Two other dark storms were discovered by the Voyager 2 spacecraft in 1989 as it flew by the remote planet, NASA said. That constant sunlight may be affecting the planet's atmosphere, forming what NASA calls a 'polar hood, ' which we see as a mass of frosty white clouds. Seasons on Neptune last for over 40 years, and it's now winter in its northern hemisphere. They are similar to clouds that appear as pancake-shaped features when air is pushed over mountains on Earth. The dark vortex, which spans approximately 6,800 miles, was located next to bright white "companion clouds", which form when the flow of ambient air is diverted upward over the dark vortex and causes gases to freeze into methane ice crystals. Hubble has observed similar clouds accompanying previous vortices. But like Jupiter's famed Great Red Spot, the dark storms seem to dredge up material from deeper levels of the planets' atmospheres.
"The images indicate that the vortices probably develop deeper in Neptune's atmosphere, becoming visible only when the top of the storm reaches higher altitudes", NASA and STScI said in their releases. The giant is sporting a wide white spot across its north pole.
Ice giants Uranus and Neptune have water-rich interiors covered with hydrogen, helium, and methane, the last of which gives these external planets their particular cyan complexion.
Scientists believe this feature is a result of Uranus' unique rotation. It's summer right now for Neptune and Uranus, and has been for years; their seasons are decades-long events.
Unlike the other planets that make up the solar system, Uranus is tilted to the side meaning that during the summer the sun shines on the north pole nearly exclusively, never setting.
NASA explained that the planet is in the middle of summer and because of its weird tilt, the north pole is always facing the sun, never setting. At the equator is another narrow cloud that encircles the planet.