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 has 17, with almost identical atmospheric compositions of hydrogen (more than 80% each), helium and methane. But now, new research suggests that a “stagnant and slow” atmosphere in Uranus allows fog to accumulate and concentrate in the gas giant, turning it into a “whiter” cyan blue than Neptune’s cerulean.
A new model, which uses wavelengths from the ultraviolet to the near infrared, investigates multiple atmospheric layers on each of the planets. The study shows that, embedded in the planetary layers of the inner atmosphere, there is even more fog than previously thought, instead of icy clouds of methane and hydrogen sulfide. According to the authors, this is the first time that a study has taken into account wavelengths from the ultraviolet to the near infrared, rather than focusing on a handful of light waves.
“He is also the first [study] to explain the visible color difference between Uranus and Neptune, “said lead author Patrick Irwin, a professor of planetary physics at Oxford University, in a statement (opens in a new tab) from the National Optical-Infrared Astronomy Research Laboratory of the National Science Foundation, or NOIRLab.
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Scientists had previously suggested that it was Neptune’s methane that made this planet so blue, as the gas absorbs a lot of red light and reflects bluer colors. But scientists could not explain what was happening in Uranus, as it has even more methane (2.3% of its atmospheric mass, compared to 1.9% in Neptune).
This discrepancy suggests that something else should be responsible for the color difference. But other key differences between the planets revealed few clues about the mystery. And both planets are little studied, each only visited by a spacecraft, NASA’s Voyager 2 in the 1980s. Since then, scientists have relied on telescopic controls to control the two blue orbs.
Uranus is a strange world, spinning sideways compared to the plane of the solar system in a way that makes extreme seasons persist for about 20 years. (A 2018 study suggested that a world twice the size of Earth hit Uranus, causing its strange orientation.) Uranus also has little or no internal heat to supplement sunlight, presenting the coldest atmosphere. of any planet in the solar system.
Infrared images of Uranus (1.6 and 2.2 microns) obtained on August 6, 2014, showing a very large storm on the planet. (Image credit: Imke of Pater (UC Berkeley) and images from the WM Keck Observatory)
One of Neptune’s distinctive atmospheric features is its storms; Neptune winds can reach up to 1,500 mph (2,400 km / h), the fastest time ever detected in the solar system. When Voyager 2 flew through Neptune, it encountered a large storm nicknamed the “Great Dark Spot,” which was large enough to contain the entire Earth. While this site has disappeared, others have appeared in observations from the Hubble Space Telescope.
Uranus has its own stormy temperament. The year 2014, for example, had an impressive spectacle when storms were quite active in the usually quiet world. Astronomers were surprised that sunlight was brightest on the planet in 2007, when sunlight fell directly on the equator. The reasons for this time gap are misunderstood.
In 2016, a storm was detected in Neptune’s atmosphere with the Hubble Space Telescope. (Image credit: NASA, ESA and MH Wong and J. Tollefson (UC Berkeley))
For the new study, astronomers used several observatories: recent work on the Gemini North telescope near the summit of Mauna Kea in Hawaii, along with archival data from the Hawaii-based NASA infrared telescope facility. the Hubble. The studies covered near-ultraviolet, visible, and near-infrared wavelengths (0.3 to 2.5 micrometers).
An especially important component of this data was spectra, the distinctive “fingerprints” that measure the amount of wavelengths of light emitted by a given object. The spectra of Gemini North allowed scientists to understand how reflective each atmosphere on the planet was between different wavelengths of near-infrared light.
The resulting model focuses on aerosols, or particles suspended in the atmosphere, suggesting three layers of aerosols at different heights in the atmospheres of the two planets.
Voyager 2 took this picture on January 25, 1986 when it left Uranus for Neptune. (Image credit: NASA / JPL-Caltech)
According to researchers, it is the middle layer of each planet that seems to be most responsible for the different shades. In this layer of the two planets, methane ice condenses into aerosols, but then the two worlds diverge.
Neptune’s most active atmosphere is likely to produce snow as it moves methane particles into the fog, which removes the fog over time. Uranus, however, has a thicker layer of fog due to its slower moving atmosphere.
Scientists also suspect that the layer of the middle atmosphere is the one that produces dark spots on each planet.
Scientists are likely to continue to rely on terrestrial and Hubble data to study the two strange worlds, as no spacecraft is yet to return so far, although a new government document suggests that a space mission ‘Uranus should be NASA’s top priority. mission and launch of planetary science in the 2030s.
Meanwhile, scientists behind the new research hope to learn more about how Uranus’ atmosphere changes before southern spring begins in 2049, as Voyager 2 examined this region in the summer.
A research-based study was published (opens a new tab) on May 23 in the Journal of Geophysical Research: Planets.
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