Comment on ‘A global system of furrows on Ganymede indicative of their creation in a single impact event’ by Hirata, N., Suetsugu, R. and Ohtsuki, K. (Icarus 352, 2020): Investigating the influence of the hypothesized ancient impactor on Ganymede's orbital eccentricity

The orbital eccentricity of a satellite may undergo significant alterations over time due to gravitational interactions with its parent planet and adjacent moons, as well as through other mechanisms, such impact by a sizable object. Ganymede is notably engaged in an orbital resonance with Io and Europa, where Ganymede completes one orbit around Jupiter for every two orbits of Europa and four orbits of Io. Nevertheless, Ganymede's free eccentricity component should have damped by the present time. It has been hypothesized that Ganymede was struck by a large impactor early in its history, generating sets of concentric furrows visible in its most ancient dark terrains (Hirata et al., 2020). In this study, we investigate the size of the impactor needed to pump Ganymede's free eccentricity up to its current value. Utilizing the constraints published on a hypothesized ancient impactor, we compare the calculated dimensions of the asteroidal or cometary impactor with the calculated requisite mass needed to excite Ganymede's orbital eccentricity. Given a head-on collision with no alteration in rotational dynamics and at a speed of 20 km/s, we deduce that pumping Ganymede's eccentricity to 0.0013 necessitates an impactor mass of 2.8274 × 1019 kg, aligning with the mass estimates for the theorized ancient impactor. As Ganymede's oldest dark terrains are believed to be about 4 Ga old, a more comprehensive analysis of the impactor's influence on Ganymede's orbital dynamics and possible effect on the resonance with Io and Europa is needed.