An enhanced image of Jupiter's Great Red Spot, as seen by NASA's Voyager 2 probe on July 7, 1979. Credit: NASA/JPL/Björn Jónsson/Seán Doran/Flickr (CC BY-NC-ND 2.0)

The red spot of Jupiter

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.
Astronomical Observations: Jupiter, by Donato Creti (1711).
Astronomical Observations: Jupiter, by Donato Creti (1711).

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.

Global view of Jupiter, with the Red Spot at 22° South. © Space Telescope Science Institute/NASA
Global view of Jupiter, with the Red Spot at 22° South. © Space Telescope Science Institute/NASA

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.
  • Satellite image of atmospheric gravity waves over the Arabian Sea. Their visibility is due to sunlight, caused by the "impression" of the atmospheric waves on the sea surface. ©Jeff Schmaltz MODIS Rapid Response Team, NASA-GSFC
    Satellite image of atmospheric gravity waves over the Arabian Sea. Their visibility is due to sunlight, caused by the "impression" of the atmospheric waves on the sea surface. ©Jeff Schmaltz MODIS Rapid Response Team, NASA-GSFC
  • 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

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.

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