A search for influences of tidal stresses on surface feature orientation on Io

As the most volcanically active body in the Solar System, Io has a surface peppered with volcanic centers (paterae) and tectonic mountains that vary widely in morphologies. The spatial relationships between these surface features have long been a subject of interest in determining the relationship between volcanism and tectonic faulting on the body, which is strongly influenced by compressive stresses due to crustal subsidence. Though they are orders of magnitude weaker than the crustal subsidence stresses, tidal stresses due to the Laplace resonance between Io, Europa, and Ganymede heat Io’s interior, and possibly influence surface feature formation. Io’s mountains generally occur as isolated linear massifs rising kilometers above the surrounding plains, while paterae are rounded, often elongate depressions; both have measurable orientations on the surface. In this study, we investigate global statistical trends in the orientations of surface features and peak tidal stresses to determine if tidal stresses may have some influence on feature formation (the most intuitive relationship being feature formation perpendicular to stress direction at maximum compressive stress).

We identified previously mapped mountains and paterae with shapes visibly elongate enough to determine a long axis with a measurable orientation from North, and then further limited the dataset to those features with a calculated aspect ratio of 2/3 or lower. The peak compressive and tensile stresses due to tides were calculated at each feature location using the SatStressGUI program, with model input parameters following common assumptions about Io’s interior structure and material properties (including the presence or absence of a magma ocean). On a global scale, mountains, and to an even greater degree paterae, exhibit a preferential East-West orientation. These measured datasets were compared to simulated datasets of randomly orientated features to demonstrate that the distribution of measured feature orientations has a statistically significant population oriented within 10 degrees of East-West. Globally, there is also a statistically significant preferential orientation among peak tidal stresses to be North-South. While there are many factors that may influence the formation of individual surface features, we are continuing to investigate this intriguing relationship to determine if tidal stresses may be influencing mountain and/or patera formation.