Bad Astronomy | Neptune’s huge dark storm has changed direction and no one knows why

A series of observations of Neptune by the Hubble Space Telescope show that a huge dark storm erupting in the northern hemisphere of the giant planet was moving south, but then inexplicably made a major U-turn, heading back north. Not only that, but it may have generated a dark storm for the baby in the process.

Neptune is what is called an ice giant, basically a giant ball of hydrogen and helium gas with charges of methane, ammonia and other molecules in it (that, for historical reasons, planetary scientists are called “frosts” even if they are gaseous). At almost four times the diameter of Earth, Neptune is the farthest planet from the Sun, 4.5 billion kilometers away.

When Voyager 2 passed Neptune in 1989, the rendered images surprised scientists; he saw a huge dark oval storm in the southern hemisphere of the planet as big as Earth itself! Called the Great Dark Spot, it had measured wind speed at an astonishing 2,100 km / h, the fastest wind ever measured in the solar system.

But when Hubble looked at Neptune in 1994, the place disappeared. Fluff. Missing. Obviously, unlike Jupiter’s Great Red Spot, which has persisted for centuries (at least), storms on Neptune evolve on smaller time scales, although they can last for several years. For example, in the same Hubble observations from 1994, a smaller dark spot was seen in the southern hemisphere of Neptune, one that must have been born in the time between Voyager’s flight and Hubble’s images.

A few other dark spots have been seen by Hubble (the only observer with enough resolution to see these features so far) since then. They form in both hemispheres of the middle latitudes and tend to move towards the equator. This is, however, a death sentence for them.

These storms are high pressure systems, supported by the Coriolis effect – the rotation of a planet has a different speed at different latitudes (with a maximum at the equator and a minimum at the poles), which means that air flows outward from a large sea pressure (or to one with low pressure) will start the system rotating as it encounters air moving at different speeds north and south.

The Coriolis effect diminishes closer to the equator, so that as these Neptune storms migrate in that direction, they tend to fall apart. This seems to be the fate of most of these storms.

But not this time. Hubble spied a dark storm in the northern hemisphere of Neptune in September 2018. It is huge, more than 7,000 kilometers wide – the entire continental United States could easily fit inside it – and was seen moving south. … but then observations made in January 2020 showed that this southern migration had reversed and the storm was heading north again. Scientists studying Neptune do not know why he did this.

But there are more: there are two other mysterious events seen associated with this storm. One is that by the time it changed its mind and began to move north again, it appears to have generated a smaller dark storm. Some computer models of the behavior of Neptune’s atmosphere predict that this may happen, especially since a large storm is beginning to break out near the equator; may shed smaller vortices. It has not been seen that this is happening directly – the remarks have been taken too far apart to witness the actual event – but perhaps that is what happened here. This may be related to the reason why he changed direction.

Also, bizarrely, this dark storm has no bright white clouds around its edges, a feature seen in almost any other dark storm. These are clouds of methane ice crystals, which are very reflective and appear white in the images. The dark storm is a high pressure system and acts as a mound of air in the atmosphere; the winds blow methane gas on the slopes of that mound where it cools and forms ice crystals. These are called orographic cloudsand are common on Earth, as air-laden air blows on the side of a mountain, cools and condenses to form clouds.

The white clouds in Neptune were seen in images taken in 2019, but disappeared earlier this year. This may be related to the strange behavior of the dark storm. Or it may not be! Neptune is difficult to observe and understand due to its distance and, despite its huge size, it still looks very small in our telescopes. Also, its atmosphere changes over short periods of time, which makes it difficult to know what is happening there.

This is why scientists are very interested in creating a dedicated mission to the outer planets, an orbiter that can spend years first on Uranus and then go into the orbit of Neptune. As I found with Cassini at Saturn, the best way to find out about a planet is to send a probe that stays there for many years. The features come and go, things change and just as important when scientists discover new phenomena that they can tell the spaceship to look more closely. Discovering new things is important, but a dedicated mission means you can stay around and realize what is causing them.

This is the lesson we also learned from Voyager flybys. Seeing Neptune up close for the first time allowed for discoveries such as dark storms, but if we want understand they have to go there to stay.