10:45 〜 11:00
[PPS01-06] Inferring Europa's resurfacing history through potential impurity layers within the ice shell
★Invited Papers
キーワード：Europa, Icy Satellites, Ocean Worlds
Europa, a Galilean moon of Jupiter, possesses an outer water ice shell 3-30 km thick that overlays a saltwater ocean ~100 km deep. The icy surface of Europa records a complex history of tectonic deformation, including the exposure of interior ice at extensional bands and removal of surface material to the interior at inferred subsumption zones. These geologic processes are critical for transporting material through the brittle ice shell exterior and understanding the redox state and astrobiological potential of the interior ocean.
Europa’s young average surface age (40-90 Myr) indicates recent or extant resurfacing processes. Observations of the cross-cutting relationships of surface features indicate that deformation style has evolved throughout Europa’s visible surface history, from forming ridged plains, to tabular bands, and finally to chaos and crack formation. Based on the inferred formation mechanisms for each of these terrains, the deformation of the ice shell has progressed from distributed to discrete. This progression of deformation, could indicate that the ice-shell has thickened throughout its visible surface history. If ice-shell thickening events are recurrent throughout Solar System history, we hypothesize that thickening events may be recorded in the distribution of non-ice materials within the ice-shell and on the surface.
The distribution of non-ice materials across Europa’s surface is non-uniform and, in most cases, higher concentrations of salt occur in discrete regions associated with geologic structures such as bands and chaos. Therefore, non-ice features on Europa’s surface may be linked to the exposure of material originating in the ice shell or subsurface ocean. In the case of ice-shell thickening, as the ice thickness changes, the amount of non-ice material incorporated into the ice from the ocean depends to first-order on how quickly the ocean freezes. As such, the distribution of non-ice materials may reflect the evolution of the ice shell as it thickened and new material froze in. Later tectonic processes may deform the ice shell, sampling compositional variations that are then exposed at the surface
In order to understand what compositional variations may arise from a thickening ice shell and the associated surface exposure, we numerically model ice shell evolution and deformation. We simulate the interaction between an outer ice shell and a mock interior ocean to create cross-sectional maps of historical freezing rate at the time of ice incorporation to the shell. Using freezing rate as an analog for non-ice incorporation, we infer the distribution of non-ice impurities within the ice shell. Following Howell and Pappalardo (2018), we extend the finite element code SiStER (Simple Stokes solver with Exotic Rheologies) to simulate the visco-elasto-plastic behavior of ice I above a simulated ocean. We include partial melting and freezing that affects the density and mechanical behavior of particles within the finite difference mesh. For particles transitioning from the ocean to the ice shell, we record the maximum freezing rate ever experienced as an indicator of potential impurity incorporation.
Here, we will show the inferred spatial and temporal changes in Europa’s ice shell composition from models of freezing rate of ocean water at the time of incorporation into Europa’s ice shell. We interpret these predictions of inferred global brine horizons to reflect the accretion history of incorporated ice. Non-ice distributions may record geologic history and interior heat flux, and might constrain whether the ice shell interior is convecting. Future robotic exploration missions to ocean world ice shells, like NASA’s planned Europa Clipper mission and ESA’s planned JUICE mission, may test whether such thickening events are recorded by compositional variations within the ice shell.
Europa’s young average surface age (40-90 Myr) indicates recent or extant resurfacing processes. Observations of the cross-cutting relationships of surface features indicate that deformation style has evolved throughout Europa’s visible surface history, from forming ridged plains, to tabular bands, and finally to chaos and crack formation. Based on the inferred formation mechanisms for each of these terrains, the deformation of the ice shell has progressed from distributed to discrete. This progression of deformation, could indicate that the ice-shell has thickened throughout its visible surface history. If ice-shell thickening events are recurrent throughout Solar System history, we hypothesize that thickening events may be recorded in the distribution of non-ice materials within the ice-shell and on the surface.
The distribution of non-ice materials across Europa’s surface is non-uniform and, in most cases, higher concentrations of salt occur in discrete regions associated with geologic structures such as bands and chaos. Therefore, non-ice features on Europa’s surface may be linked to the exposure of material originating in the ice shell or subsurface ocean. In the case of ice-shell thickening, as the ice thickness changes, the amount of non-ice material incorporated into the ice from the ocean depends to first-order on how quickly the ocean freezes. As such, the distribution of non-ice materials may reflect the evolution of the ice shell as it thickened and new material froze in. Later tectonic processes may deform the ice shell, sampling compositional variations that are then exposed at the surface
In order to understand what compositional variations may arise from a thickening ice shell and the associated surface exposure, we numerically model ice shell evolution and deformation. We simulate the interaction between an outer ice shell and a mock interior ocean to create cross-sectional maps of historical freezing rate at the time of ice incorporation to the shell. Using freezing rate as an analog for non-ice incorporation, we infer the distribution of non-ice impurities within the ice shell. Following Howell and Pappalardo (2018), we extend the finite element code SiStER (Simple Stokes solver with Exotic Rheologies) to simulate the visco-elasto-plastic behavior of ice I above a simulated ocean. We include partial melting and freezing that affects the density and mechanical behavior of particles within the finite difference mesh. For particles transitioning from the ocean to the ice shell, we record the maximum freezing rate ever experienced as an indicator of potential impurity incorporation.
Here, we will show the inferred spatial and temporal changes in Europa’s ice shell composition from models of freezing rate of ocean water at the time of incorporation into Europa’s ice shell. We interpret these predictions of inferred global brine horizons to reflect the accretion history of incorporated ice. Non-ice distributions may record geologic history and interior heat flux, and might constrain whether the ice shell interior is convecting. Future robotic exploration missions to ocean world ice shells, like NASA’s planned Europa Clipper mission and ESA’s planned JUICE mission, may test whether such thickening events are recorded by compositional variations within the ice shell.