82-11 Probing the Evolution of a Young Ocean on Mimas with the Herschel Impact Basin

Session: Friends of Hoth, Episode IX: The Rise of Icy Ocean Worlds

Presenting Author:

Authors:

Abstract:

Mimas is the smallest (r = 198 km) and innermost of Saturn’s mid-sized moons. Its heavily cratered surface, including the impact basin Herschel (D = 139 km), has been interpreted as evidence for a largely inert geologic history. However, libration and pericenter precession measurements favor the presence of an ocean [1-2], which can be reconciled with Mimas’ surficial geology if the ocean is young (~10 million years). Here, we revisit the timeline of Mimas’s putative young ocean using impact simulations [3-5] to reassess the conditions necessary to produce Herschel. While previous work determined that a Herschel-like basin could be produced in a thick (≥ 55 km) ice shell overlying an ocean [6], thermal-orbital modeling suggests ocean expansion is so rapid that the ice shell is only ≥ 55 km for ~1 Myr [7]. Although Herschel could have formed in exactly that window, such a specific timeline motivates additional study.

To assess a broader range of pre-impact conditions, and thus expand the time window in which the basin may have formed, we consider two possibilities: 1) Mimas’ ice shell was entirely frozen, but actively warming, or 2) Mimas was not entirely frozen, and instead possessed an emerging ocean with a thick (45-65-km) but relatively cold overlying ice shell. Colder shells may be less dissipative, and thus evolve slower than previous thermal-orbital predictions [7]. We find that both fully frozen and ocean-bearing scenarios can produce Herschel. However, the required thermal conditions continue to associate the timing of the basin’s formation with the onset of an ocean. Herschel either forms during the warming that precedes melting and runaway ocean growth, or during the early stages of ocean evolution, when the ice shell is >75% of Mimas’ hydrosphere. Further reconciliation of the surface geology of Mimas with the growth of its ocean can be more fully addressed through measurements by future spacecraft, as well as additional detailed modeling of the moon’s thermal-orbital evolution.

References: [1] Tajeddine, R. et al. (2014) Science 346, 322-324. [2] Lainey, V. et al. (2024) Nature 626, 280-282. [3] Amsden, A.A. et al., (1980), LANL Report LA8095:101p. [4] Collins, G.S. et al. (2004) MAPS 39(2). [5] Wünnemann, K. et al., (2006) Icarus 180(2). [6] Denton, C.A. & Rhoden, A.R. (2022) GRL 48(2), e2020GL091596.  [7] Rhoden, A.R. et al., (2024) EPSL 635, 118689.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8101

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.