Lithospheric Dilation on Europa

Lithospheric dilation on Europa has occurred at ridges, bands, and various hybrid lineaments on a global scale over a large part of the geological age of the surface. Dilational ridges (Class 2 in the R. Greenberg et al. (1998, Icarus 135 , 64–78) taxonomy) are elevated, are usually a few kilometers across, and may have a lineated or hummocky interior and a pronounced medial groove. Bands are lower and usually wider than Class 2 ridges, and may have a lineated interior with no prominent medial groove. Some lineaments have characteristics of both ridges and bands. The character of Class 2 ridges, bands, and hybrid forms suggests that they are dilational gaps in the lithosphere, filled from below, and that they constitute a morphological continuum with Class 2 ridges and bands as end-members. These relationships may be explained by a model in which external forcing superimposes a secular dilation on the tidal cycle that opens and closes cracks each Europan day, resulting in incomplete closure with accumulation and possible extrusion of new ice fill. Where the lineament ultimately falls on the morphological continuum—especially how much it is elevated above ambient terrain—depends upon the ratio of daily secular dilation to the amplitude of the cyclic tidal separation. We call this ratio the “dilation quotient”. Changes in the dilation quotient during the active life of the lineament will create variable lineament forms. One driver for dilation is tidal “walking” of strike-slip faults, which dilates linked nonparallel cracks. That process is prominent in the 800-km-long strike-slip fault Astypalaea Linea. A subsurface liquid water ocean allows the decoupling needed for horizontal displacements and is the source for the ice that fills the dilated lineaments.