Like Earth, Uranus and Neptune have season and experience changes in weather patterns as a result. But unlike Earth, the seasons on these planets last for years rather than months, and weather patterns occur on a scale that is unimaginable by Earth standards. A good example is the storms that have been observed in Neptune and Uranus’ atmosphere, which include Neptune’s famous Great Dark Spot.

During its yearly routine of monitoring Uranus and Neptune, NASA’s Hubble Space Telescope (HST) recently provided updated observations of both planets’ weather patterns. In addition to spotting a new and mysterious storm on Neptune, Hubble provided a fresh look at a long-lived storm around Uranus’ north pole. These observations are part of Hubble‘s long-term mission to improve our understanding of the outer planets.

The new images were taken as part of the Outer Planet Atmospheres Legacy (OPAL) program, a long-term Hubble project led by Amy Simon of NASA’s Goddard Space Flight Center. Every year, this program captures global maps of our Solar System’s outer planets when they are closest to Earth. One of OPAL’s key goals is to study long-term seasonal changes and comparatively transitory events, such as the appearance of dark spots.

Voyager 2 captured this image of Neptune in 1982, when it was over 7 million km (4.4 million miles) away from the planet. The Great Dark Spot in the middle of the image was the first storm ever seen on Neptune. Credit: By NASA-JPL

Spotting them is no easy task, since these dark spots appear quickly and are relatively short-lived, to the point where some may have appeared and disappeared during multi-year gaps in Hubble’s observations of Neptune. This is another goal of the OPAL program, which is to ensure that astronomers don’t miss another one.

This latest dark spot, which measures roughly 11,000 km (6,800 mi) in diameter, appears at the top center of the planet. Hubble first spotted it in September of 2018, when Neptune’s southern hemisphere was experiencing summer. This is consistent with seasonal change on the planet, where warming in the southern hemisphere causes weather patterns to become more dramatic in the north.

While it is unclear exactly how these storms form, new research by Simon and the OPAL team indicate that they form quickly, last from four to six years, and then disappear over the course of two years. 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.

In fact, Hubble observations obtained since 2016 seem to indicate that the vortices probably develop deeper in Neptune’s atmosphere and become visible only when the top of the storm reaches higher altitudes. Meanwhile, they are accompanied by “companion clouds”, which are visible in the Hubble images as bright white patches to the right of the dark feature.

This Hubble Space Telescope Wide Field Camera 3 image of Neptune, taken in Sept. and Nov. 2018, shows a new dark storm (top center). Credit: NASA, ESA/A. Simon (NASA Goddard Space Flight Center), and M. Wong and A. Hsu (University of California, Berkeley)

These clouds are composed of methane ices which freeze when the vortices cause the flow of ambient air to be diverted upward over the storm. The long, thin cloud to the left of the dark spot is a transient feature that is not part of the storm system. The same is true of Uranus, which shows a vast bright cloud cap across the north pole.

In Uranus’ case, scientists believe this is a result of Uranus’ unique orientation, where its axis is tilted over 90° relative to the Sun’s equator. Because Uranus orbits practically on its side, the Sun shines almost directly onto the north pole during summer in the northern hemisphere. At present, Uranus is approaching the middle of its summer season, which is making the polar-cap region appear more prominently.

This polar cap may be the result of seasonal changes in atmospheric flow, and is accompanied by a large, compact methane-ice cloud near its edge in the image. Also visible is a narrow cloud band that encircles the planet north of the equator. This is another mystery about Uranus and Neptune, which is how bands like these are confined to such narrow widths when the planet’s have such broad westward-blowing wind jets.

This is the fourth mysterious vortex imaged by Hubble since 1993 and the sixth since astronomers first became aware of these phenomena. The first two dark spots were discovered by the Voyager 2 spacecraft as it made its historic flyby of Neptune in 1989. Since then, only the Hubble Space Telescope has been able to track these features because of its sensitivity to blue light.

This Hubble Space Telescope Wide Field Camera 3 image of Uranus, taken in Nov. 2018, reveals a vast bright stormy cloud cap across the planet’s north pole. Credit:NASA/ESA/A. Simon (NASA Goddard Space Flight Center), and M. Wong and A. Hsu (University of California, Berkeley)

These images are part of a growing database of Hubble snapshots of Neptune and Uranus that track the planet’s weather patterns over time. Much like how meteorologists predict weather on Earth based on long-term trends, astronomers hope that Hubble’s long-term monitoring of the outer planets will help them unravel the enduring mysteries about their atmospheres.

Analyzing the weather on these worlds also will also improve our understanding of the diversity of the atmospheres in the Solar-System, as well as their similarities. In the end, this could also go a long way towards informing our understanding of extrasolar planets and their atmospheres, maybe even helping us to determine whether or not they can support life.

Further Reading: Hubblesite