Neptune and Uranus' colors sparked debate, but a new study had settled it. The two giant ice planets share the same color.
Neptune and Uranus' Colors
Previous photos of distant planets revealed Uranus as a sea-foam green and Neptune as a deep electric blue. However, new pictures collected for a recent study show that the two planets' colors are much closer. Both planets are roughly the same tint of green-blue.
"The misperception of Neptune's color, as well as the unusual color changes of Uranus, have bedeviled us for decades. This comprehensive study should finally put both issues to rest," said Heidi Hammel, a Neptune and Uranus researcher at the Association of Universities for Research in Astronomy (AURA) who did not contribute towards the study.
The NASA Voyager 2 spacecraft captured pictures of the two planets as they passed through the outer solar system, spreading the widespread belief that Neptune was blue and Uranus was green. Composite color photographs were created by combining multiple images, each taken in a different color by Voyage 2.
The "true" color of the planets was not appropriately represented by these composites, which made Neptune appear excessively blue and Uranus appear a pale green due to enhanced contrast in the Neptune photos.
For many years, astronomers have known that the two planets' hues were similar, closer to Uranus's customary hue.
Patrick Irwin, co-author of the research and professor of astronomy at the University of Oxford, claims that although the well-known Voyager 2 photographs of Uranus were published in a form closer to a 'real' hue, those of Neptune were stretched and boosted, and hence made intentionally too blue.
While planetary scientists were aware of the artificially saturated color at the time, and captions were included with the photographs to explain it, that distinction had been lost over time. By utilizing our model on the initial data, we have managed to reconstruct the most precise depiction of Neptune's and Uranus' colors to date.
The new study presented the genuine colors of the planets in images produced using data from the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope of the European Southern Observatory and the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope.
The images demonstrate how similar the planets' colors are, with Neptune's color being only slightly bluer than Uranus' due to the planet's smaller haze layer.
Uranus' Colors Changed?
Using this video loop, the research team provided an additional explanation for why Uranus appears to change color as it orbits the sun.
On Uranus, a year is equal to 84 Earth years. The globe seems greener at the summer and winter solstices, but its color turns bluer at the equinoxes.
During solstices, one of the planet's poles points toward Earth and the sun because the strange world rotates on its side.
While comparing the study's photos of Uranus, the scientists examined brightness measurements of the planet taken between 1950 and 2016 at the Lowell Observatory in Arizona.
The scientists created a model to compare light data from the polar and equatorial regions, and the results showed that the polar regions reflect more light in the green and red wavelengths. This model included the addition of a methane ice "hood," or gradually increasing haze, which is visible when the planet transitions from equinox to solstice.
They learned that the polar areas of Uranus have lower methane abundances and thicker layers of brightly reflecting methane ice particles, which contributes to the planet's greener appearance at solstices.
Hammel, vice president for science at the Association of Universities for Research in Astronomy, who has studied the ice giants for decades, said the study put to rest the issues about Uranus' changing color and the misconception behind Neptune's shade.
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