Fresh lunar crater ejecta as revealed by the Miniature Radio Frequency (Mini-RF) instrument on the Lunar Reconnaissance Orbiter

Samuel W. Bell ‘11

Submitted to the Department of Astronomy of Amherst College in partial fulfillment of the requirements for the degree of Bachelor of Arts with honors.

Advisor: Darby Dyar

Readers: George Greenstein and Peter Crowley

May 5, 2011

Abstract

On timescales of tens to hundreds of millions of years, micrometeorite, solar wind ion, and cosmic ray bombardment gradually erode the ejecta blankets that form around small lunar impact craters. Sensitive to surface roughness on the scale of its 12.6 cm wavelength, the 30 m/pixel Miniature Radio Frequency (Mini-RF) instrument provides detailed imagery of the ejecta blankets of small with unprecedented resolution and quality, allowing large numbers of ejecta blankets to be studied with a higher degree of precision than previously possible (Thompson et al., 1981; Nozette et al., 2010; Neish et al., 2011). Using well-established crater-counting techniques

(Arvidson et al., 1979; Michael and Neukum, 2010), analysis of the lifetime of the discontinuous portion of the ejecta blanket at varying crater diameters shows that the discontinuous ejecta lifetime is proportional to the square of the crater diameter.

Absolute dates of individual craters can be estimated by combining the empirically derived function for discontinuous halo lifetime with estimates of what fraction of its lifetime each discontinuous ejecta blanket has lived through. Cosmic ray exposure ages of craters visited by the Apollo missions provide confirmation of these results: The resultant lifetime model predicts that the discontinuous ejecta blanket around the 25-30

Ma Cone Crater (Turner et al., 1971) will have vanished after 8.7(+1.5/-1.7) Ma, and the discontinuous ejecta blanket is indeed absent. This method produces a radiometrically determined age estimate of 54(+39/-29) Ma North Ray Crater, consistent with the known age of 50.0±1.4 Ma (Arvidson et al., 1975).

i Table of Contents Abstract ...... i

1. Introduction ...... 1 I Dating individual craters ...... 4 II Dynamics of ejecta degradation ...... 4

2. Literature Review ...... 6 I.1 The —history and general features ...... 6 I.2 The Moon—surface water ...... 7 I.3 The Moon—erosion processes ...... 9 II.1 The impact process—the basics ...... 11 II.2 The impact process—ejecta properties ...... 14 II.3 The impact process—oblique impacts ...... 14 III.1 Radar observation—the basics ...... 17 III.2 Radar observation—S1 imagery ...... 18 III.3 Radar observation—CPR imagery ...... 21 III.4 Radar observation—topographic effects on position information ...... 23 IV.1 Optical crater dating—Trask (1971) method ...... 25 IV.2 Optical crater dating—erosion modeling method ...... 28 IV.3 Optical crater dating—space weathering ...... 30 V Radar-bright halos ...... 31

3. Methods ...... 35 I Crater counting ...... 35 I. Crater counting—R-plot analysis ...... 35 II.1 Data sets ...... 37 II.2 Datasets—initial search and highlands focus region ...... 39 II.3 Datasets—Mare Serenitatis and 2400s focus regions ...... 42 III Age quantification ...... 42 IV Image processing ...... 43 V Radial brightness profiles ...... 45

4. Results ...... 47

ii I Estimating Discontinuous Halo Lifetimes From Crater Counting ...... 47 II Estimating Ages From Discontinuous Halo Lifetimes and Diameters ...... 53 III Morphology Variation of Degrading Craters ...... 53 IV.1 Comparison With Apollo Results—Cone Crater ...... 63 IV.2 Comparisons With Apollo Results—North Ray Crater...... 67 V Thompson et al. (1981) comparison ...... 69

5. Discussion ...... 76 I.1 Secondary Cratering ...... 76 I.2 Wells et al. (2010) Ejecta Morphologies—Diagnostic of Secondaries? ...... 77 II Problems With Halo Diameter Quantification .