Perspectives of Jovian seismology: sounding the interior of the Gas Giants through oscillations

Jupiter and Saturn played a major role in the formation and evolution of the Solar System. The internal structure of the gas giants was shaped during their birth process and still contains traces of this formation. A detailed description of the internal structure would be key for understanding physical properties, Equations of State, high-pressure physics, effects of rotation, heat transfer, interaction between interiors and atmospheres, and finally would be a necessary step for the study of extra solar planets.

The measurement of gravitational moments by space missions can give a view of the internal structure. However, it is only sensitive to the external layers. Despite recent progress provided by the spacecrafts Juno and Cassini, our knowledge of the repartition of pressure, temperature and composition inside giant planets remains sparse and would not permit a unique model solution. The situation for Uranus and Neptune is even worse as these planets have not received any visit since Voyager. Among the different methods able to reveal the internal structure of planets, seismology is certainly the most powerful. A complete set of acoustic oscillations could give a profile of the sound speed and the rotation rate along the radius and latitudes. However, it requires long continuous observations, difficult to achieve from the ground.

So far, seismic measurements of Saturn have been obtained thanks to structure in the rings observed by Cassini and attributed to resonances with surface waves (f modes) of the planet. Ring seismology is very powerful as it allows recording of oscillations for many years, giving sensitivity and precision on the frequencies not achievable with other means. The analysis of these frequencies has shown the existence of stable layers inside Saturn. This fantastic result could not be extended further because of the small number of modes able to resonate with the rings. Gravity measurements from Cassini and Juno also proved the existence of acoustic modes on Jupiter and Saturn with detectable amplitudes but could not measure individual frequencies.

Many efforts were made to observe these modes through different techniques. The most promising is the monitoring of Doppler velocity images of the planet with a dedicated instrument. In 2011, a time series recorded with this method permitted the determination of a fundamental frequency present in the structure of the power spectrum and compatible with our knowledge of the Jovian internal structure. With a similar instrument, recent observations of Jupiter provided measurements of the 3 components of the wind at the surface of Jupiter, demonstrating its sensitivity. However, ground-based observations remain limited in resolution and duration. Such an instrument on a dedicated space mission would give unvaluable information.

During this talk we will present previous attempts to measure oscillations on Jupiter and Saturn from Earth and space. We will show the results and the remaining questions, including mode excitation and amplitudes. We will also review the present instrumental development and the most promising projects. Finally, we would derive a possible strategy for future observations in the frame of the international space mission context.