Hubble confirms new dark spot on Neptune

Astronomers are studying the first dark vortex to appear this century. Bill Condie reports.

Neptune photographed by the Voyager 2 probe in 1989, with the 'Great Dark Spot' (also known as GDS-89) visible at the left.
New York Public Library

Hubble space telescope has spotted a new dark spot on the face of Neptune. The vortex is one of a regular phenomenon seen on the planet, although the first observed this century.

The dark vortices are high-pressure systems, usually accompanied by bright “companion clouds”, as is the current one.

The clouds form when the flow of ambient air is diverted upward over the dark vortex, causing gases to likely freeze into methane ice crystals.

“Dark vortices coast through the atmosphere like huge, lens-shaped gaseous mountains,” said astronomer Mike Wong of the University of California, Berkeley.

“And the companion clouds are similar to so-called orographic clouds that appear as pancake-shaped features lingering over mountains on Earth.”

Similar features were seen during the Voyager 2 flyby of Neptune in 1989 and by the Hubble Space Telescope in 1994.

The clouds were first seen on Neptune in July 2015 and astronomers suspected that they might be following an unseen dark vortex.

Neptune’s dark vortices are typically only seen at blue wavelengths, and only Hubble has the high resolution required for seeing them on distant Neptune.

This new Hubble Space Telescope image confirms the presence of a dark vortex in the atmosphere of Neptune. The full visible-light image at left shows that the dark feature resides near and below a patch of bright clouds in the planet's southern hemisphere. The full-colour image at top right is a close-up of the complex feature. The vortex is a high-pressure system. The image at bottom right shows that the vortex is best seen at blue wavelengths.
NASA, ESA, and M.H. Wong and J. Tollefson (UC Berkeley)

In September 2015, the Outer Planet Atmospheres Legacy (OPAL) program, a long-term Hubble Space Telescope project that annually captures global maps of the outer planets, revealed a dark spot close to the location of the bright clouds, which had been tracked from the ground.

By viewing the vortex a second time, the new Hubble images confirm OPAL’s observations. Wong led the team which used the new data to create a higher-quality map of the vortex and its surroundings.

Neptune's dark vortices have puzzled scientists as they vary greatly in terms of size, shape, and stability. They also come and go on much shorter timescales compared to similar anticyclones on Jupiter where large storms evolve over decades.

Scientists hope the data will help to better understand how dark vortices originate, what controls their drifts and oscillations, how they interact with the environment, and how they dissipate.

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Bill Condie is publisher of Cosmos.