Possible Evidence of p-modes in Cassini Measurements of Saturn’s Gravity Field

Abstract

We analyze the range rate residual data from Cassini's gravity experiment that cannot be explained with a static, zonally symmetric gravity field. In this paper we reproduce the data using a simple forward model of gravity perturbations from normal modes. To do this, we stack data from multiple flybys to improve sensitivity. We find a partially degenerate set of normal mode energy spectra which successfully reproduce the unknown gravity signal from Cassini's flybys. Although there is no unique solution, we find that the models most likely to fit the data are dominated by gravitational contributions from p-modes between 500-700µHz. Because f-modes at lower frequencies have stronger gravity signals for a given amplitude, this result would suggest strong frequency dependence in normal mode excitation on Saturn. We predict peak amplitudes for p-modes on the order of several kilometers, at least an order of magnitude larger than the peak amplitudes inferred by Earth-based observations of Jupiter. The large p-mode amplitudes we predict on Saturn, if they are indeed present and steady state, would imply weak damping with a lower bound of Q > 10 7 for these modes, consistent with theoretical predictions.

Kronoseismology VI: Reading the recent history of Saturn's gravity field in its rings

“…However, in order for these sorts of global oscillations to produce detectable ring waves, they would have to generate perturbations in the planet's gravitational potential comparable to those associated with the fundamental normal modes or mixed fundamental and gravitational normal modes responsible for several other waves in the C ring (Hedman and Nicholson 2014;Fuller 2014;French et al 2016;Hedman and Nicholson 2016;Mankovich and Fuller 2021;Dewberry et al 2021). While the relative amplitudes of the various modes inside giant planets is currently an active area of research (Wu 2005;Markham and Stevenson 2018;Wu and Lithwick 2019;Markham et al 2020; Lin and Ogilvie 2021; Dewberry and Lai 2021), we are not aware of any explicit predictions for the likely range of gravitational signals generated by oscillations with the pattern speeds of the waves considered here. We therefore expect that additional work is needed to ascertain whether any of these oscillations could produce these waves.…”

Kronoseismology. VI. Reading the Recent History of Saturn’s Gravity Field in Its Rings

“…However, in order for these sorts of global oscillations to produce detectable ring waves, they would have to generate perturbations in the planet's gravitational potential comparable to those associated with the fundamental normal modes or mixed fundamental and gravitational normal modes responsible for several other waves in the C ring (Fuller 2014;Hedman & Nicholson 2014;French et al 2016;Dewberry et al 2021;. While the relative amplitudes of the various modes inside giant planets is currently an active area of research (Wu 2005;Markham & Stevenson 2018;Wu & Lithwick 2019;Markham et al 2020;Lin & Ogilvie 2021;Dewberry & Lai 2022), we are not aware of any explicit predictions for the likely range of gravitational signals generated by oscillations with the pattern speeds of the waves considered here. We therefore expect that additional work will be needed to ascertain whether any of these oscillations could produce these waves.…”

Gravity Investigation of Saturn’s Inner System with the Innovative Skimmer Concept

“…Because the accelerations were observed to peak at closest approach to Saturn, and decreased with distance, it can be assumed that they are gravitational in nature, due perhaps to longitudinal atmospheric effects or even internal oscillations (Iess et al 2019). In a recent work, Markham et al (2020) interpreted the unknown accelerations as normal modes. In this case, the accelerations would also largely decrease as the spacecraft moves away from the planet.…”