Possible approach to detecting the mysterious Saturnian convective dynamo through gravitational sounding

¹ State Key Laboratory of Information Engineering in Surveying, Mapping and Remote Sensing, Wuhan University, 129 Luoyu Road, Wuhan 430070, PR China
e-mail: jgyan@whu.edu.cn
² Key Laboratory of Planetary Sciences, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai, 200030, PR China
e-mail: dkong@shao.ac.cn
³ State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Taipa, Macau, PR China
⁴ Observatoire de la Côte d’Azur, Geoazur, CNRS UMR7329, 06560 Valbonne, France

Received: 13 July 2020
Accepted: 10 October 2020

Abstract

Context. Planetary dynamo research is mathematically and numerically difficult. Forward calculations are numerically expensive and subject to much uncertainty in key magnetohydrodynamics parameters. For a gaseous planet such as Saturn, even the precise location of its dynamo and typical convective strength are unknown, which further complicates studies.

Aims. We test the idea of inversely probing Saturnian convective dynamo through gravitational sounding, based on the principle that the convective fluid motion can distort the internal density distribution and hence induce the gravitational anomaly.

Methods. The Cassini Grand Finale mission has reported unprecedentedly accurate measurements of the gravitational field of Saturn. An unexplained nonaxisymmetric component of the gravitational field was detected in the data. By performing precise orbit determination (POD) simulations, we studied the possibility that the Cassini spacecraft might sense the dynamo-related nonaxisymmetric gravitational signature in the Grand Finale phase. In addition, further extensively simulated missions of various orbit configurations were carried out in order to explore promising mission strategies that might fulfill the objective of detecting the Saturnian convective dynamo.

Results. Our POD simulations show that the gravity science carried out in the Cassini Grand Finale mission is insufficient to determine weak nonaxisymmetric gravitational moments because good subspacecraft-point coverage is lacking. The origin of the unexplained Saturnian gravity remains a puzzle. However, it is positively indicated by our simulations that future gravitational sounding is probably able to detect dynamo-related gravity when the subspacecraft-point coverage of a mission is sufficient. We suggest that the mission orbits be purposely designed into a near-polar orientation with a height of about 6000 km at periapsis and a moderate eccentricity of 0.5. A total POD tracking time of five months would enable the detection of the secular nonaxisymmetric gravitational moments that are caused by the deep convective dynamo of Saturn. The orbit strategy can facilitate engineering implementation by keeping the spacecraft marginally away from the Saturn radiation belt throughout the mission.