The so-called ice giants Uranus and Neptune are the most distant giant planets in the solar system. Our knowledge of these worlds was revolutionized by his flybys of NASA’s Voyager 2 spacecraft on January 24, 1986 and August 25, 1989, respectively. Since these Voyager encounters, our knowledge of the visible appearance of these worlds has come primarily from images reconstructed from observations from Voyager 2 Imaging Science System (ISS), images were recorded with several separate filters ranging from ultraviolet to orange. In these images, Uranus appears pale green and Neptune appears dark blue, and the perception of the relative colors of these planets has become generally accepted. However, new research has revealed that the two ice giants are actually much closer in color.
Voyager 2/ISS images of Uranus and Neptune, released shortly after the Voyager 2 flybys in 1986 and 1989, respectively, were used in this study to determine the best estimates of the true colors of these planets. The filtered image was compared with the reprocessed version.Image credit: Irwin other., doi: 10.1093/mnras/stad3761.
Professor Patrick Irwin of the University of Oxford said: “While the well-known Voyager 2 image of Uranus was released in close to ‘true’ color, the image of Neptune has actually been stretched and enhanced. As a result, it was artificially too blue.”
“Although artificially saturated colors were known to planetary scientists at the time and images were published with descriptive captions, over time that distinction has been lost. I lost it.”
“By applying our model to the original data, we were able to reconstruct the most accurate representation to date of the colors of both Neptune and Uranus.”
In the study, Professor Irwin and his colleagues space telescope imaging spectrometer On board the NASA/ESA Hubble Space Telescope (STIS) Multi-unit spectroscopic explorer (MUSE) ESO’s Very Large Telescope.
This means that the STIS and MUSE observations can be processed unambiguously to determine the actual apparent colors of Uranus and Neptune.
Astronomers used these data to rebalance the composite color images recorded by Voyager 2’s camera. Hubble’s Wide Field Camera 3 (WFC3).
This revealed that Uranus and Neptune are actually quite similar shades of greenish-blue.
The main difference is that Neptune has a slight hint of additional blue. Models revealed that this is due to Neptune’s thin haze layer.
The study also provides an answer to the long-standing mystery of why Uranus’ color changes slightly during the sun’s 84-year revolution.
The authors first reached their conclusion after comparing images of the ice giant with measurements of its brightness recorded in blue and green wavelengths from 1950 to 2016 by the Lowell Observatory in Arizona.
These measurements showed that Uranus appears slightly greener during the summer and winter solstices, when one of the planet’s poles points toward our star.
However, at the vernal equinox, when the sun is above the equator, the sun takes on a somewhat blue hue.
Part of the reason for this is known to be because Uranus has a very unusual rotation.
During its orbit, it effectively rotates almost sideways. This means that during the planet’s summer solstice, either the north or south pole points almost directly in the direction of the sun and Earth.
This is therefore important because changes in reflectivity in the polar regions have a large effect on Uranus’ overall brightness as seen from Earth.
Astronomers have not been very clear about how or why this reflectance differs.
This led the researchers to develop a model that compares the spectra of Uranus’ polar and equatorial regions.
They found that in polar regions, green and red wavelengths are more reflective than blue wavelengths. Part of the reason is that red-absorbing methane is about half as abundant near the poles as it is at the equator.
But this wasn’t enough to fully explain the color change, so the researchers looked at the gradually thickening icy surface of the planet’s sunlit pole during the summer. We added a new variable to the model in the form of a haze “hood”. We move from the vernal equinox to the summer solstice.
Astronomers believe it is likely made up of particles of methane ice.
When simulated in the model, the ice particles further increased reflection in green and red wavelengths at the poles, providing an explanation for why Uranus is green at the summer solstice.
“This is the first study to match quantitative models with image data to explain why Uranus’s color changes during its orbit,” Professor Irwin said.
“Thus, we prove that Uranus at the summer solstice is greener, not only because methane abundance is reduced in the polar regions, but also because the thickness of brightly scattering methane ice particles is increased. it was done.”
“The misperceptions of Neptune’s colors and the unusual color changes of Uranus have puzzled us for decades. This comprehensive study finally puts an end to both problems. ” said Dr. Heidi Hummel, a researcher at the Association of Universities for Astronomical Research (AURA).
of result will appear in Royal Astronomical Society Monthly Notices.
_____
Patrick G.J. Irwin other. 2024. Model the seasonal cycle of Uranus’ color and size and compare it to Neptune. MNRAS 527 (4): 11521-11538; doi: 10.1093/mnras/stad3761
Source: www.sci.news
