A team led by Professor Patrick Irwin of Oxford University’s Department of Physics found that Neptune and Uranus are both pale blue-green, not deep navy blue as is commonly believed. The main purpose of their study, published in the Monthly Notices of the Royal Astronomical Society, was to explain why Uranus’s colour changes throughout its year.
Scientists have also long been puzzled by why Uranus is greener during its summer and winter but takes on a blue tinge during its spring and autumn. The planet’s spin patterns – with one of its poles pointing toward the Earth and the Sun continuously during its solstices – contributes to differences in its reflectivity.
Through comparing the spectra of Uranus’s poles to its equatorial regions, the Oxford researchers found that poles appear greener during solstices because of a lower concentration of methane and increased reflection from methane ice particles. Their study settles the question of why Uranus’s colour shifts over its 84-year orbit around the Sun.
In order to figure this out, two clashing observations about the colour similarity between Neptune and Uranus had to be sorted out. Irwin said that this endeavour ”opened up a complete rabbit hole for me as I learnt how the eye perceives colour and how sRGB monitors reproduce colour images on a screen.”
It turns out that photos showing a blue Neptune had been edited to make dark features easier to see, a fact which “became increasingly overlooked with time, and gave rise to a long-standing misunderstanding on what the true colours of these two planets actually are” according to Irwin.
Rebalancing old images using data from the Hubble Space Telescope and European Southern Observatory’s Very Large Telescope, the researchers found that both ice giants are similar in shade. Compared to Uranus, Neptune is slightly bluer due to “a thinner haze layer.”
When asked about what remains to be discovered about these two planets, Irwin pointed to our inability to explain the overall variation in absolute reflectivity depending on Uranus’s distance from the Sun. However he is hopeful about the new high-precision data from the JWST: “As soon as we’ve figured out the best interpretation of the observations we’ll let everyone know!”
