condenses so rapidly upon these haze particles that it efficiently ‘snows’ out at the base of this layer, falling to lower, warmer levels, where the methane evaporates, releasing the core haze particles.
Neptune and Uranus, the last two planets in the outer reaches of the solar system, have a lot in common including their sizes, masses and atmospheric compositions, yet they appear very different from each other. While Neptune, at a visible wavelength, has a distinctly bluer colour, Uranus is a pale shade of cyan.
The international team of researchers led by Patrick Irwin, Professor of Planetary Physics at Oxford University, developed a model to describe aerosol layers in the atmospheres of Neptune and Uranus.
“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 color between Uranus and Neptune,” explained Irwin, who is the lead author of a paper.
According to the study, the model consists of three layers of aerosols at different heights. The key layer that affects the colors is the middle layer, which is a layer of haze particles (referred to in the paper as the Aerosol-2 layer) that is thicker on Uranus than on Neptune. The team suspects that, on both planets, methane ice condenses onto the particles in this layer, pulling the particles deeper into the atmosphere in a shower of methane snow.
Because Neptune has a more active, turbulent atmosphere than Uranus does, the team believes Neptune’s atmosphere is more efficient at churning up methane particles into the haze layer and producing this snow. This removes more of the haze and keeps Neptune’s haze layer thinner than it is on Uranus, meaning the blue color of Neptune looks stronger.
The model also helps explain the dark spots that are occasionally visible on Neptune and less commonly detected on Uranus. While astronomers were already aware of the presence of dark spots in the atmospheres of both planets, they didn’t know which aerosol layer was causing these dark spots or why the aerosols at those layers were less reflective.