Have you ever noticed that these Uranus and Neptune, so alike in their size and mass, who are also similar in their basic atmospheric compositions, are somehow not the same color? We don’t have many close-up images of the pair, but the most famous pictures come from the Voyager 2 spacecraft. When you see the ice giants side by side, the difference in color is distinct. Neptune is a bright blue orb with noticeable storms; Uranus is a muted, paler blue covered in haze.

The key to that difference is the haze.

In new research published in the Journal of Geophysical Research: Planets and led by Patrick Irwin, observations across ultraviolet, visible, and near-infrared wavelengths were used to model the atmospheric layers of both planets at three different heights. Within those layers, the aerosols were analyzed, and the middle of the three was found to contain the particles causing haze. This particular layer is thicker on Uranus than on Neptune, which could partially explain the haziness differences between the two. Additionally, these aerosols give methane ice a surface onto which to condense, similar to the formation of ammonia mush balls discovered in Jupiter’s atmosphere.

Those bits of ice create a shower of methane snow deep in the atmospheres of the two planets, but then a second difference kicks in. Neptune has a more turbulent atmosphere than Uranus, which makes the brighter blue more apparent as Neptune’s storms churn up the methane particles, thinning out the hazy layer. And voila, but for that turbulence, the two planets would look the same color.

Side note: The effect that makes both of our ice giants look blue is the same that produces blue skies here on Earth – Rayleigh scattering. Methane is good at absorbing the red wavelengths of sunlight, which then leaves behind the predominantly bluer colors seen in the Voyager images.

Okay, NASA. Time for ice giant orbiters, please.

More Information

ESA Hubble press release

NOIRLab press release

“Hazy Blue Worlds: A Holistic Aerosol Model for Uranus and Neptune, Including Dark Spots,” P. G. J. Irwin et al., 2022 May 23, JGR Planets