DPS 42nd Meeting Abs..

secular spin-orbit resonance mechanism generally suffice to lt Jupiter more than is observed. Moreover, long migraon mescales which make lng Saturneasier simultaneously reduce the survival fracon (to below 20% for mescales longer than 20 Myr.) We discuss the constraints these observaons provide on thedynamical history of the giant planets, and the remaining possibility of lng Saturn during a late very slow migraon of Neptune to its present locaon aer themain phase of migraon is complete. [This work was supported by Hong Kong RGC grant HKU 7024/08P.]04.05: A Crical Lunar and Planetary Period ~ 4 Gy Ago - Independent of Dynamical Cataclysm ModelsAuthor Block: William K. Hartmann 11 Planetary Science Instute.Presentaon Time: 10/4/2010 11:10 AM - 11:20 AMLocaon: Ballroom DAbstract: Lunar crater counts at various landing sites show that the lunar cratering rate was declining from about 3.8 to 3.3 Gy ago, as found independently bydifferent workers [1,2,3]. This means saturaon cratering was reached in much shorter intervals around 3.8 Gy ago (and probably 3.9-4.1 Gy) than today. Thiscreates a crical period in terms of surface sample properes.Geometric consideraons alone show that as crater densies approach saturaon levels, the depth of pulverizaon, reworking, and regolith producon increasesexplosively [4,5]. Thus, while some 5 to 20 meters of regolith have been produced on mare surface in the last 3.5 Gy, such depths would have been reworkedmany mes over in intervals as short as 30 My at mes around 3.8 Gy and probably before - whether a Nice-style cataclysm happened or not.The conclusion is that rocks placed on the surfaces of airless inner solar system worlds before 3.8-4.1 Gy ago have much lower probability of surviving intact unltoday, than rocks delivered aer 3.8 Gy ago. This statement explains some properes of rock collecons from surface sites of different ages.References: [1] Hartmann, W.K. 1972. Astrophysics and Space Sci. 12:48-64. [2] Neukum, Gerhard 1983 Habilitaon Dissertaon, Ludwig-Maximilians-University,Munich. [3] Neukum, G., Boris Ivanov, and W. K. Hartmann 2001. Space Sci. Rev., 96:55-86. [4] Hartmann, W. K. 1980. In Proc. Conf. Lunar Highlands Crust, ed. J.Papike and R. Merrill. (N.Y.: Pergamon Press), pp. 155-171. [5] Hartmann, W. K. 2003. Meteorics and Planet. Sci. 38:579-593.04.06: The Structure of the Pre-Lunar DiskAuthor Block: William R. Ward 11 Southwest Research Inst..Presentaon Time: 10/4/2010 11:20 AM - 11:30 AMLocaon: Ballroom DAbstract: In the giant impact model of lunar formaon (Cameron & Ward 1976), the Earth is surrounded by a debris disk composed primarily of silicate materialimmediately following the impact event (e.g., Canup 2004). The dynamical evoluon of the disk is driven by gravitaonal instabilies that generate an effecveviscosity, causing the disk to spread (Ward & Cameron 1978; Takeda & Ida 2001). A poron of the material expands across the Roche boundary where it caneventually accrete into the Moon, while the remaining material is re-accreted by the Earth. The spreading rate is regulated by the disk's radiaon budget througha complex two-phase gas/magna equilibrium state that maintains the radiaon temperature near 2000 K (Thompson & Stevenson 1988). The vercal structure ofthe disk must sasfy the Clausius-Clapeyron phase equilibrium equaon as well as equaons of hydrostac and convecve equilibrium. The two phase disk has amodified sound speed that depends on the mass fracons of gas versus liquid, and consequently, there is a range of possible structures that depend on thesefracons. We will examine under what condions different disk structures are likely to have prevailed during the early history of the system. The resulngconsequences of various disk histories on the dynamics and ming of the Moon's accreon will also be considered.This research is supported by funds from the NASA Lunar Science Instute (NLSI) and from NASA's LASER program.04.07: Minimal Lunar Cataclysm from Tidal Disrupon of Mesosiderite Parent BodyAuthor Block: Maja Cuk 11 Smithsonian Astrophysical Observatory.Presentaon Time: 10/4/2010 11:30 AM - 11:40 AMLocaon: Ballroom DAbstract: The Lunar Cataclysm 3.9 Gyr ago is sll a controversial subject more than 30 years since its discovery. Researchers are sll divided over theinterpretaon of crater counts and the provenance of Apollo samples. Surprisingly, lunar basin magnezaon has not been used yet to constrain basin formaonhistory. I will show that basin magnezaon data strongly support a minimal cataclysm (which includes only Hertzsprung and Imbrian basins). Available craterSFD data are also consistent with this picture. This event is not consistent with delayed planetary migraon but may indicate a disrupon of a single body. Wecalculate that a survival of a Vesta-sized primordial Mars-crosser unl this era is possible if the inial populaon was about 0.01 Earth masses (Boke et al.2007, in contrast, studied only bodies with q