Why does Neptune look blue?

Neptune's upper atmosphere is mostly hydrogen and helium with a few per cent methane. Methane absorbs red and orange wavelengths of sunlight, so the light scattered back to space is mainly blue-green. For decades, Voyager 2's 1989 images made Neptune look a rich, deep blue, but those images had been contrast-enhanced to show cloud details. A 2024 study led by Patrick Irwin at the University of Oxford recalibrated the data and found Neptune is only slightly bluer than Uranus, both being a pale greenish-blue. The small colour difference is linked to a thinner haze layer on Neptune.