Turbulent drag at the ice-ocean interface of Europa in simulations of rotating convection: Implications for nonsynchronous rotation of the ice shell

Europa’s geologically scarred surface shows significant evidence that the ice shell may have rotated nonsynchronously in the past. The long-term spin-state of the ice shell is controlled by the time-mean torques acting on upon it. A torque that has not been previously considered is that exerted due to drag from oceanic currents beneath the ice. We estimate this torque for the first time by performing global, non-hydrostatic, three-dimensional simulations of Europa’s ocean, including nonlinear turbulent boundary layer drag at the seafloor and ice-ocean interface. Our simulations show that ocean dynamics which manifest in alternating east-west jets result in a net torque on the ice shell. The torque can act to either spin-up or spin-down the ice shell depending on the strength of convection, suggesting that a torque reversal can occur as Europa’s interior thermally evolves. Scaling analysis indicates that an average jet speed of at least ∼1 cm/s is required for the ice-ocean torque to be comparable to the tidal torque acting to spin-up the ice shell. Our results suggests that ocean currents may contribute to any nonsynchronous rotation of the ice shell. Consequently, Europa’s present day spin-state may hold information about the dynamics of its subsurface ocean.

木卫二（Europa）地质上布满疤痕的表面，提供了冰壳在过去可能存在非同步旋转的显著证据。冰壳的长期自旋状态由作用于其上的时间平均力矩控制。此前未被考虑的力矩是冰下洋流拖曳产生的力矩。我们首次通过对木卫二海洋进行全球非静压三维模拟（包含海床与冰-海界面的非线性湍流边界层拖曳）来估算该力矩。模拟结果表明，以东西交替射流为表现形式的海洋动力学过程会对冰壳产生净力矩。该力矩可根据对流强度加速或减速冰壳旋转，表明随着木卫二内部的热演化，力矩方向可能发生反转。尺度分析显示，当平均射流速度至少达到约1cm/s时，冰-海力矩才能与加速冰壳旋转的潮汐力矩相当。研究结果表明，洋流可能是冰壳非同步旋转的成因之一。因此，木卫二当前的自旋状态或可反映其地下海洋的动力学特征。