The biennial libration of Io

Abstract:

The melt fraction in Io's mantle is unknown. One way to infer the existence of a fully molten layer, or magma ocean, and constrain the thickness and rigidity of the overlying crust, is to measure Io's libration – a modulation of its spin rate resulting from Jupiter's variable gravitational torque. Although the major perturbation is diurnal and takes place at Io's orbital period, or one Ionian day (1.76 solar days), a weaker, subharmonic perturbation arises from the unique resonant configuration of the Galilean system, where Io-Europa conjunctions occur once every two orbital revolutions of Io, or two Ionian years (3.53 solar days). We show that the peak-to-peak amplitude of this biennial libration could reach 250 meters, thanks to a stronger resonant amplification with the out-of-phase free libration (5-9 solar days). If detected by IVO, Io's biennial libration would provide additional constraints on its interior.