A series of observations of Neptune by the Hubble Space Telescope show that a massive dark storm that raged in the giant planet’s northern hemisphere moved south, but then inexplicably made a large U-turn back north. Not only that, but it could have spawned a dark baby storm 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 (which for historical reasons are referred to by planetary scientists as “ice creams,” even if they are gaseous). At nearly 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 images it sent back surprised scientists; it saw an immense oval dark storm in the southern hemisphere of the planet the size of Earth itself! Called the Great Dark Spot, it had measured wind speeds of a staggering 2,100 mph, the fastest wind ever recorded in the solar system.
But when Hubble looked at Neptune in 1994, the spot was gone. Poof. Gone. Obviously, unlike Jupiter’s Great Red Spot, which lasts (at least) for centuries, storms on Neptune evolve on smaller timescales, although they can last for several years. For example, the same Hubble observations from 1994 saw a smaller dark spot in Neptune’s Southern Hemisphere, one that must have been born in the intervening time between the Voyager flyby and the Hubble images.
Since then, Hubble has seen several other dark spots (the only observatory with enough resolution to see these features from so far away). They form on both hemispheres at the mid-latitudes and tend to drift towards the equator. However, that’s a death sentence for them.
These storms are high-pressure systems, supported by the Coriolis effect – the rotation of a planet is at different speeds at different latitudes (with a maximum at the equator and a minimum at the poles), meaning air flows out from a great height . pressure system (or in the direction of a low pressure system) will make the system run when it encounters air moving at different speeds north and south.
The Coriolis effect diminishes closer to the equator, so when these Neptunian storms migrate in that direction, they tend to disintegrate. That seems to be the fate of most of these storms.
But not this time. Hubble detected a dark storm in Neptune’s northern hemisphere in September 2018. It’s huge, over 4,000 miles wide – the entire continental United States could easily fit in – and was seen moving south … but then January 2020 observations showed that this southern migration had reversed and the storm was moving north again. Scientists studying Neptune don’t know why it did that.
But there is more: there are two other mysterious events related to this storm. One is that around the time it changed its mind and started going north again, it appears to have spawned a smaller dark storm. Some computer models of the behavior of Neptune’s atmosphere predict that this could happen, especially if a major storm erupts near the equator; it can throw off smaller eddies. This didn’t happen right away – the observations were scattered too far apart to witness the actual event – but it could be what happened here. That may have to do with why it changed direction.
Strangely, this dark storm also doesn’t have bright white clouds around the edges, a feature seen in almost every other dark storm. These are clouds made of methane ice crystals, which are highly reflective and appear white in images. The dark storm is a high-pressure system and acts like a mountain of air in the atmosphere; winds blow methane gas up the slopes of that hill where it cools and forms the ice crystals. These are called orographic clouds, and they are very common on Earth, because water-laden air blows up the side of a mountain, cools and condenses to form clouds.
The Neptunian white clouds featured in images taken in 2019, but disappeared early this year. This may have something to do with the dark storm’s strange behavior. Or maybe not! Neptune is difficult to perceive and understand because of its distance, and despite its enormous size, it still appears very small in our telescopes. Also, the atmosphere changes on short time scales, making it difficult to know what’s going on there.
This is why scientists are very interested in creating a dedicated mission to the outer planets, an orbiter that can first spend years on Uranus and then continue to orbit Neptune. Just as we discovered with Cassini near Saturn, the best way to learn more about a planet is to send a probe that stays there for many years. Functions come and go, things change, and just as importantly, when scientists discover new phenomena, they can tell the spacecraft to take a closer look. Discovering new things is important, but a dedicated mission means you can linger and maybe figure out what caused them.
That’s the lesson we learned from the Voyager flybys too. Seeing Neptune up close for the first time allowed discoveries like the dark storms, but if we want to understand them we have to go there to stay.