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The Depths of Jupiter’s Winds

Jupiter, which has a radius of 70,000 km (11 times larger than Earth), is a so-called gas giant. It is mainly composed of hydrogen and helium, just like our sun. Thus, the planet does not have a solid surface and probably does not have a well-defined core, either. Indeed, until very recently, our knowledge of this fascinating celestial body has been rather scant. And scientists have mainly been involved in observations of the planet’s surface, its coloured stripes, and the massive cyclonic and anti-cyclonic storms – like the Great Red Spot – that ravage the planet's surface with winds up to 360 km/hr.

As the planet has no hard surface to block them, how deep into the gas giant do these winds reach? And what is the size of the planet’s core? These and other questions have been tackled by NASA’s Juno mission.

The initial gravity measurements made by Juno indicated that the planet’s heavy elements are evenly distributed throughout half the planet’s radius. However, there was no indication of the winds’ depth. Did it extend for 100-300 or 10,000-20,000 kilometers? The research group led by Professor Luciano Iess from the Sapienza Department of Mechanical and Aerospace Engineering, in collaboration with Daniele Durante and Paolo Racioppa, as well as other researchers from both Italian and foreign universities, used the data collected by the probe to determine that Jupiter’s surface winds reach as far as 3000 km under the cloud level.

Jupiter’s winds are governed by the same laws that regulate atmospheric circulation on Earth, where high- and low-pressure areas, associated with different atmospheric densities, drive the movement of large masses of air. The depth of the winds affects not only the magnitude of the atmospheric masses that they move, but also the generation of variations in gravity. And this – the variations in gravity associated to different atmospheric density and the speed of the wind in the two hemispheres (North and South) – was the key to understanding the depths of the planet.

“The precise data collected by the Juno Probe,” explains Professor Iess, “was measured by a high precision tool that guarantees our radio transmissions with the Juno. The Ka-band Translator (KaT) was developed by Thales Alenia Space Italia with significant contributions by Sapienza Researchers and funded by the Italian Space Agency (ASI). The KaT allows us to determine the speed of the probe with great precision (an order of thousands of a millimetres/sec.) Thus, by following Juno’s passage above Jupiter’s clouds (at about 70 Km/sec.), we calculated the fine structure of the planet’s gravitational field and were able to delve into the invisible depths of its atmosphere.”

The results of the study, coordinated by Sapienza Professor Luciano Iess, Yohai Kaspi from the Weizmann Institute (Rehovot, Israel) and Tristan Guillot from the Observatoire de la Cote d’Azur (Nice) were published on March 8 on Nature as three distinct articles.


The Measurement of Jupiter’s Asymmetric Gravity Field  – Authors: L. Iess, W.M. Folkner, D. Durante, M. Parisi, Y. Kaspi, E.Galanti, T. Guillot, W. B. Hubbard, D.J. Stevenson, J.D. Anderson, D. R. Buccino, L. Gomez Casajus, A. Milani, R. Park, P. Racioppa, D. Serra, P. Tortora, M. Zannoni, H. Cao, R. Helled, J.I. Lunine, Y. Miguel, B. Militzer, S. Wahl, J.E.P. Connerney, S.M. Levin, S.J. Bolton. Nature 8 marzo 2018 - DOI: 10.1038/nature25776