THE FORMATION AND IMPLICATIONS OF CRATERED ISLANDS ON ENCELADUS

Enceladus has a complex geological history dominated by partial resurfacing episodes. The South Polar Terrain (SPT) is the youngest manifestation of these resurfacing episodes, as it is currently active; however, there is also evidence for past partial resurfacing in the satellite's leading and trailing hemispheres. In the southeastern region of the leading hemisphere (near 30° S, 90° W), there are relic fragments of cratered terrain encircled by younger ridged terrain. These “crater islands” exhibit evidence of rotation around vertical axes, from which we estimate the viscosity of the ice at the time of block rotation. We use the ice viscosity to assess potential heat flux and to provide constraints on other properties of the ice shell.

Our initial estimate of the rotation angle is ~30^o, based on the angle of the deflected ridges with respect to the overall sub-horizontal trend. We use 10 Ma, the youngest potential age for the Leading Hemisphere Terrain, for the time of formation. The diurnal stresses on Enceladus have a predicted maximum of ~100 kPa, so we took the stress to range from 10-100 kPa, resulting in the viscosity range 10¹⁷-10¹⁹ Pa s. This viscosity implies ductile ice at or close to the surface, which in turn suggests a high heat flow in this region at the time of formation. We can also solve for the depth of the crater island lithospheric layer (z), assuming the frictional force must be equal to the shear stress at the ductile base of the rotated block. As such, we calculate the layer thickness z ~ 1 km and the resulting heat flux ranging from 0.2 – 0.7 W/m². To keep the heat flux within a value inferred at the SPT today (~0.2 W/m²) the surface must be topped with an insulating layer to create an effective surface temperature of ~110 K instead of the ~70 K observed.

We can also calculate the average grain size of the ice using a power law rheology, which can further indicate a deformation process (e.g. sluggish lid convection). In addition to refining our calculations, we are working to employ a physical analogue model. These analogue experiments will aid in verifying our results, as we will be able to test the depth and viscosity ratio of the layers, as well as the potential existence of an overlying insulating layer.