The depth of Jupiter’s winds measured by the probe Juno
Jupiter is a gas giant with a radius 11 times larger than the Earth (70000 km) and a solar-like chemical composition, largely hydrogen and helium. While extensive observational campaigns have provided a wealth of data on the upper layers of the atmosphere, characterised by a banded structure of different colours and powerful cyclones and anticyclones like the Great Red Spot, the structure of the deep interior remained largely undetermined. How massive is the heavy element core? To what depth do the strong winds observed on the surface extend? NASA’s mission Juno, carrying aboard an important Italian contribution, has answered all these questions.
The first Juno’s gravity measurements provide a strong indication that Jupiter does not possess a well-defined core. Rather, its heavy elements are distributed in a much larger volume, extending out to almost half the planet’s radius. The vertical extension of the winds, which at the surface reach speeds as high as 360 km/h, remained completely unknown. On Jupiter, where, unlike the Earth, there is no solid surface blocking the flows, the winds could extend to very large depths (even 10000-20000 km), or be very shallow (100-300 km), that is just below the visible cloud level.
Juno’s gravity measurements indicate that atmospheric flows extend to a depth of about 3000 km. Jupiter’s winds are determined by the same laws governing the atmospheric circulation on Earth, where high and low pressure zones associated with different air densities force the movement of large air masses. The tiny gravity variations associated with different atmospheric densities and velocities of the flows, different in the two hemispheres, have been the key to plumb the depths of the planet. Indeed, the deeper the flows, the larger are the atmospheric masses in motion, and the larger the gravity signature is.
“Juno’s very precise measurements – Luciano Iess of Sapienza, explains – have been possible thanks to a high precision instrument onboard Juno, enabling a radio link to and from the spacecraft. The Ka-band Translator (KaT), funded by the Italian Space Agency (ASI), was designed and built by Thales Alenia Space Italy with the contribution of Sapienza’s researchers. The KaT enables velocity measurements of exquisite precision (up to 0.003 mm/s). Tracking the motion of Juno as it skims the clouds at a speed of 70 km/s, Juno’s been able to determine the structure of Jupiter’s gravity and to plumb the hidden depth of its atmosphere".
The research group is led by Luciano Iess, from the Depatment of Mechanical and Aerospace Engineering of Sapienza University of Rome, in collaboration with Daniele Durente and Paolo Racioppa. Other scientists and engineers from universities and scientific institutions in Italy and abroad contributed to the research.
'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
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