Other
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p31d..01d&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P31D-01
Other
[5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5450] Planetary Sciences: Solid Surface Planets / Orbital And Rotational Dynamics, [6280] Planetary Sciences: Solar System Objects / Saturnian Satellites
Scientific paper
The ridge on Saturn's satellite Iapetus is truly peculiar. Up to ~20 km tall and ~100 km wide, the ridge spans > 75% of the circumference of the moon, is very straight, and sits exactly on the equator. Endogenic and exogenic formation mechanisms have been proposed, with most effort being directed to endogenic models. In these, a global to regional stress field exists and ostensibly results in the formation of the ridge; however, these models suffer from the lack of a physically plausible mechanism that can produce extreme tectonic localization, yielding such a narrow feature but leaving the rest of the surface largely undeformed. There exists one exogenic model: a collapsed primordial ring system around Iapetus (due to Ip). Although this model naturally explains the location, it requires a relatively massive ring. Here, we propose an alternate exogenic model: formation by means of a "giant" impact. Given the examples of the Moon and Charon, satellite formation by giant impact is conceivable. Hydrocode simulations by Canup of the formation of Charon via a relatively low-speed collision of roughly co-orbital bodies (a likely scenario for Iapetus during the satellite forming epoch) are generally consistent with intact capture, although accretion from a debris disk is also possible. Thus we propose intact capture or accretion from a debris disk of a "sub-satellite" (possibly more than one) around Iapetus. (Formation via direct descent of a close-in debris disk, without coalescence of a sub-satellite, may also be possible, although the debris would land on a warmer post-impact surface, with a lithosphere conceivably too thin to support the ridge.) Such a sub-satellite would be tidally drawn towards Iapetus, once Iapetus is sufficiently despun by tidal interaction with Saturn (or if the sub-satellite forms in a retrograde orbit). Orbital decay times range from ~105 yr at Iapetus' Roche limit to ~109 yr four times farther out, for a sub-satellite/Iapetus mass ratio of 10-3 and Q for Iapetus of 100. Once deep inside the Roche limit, tides would then tear the sub-satellite apart. The resulting debris would collisionally evolve to the equatorial plane, dissipating orbital energy and ultimately raining down on the equator (deorbiting) at subsonic speeds. The lack of numerous oblique impact craters on the ridge in even the highest resolution images indicates either very fine comminution of the sub-satellite or substantial post-emplacement modification. This model naturally explains the location of the ridge on and only on the equator of Iapetus, and provides the necessary mass. Assuming the density of ice and a high porosity, rubble-pile structure, all the mass in the ridge can be supplied by a sub-satellite ~100 km in radius, a size relative to Iapetus far smaller than the relative sizes of the Moon to the Earth or Charon to Pluto. It also is consistent with the fact that no other outer planet satellite is known to possess such a ridge. Iapetus has by far the largest Hill Sphere, relative to the size of the satellite, of any major gas-giant moon. Thus, impact formation of a sub-satellite may be facilitated by the relatively wide dynamical space around Iapetus.
Cheng Andrew F.
Dombard Andrew J.
Kay Jonathan P.
McKinnon William B.
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