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AGU: Journal of Geophysical Research, Planets



  • Iapetus
  • Saturnian satellites
  • dynamics
  • impact phenomena

Index Terms

  • 5420 - Impact phenomena, cratering
  • 5450 - Orbital and rotational dynamics
  • 6280 - Saturnian satellites

Paper in Press


Delayed formation of the equatorial ridge on Iapetus from a sub-satellite created in a giant impact

Key Points
  • Past models do not explain 3 critical observations of Iapetus' Equatorial Ridge
  • A giant impact created a sub-satellite that tidally evolved to build the Ridge
  • Ridge formation is delayed, facilitating preservation and the lack of flexure


Andrew J. Dombard

Andrew F. Cheng

William B. McKinnon

Jonathan Kay

The great Equatorial Ridge on Saturn's moon Iapetus is arguably the most perplexing landform in the Solar System. A mountain range up to 20 km tall and sitting on the equator of Iapetus, explaining its creation is an unresolved challenge. Models of its formation must satisfy 3 critical observations: why the Ridge 1) sits exactly on the equator, 2) is found only on the equator, and 3) is thus far only found on Iapetus. We argue that all previously proposed models fail to satisfy these observations, and we expand upon our previous proposal [Dombard et al., Abstract P31D-01 presented at 2010 Fall Mtg., AGU] that the Ridge ultimately formed from an ancient giant impact that produced a sub-satellite around Iapetus. The orbit of this sub-satellite would then decay, once Iapetus itself had despun due to tides raised by Saturn, until tidal forces from Iapetus tore the sub-satellite apart. The resultant debris formed a transient ring around Iapetus, the material of which rained down on the surface to build the Ridge. By sequestering the material in a sub-satellite with a tidally evolving orbit, formation of the Ridge is delayed, which increases the likelihood of preservation against the high impact flux early in the Solar System's history and allows the Ridge to form on thick, stiff lithosphere (heat flow likely <1 mW m-2) required to support this massive load without apparent flexure. This mechanism thus explains the 3 critical observations.

Received 20 October 2011; accepted 6 January 2012.

Citation: Dombard, A. J., A. F. Cheng, W. B. McKinnon, and J. Kay (2012), Delayed formation of the equatorial ridge on Iapetus from a sub-satellite created in a giant impact, J. Geophys. Res., doi:10.1029/2011JE004010, in press.