Geometrical Characteristics of Microchaos on Europa and Relation with Regional Stress Field

Abstract:

Microchaos has been hypothesized to formed via cryovolcanism, diapirism, and melt-through of the icy shell on Europa. Here we use the microchaos map completed by Noviello et al. [2019] to conduct a geometrical analysis of these features, focusing on the area, aspect ratio (ellipticity) and orientation of microchaos. The mean orientation of the microchaos identified within the -60° to 60° latitude and 110° and 130° longitude area (trailing hemisphere) is N58°E ± 42°. Most microchaos in the northern trailing hemisphere have orientations close to that average and are rather small. Microchaos in the southern trailing hemisphere are fewer and tend to be larger. In that region the mean microchaos orientation N110°E ± 40°. Microchaos size is a powerful discriminant of microchaos orientation: Microchaos with an area less than 50 km² are preferentially oriented N50°-70°E whereas microchaos with an area larger than 50 km² are often oriented between N160°E and N180°E. Microchaos orientation were then compared to the orientation of bands within the same area. Bands were determined by the traits described by Prockter et al. [2002]. Misorientation, defined as the difference between the azimuth of the long axis of microchaos and its nearest band is on average less than 20°, and there is no systematic relationship between misorientation and the distance between microchaos and the nearest band. Thus microchaos orientation appears to generally obey the same stress field as the bands, although some microchaos-band pairs have very high misorientation. As microchaos are typically aligned parallel to the bands, diffuse strain or breakout of a semicircular vertical conduit cannot explain their elongation. Instead is it possible that the microchaos follows a band-parallel crack, whether preexisting (tectonic origin) or form simultaneously to microchaos emplacement (volcanic dike). In one example, a chain of microchaos is clearly linked by a fracture, although it is impossible to identify the origin of the fracture. The presence of two microchaos populations, one consisting of small, ENE-oriented objects and the other of larger, N-S oriented objects, may be explained if each population corresponds to different volcano tectonic episodes with different stress fields and different ice shell structure.