Neptune Corrected

The best-known photograph of Neptune shows a deep azure planet, and the caption NASA released with it said the colour was not accurate.³

A reanalysis published in January 2024 reconstructed the planet's true colour from spectral data and found a pale greenish blue, close to that of Uranus.^{1, 2}

Voyager 2 acquired the images on 16 and 17 August 1989 using its narrow-angle camera.³ The spacecraft did not record colour directly: it imaged through separate filters, and those frames were combined into composites afterwards, a process that was not always balanced accurately.² The composite was then stretched to bring out atmospheric structure, and the JPL caption states plainly that the image was processed to enhance the visibility of small features at some sacrifice of colour fidelity.³ Planetary scientists were aware of this at the time; the distinction was lost as the image passed into general circulation.²

The reanalysis was led by Patrick Irwin of the University of Oxford and published in Monthly Notices of the Royal Astronomical Society.¹ Its method relies on instruments that capture a continuous spectrum at every pixel rather than a handful of filtered channels: the Space Telescope Imaging Spectrograph on Hubble, and the Multi Unit Spectroscopic Explorer on the European Southern Observatory's Very Large Telescope.² With the true spectral balance established, the team rebalanced the Voyager 2 and Hubble WFC3 composites.² Both planets resolve to a similar greenish blue.² Neptune retains a slight additional blue tint, which the authors attribute to its thinner haze layer; the paper frames the same relationship from the other side, explaining that Uranus is the less blue of the two because of its thicker aerosol layer, with the difference greatest at the solstices.^{1, 2}

The corrected image is harder to read. The dark oval the 1989 caption describes at 22 degrees south, circling the planet roughly every 18.3 hours, loses contrast against the paler disc.³ That feature, later called the Great Dark Spot, had disappeared by the time Hubble observed Neptune in 1994, so the Voyager frames remain the only close record of it.⁴ The error was documented at source, in the caption, and propagated anyway.