I learned this today. Jupiter’s Great Red Spot is a high-pressure region in the atmosphere that causes the largest anticyclonic storm in the Solar System.
An anticyclonic storm is a weather system with very high pressure in the center. The air around the high-pressure center circulates clockwise in the northern hemisphere and anticlockwise in the southern hemisphere.
The Great Red Spot is in Jupiter’s southern hemisphere, and it rotates anticlockwise in the atmosphere. The atmosphere on Jupiter is made of hydrogen, helium, and some methane and ammonia. It is basically made of the same elements that make up the sun. The wind speeds in the Great Red Spot can get as high as 432 km/h. It is difficult to think of the Great Red Spot as a storm because it is almost 2 times bigger than our entire planet. It takes six Earth days for the storm to rotate once and the tops of the clouds in the Great Red Spot are 8 km higher than the surrounding clouds.
Nobody knows how long the Great Red Spot has existed, but it was possibly first observed in 1665. There is no way of telling if that was the Great Red Spot that exists today, but it seems likely. It also seems likely that the Great Red Spot existed even before then.
The Great Red Spot has been shrinking in size over the last century. By 2004, it was down to a diameter of 40,000km and recently, bits of it have started to flake off. There is some speculation that it may disappear over the next twenty years.
There are two questions we need to look at here: What causes the Great Red Spot, and why is it red?
The leading theory on its redness is that a colorless layer of ammonium hydrosulfide is deep under the outer clouds of Jupiter. This layer could be reacting with the UV radiation from the sun and the organic compound acetylene to make the red color.
What causes it? The storm is caused by a high-pressure region in the atmosphere. It is then trapped between two powerful jet streams of gas that are moving in opposite directions. There is a mild eastward jet stream to its south and a very strong westward jet stream to its north. These two streams pull the storm in different directions and keep it spinning. On Earth, a storm would wear itself out and dissipate because of friction with the ground. Jupiter doesn’t have a ground and so there is no friction for the storm to wear itself out on. It will keep spinning for as long as it has fuel and energy.
Scientists thought that the Great Red Spot was just a storm on the surface of Jupiter until recently. Data from the Juno spacecraft that is currently orbiting Jupiter has shown that the Great Red Spot extends as much as 500km underneath Jupiter’s cloud tops. It is then linked to jets of gas that go even deeper, linking the upper and lower atmospheres together.
This was all calculated by using Juno’s gravity science instrument and its microwave radiometer.
Juno flew over the Great Red Spot at 210,000 km/h. As it flew over the planet, the gravity of Jupiter pulled Juno ever so slightly off course. In areas of strong gravity it was pulled off course more than in areas of weak gravity. This slight move in its course changed the radio waves coming from Juno to Earth. It caused a Doppler shift, which is where the radio waves get squeezed and stretched. Even though Earth is 645 million km away from Jupiter, movements of 0.01mm could be detected and scientists can use this information to measure the relative gravity across Jupiter’s surface. What the gravity showed is that the storm descends to a maximum of 500 km under the surface clouds of Jupiter. The gravity also revealed the great circling jets of gas that reached over 3,000km farther down into the atmosphere of Jupiter. The microwave radiometer measurements showed the storm extended to at least 300 km under the surface. So, using the two measurements, scientists know that it is between 300 and 500 km deep.
This was surprising, but nobody seems to know why it stops at this depth. The jet streams that surround the Great Red Spot extend about 3,000 km into the cloud layer. What happens at 500 km to dampen the Great Red Spot and stop it going any deeper? Nobody knows.
So, the Great Red Spot is an anticyclone rotating anticlockwise in the atmosphere of Jupiter. It has been around for at least 400 years and is twice the size of our planet. Calculations made from the spacecraft Juno have shown that it goes far deeper into the planet than previously thought. And this is what I learned today.
Photo By NASA – http://photojournal.jpl.nasa.gov/catalog/PIA00014
Sources:
https://www.nasa.gov/feature/goddard/jupiter-s-great-red-spot-a-swirling-mystery
https://wonderopolis.org/wonder/why-is-there-a-great-red-spot-on-jupiter
https://en.wikipedia.org/wiki/Great_Red_Spot
https://www.space.com/jupiter-great-red-spot-deeper-than-thought
https://www.nasa.gov/image-feature/jpl/jupiter-s-great-red-spot-a-rose-by-any-other-name
https://www.theverge.com/2021/10/28/22749095/nasa-juno-jupiter-great-red-spot-depth