An Intrusive Dike Origin for Iapetus’ Enigmatic Ridge?

Introduction: The striking equatorial ridge of Iapetus was discovered by the Cassini mission [1]. 18 km high, 100-200 km wide and 1600 km long, this ridge accurately encircles almost 1/3 of the satellite’s equator. Iapetus is also unusual in that its shape is out of equilibrium with its current slow rotation rate: Best fits to its second order harmonic shape indicate that its equatorial bulge is consistent with a 16 hour rotational period, not its current 79.33 day rotation. Several investigators have connected these two facts and suggested that the ridge is, in some way, connected with despinning [2,3]. However, the standard solution for the stresses in the lithosphere of a despun planet are consistent only with an equatorial band of strike-slip faults [4]. Compression: Compressional stresses do reach a maximum at the equator of a despun body, but their orientation is east-west, not north-south. The morphology of the Iapetus equatorial ridge, moreover, is not consistent with other compressional features observed in the solar system, such as Mercurian lobate scarps or mare ridges: These features are typically lobate in plan, asymmetric in profile and display elevation offsets from one side to another. None of these characteristics apply to the ridge of Iapetus. Extension: The features most similar to Iapetus’ ridge in length, linearity and symmetry of profile are dikes. On Earth, dikes in the MacKenzie swarm stretch thousands of km across the Canadian Shield in nearly straight lines [5]. Ridges on Europa have also been described as the result of long linear intrusions and exhibit similar forms [6]. Indeed the Cassini September 10, 2008 close flyby of Iapetus revealed hints of bifurcation in its ridge suggestive of the well bifurcated ridges on Europa (Fig. 1). The somewhat discontinuous character of the ridge crest is also similar to terrestrial dike exposures. The problem with a dike origin of the ridge is the stress field: Although the despinning stress pattern predicts north-south extension at the equator, the extensional stress in a shell of uniform thickness is actually a minimum at the equator and maximum in the polar regions, so it is difficult to see how an intrusion could have been guided so accurately to the equator. There is also the problem of how melts to fill a putative dike might have been generated. We suggest that the solution to both these problems might be related. Tidal despinning itself generates heat that, near the surface, is concentrated toward the equator [3]. Similarly, convection might also lead to concentration of rising warm currents near the equator [7], although neither mechanism concentrates heat in a zone as narrow as the ridge itself.