Near the dark storm now on Neptune, Hubble spotted another atmospheric feature: sparkling white "companion clouds", which scientists have spotted around dark storms in the past. This suggests that the systems take a while to build and likely find their roots deeper in the planet's atmosphere, and perhaps even deeper than that in its superheated ocean-like mantle of water, ammonia, and methane-ices.
'Because of this extreme tilt, during the planet's summer the Sun shines nearly directly onto the north pole and never sets.
In contrast to Earth, where seasons last only a couple of months, Neptune and Uranus experience seasons that keep going for quite a long time, bringing about weird and intense atmospheric phenomena.
Studying evolving weather systems on planets 2.7 billion and 1.6 billion miles away (respectively, at their closest) is bewildering to say the least. Scientists think this near constant exposure of the northern pole creates atmospheric conditions ripe for enormous polar storms like the one Hubble captured in November 2018.
The snapshot of Uranus, like the image of Neptune, reveals a dominant feature: a vast bright stormy cloud cap across the north pole.
"Back in 2007, there didn't appear to be anything like this polar cap over the springtime pole". At the equator is another narrow cloud that encircles the planet. "And now, 10 years on, that band has turned into a thick polar cap of aerosols that's hiding the deeper polar region from view". This polar hood may have formed by seasonal changes in atmospheric flow.
Storms like this appear every four to six years in different parts of the planet and disappear after about two years, NASA reported.
The methane gas then freezes to form ice crystals in the upper atmosphere, similar to the way clouds form as air is pushed over mountains here on Earth.
Since its launch into orbit in 1990, the NASA/ESA Hubble Space Telescope has helped astronomers to amass an album of outer planet images.
It's unclear how these storms form. However, scientists expect the large dark vortex on Neptune to last about two years like other similar storms on the planet have.
Taken together, these observations affirm the transient and recurring nature of these storms.
Hubble has discovered a new and mysterious "dark tempest" on Neptune that's 10,944 km across.
Amy Simon, a scientist at NASA's Goddard Space Flight Center who leads the OPAL mission told Gizmodo, "The yearly observations are helping us to understand the frequency of storms, as well as their longevity".
Often overshadowed by their larger cousins, Jupiter and Saturn, these distant ice giants are proving to be fascinating in their own right.