Lineament azimuths on Europa: Implications for obliquity and non-synchronous rotation

Lineaments are thought to form as tensile cracks due to tidal stress, which is driven mainly by Europa's eccentric orbit. However, this model would not produce the wide range of lineament azimuths observed on Europa unless the stress in a given region, or the conditions for fault failure, change over time. In this work, we test the ability of two mechanisms that would alter the stress field over time to account for the observed lineament azimuths: non-synchronous rotation and spin pole precession. First, we revisit previous analysis of lineaments and find that an underlying assumption used to predict their azimuths was inconsistently applied. After revising these predictions, we incorporate the effects of a non-zero obliquity, which has been shown to influence the formation of other tidal-tectonic features. We then expand our analysis to include the effects of the time-variable phenomena, spin pole precession and non-synchronous rotation of the ice shell. We also consider additional failure assumptions to those used in previous work on lineament azimuths. We test our models against the azimuths of observed lineaments in the Bright Plains region of Europa. Without obliquity, we find that non-synchronous rotation is insufficient to explain the wide range of azimuths observed in this region. In the presence of obliquity, we find that either spin pole precession or non-synchronous rotation could produce wide variations in lineament azimuths. However, neither model can independently account for the observed distribution of azimuths in the Bright Plains region. In fact, a model in which all of the lineaments are assumed to form at random orientations outperforms the non-synchronous rotation model in our statistical tests. The model with the highest likelihood of producing the observations is one in which 45% of the lineaments formed as predicted in the precession model, 55% formed at random orientations, and older lineaments are less represented in the tectonic record. Given the relative timescales expected for spin pole precession and non-synchronous rotation, it makes sense that the signal of precession is more apparent in the tectonic record. These results are in good agreement with assessments of strike-slip faults; modeling of both types of features indicates a large obliquity, the same spin pole direction during the most recent epoch of tectonic activity, a similar percentage of features that are not well-explained by the tidal model, and little evidence of non-synchronous rotation.