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

Burkhard, Liliane M.L.; Thomas, Nicolas (2024). 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. Icarus, 416 Elsevier 10.1016/j.icarus.2024.116088

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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 tim
e. 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.

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