Thicknesses of and Primary Ejecta Fractions in Basin Ejecta Deposits Larry A. Haskin and William B. McKinnon Department of Earth and Planetary Sciences,

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Thicknesses of and Primary Ejecta Fractions in Basin Ejecta Deposits Larry A. Haskin and William B. McKinnon Department of Earth and Planetary Sciences, Washington University, St. Louis

Why would a geochemist attempt to do ejecta deposit modeling? From where on the Moon did the materials sampled by the Apollo and Luna missions come? Mostly beneath the sites? Or mostly from a long way off? Did Th-rich KREEP form as a global layer on the Moon? Or was most of the Th we find at the Moon’s surface ejected from the Procellarum KREEP Terrane when the Imbrium basin formed? Which basins did the samples of crystalline breccia dated by geochronologists come from? Several? Or mainly from Imbrium?

Our approach to ejecta deposit modeling: Desired output: ejecta deposit thickness and the fraction of ejecta in the deposits. Assume ballistic cratering (Oberbeck, Morrison, Hörz). Concatenate results from several types of cratering studies to estimate average properties of ejecta deposits.

Steps in the modeling: 1.Select a basin, select a sampling site, and find the distance between them. 2. Estimate the total ejected volume as that of a paraboloid using the transient crater radius of the basin and d/D = 0.1, less ~10%, e.g., Melosh. 3. Estimate the ejecta thickness at the sampling site: Housen et al.; map to sphere using ejecta angle and velocity. 4. Estimate the mass distribution of primary fragments: M T -0.85, from Hartmann, Melosh, Turcotte. 5. Constrain the largest fragment size: M T 0.8, O’Keefe & Ahrens; decrease with distance: v -2, Vickery.

Steps, continued: 6. Calculate the mass and number of primary fragments in each size range. 7. Secondary crater diameters from Schmidt-Holsapple scaling; excav. volumes as paraboloids with d/D = Determine the fraction of the area excavated as craters of each size range; Garwood (bomb craters). 9. Estimate excavation efficiency on the basis of the largest primary fragment to excavate in any spot; calibrate to data for Orientale and Ries. 10. Result: the areal distribution of deposit thicknesses and % of primary material in deposits around the site of interest

Two points per crater on this diagram; they do not mutually agree.

Craters near the Apollo 16 site (from Jeff Gillis) Crater diam. (km) fill (m)Crater diam. (km)fill (m) Abulfeda Kant G26884 Kant D Zollner D24547 Descartes Unnamed17992 Zollner47 627Abulf. C Taylor Kant B Taylor A40 109Dolland Y Andel Andel A Dolland B Unnamed Lindsey For fresh cratersAverage 1500  700 For degraded cratersAverage 750  350 Modeled: 2.2 km (CL10%) 1.1 km (CL50%) <0.50 km (CL90%)

Conclusions: 1.The model gives reasonable deposit thicknesses (after empirical calibration). 2. The model gives reasonable estimates of the fraction of ejecta in those deposits. 3. The results of the modeling are somewhat sensitive to ejection angle and to the size distribution exponent. 3. The model overpredicts the density of observed secondary craters and underpredicts their size range.