2024-01-22 14:01:00
Voyager 2/ISS images of Uranus and Neptune released shortly after the Voyager 2 flybys in 1986 and 1989, respectively, compared to a reprocessing of the individual filter images in this study to determine the best estimate of the true colors of these planets. Photo by Patrick Irwin. Kerry Taylor-Smith Meteored United Kingdom 01/22/2024 11:01 7 min
The images of Neptune They usually show the gas giant as an intense sky blue, while its neighbor Uranus is a pale cyan green, but a new study has revealed that They are actually more similar in color: a similar shade of blue-green.
Most modern images of the planets do not accurately reflect their true colors. The first images, including those taken by Voyager 2, the only spacecraft to pass by these planets, recorded images in separate colors. These single color images were recombined later to create composite color images, but were not always precisely balanced.
too blue
“Although the well-known images of Uranus taken by Voyager 2 were published in a form closer to ‘true’ color, those of Neptune were, in fact, stretched and enhanced, and therefore artificially made too blue“explains Professor Patrick Irwin from the Department of Physics at the University of Oxford, who led the study published in the scientific journal Monthly Notices of the Royal Astronomical Society.
“Although planetary scientists knew artificially saturated color At the time—and the images were published with captions explaining this—that distinction had been lost over time. Applying our model to the original data, we have been able to reconstitute the most accurate representation so far of the color of Neptune and Uranus”.
Uranus: one of the most mysterious planets in our solar system
The study used data from the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope and the Multi-Unit Spectroscopic Explorer (MUSE) on ESO’s Very Large Telescope to rebalance composite color images recorded by Hubble’s Voyager 2 camera and Wide Field Camera 3 (WFC3).
STIS and MUSE record each pixel as a continuous spectrum of colorsmeaning that your observations can be processed unambiguously to determine the true apparent color of Uranus and Neptune. The data revealed that Uranus and Neptune have a similar shade of blue-greenalthough Neptune does have an extra hint of blue, which the model says is due to a thinner layer of haze on the planet.
The color change of Uranus
The study also explains why Uranus’s color changes slightly during its 84-year orbit around the Sun. Images of the planet were compared to brightness measurements taken by the Lowell Observatory in Arizona between 1950 and 2016, at blue and green wavelengths; this showed that Uranus appears greener on its solstices when one of the planet’s poles points towards the Sun, but bluer during its equinoxeswhen the Sun is over the equator.
Uranus’ highly unusual spin is partly responsible for this; the planet turns almost sideways during its orbit, so during its solstices, one of its poles points almost directly towards the Sun and the Earth. This is significant because any variation in the reflectivity of the polar regionss would have a large impact on the overall brightness of Uranus when viewed from Earth; however, how or why this reflectivity differs is a mystery.
Uranus as seen by HST/WFC3 between 2017 and 2022. During this sequence, the north pole, which is a paler green color, swings toward the Sun and Earth. In these images the equator and latitude lines are marked at 35N and 35S. Photo by Patrick Irwin.
A new model comparing the spectra of the polar regions of Uranus with its equatorial regions found that polar regions are more reflective at green and red wavelengths than at blue wavelengths, partly because methanewhich absorbs red, is about half as abundant near the poles as the equator.
But this didn’t fully explain the color change, so the researchers added a new variable to the model: a gradually thickening “hood” of fog of suspected methane ice particles that have been observed during the summer, a pole illuminated by the Sun as the planet transitions from the equinox to the solstice.
When simulated in the model, ice particles further increased reflection at green and red wavelengths at the poles, offering an explanation for why Uranus is greener on the solstice.
Uranus: the coldest planet in the solar system
“This is the first study to compare a quantitative model with imaging data to explain why Uranus’s color changes during its orbit“explains Irwin. “In this way, we have shown that Uranus is greener on the solstice Because the polar regions have a lower methane abundance but also a greater thickness of brightly dispersing methane ice particles.”
News reference:
Irwin, P.; Dobinson, J.; James, A.; et al.; Modelling the seasonal cycle of Uranus’s colour and magnitude, and comparison with NeptuneMonthly Notices of the Royal Astronomical Society (2024).
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