The Mysterious Dark Vortices of Neptune

Dark spots on 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 dark spot measures roughly 3,000 miles (4,800 kilometers) across. Other high-altitude clouds can be seen at the planet's equatorial region and polar regions. The image at right shows that Neptune's dark vortices are typically best seen at blue wavelengths. Only Hubble has the high resolution required for identifying such weather features on distant Neptune. NASA/ESA STSCI

Neptune is the 8th planet out from the Sun and the most distant one (even counting Pluto, whose orbit takes inside Neptune's). The only way we have to study it is by using ground-based or space-based telescopes. No spacecraft has visited it since Voyager 2 in 1989.

The Hubble Space Telescope has studied Neptune quite a bit, discovering a giant dark vortex in the upper atmosphere of Neptune. This isn't the first time such dark spots have been seen on the planet. The Voyager 2 mission spotted a couple, which eventually dimmed and went away. Hubble Space Telescope and other ground-based telescopes kept careful watch on Neptune, and finally found another one in 2016. It was the first vortex observed on Neptune in the 21st century.

What are Neptune's Vortex Spots?

The planet's mysterious dark spots are phenomena familiar to us here on Earth — high-pressure systems. Normally these Neptunian systems also have bright "companion clouds". Those brighter ones form as ambient air flows are disturbed, and they get diverted up over the dark vortex. The gases in the clouds freeze into ice crystals, usually made of methane. The vortices themselves more or less float — coasting through the upper layers of the atmosphere. The companion clouds are similar to so-called orographic clouds that appear as pancake-shaped features lingering over mountains on Earth, often referred to as "lenticular" clouds. (Some people joke that they appear like UFOs.)

Those bright clouds started showing up in July 2015, and they were easily spotted by amateur and professional observers. They were a clue that a dark vortex or two might be forming — although the dark spots couldn't be seen visually. However, they can be detected in blue wavelengths of light. So, planetary scientists got time on and used Hubble Space Telescope to look for a vortex. HST is equipped with instruments sensitive to the extreme blue and it has a very sharp eye that allows it to spot such a dark, but distinct phenomenon on the planet. Eventually, it found the vortex, accompanied by its bright clouds. 

Neptune's dark vortices have each been very different in terms of size, shape, and stability. They wander around the planet, changing their latitudes and their speeds seem to change at the whim of the winds. They also come and go very fast, in fact much faster than similar anticyclones seen on Jupiter, where large storms take decades to form and evolve as they spin around in the planet's upper atmosphere.

What Causes The Vortices on Neptune?

The planetary vortices on Neptune still pose a lot of questions: how do they originate? What controls their motions — their meandering drift? Do they interact with their near environment, and how? Why do they seem to fade out and go away, only to return years or decades later?

Is there something going on inside Neptune that causes these whirling vortices to form? To answer that, planetary scientists need to understand more about all aspects of the planet itself. Its interior is a lot like the inside of Uranus, the next closest ice giant planet to Neptune. There's a tiny core made of rock and ice, all covered over by a mantle made of water, ammonia and methane ices. (This is why it's called an ice giant.) The heavy atmosphere smothers the core and mantle, and it's made of hydrogen, helium, and methane gases. The lower part of the uppermost atmosphere is where the vortices exist.

One interesting point about Neptune is that its thermosphere (the lower portion of its atmosphere) is really quite hot — 750 K (about 900 F, or 476 C). That's hotter than the surface of Earth's "sister" planet Venus!! (Think hotter than a pizza oven!). That's pretty hot for a cold planet out in the solar system's deep freeze. Could whatever is heating that region of the atmosphere play some role in the formation of vortices high up in the atmosphere? Perhaps as a method of conveying heat from the interior?

Or, could the heating of Neptune's poles do the trick? Or are there other physical and chemical mechanisms at work in Neptune's atmosphere that cause the vortices? Could activity and interactions between the atmosphere and Neptune's magnetic field play a role? All good questions. Studies like the ones uncovering the dark spots will help planetary scientists unravel the mystery of Neptune's whirling vortices as they consider all the factors at play on this giant planet.