If it were a movie or novel of questionable taste, it might be called “50 Shades of Blue”. Fortunately, however, it’s just a scientific study, one that may finally shed light on why Uranus and Neptune have slightly different colors.
The seventh and eighth planets in the Solar System are the closest we come in these parts to having twin worlds. The two are classified as icy giants, planets in an intermediate category between rocky, like Earth, and gas giants, like Jupiter and Saturn.
In practice, by the way, they are also gas giants, but they gathered less gas and have a proportionately greater presence of ice, such as methane and hydrogen sulfide. Both are about 50,000 km in diameter (Uranus a little more, Neptune a little less). And, being so similar, it was always a little surprising that they had distinct personalities, one looking pale blue, almost green, and the other a more intense blue.
The generic appearance of the color is the easy part to understand: this is for the same reason that the sky on Earth is blue. The scattering of light by molecules in the atmosphere is most efficient in this range of the spectrum. The novelty here was to understand the difference between the tones and the general appearance of the two distant worlds.
The key came with creating a computer model of three layers of aerosols distributed at different heights in the planets’ atmospheres. The work was only possible thanks to the analysis of archival data from the Hubble Space Telescope, as well as the realization of new observations with the Gemini North telescope in Hawaii.
Developed by the team of Patrick Irwin, from the University of Oxford, in the United Kingdom, and presented in an article published in the Journal of Geophysical Research: Planets, the model solved the mystery: a layer of concentrated haze that exists in the atmosphere of the two planets is more thick on Uranus, which makes it pale.
The presence of the fog, according to the researchers, represents the formation of methane crystals that then fall like snowflakes. If this middle layer did not exist on either planet, both would be equally blue, reflecting only the scattering of color in the atmosphere.
The work also helped to clarify where the dark spots that occasionally appear, mainly on Neptune, come from: they are the result of a darkening of aerosols in the deepest layer of the model, where the condensation of hydrogen sulfide present in the atmosphere occurs.
The three layers are distributed over a strip of 200 km in height, in the region immediately below the stratosphere of the two planets, with pressures that vary between 1 and 10 times that found on Earth at sea level. And a major breakthrough is that the model reflects observations made of the two planets at various wavelengths: not just visible light, but also ultraviolet and infrared.
This column is published on Mondays, in Folha Corrida.
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