Tortuous Ridges on Europa

Double ridges are the most common surface feature on Europa and account for both some of the oldest and newest surface features on the moon. Ridges with linear and cycloidal trajectories have been well-studied, with established hypotheses for their formation and trajectories. We find that a number of ridges consistently deviate from either linear or cycloid trajectories in an irregular pattern. Tentatively described as “tortuous” ridges, these have not been previously classified or studied in detail. To gain a better understanding of their distribution and context, we mapped ridges with tortuous, linear, and cycloid trajectories across four regions of Europa using regional Galileo SSI images having resolutions < 1 km/px and analyzed ridge trajectory using fractal dimension analysis. Fractal dimension, D, is used to characterize patterns in nature by quantifying their self-similarity or self-affinity. Through fractal analysis, we quantify the tortuosity of ridges on Europa and classify some ridges as distinctly non-linear and non-cycloidal (i.e., as tortuous). The fractal dimension of a line is D ~ 1 and Brownian noise is D ~ 1.5, so we would expect a higher fractal dimension for tortuous trajectory (i.e., 1 < D < 1.5) compared to both linear and cycloid. This analysis explores a quantitative means for classifying tortuous ridges on Europa, which would enable more focused research on the topic that could inform future models for ridge and fracture formation on Europa, and lend insight into the moon’s ice shell properties.