Juno probe, recapturing Jupiter
Jupiter is one of the most studied planets in our solar system. Visible to the naked eye in the night sky, 7 missions flew over it and a probe even plunged into its atmosphere during its survey by the American ship Galileo between 1995 and 2003.
However, it was not until JUNO arrived in the gas giant's polar orbit (July 2016) that the research teams had solid data on the three main themes of the mission: Jupiter’s internal composition, its magnetosphere and its atmosphere. Juno is on an elliptical trajectory, which brings it every 53 days to less than 4 000 km of powerful cloud currents that encircle Jupiter on several "bands" already well known, rotating at different speeds (sometimes with a difference of more than 300 km/h).
The probe has only achieved 11 orbits so far, but that did not prevent it from performing measurements studied in research units around the world. "As it passes very close to the poles and cloud formations, Juno's special orbit is essential for gravitational field research. It is a unique mission producing data for generations of scientists," Tristan Guillot1 praises, whose results are published in the columns of Nature on 7 March 2018.
The precise measurement of Jupiter's gravity is at the base of this work. During its stay at the perijove, as close as possible to Jupiter's atmosphere, Juno receives a series of messages transmitted from the Earth, which he immediately sent back to X and Ka band. But the variations of the giant planet’s gravitational field induce tiny displacements which translate into frequency shifts between the messages. Thanks to this application of the Doppler effect, a map of gravity 100 times more accurate than the previous measurements could be established.
And this is the first surprise: unlike what could predict a mathematical model for a gas planet, the attraction is greater in the northern hemisphere than in the southern hemisphere2! "Jupiter has no surface, and its rapid rotation (10 terrestrial hours) suggested a symmetrical north-south gravitational field, but this is not the case." continues Tristan Guillot. The cause of this phenomenon is still unknown. The heart of the planet is quickly exonerated because despite its phenomenal density, all models predict that it is homogeneous and in a state of equilibrium.
The winds, Jupiter’s sculptors
What if it was the winds? Currents so powerful and so deep that the mass of gas they move would be sufficient to create variations in the harmonics of the gravitational field? This is what the researchers sense: the cloud bands of the northern hemisphere do not have the same characteristics as those of the south of Jupiter.
Even if these variations are minimal, JUNO's data are consistent and can be explained if atmospheric winds penetrate up to 3 000 km deep. Impressive results, which show that the currents move up to 1% of the total mass of Jupiter, almost 3 times the mass of the Earth! However, there is no doubt: alongside this analytical method (whose first author is Professor Yohai Kaspi3), Tristan Guillot’s team provided similar results by comparing the results of JUNO to those of hundreds of thousands of models from inside the gas giant.
But validating these results is of paramount importance "This is the next step. Now that we have a faithful representation of the wind currents on Jupiter, this analysis will constrain the composition of the inner layers of the planet." Tristan Guillot confirms. Because deeper, temperatures and pressures increase. And the magnetic field, also more complex than expected, interacts with the winds rotating at different speeds. Even though the deep interior of Jupiter turns uniformly, the mechanisms are complex.
Internal structure of Jupiter © Tristan Guillot, Observattoire de la Côte d'Azur, France
"We are only beginning to understand Jupiter's internal rotation and the distribution of atomic elements heavier than hydrogen and helium" Yamila Miguel4 says. However, the precise study of winds, vortex, and storms on the surface of Jupiter is only beginning. "At each passage of Juno we increase our knowledge on the gravitational field, and this allows us to better apprehend the currents".
The JIRAM infrared spectrometry instrument on board the probe will refine the models, revealing the highest zones of these complex structures (up to approximately 70 km deep). The fourth and last publication of Nature highlights two groups of gigantic cyclones at Jupiter’s North and South Poles, observed for the first time thanks to the probe. A central vortex and 8 related tornadoes, almost touching each other in the north, and 6 vortexes to the south! A unique observation for the solar system, Alberto Adriani5 explains in his article, especially since it would not be temporary storms but stable phenomena over time despite the strength of winds that reach 350 km/h. And other questions emerge. These polar formations have almost nothing in common with those observed by CASSINI during its last year of measurement on Saturn, the other gas giant.
"Combining our research with Cassini's measurements allows us to say, for example, that the winds on Saturn have an influence that extends up to 9 000 km deep. By deciphering Jupiter, its winds, its composition and its magnetic field, we obtain keys on the origin of the Solar system." Mr. Guillot explains. But continuing to solve this puzzle of our distant past is also refining our analysis of other systems: the discovery of exoplanets and brown dwarfs benefits from this research. It is even a complementary approach. Tristan Guillot concludes: "The dynamics of Jupiter and Saturne can be studied in great detail, but they are the only ones available to us. On the contrary, there are hundreds of known gas exoplanets, but the data we can collect is much less precise and detailed!"
1 Tristan Guillot is the first author of the article "A suppression of differential rotation in Jupiter’s deep interior" published in Nature, and co-author of two other articles published in the same journal. Université Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, Nice, France.
2 Luciano Iess is the first author of this article of Nature, entitled "Measurement of Jupiter’s asymmetric gravity field" and co-author of two other articles published in the same journal. Sapienza University, Rome, Italy.
3 Yohai Kaspi is the first author of the article "Jupiter’s atmospheric jet streams extend thousands of kilometres deep" published in Nature, and co-author of two other articles published in the same journal. Weizmann Institute of Science, Rehovot, Israel.
4 Yamila Miguel is co-author of three out of four articles published in Nature. Leiden Observatory, Netherlands.
5 Alberto Adriani is the first author of the article "Clusters of cyclones encircling Jupiter’s poles" published in Nature. Institute for Space Astrophysics and Planetology, Rome.
- Tristan Guillot, director of research at CNRS at the Observatoire de la Côte d'Azur: tristan.guillot at oca.eu
- Yamila Miguel, assistant professor at the Leiden Observatory, Netherlands: ymiguel at strw.leidenuniv.nl
- Francis Rocard, director of the solar system exploration programmes: francis.rocard at cnes.fr