Thicknesses of and Primary Ejecta Fractions

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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
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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 Moons 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?
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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.
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• Steps in the modeling
• 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
• MT-0.85, from Hartmann, Melosh, Turcotte.
• 5. Constrain the largest fragment size MT0.8,
  OKeefe Ahrens decrease with distance v-2,
  Vickery.

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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 0.10 8. 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
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Two points per crater on this diagram they do
not mutually agree.
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Craters near the Apollo 16 site (from Jeff
Gillis) Crater diam. (km) fill
(m) Crater diam. (km) fill (m) Abulfeda 65
1668 Kant G 26 884 Kant D 50
1889 Zollner D 24 547 Descartes 48
2628 Unnamed 17 992 Zollner 47
627 Abulf. C 17 2049 Taylor 42
2156 Kant B 16 1705 Taylor A 40
109 Dolland Y 14 1310 Andel 35
1544 Andel A 14 1810 Dolland B 33
1561 Unnamed 13 2059 Lindsey 32
1463 For fresh craters Average
1500 ? 700 For degraded craters Average
750 ? 350 Modeled 2.2 km (CL10) 1.1
km (CL50) lt0.50 km (CL90)
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• Conclusions
• 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.

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