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A series of observations of Neptune by the Hubble Space Telescope shows that a huge dark storm raging in the northern hemisphere of the giant planet was moving south, but then inexplicably took a major halftone turn, heading north. Not only that, but it also maybe spawned a baby black storm in the process.
Neptune is what is known as an ice giant, essentially a gigantic ball of hydrogen and helium gas containing loads of methane, ammonia, and other molecules (which for historical reasons planetary scientists call “Ice creams” even if they are carbonated). At nearly four times the diameter of Earth, Neptune is the largest planet farthest from the Sun, at 4.5 billion kilometers.
When Voyager 2 passed through Neptune in 1989, the images it returned surprised scientists; he saw a huge black oval storm in the southern hemisphere of the planet as large as the Earth itself! Called the Great Dark Spot, it had measured wind speeds of 2,100 km / h, the fastest wind ever measured in the solar system.
But, when Hubble watched Neptune in 1994, the place was gone. Pouf. Faded away. Obviously, unlike Jupiter’s Great Red Spot, which has persisted for centuries (at least), storms on Neptune evolve on shorter timescales, although they can last for several years. For example, in the same Hubble observations in 1994, a smaller dark spot was seen in the southern hemisphere of Neptune, one that must have arisen in the interval between the Voyager flyby and the Hubble images.
Several other dark spots have been seen by Hubble (the only observatory with sufficient resolution to see these features from so far away) since. They form in both hemispheres at mid-latitudes and tend to drift towards the equator. It is a death sentence for them, however.
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 outward from a large high pressure system (or to a low pressure system) will start the system rotating when it encounters air moving at different speeds north and south .
The Coriolis effect decreases closer to the equator, so these Neptunian storms migrate in that direction, they tend to break up. That seems to be the fate of most of these storms.
But not this time. Hubble spotted a black storm in Neptune’s northern hemisphere in September 2018. It’s huge, over 7,000 kilometers wide – the entire continental United States could easily fit in – and it’s been seen moving around south … but observations taken in January 2020 showed that this southward migration had reversed and the storm was moving north again. Scientists who study Neptune don’t know why he did this.
But there is more: there are two other mysterious events associated with this storm. The first is that around the time he changed his mind and started heading north again, it appears to have spawned a smaller black storm. Some computer models of the behavior of Neptune’s atmosphere predict that this can happen, especially as a large storm begins to erupt near the equator; it can spread smaller vortices. This was not seen to happen directly – the sightings were too far away in time to witness the actual event – but maybe that is what happened here. It may have something to do with why he changed direction.
Plus, oddly, this dark storm doesn’t have bright white clouds around its edges, a feature seen in almost every other dark storm. These are clouds made up of methane ice crystals, which are highly reflective and appear white in images. The black storm is a high pressure system and acts like a mound of air in the atmosphere; the winds blow methane down the slopes of this mound where it cools and forms ice crystals. These are called orographic clouds, and they are common on Earth because the water laden with water blows on the side of a mountain, cools and condenses to form clouds.
The Neptunian white clouds were seen in images taken in 2019, but disappeared earlier this year. It may have something to do with the strange behavior of the Black Storm. Or maybe not! Neptune is difficult to observe and understand due to its distance, and despite its enormous size, it still looks very small in our telescopes. Plus, its atmosphere changes on short time scales, making it hard 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 spend years first on Uranus and then orbit around Neptune. Just as we saw with Cassini at Saturn, the best way to learn more 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 ask the spacecraft to take a closer look at them. Discovering new things is important, but a dedicated mission means you can stick around and maybe figure out what’s causing them.
This is also the lesson we learned from the Voyager flybys. Seeing Neptune up close for the first time allowed for discoveries like Dark Storms, but if we want to understand we have to go there to stay.
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