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Why Neptune’s azure was an error: an introduction to false-colour imagery


Neptune – the outermost planet in our solar system – has always been imagined with a clear shade of azure. Yet as new research is published, many are surprised to realise that its true colour is a lighter blue, not unlike Uranus’ – and the canonical azure impression of the planet has always been a “false colour image”. 

 


The research (link to the paper under References) which brought Neptune to the light was initially intended to study Uranus. Because Uranus orbits the sun with an axis at a unique 95-degree tilt, its seasons are very extreme and drastically different to other solar system planets. A single night on Uranus lasts for 21 Earth years, which means that observatories on Earth can only view one side of Uranus in the two-decade period, limiting our observations of the planet.


The purpose of the research was to create a comprehensive outlook of Uranus’ atmosphere. The researchers, Irwin et al., modelled observations of Uranus using data from 1950 to 2016. The long period enables them to fully understand all of Uranus’ atmosphere.


The researchers later applied the same method to analyse data from Neptune, and compared the spectral data to Uranus’. It was this which led to their discovery that Uranus and Neptune’s colours were not as distinct as commonly believed, and thus the rediscovery of Neptune's true appearance to the human eye.


In short, their primary focus had been to study Uranus, not Neptune. However, in the next parts of this article, we will discuss the rediscovery regarding Neptune, to understand more about false-colour imaging which has caused the illusion.



Why false colours?


Longtime astronomers and enthusiasts may remember that when the research by Smith et al. titled “Voyager 2 at Neptune: Imaging Science Results” was released in 1989, it was mentioned that Neptune’s image was in false colour. Indeed, Neptune was portrayed with a deeper blue because blue light helped to highlight its atmospheric features such as clouds, bands and winds; meanwhile, Uranus was portrayed with a plain sky-blue because its atmosphere had few features of interest to show.


Pictures of planets are only one example where false colours are useful. False colours are often assigned by astronomers to images of objects in order to amplify and help us study certain properties. This means that even though the objects don’t emit visible light, their images can be processed to appear colourful.


In images of such objects like nebulae and galaxies, the distinction between the image and the true appearance is much greater. In these cases, false colours are not used to show surface features but rather to reflect its chemical composition. For example, in the Crab Nebula image (left), blue indicates neutral oxygen, green, singly ionised sulphur and red, doubly-ionised oxygen. In the Helix Nebula image (right), blue indicates oxygen and red indicates hydrogen and nitrogen.




Telescope observations and image processing


Most of the time, the choice of colour spectrum is already embedded in the telescope’s observation process, instead of being assigned after data of various wavelengths of light is collected. This is because cameras are made to specify in receiving a certain range of the light spectrum. The choice is made with spectral filters which allow desired wavelengths to pass through.


This is true for Voyager 2’s observations of Uranus and Neptune. According to NASA’s site, Voyager 2’s Imaging Science Subsystem (ISS) “consists of two television-type cameras, each with 8 filters in a commendable Filter Wheel mounted in front of the vidicons.” This means that a series of images are taken with spectral filters, and then the filtered images are combined to produce a full colour picture. 



For Uranus and Neptune


In the below graphs from the research paper, the luminous flux (luminosity i.e. perceived brightness) of received light from Uranus at different wavelengths is shown. They indicate how strongly each observer would receive the planet’s light of each wavelength. On both graphs, the black line represents Uranus’ full spectrum. The top graph compares light that the human eye’s cone cells would receive (which is divided into red, green and blue light) to what the Hubble Space Telescope’s spectral filters received. The bottom graph compares light that Voyager 2’s ISS received to what the HST received.



Notice that in the top graph, blue, green and red light are present at approximately equal luminosities, while in the bottom graph, after going through the ISS’s filters, the blue-green-red ratios are significantly different – with the blue and violet colours become more luminous than the green and red colours, causing a darker blue colour which does not reflect what the human eye would have seen if directly observing Uranus.


One could imagine a similar effect the ISS had on Neptune’s observations. In a direct comparison of Uranus and Neptune imaging colours using ISS and HST data for Neptune, the researchers found that “although Neptune appears slightly more blue than Uranus when the intensities of both planets are scaled to their maximum, the difference in colour is not very great.” The below figure from the paper “shows that Neptune would appear bluer when the planet images are scaled to the correct relative size and intensity when viewed from Earth, but the difference in colour is still not very strong.”




References


Modelling the seasonal cycle of Uranus’s colour and magnitude, and comparison with Neptune (Irwin et al. 2024)


NASA’s description of Imaging Science Subsystem used on Voyager 2



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