# A model to describe them all – Well, ice giants anyway

Physics 15, 89

The spectral responses from the atmospheres of Uranus and Neptune can now both be completely characterized with the same model, an achievement that has implications for characterizing the atmospheres of exoplanets.

When it comes to space missions, the focus has largely been on planets near Earth, where they have been left outside in the cold. But that is about to change with a dedicated mission to Uranus planned for the 2030s. Scientists say this trip will help them better understand the atmosphere in both Uranus and Neptune, our solar system’s two ice giants – cold gas storms that mostly consist of elements heavier than hydrogen and helium. Now Patrick Irwin from the University of Oxford, UK, and colleagues, for the first time, simultaneously analyzed the reflectance spectra of both ice giants with the same model [1]. Thus, the team made two incredible discoveries about the visual appearance of both worlds, and unlocked the reason why they shimmer in different shades of blue and why Neptune has dark spots. The team says that the findings could have consequences for the study of the atmosphere on planets beyond our solar system.

Although there have been no previous independent missions to Uranus and Neptune, the planets have not been completely ignored. For example, when NASA’s Voyager 2 passed the ice giants in the late 1980s, it gathered information about the reflections of both worlds. In 1994, the Hubble Space Telescope took its first images of the planets’ icy exterior.

Most of the data collected from these and other terrestrial observations look at small spectral regions of light that Uranus and Neptune reflect and emit. These limited data make it difficult for scientists to determine the properties of certain aerosol particles in the planets’ atmospheres. This problem is especially acute for Neptune because its small size in the sky makes it difficult to collect the longer wavelengths that the planet emits.

To solve that problem, Irwin and colleagues adapted a model that has previously been used to explore the spectral spectra of almost every other planet in our solar system, as well as some exoplanets. They adapted the model to work over a wide wavelength range, from 0.3 to 2.5

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. The team then used the model to simultaneously analyze available observational data for both ice giants.

The team’s analysis reveals the presence of what they believe are hydrocarbon-based aerosols high up in the stratospheres of both Uranus and Neptune. These particles fall through the planets’ atmospheres, mix and react with gas, such as methane, which simultaneously moves upwards. This process creates a haze around each planet. The team shows that Neptune’s atmosphere is more dynamic, which gives Neptune a thinner haze – and darker hue – than Uranus. The team also finds that these aerosols may also be behind Neptune’s previously unexplained dark spots. After interacting with methane, the aerosols encounter hydrosulfide, which is found deeper in the atmosphere of both Neptune and Uranus. The team believes that the dark spots develop in places on Neptune that have a lower density of this material, with spots that come from the dynamics of its enveloping gas.

Understanding the atmosphere of ice giants can be important for future exoplanet research, as it is possible that this type of planet is among the most common in the Milky Way, says Erich Karkoschka, a planetary researcher at the University of Arizona. That thinking comes because so far most exoplanets that have been found in our galaxy have masses on the order of Uranus and Neptune.

Karkoschka notes that because Uranus and Neptune have such similar sizes, masses and composition, Irwin and his colleagues were able to adapt their data with the same model, but it is unlikely that a similar analysis could be done with any other two of our solar system planets. That ability has implications for characterizing other ice giants. “If you had one [model] for Uranus and another for Neptune, then you would not know which one to use “for another ice giant,” he says. But since this model works for both planets, he believes there is “validity” in using it on related systems.

–Allison Gasparini

Allison Gasparini is a freelance science writer based in Santa Cruz, California.

## References

1. PGJ Irwin et al.“Hazy blue worlds: A holistic aerosol model for Uranus and Neptune, including dark spots,” JGR Planets 127 (2022).

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