Estimates for Tethys' Moment of Inertia, Heat Flux Distribution, and Interior Structure From Its Long‐Wavelength Topography. Issue 2 (30th January 2023)

• Record Type: Journal Article
• Authors: Gyalay, Szilárd; Nimmo, Francis
• Is Part Of: Journal of geophysical research. Volume 128:Issue 2(2023)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2023-01-30
• Subjects: Tethys -- tidal heating -- isostasy -- heat flow; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2022JE007550
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: We examine if Saturn's moon Tethys may be an ocean world by assuming that spatial variations in tidal heating are responsible for thickness or temperature variations in an isostatic ice shell, which manifests as surface topography. Because patterns of tidal heating depend on average ice shell thickness and whether the shell overlies a rigid or liquid layer, we can use Tethys' long‐wavelength topography to infer its interior structure. We test a wide range of assumed parameters to hone in on the characteristics of Tethys that produce self‐consistent and physically plausible interior models. To verify our technique, we apply it to Enceladus and recover the signature of a sub‐surface global ocean with an appropriately thick ice shell and moment of inertia. Our best‐fit Tethys models require Pratt isostasy and obliquity tides, with a normalized moment of inertia 0.340–0.345 and an average surface heat flux 1–2 mW m −2 . The best‐fit basal heat flux distribution indicates that Tethys does not have an ocean. The total power inferred (4–8 GW) to produce Tethys' shape from tidal heating indicates either a highly dissipative interior or an obliquity higher than previously estimated. The topography may also be a relic of a warmer past when the obliquity was higher. Plain Language Summary: Oceans have been discovered beneath the surfaces of many icy moons, the most famous among them being Europa and Enceladus. We examine if Tethys, a moon of Saturn's, may also have a sub‐surface ocean. We cannot answer this question using the techniques employed at other worlds, as those required close flybys of spacecraft. However, the movement of Tethys within Saturn's gravity field causes tidal heating in Tethys' interior. The exact spatial distribution of tidal heating can indicate if there is a liquid or rigid layer beneath Tethys' ice shell, as well as how thick this ice shell is. This tidal heating can be inferred from the large scale topography (essentially, the shape) of Tethys. We iterate through assumed values for a series of parameters to find a scenario that best explains Tethys' interior, finding that it does not have an interior ocean. However, the amount of tidal heating we infer indicates that Tethys should have a higher tilt relative to its orbit than is currently predicted. Given how long heat takes to conduct through ice, we may be seeing the effect of a high tilt from a billion years ago. Key Points: We can infer patterns of tidal heating from long wavelength topography of an icy satellite The tidal heating pattern can indicate a moon's internal structure Tethys has no sub‐surface ocean, but could have had a larger obliquity in the past