Since its launch into orbit in 1990, the NASA/ESA Hubble Space Telescope has helped astronomers to amass an album of outer planet images. Yearly monitoring of these giant worlds is now allowing scientists to study long-term seasonal changes, as well as capture transitory weather patterns. One such elusive event is yet another dark storm on Neptune, shown in the latest Hubble image of the planet. Hubble’s new image of Uranus shows that the ice giant is not a planetary wallflower; a vast bright polar cap across the north pole dominates the image. The cap, which may form due to seasonal changes in atmospheric flow, has become much more prominent than in previous observations dating back to the flyby of NASA’s Voyager 2 spacecraft, when the planet, in the throes of winter, looked bland.
Like Earth, Uranus and Neptune have seasons, which likely drive some of the features in their atmospheres.
But their seasons are much longer than on Earth, spanning decades rather than months.
The new Hubble view of Neptune shows the dark storm, seen at top center.
Appearing during the planet’s southern summer, the feature is the fourth and latest mysterious dark vortex captured by Hubble since 1993.
Two other dark storms were discovered by Voyager 2 in 1989 as it flew by the remote planet. Since then, only Hubble has had the sensitivity in blue light to track these elusive features, which have appeared and faded quickly.
Hubble uncovered the latest storm in September 2018 in Neptune’s northern hemisphere. The feature is roughly 6,800 miles (10,944 km) across.
To the right of the dark feature are bright white ‘companion clouds.’
These bright clouds form when the flow of ambient air is perturbed and diverted upward over the dark vortex, causing gases to freeze into methane ice crystals. They are similar to clouds that appear as pancake-shaped features when air is pushed over mountains on Earth.
The long, thin cloud to the left of the dark spot is a transient feature that is not part of the storm system.
It’s unclear how these storms form. But like Jupiter’s Great Red Spot, the dark vortices swirl in an anti-cyclonic direction and seem to dredge up material from deeper levels in the ice giant’s atmosphere.
The Hubble observations show that as early as 2016, increased cloud activity in the region preceded the vortex’s appearance. 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.
The new image of Uranus reveals a dominant feature: a vast bright cloud cap across the north pole.
Scientists believe this feature is a result of Uranus’ unique rotation. Unlike every other planet in the Solar System, Uranus is tipped over almost onto its side. Because of this extreme tilt, during the planet’s summer the Sun shines almost directly onto the north pole and never sets.
Uranus is now approaching the middle of its summer season, and the polar-cap region is becoming more prominent. This polar hood may have formed by seasonal changes in atmospheric flow.
Near the edge of the cloud cap is a large, compact methane-ice cloud, which is sometimes bright enough to be photographed by amateur astronomers.
A narrow cloud band encircles the planet north of the equator.
It is a mystery how bands like these are confined to such narrow widths, because Uranus and Neptune have very broad westward-blowing wind jets.