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Thermal Segregation as a Mechanism for Darkening Ridge Troughs on Europa

Presentation #210.03 in the session Ocean Worlds: Tectonics, Surfaces, and Ionospheres (Oral Presentation)

Published onOct 23, 2023
Thermal Segregation as a Mechanism for Darkening Ridge Troughs on Europa

The surface of Jupiter’s moon Europa is heavily marked with tectonic features. Many of these features, such as the lineae in the Rhadamanthys region, display distinct reddening and darkening in the central troughs within ridges. Several theories have been proposed as possible mechanisms for this reddening, including shear heating on active faults [1] or thermal segregation [2]. In areas where topography induces significant self-heating, increased ice sublimation from warmer areas may concentrate non-ice materials in the form of lag deposits that could cause visible surface darkening and further increase temperatures due to lower albedo [3]. We model the temperatures in ridge troughs using realistic topography and use these temperatures to determine the ice sublimation and lag formation rates. We investigate whether preferential self-heating due to topography may cause sufficient thermal segregation and ice loss to significantly increase concentration of non-ice materials in troughs on Europa.

We apply a 3-d thermophysical model initially developed for the Janus mission to binary asteroids [4]. Using digital elevation models, we model the temperatures in prominent ridge structures and investigate the effect of ridge orientation as well as trough depth. Though significant, we assume sputtering is sufficiently isotropic that it does not cause albedo variations at the scale of the 0.1 – 1-km wide ridges [5]. Using modeled temperatures, we determine the ice sublimation rates at each timestep. Preliminary results suggest that trough topography can lead to higher rates of sublimation, with deeper troughs producing a larger effect. We calculate regolith lag layer thickness, and determine whether that layer is thick enough to be visible on the surface. We estimate that within steeply sloping troughs and on steep ridge slopes, the timescale of sublimating sufficient ice to achieve an optically thick layer of non-ice material is about 104 - 105 years, about 2-3 orders of magnitude shorter than non-ridged terrain. Measurements from the Europa Thermal Emission Imaging System (E-THEMIS) onboard NASA’s Europa Clipper are anticipated to yield new insights into the temperatures and thermal histories of ridge features.

References:[1] Nimmo, Francis, & Eric Gaidos. Journal of Geophysical Research: Planets 107.E4 (2002): 5-1. [2] Spencer, John R. Icarus 69.2 (1987): 297-313. [3] Bramson, A. M., et al. Journal of Geophysical Research: Planets 124.4 (2019): 1020-1043. [4] Sorli, K. C., et al. (2022). LPI Contributions, 2678. [5] Cassidy, T. A., et al. Planetary and Space Science 77 (2013): 64-73.

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