Constraining the Ephemeris and Interior Structure of Io using Space-Based Astrometry by JUICE

Abstract

Owing to the assumed presence of sub-surface oceans, the Galilean satellites - Io, Europa, Ganymede, and Callisto - are among the most promising candidates for potential extraterrestrial habitats within our Solar System. To this end, the moons are going to be extensively studied by the upcoming JUICE and Europa Clipper missions. Ultimately, understanding the dynamics of the Jovian system is a means to shed light on the general existence and stability of these presumed habitable worlds as well as the formation and evolution of the entire Solar System. Yet, while the dynamics of Ganymede (in particular via the orbital phase of JUICE) and Europa (mainly via the various flybys of Europa Clipper) are going to be observed to an unprecedented level of accuracy, the absence of flybys of Io due to its harsh radiation-environment results in a significantly imbalanced data set. To stabilise the numerical data inversion, spaced-based imaging by the camera subsystem of JUICE is crucial to constrain the dynamics of Io.

However, while the analysis of orbital dynamics is usually performed with respect to the centre-of-mass (COM) of natural satellites, optical space-based astrometry provides measurements of the position of a body's centre-of-figure (COF), introducing a discrepancy between the dynamical and observational model. Explicitly accounting for the offset between the observed centre-of-figure and the propagated centre-of-mass during ephemeris estimation, however, ensures the consistency of the dynamical and observational model. In turn, this allows us to assess the extent to which optical space-based astrometric observations might either validate the merely indirectly obtained radio science data or contribute to the overall orbital solution of Io. Finally, obtaining a measure of the offset between the centre-of-figure and centre-of-mass yields an entirely new constraint on the interior structure and composition of Io.

In order to quantify the COF-COM-offset, we have simulated optical astrometric observations by JUICE and subsequently determined the formal uncertainties of the estimated offset using covariance analyses. Using suitably computed extit{a prioir} covariance matrices, we have constrained our analyses to the averaged propagated formal errors of Io that would arise from the radiometric tracking set-up of JUICE and Europa Clipper. We have found that the contribution of optical space-based astrometry to the COF-COM-offset of Io and its estimated state highly depends on the observations' quantity, quality, and geometry. Thus, an algorithm for the selection epochs at which images are to be simulated based on the main drivers - the absolute uncertainties and relative geometries of a series of observations - of the formal errors COF-COM-offset has been developed. However, owing to the largely equatorial alignment of JUICE with respect to Io - observations of the in-plane contribution have been found to be obstructed by the brightness of Jupiter. To maximise the scientific return of optical space-based astrometry, in particular, astrometry during the high-inclination phase has proven beneficial.

Overall, significant constraints of the discrepancy between the centre-of-figure and centre-of-mass and the orbital solution of Io have been obtained. For an expectable number of about 1300~images being taken of Io, realistically attainable formal uncertainties in the estimated COF-COM-offset of no more than 300~metres have been obtained. Furthermore, since notable contributions to the orbital solution already occur for reasonable radio science true-to-formal-error ratios between two and five, we have concluded a high likelihood of space-based astrometry contributing to the orbital solution. This potential of space-based imaging to balance and contribute to the orbital solution of Io thus motivates future research concerning the offset between the centre-of-figure and centre-of-mass.