Critical Spin as a Possible Origin of the Iapetus Ridge

Details Critical Spin as a Possible Origin of the Iapetus Ridge Critical Spin as a Possible Origin of the Iapetus Ridge

Oct 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.0103k&link_type=abstract

American Astronomical Society, DPS meeting #42, #1.03; Bulletin of the American Astronomical Society, Vol. 42, p.941

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Iapetus has a significant non-hydrostatic oblateness ("equatorial bulge") and an enigmatic ridge that encircles a significant portion of the equator. A number of hypotheses involving a higher past spin rate of Iapetus have been proposed to simultaneously explain both features, but they have difficulty accounting for the exceptional straightness and regularity of the ridge.
We suggest that by the end of its accretion Iapetus gained an extremely high spin rate, close to the critical rate, when gravity at the equator is equal to centrifugal acceleration. The small critically-spinning asteroid 66391 is known to have an oblate shape with a prominent equatorial cusp. A model of a cohesionless critically-spinning rubble pile with dry friction [e.g., Harris et al., Icarus 199, 310-318, 2009] accurately reproduced this shape. This model cannot be quantitatively applied to a critically-spinning Iapetus, because for a large body the implicit assumption of infinite shear strength of rubble-pile-forming particles fails; the overall shape of a critically-spinning Iapetus would be closer to the hydrostatic solution. However, near the surface, especially in the equatorial region, the material behavior would likely be described by cohesionless dry friction, and an equatorial cusp would form by the same sliding mechanism as for rubble pile asteroids. Tides would then reduce the spin rate down to synchronous, but, due to lithospheric strength, the relaxation stopped before it reached the hydrostatic state, accounting for the present-day relic equatorial bulge [Castillo-Rogez et al., Icarus 190, 179-202, 2007]. The equatorial cusp would remain as a ridge, be disrupted by a few early basin-forming impacts, degrade due to small-impact-initiated surface creep, but preserve its original unique straightness. Other satellites would not have similar ridges, because they are closer to Saturn, hence their despinning is quicker, and they had no chance to gain an extreme spin rate during accretion.

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