Uranus and Neptune
Uranus and Neptune are the two exoplanets of the solar system. At the end of the 80s of the 20th century, Voyager 2 sent pictures of Uranus (1986) and Neptune (1989) to Earth after a long journey. The photos show that the two planets have distinctly different colors, with Neptune appearing blue and Uranus pale cyan. In most modern images of Uranus and Neptune, they are rendered in such colors.
A new study published in the Monthly Bulletin of the Royal Astronomical Society shows that the two icy giants actually have a very similar blue-green color, except that Neptune appears slightly bluer.
Images of Uranus and Neptune released shortly after Voyager 2's flyby of Uranus and Neptune in 1986 and 1989, compared with images reprocessed in the new study, identified more realistic color differences between the two planets. (Photo: Patrick Irwin, University of Oxford.)
This difference in color can be attributed to the different opacities of the haze layer and methane of the two planets. Neptune's haze layer is thinner, so it allows more sunlight to reach deep into the atmosphere. At such depths, sunlight can be absorbed by methane, which absorbs red light, making Neptune appear a little bluer.
Reconstruct the color
Could it be that the colors of Uranus and Neptune changed in the late 80s and early 21st centuries, or do we need to think more carefully about how to translate our observations of planets into the "true" colors that ordinary human observers can observe?
Color images of the planets are highly processed. The three colors, red, green, and blue, are usually recorded separately by spacecraft, and then they are transmitted back to Earth in the form of black and white images, which are then combined into color images. However, such an image may differ from the true colors that the human eye can see.
Even light that is outside the visible wavelength range, such as ultraviolet light, turns red, green, or blue when displayed. There are several steps involved in this process, and depending on the choices made, planetary images can take on a wide variety of appearances.
In fact, astronomers have long known that most modern images of these two planets do not accurately reflect their true colors. Because when Voyager 2 took images of these two planets in the late 80s of the 20th century, it used this different color to record the images.
To determine the truest colors of Uranus and Neptune to date, the researchers combined observations from the Hubble Space Telescope with the latest observations from the Very Large Telescope in Chile. In the images recorded by these two telescopes, each individual pixel is a complete, continuous spectrum that covers all the colors that the human eye can see, which makes them more accurate in terms of color.
The researchers used this data to reprocess the synthetic color images recorded by Voyager 2. When comparing the reprocessed Voyager 2 images with some of the earlier images, it is clear that the early images of Uranus largely matched what we now think of as Uranus, but that the early images of Neptune were much darker than they really were.
Uranus that changes color
Comparing the true color of Uranus taken in 1986 with recent observations, it is clear that Uranus in 1986 is actually slightly greener than Uranus in the early 21st century. The researchers sought to find out why this was the case, so they looked at data from the Lowell Observatory in Arizona from 1950 to 2016 that included the overall brightness of Uranus and Neptune at two wavelengths, green and blue, almost every year.
The color change of Uranus observed by the Hubble Space Telescope. (Photo/CC BY-SA)
They found that Uranus does change color, and that it is greener at the solstice than at the equinox. This color change is partly due to Uranus' 84-year orbital cycle, when its spin direction was almost sideways. This means that at the solstice of this planet, its north or south pole points almost directly to the Sun and Earth. Thus, the extreme yellow latitude dominates the total reflectance.
To do this, the researchers developed a model that compares the spectra of Uranus' polar regions with those of the equatorial regions. They found that the polar regions reflected more strongly at the wavelengths of the green and red parts than the blue wavelengths, in part because the methane content near the polar regions was half that of the equator.
However, this does not fully explain the change in color. To match the Lowell Observatory data, the researchers believe that an additional layer of haze would need to be added during the summer months as a correction. The revised model can largely reproduce Lowell's observations, explaining how Uranus's overall color changes during its orbit around the Sun.
Remember the truth
In fact, regarding the difference in Uranus and Neptune's colors, the Voyager 2 imaging team annotated the caption when the image was released. However, since the purpose of these images is to represent the achievements of astronomical missions, it would be "sensible" to highlight these discoveries with enhanced images, rather than showing them in "real" colors where some of the planet's features have been diluted.
However, as time went on, this episode in the process was gradually forgotten. Now, most people accept that Neptune is much bluer than Uranus, and that's not true.