Groundbreaking Discovery: Uranus and Neptune are Not That Different in Color
English astronomers have recently discovered that the color difference between Uranus and Neptune, two planets that usually appear different in images – the former as a soft bluish-green and the latter as a deep blue – is not as significant as previously thought.
The research findings, which were published in the Royal Astronomical Society’s monthly journal on January 4th, challenge the long-held belief about the colors of these two planets.
The image above, shared by NASA, is a composite picture of Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune (from top to bottom). Xinhua News Agency.
The commonly recognized images of Uranus and Neptune are derived from photos taken by the American Voyager 2 spacecraft in 1986 and 1989 respectively. These images were subsequently processed and combined into a single picture. Scientists then enhanced the color of Neptune in these images to highlight its white clouds and air flow, says Patrick Irwin, a professor of planetary physics at Oxford University and one of the authors of the study.
He explains: “The color of the image of Uranus, taken by Voyager 2, was close to ‘real’, but the Neptune’s color was artificially darkened and made too blue.” He recounts how, despite the fact that the published images had notes attached specifying the artificial color enhancement of Neptune, as time passed, the color difference between Neptune’s actual color and its artificially enhanced version gradually faded away.
A more accurate color depiction was obtained by combining data captured by the Hubble Space Telescope’s imaging spectrometer and the Multi Unit Spectroscopic Explorer instrument on the Very Large Telescope in Chile. Through this combination of data, Irwin and his team determined that both Uranus and Neptune are bluish-green in color, with Uranus appearing slightly lighter due to its thicker outer haze layer.
The study also reveals why Uranus changes color with the seasons – appearing greener during solstices and bluer during equinoxes. The phenomenon is due to its 84-year-long orbit around the Sun.
By comparing observation data of both planets from the Lowell Observatory in the US from 1950 to 2016 and data gathered about the light in the polar regions and near the equator of Uranus, the research confirms previous findings that Uranus’s color change is related to the amount of methane gas in its atmosphere. As methane absorbs red and green light, the Uranus equator area, which is rich in methane, reflects more blue light, while the areas near the poles with fewer methane reflects more green light.
The researchers also reveal that the Uranus areas near the poles have a higher concentration of methane ice crystals in the atmosphere, which enhances the green light reflection. Due to its unique axial tilt of 98 degrees relative to its orbital plane – it’s often referred to as a “lying” planet – during the solstices, we can observe Uranus’s poles from Earth, making the planet appear greener than usual. (Yuan Yuan)