A series of observations of Neptune by the Hubble Space Telescope shows that a huge dark storm raging in the giant planet’s northern hemisphere was moving south, but then inexplicably took a large U-turn, returning north. Not only that, but it could also have generated a little dark storm in the process.
Neptune is what is called an ice giant, basically a giant ball of hydrogen and helium loaded with methane, ammonia and other molecules (which for historical reasons, planetary scientists call “ice” even though they are gaseous). At nearly four times the diameter of the Earth, Neptune is the farthest planet from the Sun, 4.5 billion kilometers away.
When Voyager 2 passed through Neptune in 1989, the images returned surprised scientists; saw an immense oval dark storm in the southern hemisphere of the planet as big as the earth itself! Called the Great Dark Spot, it measured wind speeds of 2,100 km / h, the fastest wind ever measured in the solar system.
But when Hubble looked at Neptune in 1994, the point was gone. Poof. Disappeared. Clearly, unlike Jupiter’s Great Red Spot, which has lasted for centuries (at least), storms on Neptune evolve on smaller time scales, although they can last for several years. For example, in the same 1994 Hubble observations a smaller dark spot was observed in the southern hemisphere of Neptune, one that must have arisen in the time between the Voyager flyby and the Hubble images.
Several other dark spots have been seen by Hubble (the only observatory with enough resolution to see these features from this far) since then. They form in both hemispheres in mid-latitudes and tend to move towards the equator. For them, however, it is a death sentence.
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 the air flowing towards the outside from a great height the pressure system (or towards a low pressure one) will begin to rotate the system when it encounters air moving at different speeds north and south.
The Coriolis effect decreases closer to the equator, so when these Neptunian storms migrate in that direction they tend to break apart. This appears to be the fate of most of these storms.
But not this time. Hubble spotted a dark storm in Neptune’s Northern Hemisphere in September 2018. It’s huge, over 7,000 kilometers wide – the continental United States could easily fit into it – and it was seen moving south … but observations carried out in January 2020 showed that this southern migration had reversed and the storm was moving north again. Scientists who study Neptune don’t know why it did this.
But there’s more: there are two other mysterious events seen associated with this storm. One is that by the time he changed his mind and started heading north again, it appears to have generated a smaller dark storm. Some computer models of the behavior of Neptune’s atmosphere predict that this could happen, especially when a large storm begins to erupt near the equator; it can release smaller eddies. This was not seen to happen directly – the observations were dismantled too in time to witness the actual event – but it could be what happened here. This may have something to do with why it changed direction.
Also, oddly enough, this dark storm does not have bright white clouds around the edges, a feature seen in almost all other dark storms. These are clouds made of methane ice crystals, which are highly reflective and appear white in the images. The dark storm is a high pressure system and acts as a heap of air in the atmosphere; the winds blow methane gas up the slopes of that mound where it cools and forms ice crystals. These are called orographic cloudsand they are common on Earth when water-laden air blows over a mountainside, cools and condenses to form clouds.
The white Neptunian clouds were seen in images taken in 2019, but they disappeared earlier this year. This may have something to do with the strange behavior of the dark storm. Or not! Neptune is difficult to observe and understand due to its distance, and despite its enormous size, it still appears very small in our telescopes. Plus, its atmosphere changes quickly, making it difficult to know what’s going on there.
This is why scientists are very interested in creating a mission dedicated to the outer planets, an orbiter that can spend years first on Uranus and then move to Neptune’s orbit. Just as we discovered with Cassini on Saturn, the best way to learn about a planet is to send a probe that stays there for many years. Features come and go, things change, and just as importantly, when scientists discover new phenomena, they can then tell the spacecraft to take a closer look. Discovering new things is important, but a dedicated mission means you can stick around and maybe figure out what’s causing them.
This is the lesson we also learned from the Voyager flybys. Seeing Neptune up close for the first time allowed discoveries such as dark storms, but if you will to understand they have to go there to stay.