by Alexandra Pais
Jupiter hosts the most wide and strong windstorm known in the solar system. The Great Red Spot is a huge anticyclone of hydrogen and helium winds, powered from inside the planet. It is probably the same storm that has been seen since the first telescopes were targeted to Jupiter’s surface, more than 350 years ago.
Galileu Galilei was the one to catch the first glimpse on Jupiter’s 4 largest moons (Io, Europe, Ganymede, Callisto), but it was Robert Hooke who, in 1664, discovered the sign of a large spot on the planet. Soon after, in 1665, Gian Domenico Cassini described the spot in detail. The spot was then observed from time to time during the 18th century and has been in continuous observation since 1879. Was it always the same storm, which today we call Jupiter Great Red Spot (GRS)? We think it was, and we can be sure of it for the last 144 years.
Jupiter clouds are distributed over latitudinal bands, which run parallel to Jupiter’s equator: the belts (light bands) and the zones (darker, reddish bands). Consecutive bands are separated by strong east-west winds, called zonal jets. The GRS eye is located 22º latitude south. The winds at the large storm external boundary turn counterclockwise with values reaching more than 430 km/h around a high-pressure centre (in the opposite sense to hurricanes on Earth). To understand how strong this is, just note that the strongest winds ever registered on Earth were 345 km/h, and with only one-minute duration.
Recent data from NASA Juno mission (in orbit since 2016) show that Jupiter GRS extends deep down into the planet interior, possibly more than 500 km beneath Jupiter’s cloud tips. This is a distance larger than what separates us from the International Space Station orbit! On Earth, hurricane vortices extend down to the solid surface, never more than about 15 km. But inside Jupiter there may be no solid surface… This is also the reason why Jupiter GRS has remained active for centuries: no slow down due to friction action near a solid boundary. Well, there must be a braking effect, after all, since the centre of Jupiter is further down at about 70 000 km… But this is thought to be due to magnetic forces.
Results on the depth of the GRS were obtained from Juno, by monitoring fluctuations of the gravitational field of Jupiter over the large storm and explaining them in terms of mass displacement of the winds below. Also, Juno’s microwave radiometer provides information from different microwave wavelengths, allowing to retrieve the temperature distribution in depth. Microwave data showed that the GRS temperature is warmer in depththan at the top.
Very interesting! The heat below is probably driving the storm’s strong winds. But why then at that location in particular? Lots of mysteries inside Jupiter, that ESA space mission JUICE will help to solve!
See more in:
Great Red Spot https://en.wikipedia.org/wiki/Great_Red_Spot#cite_note-30
May 1664: Hooke vs. Cassini: Who Discovered Jupiter’s Red Spot? https://www.aps.org/publications/apsnews/202005/history.cfm
What’s going on with Jupiter’s Red Spot? https://earthsky.org/space/jupiters-great-red-spot-shrinking-anticyclone/
Jupiter’s atmospheric jet streams extend thousands of kilometres deep https://www.nature.com/articles/nature25793