Impact-induced Contamination of Ganymede’s Deep Ice Shell

Abstract Juno flew by Ganymede in 2021 June and measured the microwave brightness temperatures of the ice shell at six frequencies from 0.6–22 GHz. Based on the increasing brightness temperature with wavelength, S. Brown et al. (2023) derived a geothermal temperature gradient of 1 K km −1 . However, this assumes a very high purity ice shell up to ∼20 km thick. In this work, we estimate the amount and extent to which impurities introduced by cometary impactors into Ganymede’s ice shell will affect the attenuation, which has implications for the temperature gradient. We assume that in each impact event, the broken-up impactor and target material will reaccrete onto the surface. Many impact events thus emplace a surface layer that is a nearly homogeneous mix of the impactor (∼50/50 ice/rock by mass) and disrupted target material. We use estimates of impactor flux and size spectrum from the literature to calculate that this globally emplaced layer is 3.8–107.0 km in thickness with impurity (silicate) concentration 3.6−5.2 mass%. The consequent increase of attenuation implies a temperature gradient ≫1 K km −1 , which is unrealistically large given the current heat source of Ganymede is the radiogenic heat production of the rocky component. Therefore, Juno microwave results and our calculations of impact-induced dirtiness of the ice shell together place a strong constraint on the upper bound of the dirt in the ice shell and, hence, an upper bound on the total impactor flux on Ganymede from the time of the late heavy bombardment period to be <5.5 × 10 19 kg.