A series of observations of Neptune by the Hubble Space Telescope show that a huge dark storm raging in the northern hemisphere of the giant planet was moving south, but inexplicably made a large U-turn, north. Not only that, but it can also have generated a dark baby storm in the process.
Neptune is what is called an ice giant, basically a giant ball of hydrogen and helium with lots of methane, ammonia and other molecules (which for historical reasons planetary scientists call “ice cream” even if they are gaseous). With almost four times the diameter of Earth, Neptune is the largest planet in the Sun, 4.5 billion kilometers away.
When Voyager 2 passed through Neptune in 1989, the images that returned surprised scientists; saw an immense dark oval storm in the southern hemisphere of the planet as large as Earth itself! Called the Great Dark Spot, it had measured wind speeds of an impressive 2,100 km / h, the fastest wind ever measured in the solar system.
But by the time Hubble looked at Neptune in 1994, the place was gone. Poof. Disappeared. Clearly, 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, the same Hubble observations of 1994 showed a smaller dark spot in the southern hemisphere of Neptune, which must have been born in the time between the Voyager’s overflight and Hubble’s images.
Since then, Hubble (the only observatory with enough resolution to see these features from afar) has seen several other dark spots. They form in both hemispheres at mid-latitude and tend to drift toward the equator. But it is a death sentence for them.
These storms are high-pressure systems, sustained 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 flows outward from a great height. the pressure system (or toward a low pressure one) will begin to rotate the system as it encounters air moving at different speeds north and south.
The Coriolis effect decreases closer to the equator, so as these Neptune storms migrate in this direction, they tend to break. 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 miles wide (the entire continental United States could easily fit inside it) and was seen moving to the south … this migration from the south had been reversed and the storm was moving north again. Scientists studying Neptune do not know why he did it.
But there is more: there are two other mysterious facts seen associated with this storm. One is that around the time he changed his mind and started heading north again, it seems to have generated a smaller dark storm. Some computer models of Neptune’s atmospheric behavior predict that this may happen, especially when a large storm begins to break near the equator; may spill smaller vortices. This was not seen to happen directly (observations were made too far apart in time to witness the actual event), but it may be what happened here. This may have something to do with the reason he changed direction.
Also, strangely, this dark storm has no bright white clouds around its edges, a feature seen in almost every other dark storm. These are clouds formed by 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 pile of air into the atmosphere; the winds blow methane gas down the slopes of that mound where it cools and forms ice crystals. These are called orographic clouds, and are common on Earth as water-laden air blows up the side of a mountain, cools, and condenses to form clouds.
Neptune’s white clouds were seen in images taken in 2019, but disappeared earlier this year. This may have something to do with the strange behavior of the dark storm. Or maybe not! Neptune is difficult to observe and understand because of its distance, and despite its enormous size, it still appears very small in our telescopes. Also, its atmosphere changes in the short term, making it difficult to know what’s going on there.
That is why scientists are very interested in creating a mission dedicated to the outer planets, an orbiter that can spend years first in Uranus and then move into the orbit of Neptune. As we found with Cassini on Saturn, the best way to learn about a planet is to send a probe that would stay there for many years. Functions come and go, things change, and in the same way, it’s important when scientists discover new phenomena that they can then tell the ship to analyze closely. Discovering new things is important, but a dedicated mission means you can stay and maybe find out what causes them.
This is the lesson we also learned from Voyager flybys. Seeing Neptune up close for the first time allowed for discoveries like dark storms, but if we will to understand we have to go there to stay.