During its annual routine weather surveillance on the outer planets of our solar system, NASA's Hubble Space Telescope has discovered a new mysterious dark storm on Neptune and re-illuminated a long-lasting storm around the North Pole region on Uranus.
Like the Earth, Uranus and Neptune have seasons that are likely to affect some of the characteristics in their atmosphere. But their seasons are much longer than on Earth, extending over decades instead of months.
The new Hubble view of Neptune shows the dark storm that can be seen in the middle above. The feature released during the planet's southern summer is the fourth and final mysterious dark vortex Hubble has captured since 1993. Two more dark storms were discovered in 1989 by the Voyager 2 probe as it flew across the distant planet. Since then, only Hubble had the sensitivity in the blue light to track these elusive features that had surfaced and disappeared quickly. A study by the University of California, Berkeley, by student Andrew Hsu, estimated that the dark spots appear every four to six years in various latitudes and disappear after about two years.
Hubble discovered the last storm in September 2018 in the northern hemisphere of Neptune. The feature has a diameter of approximately 8,800 miles.
To the right of the dark feature are bright white "clouds". Hubble has observed similar clouds that accompanied previous eddies. The light clouds form when the ambient air flow is disturbed and deflected upwards through the dark vortex, causing the gases in methane ice crystals to freeze. These clouds resemble clouds that appear as pancake-like features when air is pushed over mountains on the earth (although Neptune does not have a solid surface). The long, thin cloud to the left of the dark spot is a transitory feature that is not part of the storm system.
It is not clear how these storms form. But like Jupiter's Great Red Spot, the dark vortex swirls in an anticyclonic direction and seem to hurl material from deeper levels in the ice giant's atmosphere.
Hubble's observations show that as early as 2016, cloud activity in the clouds rose in the region before the appearance of the vortex. The images show that the vortices in the atmosphere of Neptune are likely to develop deeper and become visible only when the top of the storm reaches higher altitudes.
The snapshot of Uranus shows as the image of Neptune a dominant feature: a huge stormy cloud cap over the North Pole.
Scientists believe that this new function is a result of the unique rotation of Uranus. Unlike any other planet in the Solar System, Uranus is almost tipped to one side. Because of this extreme inclination, the sun shines almost directly on the North Pole during the summer of the planet and never goes under. Uranus is now approaching the middle of the summer season, and the polar cap region is becoming more important. This polar cap could have been formed by seasonal changes in the atmospheric flow.
Near the edge of the Polarstorm is a large, compact methane ice cloud that is sometimes bright enough to be photographed by amateur astronomers. A narrow band of clouds surrounds the planet north of the equator. It's a mystery how such bands limit themselves to such narrow latitudes, because Uranus and Neptune have very broad, west-wafting windjets.
Both planets are classified as giant ice planets. They have no solid surface, but shells of hydrogen and helium, which surround a water-rich interior and are perhaps wrapped around a rocky core. Atmospheric methane absorbs red light, but leaves blue-green light back into space, giving each planet a cyan hue.
The new Neptune and Uranus images are from the Outer Planet Atmospheres Legacy (OPAL) program. The Hubble project, led by Amy Simon of the NASA's Goddard Space Flight Center in Greenbelt, Maryland, annually captures global maps of the World outer planets of our solar system when they are closest to Earth's orbit. The key objectives of OPAL are to study long-term seasonal changes and to record relatively transitory events, such as the appearance of Neptune's dark spot. These dark storms may be so fleeting that in the past some of them had appeared and faded during several years of gaps in Hubble's observations of Neptune. The OPAL program ensures that astronomers will not miss another one.
These images are part of a scrapbook of Hubble snapshots of Neptune and Uranus tracking the weather patterns on these distant, cold planets over time. Just as meteorologists can not predict the weather on Earth by snapshots, astronomers can not track atmospheric trends on solar system planets without regular observations. Astronomers hope that Hubble's long-term observation of the outer planets will help them solve the puzzles that still exist over these distant worlds.
The analysis of the weather on these worlds will also help scientists better understand the diversity and similarities of the solar atmosphere-planet, including Earth.
Image: Hubble sees a new dark spot on Neptune