Implications of Europa's global cycloid population

Abstract : Introduction: Cycloids are arcuate features observed on the surface of Europa proposed to record the stress changes that occurred during their formation [1,2]. These features are interpreted to be tensile cracks that form due to diurnal stresses from Europa's orbital eccentricity [1,2,3]. The shapes of cycloids can, thus, be used to constrain parameters that contribute to tidal stress, such as the interior structure and rotation history of Europa. Cycloids have been mapped regionally from Voyager and Galileo images [4,5], but the only published global map of cycloids [3] did not include a database or digital map, limiting its usefulness. For this study, we completed a global map of cycloids and generated a database of cusp angle measurements to obtain new constraints on the thickness and rheology of Europa's icy shell and on the formation of different fracture types observed on Europa Background: Physical models have been developed to successfully explain the orientations and locations of many fractures observed on Europa's surface, including the arcuate paths of cycloids [6]. Early models were heavily based on an eccentricity-driven stress field; other parameters have also been considered, including stress from non-synchronous rotation (NSR) of Europa's ice shell and the possibility of Europa having a forced obliquity due to interactions with Jupiter's other large moons [1,7]. A stress field that includes obliquity and spin pole precession provides the best matches to individual cycloids [1], the orientations of lineaments [7], and the global distribution of strike-slip faults [10]. However, cycloids and lineaments have different implications for NSR; cycloids record substantial longitudinal reorientation while lineaments do not. Both cycloids and lineaments are thought to form through tensile failure, at orientations perpendicular to the maximum tensile stress direction [2]. Hence, the reason for this discrepancy is not obvious. It could indicate a change in tidal stress conditions with time, perhaps due to a change in ice shell thickness or rheology. Unfortunately, the challenging nature of cycloid modeling has limited its application to only six features whereas more than 100 lineaments have been mapped and analyzed. To further investigate the formation conditions of cycloids, and their relationship to lineaments, we measure the orientations of all observed cycloids at their cusps, when the physical model of their formation would be most similar to that of a lineament. We then compare the results to the orientations predicted by the tidal stress model most compatible with observed lineaments [7]. Our preliminary results indicate a lack Figure 1. The global distribution of cycloids on Europa, which combines features from a global mapping study (red) and features identified during the measurement phase of this study (blue), reveals two main clusters of cycloids that are offset from the equator. Green circles represent cusps whose orientations we measured; no cusps were found in areas shaded in red.
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K.J. Mohr, A.R. Rhoden, T.A. Hurford, David Dubois. Implications of Europa's global cycloid population . LPSC 2016 - 47th Lunar and Planetary Science Conference, Mar 2016, Houston, United States. ⟨hal-01274501⟩

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