Gravity measurements made by the Juno spacecraft (NASA) during the first five years in orbit around Jupiter have shown that the planet's interior and surface oscillate. 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, which have been measured by the spacecraft's sensitive instruments orbiting the planet.
The new study has revealed how the gravity field of Jupiter is perturbed by internal oscillations, i.e., real waves that propagate from one part of the planet to the other, passing through the innermost regions. The most energetic modes have oscillation periods of the order of 15 minutes and cause waves having an amplitude in the range of 15 to 80 meters on the surface.
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.
The Juno spacecraft has been orbiting Jupiter in a highly eccentric orbit. That is, approximately every 52 days, Juno makes close passages of the planet, going as low as to 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, preserving phase coherence, 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. The gravity science experiment is enabled by the KaT and the Deep Space Transponder (DST), which allow to collect simultaneous Doppler data at both X-band and Ka-band, which can then be analyzed to accurately determine the gravity field of the planet.
Juno’s gravity measurements had already led to other important discoveries related to the internal structure of the planet. The discovery of the large north-south asymmetry of Jupiter’s gravitational field has been used to determine the depth of strong east-west, zonal winds (having speeds up to 360 km/h), which penetrate down to about 3,000 km below the cloud level. On the contrary, its deep interior rotates as a rigid body. Furthermore, the gravity measurements carried out during two close flyovers of Jupiter’s iconic anticyclonic storm, the Great Red Spot, enabled the determination, for the first time, of the mass involved in the circulation and thus of its penetration 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.
According to the scientific team, 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 (Jupiter completes a complete revolution around its axis in 10h and 55m), but also inside it. The results show an amplitude spectrum with a peak radial velocity of 10–50 cm/s at frequencies in the range 900–1200 mHz, while lower frequency, fundamental (f-)modes have to be smaller than 1 cm/s. This means that 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 a much more detailed description of the internal structure of the planet than is possible nowadays. The detection of normal modes inside Jupiter (and Saturn!) is a crucial step for the future exploration of gas giants, effectively paving the way to the discipline of seismology applied to this class of bodies.
This research has been funded by the Italian Space Agency.