by In many images of the two outer gas giants of the Solar System, Neptune usually appears a rich blue with Uranus coming across as a light green. But now our new study, published in the Monthly Notices of the Royal Astronomical Society, has shown that these two ice giants are actually very similar in blue-green colors.
The study follows our previous work in 2022 that analyzed the spectrum (light broken down by wavelength) of light reflected from Uranus and Neptune from various sources, including the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. These were recorded in 2002 (Uranus) and 2003 (Neptune).
We found that the colors of Uranus and Neptune were remarkably similar, with Neptune appearing only slightly brighter – see image below. The difference in color was attributed to the difference in the opacity of the coal and methane ice layers.
Ultimately, Neptune has a thinner layer of nebula, allowing sunlight to reach deeper into the atmosphere. At such a depth, it can be absorbed by methane gas, which absorbs red light – making the planet appear slightly bluer.
Reinventing the colors
Our reconstructed colors of Uranus and Neptune look very different from previous images, which come from the Voyager 2 spacecraft’s encounters with these planets in 1986 and 1989 respectively.
So, did the colors of Uranus and Neptune change between the late 1980s and early 2000s? Or do we need to consider more carefully how observations of planets are converted to the “true” color that a normal human observer would observe? The answer, it turns out, is a bit of both.
Color images of planets are highly processed. Spacecraft usually record the red, green and blue components separately. They are then sent back to Earth as black-and-white images, where they can be combined into color. However such images may not reflect the true color that the human eye would see.
Even light recorded in channels beyond the visible range, for example in ultraviolet, becomes red, green or blue when reflected. This process involves a number of steps and, depending on the choices made, a planet image can take a wide variety of looks.
To find the best color for Uranus and Neptune to date, we combined our Hubble data with later observations at the Very Large Telescope in Chile. Both of these instruments record images in which each individual pixel is a full, continuous spectrum covering all colors visible to the human eye – making them more accurate than spacecraft when it comes to color.
This allowed us to unambiguously determine the actual color that the human eye would see for Uranus and Neptune. We could then reprocess observations made by imaging cameras on Voyager 2 and Hubble with this in mind.
When Voyager 2’s reprocessed views of Uranus and Neptune are compared to some of the images released early, it is clear that the early images of Uranus correspond fairly well to the color we now believe. The early images of Neptune, however, are a much darker blue than their true color.
This difference was known to the Voyager imaging team at the time, and the captions released with the images explained this fact. However, since the purpose of these images is to communicate the mission’s exciting new discoveries, it was wisely considered that an enhanced version of the images that highlighted the discoveries would be preferable to a “true” color version. where the features appeared washed. out.
However, the processing differences have been forgotten over time and so now most people, including planetary researchers, assume that Neptune is much bluer than Uranus, which is not the case.
Uranus changes color
Comparing the actual color of Uranus in 1986 with later observations, it became clear that Uranus in 1986 was actually slightly greener than it was in the early 2000s. We tried to find out why this was so by tackling observations made between 1950 and 2016 at the Lowell Observatory in Arizona. These observations included the total brightness of Uranus and Neptune almost every year at two wavelengths: green and blue.
This showed that Uranus changes colour, becoming greener at the solstices (when the Sun’s path in the sky is furthest north or south of the planet’s equator) than at the equators (when the path of the the planet’s equatorial Sun).
Part of the reason for this color change is that Uranus turns almost on its side during its 84 year orbit around the Sun. This means that during the planet’s solstice, its north or south pole points almost directly towards the Sun and the Earth. Therefore, total reflectance is greatest at polar latitudes.
As a result we developed a model that compared the spectrum of Uranus’ polar regions to its equatorial regions. We found that polar regions are more reflective at green and red wavelengths than at blue wavelengths, in part because methane is half as abundant near the poles than at the equator.
However, this did not fully explain the color change. To match the Lowell Observatory data, we found that we need to add a “hood” of icy haze during the summer. This revised model substantially reproduced Lowell’s observations and thus explains how the overall color of Uranus changes during its orbit around the Sun.
So, the next time you see an old image of the two gas giants, keep in mind that you probably aren’t seeing their “true” color.
This article from The Conversation is republished under a Creative Commons license. Read the original article.
Patrick Gerard Joseph Irwin has received funding related to this study from the UK Science and Technology Facilities Council.
