Jupiter is a gaseous planet, and its internal masses can move, generating oscillations similar to sea waves and earthquakes. These mass shifts cause small variations in the planet's gravity.

A new study, coordinated by Daniele Durante of the Department of Mechanical and Aerospace Engineering at Sapienza, has revealed how the gravity field of Jupiter is perturbed by internal oscillations, i.e., real waves that spread from one part of the planet to another. This result has been obtained after analyzing the gravity measurements carried out by NASA's Juno spacecraft. In particular, the highly sensitive instrumentation onboard the spacecraft enabled the authors to determine the oscillation periods of the most energetic modes. They generate waves having an amplitude in the range of 15 to 80 meters on the surface, oscillating every 15 minutes or so.

The research results, carried out by an international team and partly funded by the Italian Space Agency, were published in the prestigious scientific journal Nature Communications.

The Juno mission, in orbit around Jupiter since 5 July 2016, has as its main objectives the study of the formation processes of its internal structure, the magnetic field, and the atmosphere of the planet. Jupiter, which alone has a mass two and a half times greater than that of all the other planets combined, is almost exclusively composed of hydrogen and helium. Since its interior is not directly observable, to understand its deeper structure accurate measurements of the gravitational field, being expression of the distribution of the planet's internal masses, are used.

“Approximately every 52 days - Daniele Durante, first author of the study, explains - the Juno probe makes close passages of the planet, about 4,000 km from the edge of the clouds. At these distances, Juno experiences small but measurable accelerations exerted by the internal oscillations of the planet”.

The onboard radio science instrument KaT (Ka-Band Translator, developed by Thales Alenia Space Italy and financed by the Italian Space Agency) is the heart of the gravity experiment that made it possible to measure the perturbations in the gravitational field caused by Jupiter’s internal oscillations. The KaT receives and retransmits the radio signal sent by a dedicated ground antenna located in the California desert, allowing it to measure the relative speed of the probe with accuracies of hundredths of a millimeter per second, and variations in the gravitational acceleration 60 million times smaller than Earth's gravity.

Juno’s gravity measurements had already led to other important discoveries related to the internal structure of the planet, including the depth of strong east-west winds (having speeds up to 360 km/h), which penetrate down to about 3,000 km below the cloud level. Furthermore, the gravity measurements carried out during two close flyovers of Jupiter’s Great Red Spot enabled the determination, for the first time, of its depth, which resulted equal to about 300 km, much lower than that of the east-west winds.

In the 22 orbits performed in the first 5 years of the mission and dedicated to the study of the gravitational field of Jupiter, the Juno spacecraft flew over the planet at distances as close as 4-5,000 km above the level of the clouds (since Jupiter does not have a real surface), very accurately measuring the planet's gravity field at each pass. Thus, it was possible to observe that the gravity of the gas giant changed slightly over time.

"Our scientific team - continues Daniele Durante - the presence of dynamic phenomena such as oscillation modes is by far the most convincing interpretation of Juno’s data. The measurements show a constantly moving planet, not only around its axis of rotation but also inside it”.

The outermost layers oscillate vertically for 15–80 meters every about 15 minutes. This phenomenon is similar to what happens with terrestrial tides. These oscillation modes are called ‘p-modes’ since the restoring force is the internal pressure.

"Similarly to what happened for the Sun with the research field known as helioseismology, the measurement of these oscillations with dedicated instruments will be able to provide in the future - concludes Durante - a much more detailed description of the internal structure of the planet than is possible nowadays".

References:

Juno spacecraft gravity measurements provide evidence for normal modes of Jupiter - Daniele Durante, Tristan Guillot, Luciano Iess, David J. Stevenson, Christopher R. Mankovich, Steve Markham, Eli Galanti, Yohai Kaspi, Marco Zannoni, Luis Gomez Casajus, Giacomo Lari, Marzia Parisi, Dustin R. Buccino, Ryan S. Park, Scott J. Bolton - Nature Communications (2022) https://doi.org/10.1038/s41467-022-32299-9

Further Information

Daniele Durante
Department of mechanical and aerospace engineering
daniele.durante@uniroma1.it

CAPTIONS

Figure 1: Artistic representation of the Juno spacecraft around Jupiter.
Credits: NASA/JPL-Caltech.

Figure 2: Artistic representation of Juno orbiting Jupiter. Since Jupiter is a gaseous planet, its gravity field is perturbed by the presence of internal oscillations, which move a large amount of mass.

Credits, Juno image: NASA/JPL-Caltech.

Credits, Jupiter image: NASA, ESA, A. Simon (Goddard Space Flight Center), and M. H. Wong (University of California, Berkeley) and the OPAL team.

Figure 3: Recovered radial velocity profile of Jupiter’s upper layer as a function of oscillation frequency of internal normal modes. Each line corresponds to a different model whose opacity is proportional to that model’s probability: darker lines are for more likely models, conversely, lighter lines are for gradually less probable models. The spread of the lines provides an indication of the uncertainty of the recovered amplitude spectrum.

Video (downloadable at the link https://we.tl/t-cmKehxuSie): The gravitational acceleration at Jupiter’s surface changes over time due to internal oscillations. The surface oscillates up and down by about 15-80 meters every 15 minutes, very similarly to what happens with terrestrial tides.