Why are Uranus and Neptune two different shades of blue?
The two worlds are quite similar in mass to each other – Uranus is about 15 times the mass of Earth, while Neptune is 17 – with nearly identical atmospheric compositions of hydrogen (over 80% each) , helium and methane. But now new research suggests that a ‘stagnant and slow moving’ atmosphere on Uranus allows haze to build up and concentrate on the gas giant, making it a ‘whiter’ cyan blue than the cerulean of Neptune.
A new model, using wavelengths ranging from ultraviolet to near infrared, studies several atmospheric layers on each of the planets. The study shows that embedded in the inner atmospheric planetary layers is even more haze than expected, rather than just icy clouds of methane and hydrogen sulfide. It’s the first time a study has considered wavelengths ranging from ultraviolet to near-infrared, rather than focusing on a handful of light waves, the authors said.
“It is also the first [study] to explain the visible color difference between Uranus and Neptune,” lead author Patrick Irwin, professor of planetary physics at the University of Oxford, said in a statement. (opens in a new tab) from the National Science Foundation’s National Optical-Infrared Astronomy Research Laboratory, or NOIRLab.
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Previously, scientists had suggested that it was Neptune’s methane that made this planet so blue, because the gas absorbs a lot of red light and reflects bluer colors. But scientists were unable to explain what was happening to Uranus, given that it contains even more methane (2.3% of atmospheric mass, compared to 1.9% in Neptune).
This discrepancy suggests that something else must be responsible for the color difference. But other key differences between the planets have revealed few clues to the mystery. And both planets are understudied, each having only been visited by a single spacecraft, NASA’s Voyager 2 in the 1980s. Since then, scientists have relied on telescopic recordings to monitor the two blue orbs .
Uranus is a strange world, spinning sideways with respect to the plane of the solar system in a way that leads to extreme seasons persisting for about 20 years. (A 2018 study suggested that a world twice the size of Earth would have slammed into Uranus, causing its odd orientation.) Uranus also has little or no internal heat to supplement sunlight, presenting the most cold of all the planets of the solar system.
One of Neptune’s distinctive atmospheric characteristics is its storms; Neptune’s winds can reach up to 1,500 mph (2,400 km/h), the fastest ever detected in the solar system. When Voyager 2 flew by Neptune, it found a large storm dubbed the “Great Dark Spot”, which was large enough to contain the entire Earth. While this place has disappeared, others have appeared in Hubble Space Telescope observations.
Uranus has its own stormy temper. The year 2014, for example, saw an awe-inspiring spectacle when storms were quite active over the usually quiescent world. Astronomers were amazed given that sunlight was strongest on the planet in 2007, when sunlight fell directly on the equator. The reasons for this time lag are poorly understood.
For the new study, astronomers used multiple observatories: new work from the Gemini North Telescope near the summit of Mauna Kea in Hawai’i, as well as archival data from NASA’s Hawaii-based Infrared Telescope Facility ‘i and Hubble. Studies have focused on ultraviolet, visible and near infrared wavelengths (0.3 to 2.5 micrometers).
A particularly important piece of this data was spectra, the distinctive “fingerprints” that measure the amount of wavelengths of light emitted by a given object. Spectra from Gemini North allowed scientists to understand how reflective each atmosphere was across the planet among different wavelengths of near-infrared light.
The resulting model focuses on aerosols, or airborne particles, suggesting three layers of aerosols at different heights in the atmospheres of the two planets.
According to the researchers, it is the middle layer of each planet that seems to be most responsible for the different hues. In this layer on both planets, methane ice condenses on aerosols, but then the two worlds diverge.
Neptune’s more active atmosphere likely produces snow as it displaces methane particles in the haze, which clears the haze over time. Uranus, however, has a thicker haze layer due to its slower atmosphere.
Scientists also suspect that the middle layer of the atmosphere is what produces dark spots on each planet.
Scientists will likely have to continue to rely on Earth and Hubble data to study the two strange worlds, as no spacecraft are yet targeted to go that far, although a new government document suggests a Uranus mission should to be NASA’s highest priority mission. planetary science mission and launch in the 2030s.
In the meantime, the scientists behind the new research hope to learn more about how Uranus’ atmosphere is changing before the onset of the austral spring in 2049, as Voyager 2 examined this region in summer.
A research-based study has been published (opens in a new tab) May 23 in Journal of Geophysical Research: Planets.