Scientists have explained why, despite their many similarities, Uranus and Neptune look very different.
The two outermost planets in the solar system have similar masses, sizes, and atmospheric compositions, but Neptune looks noticeably bluer than its neighbor.
New research, led by Professor Patrick Irwin of the University of Oxford, suggests that a layer of haze on the two planets is to blame for the different hues.
Both would appear almost equally blue if there was no haze in their atmospheres, the study suggests.
Explaining the color difference between Uranus and Neptune was an unexpected bonus
Dr. Mike Wong, University of California
Using observations from the Hubble Space Telescope, the Nasa Infrared Telescope Facility and the Gemini North Telescope, an international team of researchers has developed a model to describe the aerosol layers in the atmospheres of the two planets.
Professor Irwin, lead author of the paper, said: “This is the first model to simultaneously fit observations of reflected sunlight from ultraviolet to near infrared wavelengths.
He added: “It is also the first to explain the visible color difference between Uranus and Neptune.”
The model involves three layers of haze at different heights in the atmospheres of the two planets.
The middle layer of haze particles is thicker on Uranus than on Neptune, which affects the visible color of both planets, scientists say.
On both planets, methane ice condenses on midlayer particles, forming a shower of methane snow that carries haze particles deeper into the atmosphere.
Neptune has a more active and turbulent atmosphere than Uranus, which suggests that its atmosphere is more efficient at stirring gas into the haze layer where it can condense on haze particles and produce this snow.
This removes more haze and keeps Neptune’s haze layer thinner.
As a result, Neptune appears bluer, while excess haze over Uranus collects in the planet’s stagnant, slow-moving atmosphere, giving it a lighter tone.
The research also showed the presence of a second, deeper layer in the model.
When darkened, this could explain the dark spots occasionally observed on Neptune and more sporadically on Uranus, such as the famous Great Dark Spot (GDS-89) on Neptune observed by Voyager 2 in 1989.
Dr Mike Wong, an astronomer at the University of California, Berkeley, said: “We hoped that developing this model would help us understand clouds and haze in the atmospheres of ice giants.
“Explaining the color difference between Uranus and Neptune was an unexpected bonus.”
The results are published in the Journal of Geophysical Research: Planets.