Exploring the Interior of Europa with the Europa Clipper

Abstract

The Galileo mission to Jupiter revealed that Europa is an ocean world. The Galileo magnetometer experiment in particular provided strong evidence for a salty subsurface ocean beneath the ice shell, likely in contact with the rocky core. Within the ice shell and ocean, a number of tectonic and geodynamic processes may operate today or have operated at some point in the past, including solid ice convection, diapirism, subsumption, and interstitial lake formation. The science objectives of the Europa Clipper mission include the characterization of Europa's interior; confirmation of the presence of a subsurface ocean; identification of constraints on the depth to this ocean, and on its salinity and thickness; and determination of processes of material exchange between the surface, ice shell, and ocean. Three broad categories of investigation are planned to interrogate different aspects of the subsurface structure and properties of the ice shell and ocean: magnetic induction, subsurface radar sounding, and tidal deformation. These investigations are supplemented by several auxiliary measurements. Alone, each of these investigations will reveal unique information. Together, the synergy between these investigations will expose the secrets of the Europan interior in unprecedented detail, an essential step in evaluating the habitability of this ocean world.

Keywords: Europa Clipper; Ice-penetrating radar; Interior; Magnetic induction; Subsurface ocean; Tidal deformation.

Fig. 1

The left-hand side of the figure shows a cutaway view of Europa’s interior. From this image it is evident that despite the surficial appearance, Europa is not truly an icy moon. Rather, it is a rocky body covered in ice. The ice shell and underlying ocean form a thin (∼100 km thick) veneer of volatiles overlying a rocky mantle and metallic core. On this global scale, the key physical process that occur are the tidal dissipation in the lower ice shell and mantle, zonal and meridional flows in the ocean, and the induced magnetic field caused by the body’s passage through Jupiter’s variable magnetic field (shown in the background). Locally, most of the important features and processes occur in the ice shell and ocean, shown in the inset. Here, the ice shell is shown not as homogeneous, but highly variable. The cold brittle ice near the surface lies on top of warmer, ductile material below that is heated unevenly by tides. This may drive subsolidus convection in the ice shell resulting in upwelling ocean ice diapers, formation and re-freezing of melt lenses, and diurnal stresses. These processes manifest at the surface in the form of cycloids, double ridges, and chaos terrains

Fig. 2

Map of Europa from USGS (Image source: https://astrogeology.usgs.gov/maps/europa-voyager-galileo-global-mosaics) with Europa Clipper groundtracks and closest approach points for the baseline trajectory at the time of publication (Rnd7_T1_E49)

Fig. 3

Geometry of Jovian magnetic field and induced magnetic field at Europa. (Top) As Jupiter rotates, the magnetic field in Europa’s frame varies at the 11.2 hour synodic period of Jupiter (i.e., the time required for Jupiter to return to the same geographic longitude as observed from Europa) due to the 9.6° tilt of Jupiter’s magnetic axis with respect to its rotation axis. (Bottom left) Because the synodic variation is primarily confined to Europa’s orbital plane, the induced magnetic moment, and the associated dipolar field, rotates approximately in Europa’s equatorial plane at the 11.2 hour period. (Bottom middle) Europa’s orbital eccentricity causes additional variation in the magnetic field at its 85 hour orbital period, which generates an induced magnetic moment, and associated dipolar field, approximately aligned with or against Europa’s spin axis. (Bottom right) The total magnetic field at Europa consists of Jupiter’s strong time-varying magnetic field and Europa’s induced dipolar magnetic field at multiple frequencies. Not pictured is the magnetospheric plasma corotating with Jupiter near the magnetic equator, which sweeps past the moon from the trailing side and complicates the interpretation of the measured magnetic field

Fig. 4

Graphical representation of the Doppler tracking of the Europa Clipper flight system using the DSN, including sources of radio noise

Fig. 5

Spectrum of the forced libration of Europa. Adapted from Rambaux et al. (2011)

Fig. 6

The amplitude response as a function of ocean conductivity, ocean thickness, and ice shell thickness for a three-layer model. The range of response factor deduced by Schilling et al. (2004) are marked by horizontal dotted lines. The upper limit imposed on the conductivity of the solution from saturation effects are marked by the two vertical lines. Figure reproduced from Hand and Chyba (2007)

Fig. 7

The dipolar surface induction field created by the interaction of Europa with Jupiter’s varying field at the two principal frequencies (T = 11.1 h and T = 85.2 h) for a range of conductivities and ocean shell thicknesses.

Fig. 8

Overlapping measurements combine to constrain the ice shell and ocean thickness. In the example illustrated here, the “true” mean ice shell thickness is 20 km, the upper 6 km of which is rigid. The subsurface ocean is 60 kim thick and has a conductivity of 1 S/m. See text for additional details.