TIDAL‐TECTONIC PROCESSES AND THEIR IMPLICATIONS FOR THE CHARACTER OF EUROPA'S ICY CRUST

Abstract

Jupiter's satellite Europa is covered by an icy crust, likely overlying a deep, global liquid water ocean. The mutually dependent relationship between the orbit and tidal processes controls Europa's rotation, heating, and stress. Observations of the surface and investigation of tidal‐tectonic processes indicate that the dominant types of surface terrain have been created by frequent repeated exposure of an underlying ocean to the surface. Surface lineaments, including ubiquitous cycloidal (chains of arcs) features, are correlated with tidal stress patterns, demonstrating that they form by crustal cracking but only if a substantial ocean is present to give adequate tidal amplitude. Stratigraphy of tectonic features, combined with models of how and where they formed, demonstrates a moderate rate of nonsynchronous rotation. Tidal driving of strike‐slip displacement (in which surface areas shear passed one another) by daily (85 hours on Europa) tidal stresses suggests that cracks penetrate to a liquid layer. The characteristic ridge sets that cover tectonic terrain may be built by tidal pumping of fluid and slush to the surface on a daily basis. Widespread tectonic dilation creates new surface as material rises from below. In addition to those tectonic terrains, nearly half of the surface is chaotic terrain, with morphology and other characteristics indicative of melt‐through from below, consistent with the tectonic indications of thin ice and plausible tidal heating rates. Formation of both chaotic and tectonic terrains has continually resurfaced the satellite, while connecting the ocean to the surface. Surface colorants correlate with locations where ocean water reached the surface, such as along large‐scale ridge systems and around chaotic terrain. As a result of tides, liquid water may have bathed crustal cracks and surfaces with heat, transporting and mixing substances vertically. Such daily transported materials would include substances from the oceanic reservoir, oxidants, and fuels created at the surface, as well as any organisms and their chemical products. Thus a variety of habitable environments likely exist in the crust. Deactivation of individual cracks after thousands of years (due to the tidally driven nonsynchronous rotation) constrains processes like ridge building and strike‐slip displacement and would require any living organisms to adapt to such change.