Based on observational data from ground-based and space telescopes, astronomers created a three-layer model of the distribution of aerosols in the atmospheres of Uranus and Neptune, which made it possible to explain the difference in their color and the observed elements of the atmospheres.
It turned out that the pale blue color of Uranus can be explained by a highly opaque layer of haze at a pressure of 1–2 bar, while Neptune has a thin layer of haze of methane ice particles occurring at a pressure of 0.2 bar. Dark spots on Neptune can be explained by the darkening or brightening of the lowest layer of aerosols.
Uranus’ and Neptune’s colors
Uranus and Neptune appear blue or bluish-green when observed at optical wavelengths, in contrast to the more yellow-reddish Jupiter and Saturn. This color gamut is due to the similar atmospheres of Uranus and Neptune, which have similar tropospheric temperature profiles and He/H2 ratios, as well as a high content of methane, which absorbs infrared and red optical radiation.
In addition, in the case of ice giants, Rayleigh scattering is strongly pronounced in an atmosphere with a low content of aerosols.
However, the color of both planets is still different from each other. There are differences in the observed details of the atmospheres. For example, methane clouds can be found on Uranus and Neptune at different heights.
In addition, several dark spots have been seen in the atmosphere of Neptune, the most famous of which is the Great Dark Spot anticyclone, and only one dark spot has been observed on Uranus. Neptune also appears less bright at longer wavelengths and reflects more light in the ultraviolet than Uranus.
Models of the atmospheres of ice giants, designed to explain the observational data, mainly contain a thick aerosol layer at a pressure of 2–4 bar (presumably a haze of photochemical origin and, possibly, hydrogen sulfide mixed with ice) and a haze above it.
However, scientists still do not have reliable information about what the aerosols of ice giants are made of and what their spectral properties are, which is why there is a problem of checking current models for correctness.
Scientists might have an answer
Patrick Irwin of the University of Oxford and his colleagues published the results of an analysis of observations of Uranus and Neptune from the ground-based IRTF and Gemini telescopes, the Hubble Space Telescope and Voyager 2, covering the wavelength range of 0.3– 2.5 micrometers and their comparison with the data of simulations of the reflection spectra of ice giants.
The aim of the scientists was to obtain a unified model of the distribution of aerosols in the atmospheres of Uranus and Neptune, which would be consistent with the observations.
The final aerosol distribution model is as follows. In the deep layers of the atmosphere (at a pressure of more than 7 bar) lies the first layer of aerosols, consisting of submicron particles of fog and ice based on hydrogen sulfide, created by photochemical processes.
These particles strongly scatter light at a wavelength of 500 nanometers but absorb radiation more strongly at both shorter and longer wavelengths. The second thin layer of aerosol lies near the level of methane condensation (at a pressure of 1–2 bar) and consists of micron particles of haze of photochemical origin, which have a lower reflectivity in the optical wavelength range than particles from the first layer, and also absorb radiation more strongly, both at a higher temperature. short and longer wavelengths.
Thus, the photochemical haze formed in the upper layers of the atmosphere of both planets steadily mixes with the lower layers, where it is concentrated in a vertically thin and statically stable layer near the level of methane condensation.
Methane condenses on these haze particles so quickly that it falls as snow at the base of this layer, sinking to lower, warmer levels where it evaporates, releasing haze particles that initiate the formation of hydrogen sulfide ice crystal clouds.
In the case of Neptune, the scientists concluded that a thin layer of micron-sized particles of methane ice deposited at a pressure of 0.2 bar must be added to the model to explain the enhanced reflection at longer wavelengths. In addition, the spectral characteristics of dark spots can be explained by the darkening or brightening of the lowest layer of aerosols.
In the case of Uranus, the opacity of the second layer of aerosols is 2 times greater than that of Neptune, which explains the lower reflectivity of Uranus in the ultraviolet range and why Uranus appears pale blue to the human eye.
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• Davis, M. (2022, February 1). Scientists explain why ice giants uranus and Neptune have different shades of Blue. Science Times.
• Irwin, P. G. J., Teanby, N. A., Fletcher, L. N., Toledo, D., Orton, G. S., Wong, M. H., Roman, M. T., Perez-Hoyos, S., James, A., & Dobinson, J. (2022, January 12). Hazy Blue Worlds: A holistic aerosol model for uranus and Neptune, including dark spots. arXiv.org.
• Luntz, S. (2022, January 31). Why Neptune and Uranus are different shades of Blue. IFLScience.
• Starr, M. (n.d.). Uranus and Neptune aren’t the same color. A new study could finally explain why. ScienceAlert.
• Yirka, B. (2022, February 1). A possible explanation for the difference in the blue hues of Uranus and Neptune. Phys.org.
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