Neptune’s atmosphere is made mostly of hydrogen and helium, with trace amounts of methane, water, ammonia and other ices. It’s the methane in the planet’s upper atmosphere that give it its bright blue color.
In the Voyager 2 high resolution color image below, you can see Neptune’s bright cloud streaks.
“The bright sides of the clouds which face the Sun are brighter than the surrounding cloud deck because they are more directly exposed to the sun. Shadows can be seen on the side opposite the sun. These shadows are less distinct at short wavelengths (violet filter) and more distinct at long wavelengths (orange filter). This can be understood if the underlying cloud deck on which the shadow is cast is at a relatively great depth, in which case scattering by molecules in the overlying atmosphere will diffuse light into the shadow. Because molecules scatter blue light much more efficiently than red light, the shadows will be darkest at the longest (reddest) wavelengths, and will appear blue under white light illumination.”
High-altitude cloud streaks in Neptune’s atmosphere. Credit: NASA
Neptune’s Dark Spot:
The Great Dark Spot (also known as GDS-89) was one of a series of dark spots on Neptune similar in appearance to Jupiter’s Great Red Spot. Like Jupiter’s spot, Great Dark Spots are anticyclonic storms. However, their interiors are relatively cloud-free, and unlike Jupiter’s spot, which has lasted for hundreds of years, their lifetimes appear to be shorter, forming and dissipating once every few years or so. Based on observations taken with Voyager 2 and since then with the Hubble Space Telescope, Neptune appears to spend somewhat more than half its time with a Great Dark Spot. The Great Dark Spot is thought to represent a hole in the methane cloud deck of Neptune.