The Hubble Space Telescope has confirmed a “dark spot” on Neptune, the first vortex seen on the gas giant this century, and the first since the Voyager 2 spacecraft flew past in 1989.
Images obtained on May 16 confirmed its existence, following up on observations from July 2015. It was discovered by a team led by researcher Mike Wong from the University of California, Berkeley.
Vortices on Neptune are high-pressure systems that can have bright “companion clouds” with them. They can range in size and shape, with air flowing high into the atmosphere and freezing, making the crystals synonymous with the bright clouds. Some of these bright clouds were spotted in 2015, but it is only now that the accompanying dark spot has been seen.
"Dark vortices coast through the atmosphere like huge, lens-shaped gaseous mountains," Wong said in a statement. "And the companion clouds are similar to so-called orographic clouds that appear as pancake-shaped features lingering over mountains on Earth."
Views of the vortex were also captured in September 2015 by the Outer Planet Atmospheres Legacy (OPAL) program, which captures global maps of the outer planets. The later views in July confirmed that this was a vortex.
Interestingly, vortices on Neptune appear to come and go on much shorter timescales than those seen on Jupiter, which can rage for decades. But astronomers still know very little about how these features form, and how they move and evolve. Discoveries like this help clarify the picture.