Hi there! Today: the red spot of Jupiter. When you observe Jupiter with a common telescope, you just cannot miss it (if it is on the visible side, of course, since Jupiter rotates in about 9.5 hours). It is a red oval, located in the southern equator of Jupiter, as large as 3 Earth. It is actually an anticyclonic storm, which persists since at least 1830. The different space missions have permitted to observe its evolution and measure the winds composing it. Today I present the result of observations by the spacecraft Juno. The study, The rich dynamics of Jupiter’s Great Red Spot from JunoCam: Juno images, has recently been published in The Astronomical Journal.

The Red Spot

This Red Spot has been continuously observed since 1830. To be honest, I don’t know who observed it at that time, but the fact is that it is stable since at least 188 years. Before that, several astronomers, including Giovanni Cassini, claimed to have observed it between 1665 and 1713. It is even depicted by Donato Creti in 1711. But, because of the absence of observations between 1713 and 1830,

1. we do not know whether it is the same spot or not,
2. it could have disappeared and reappeared during the 18th century.

And this is possible, since the red spot is currently shrinking. We know it thanks to the different spacecraft having met Jupiter (Pioneer, Voyager, Galileo, Cassini, and now Juno) and the Hubble Space Telescope. It attained its maximal known width by the end of the 19th century, some 25,000 miles (40,000 km), while it is a little more than 10,000 miles (16,000 km) by now. At this rate, it should become a circle by 2040.

It rotates counter-clockwise with a period of 6 days, while the atmosphere of Jupiter rotates clockwise. The top of the spot is higher of 8 kilometers than the surrounding clouds, which makes it colder.

The spacecraft JUNO orbits Jupiter since July 2016, and permits a new analysis of the Red Spot.

The spacecraft JUNO

The NASA spacecraft JUNO, for JUpiter Near-polar Orbiter, has been launched to Jupiter from Cape Canaveral in August 2011. It orbits Jupiter since July 2016, on a polar orbit. This means that it flies over the poles of Jupiter. Its orbit is very eccentric, with a period of 53 days.

Contrary to Galileo, it is interested only in the planet itself, not in its satellites. Its payload is composed of 9 instruments, and among its objectives are the map of the magnetic field of Jupiter, the map of its gravitational field, which contains information on the solid core which is beneath the atmosphere, and a better knowledge of the chemical composition of the atmosphere.

Among the nine instruments is the camera JunoCam, which provided the data permitting this study.

The data: JunoCam images

JunoCam has not been conceived as a science, but as an outreach instrument, i.e. designed to give beautiful images. And it does.
But in this case, it appears that its data can be used for science. You can find below some images of the Red Spot by Juno, this video having been made by Gerald Eichstädt, one of the authors of the study. You can find more of them on its Youtube channel.

JunoCam has a field of 58°, and 4 filters:

• Blue at 480.1 nanometers (nm),
• Green at 553.5 nm,
• Red at 698.9 nm. These three filters are in the visible spectrum,
• while the fourth one is centered in the methane absorption band at 893.3 nm. This last one belongs to the near infrared spectral domain.

The authors used the images taken in visible light, i.e. with the first three filters, during a close fly-by of the Red Spot on 2017 July, 11.

From raw data to measurements

To make good science from raw data, you have to treat them. In particular, the authors needed to

• consider the exact location and orientation of the spacecraft,
• correct the images from distortion. For that, they assumed that the camera had Brown-Conrady radial distortion, or decentering distortion, which would be due to physical elements in a lens not being perfectly aligned.

Once they made these corrections, they got 4 images, distributed over 581 seconds. In comparing the location of the cloud features on these four images, they got the wind velocities in the upper level of the spot.

5 features in the spot

And from these velocities, they identified 5 structures, which are listed in the Table below.

	Location	Size	Winds’ velocity
Compact cloud clusters	Northern part	500 x 250 km	30-50 m/s
Mesoscale waves	Northern boundary	2,000 x 500 km	50 m/s
Internal spiraling vortices	South-West	1,000 x 1,000 km	75 m/s
Central nucleus	Center	5,200 x 3,150 km	10-20 m/s
Large dark thin filaments	Border	2,000-7,000 x 150 km	2-4 m/s

In particular,

• the compact cloud clusters are composed of between 50 and 60 single wind cells, each with a size between 50 and 70 kilometers. This size suggests ammonia condensation.
• The mesoscale waves could either be atmospheric gravity waves, i.e. when buoyancy tries to restore equilibrium between two media (see picture below, of atmospheric gravity waves observed on Earth), or shear instability waves, due to high wind.



• The cause of the internal spiraling vortices still needs to be understood.
• The central nucleus is probably a cyclonic region, with turbulent winds.
• It is not clear whether the large dark thin filaments are traced by darker aerosols, or represent areas with differetn altitudes and particles densities. They could be Vortex Rossby Waves, which accelerate the tangential winds, and play an important role in hurricanes. You can find more details here.

The study and its authors

• You can find the study here. The access to it is free, i.e. the authors paid extra fees, so that you could read their paper. Many thanks to them!
• This study is also presented on AAS Nova. And now, the authors!
• The IAU page of Agustin Sánchez-Lavega,
• the webpage of Ricardo Hueso Alonso,
• the one of Gerald Eichstädt. You can also visit his Youtube channel,
• the webpage of Glenn Orton,
• the one of Candice J. Hansen,
• the one of Thomas Momary,
• the one of Fachreddin Tabataba-Vakili,
• and the one of Scott Bolton.

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter, Facebook, Instagram, and Pinterest.