Jupiter, Saturn and Pluto have received the most attention from astronomers in the recent years, given the Juno, Cassini and New Horizons missions to explore these celestial bodies. Now, the focus is on Uranus and Neptune – at least as far as the Hubble Space Telescope is concerned.
Both Uranus and Neptune are ice giants, without a solid surface beneath the atmosphere. Their rocky core is surrounded by a form of supercritical fluid, which is in turn surrounded by mantles of hydrogen and helium. Each planet appears to have a blue tint because the atmospheric methane absorbs the red light and scatters the blue-green light back into space. Both appear to be active, dynamic planets, with massive storms and clouds.
Uranus and Neptune were last visited by Voyager 2 in 1986 and 1989, respectively. Although missions to these worlds have been proposed, nothing is actually in the works. Until that happens, Hubble is the best source of information about the two ice giants. Last week, it released new images as part of its Outer Planet Atmospheres Legacy (OPAL) program.
Images of Uranus show a northern cloud cap that is believed to be the result of the planet’s rotational axis – compared to other planets, Uranus rotates on its side. Images of Neptune captured a large dark storm near the top of the planet – which, according to researchers, appears every 4-6 years. This storm is the fourth captured since 1993.
Bright and dark clouds and storms on Uranus (left) and Neptune (right) as seen by the Hubble Space Telescope in September 2018. Image via NASA/ ESA/ A. Simon (NASA Goddard Space Flight Center) and M.H. Wong/A. Hsu (University of California, Berkeley)/HubbleSite
“The Neptune dark spot is much larger than the one we saw a few years ago, and is comparable in size to the Voyager Great Dark Spot seen in 1989,” said Amy Simon, a NASA scientsits at the Goddard Space Flight Center. “This is also the first time we could see the region before a storm of that size formed, so that will help us in modeling the formation process.”
Similar to Jupiter, dark storm vortices follow an anti-cyclonic direction and resemble pancake-shaped, or lenticular, clouds on Earth, such as those that often appear over the Lítla Dímun island in Denmark.
Hubble saw the new dark storm in September of last year, and some cloud activity in 2016. Researchers think there’s a possibility that storms develop deep in the atmosphere long before they become visible. Other storms on Neptune include bright white “companion clouds,” which form when air is disturbed and diverted upward, where gases freeze and become methane ice crystals.
The new images will help OPAL observe and predict whether phenomena in the distant worlds, but more repeated observations are needed. “The yearly observations are helping us to understand the frequency of storms, as well as their longevity,” said Simon.