A SHEAR HEATING ORIGIN FOR RIDGES ON TRITON

Voyager 2 images of Triton and Galileo images of Europa show linear ridges of analogous morphologies, apparently unique to these satellites. Although much sparser in distribution, Triton’s ridges have remarkably similar morphological forms and topographic expressions to those on Europa, and both span a range from isolated troughs to double ridges to multi-ridged structures. In general, Triton’s ridges tend to be morphologically subdued compared to Europa’s ridges, but are much larger in scale.

Triton is believed to be a captured satellite based on its retrograde orbit. Following capture in a highly eccentric orbit, tidal dissipation would have reduced Triton's semi-major axis and eccentricity over time. Modeling of the circularization of Triton's orbit, Ross and Schubert (1990) show that there is a peak in dissipation during a brief period when the semi-major axis is reduced but eccentricity remains relatively high. The resulting high diurnal stresses can lead to significant shear motion, which in turn may generate shear heating. It has previously been suggested that shear heating on Europa may be responsible for the development of double ridges. We apply the shear heating model to the case of Triton, and find that the morphological expression of ridges on Triton can be reproduced using reasonable parameters, and provides a depth to the brittle-ductile transition of ~22 km.

Because diurnal stressing was important when Triton began to circularize its orbit (not too long after capture) and crater density indicates the surface is young, capture must be relatively recent (i.e., within the last billion years) if diurnal stresses are an important mechanism for ridge formation. This argues for impact-related capture rather than gas drag capture in the Neptunian nebula.