G. EJECTA DISTRIBUTION MODEL, SOUTH RAY CRATER by GEORGE E
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G. EJECTA DISTRIBUTION MODEL, SOUTH RAY CRATER By GEORGE E. ULRICH, HENRY J. MOORE, V. STEPHEN REED, EDWARD W. WOLFE, and KATHLEEN B. LARSON CONTENTS Page Introduction …………………………………………………………………………………………………… 160 Definition of South Ray ejecta ………………………………………………………………………………. 160 Mapping techniques ………………………………………..……………………………………………. 161 Measurement of ejects- and ray-covered areas……………………………………………………… 161 Measurement of rim height and crater volume…….………………………………………………… 161 Surface observations ……………………………………………………………………………...……… 163 Distribution mod ……………………………………………………………………………………………... 166 Summary ……………………………………………………………………………………………………... 173 Acknowledgments ……………………………………………………………………………………………. 173 ILLUSTRATIONS Page FIGURE 1. Premission photomosaic map of Apollo 16 landing site ………………………………………………………………………………… 162 2. Photograph of Apollo 16 landing site and traverse locations in high-sun illumination ………………………………………………… 163 3. Digitally enhanced regenerations of photograph in figure 2 ……………………………………………………………………………. 164 4. Topographic map of South Ray crater and approximation of pre-South Ray surface ………………………………………………….. 165 5. Outline of hummocky ejects around South Ray crater rim and rim heights above outer rim margin ………………………………….. 166 6. Map of concentric rings centered on South Ray crater………………………………………………………………………………….. 167 7. Histogram of area covered by mappable ejects and rays within each annulus shown in figure 6 ………………………………………. 168 8. Histogram of percentage of area covered by ejects and rays in annuli of figure 6………………………………………………………. 169 9. Illustration of terms used for crater measurements discussed in text ……………………………………………………………………. 169 10. Ray-covered areas photographed from the Lunar Roving Vehicle …………………………………………………………………….. 170 11. Semilogarithmic plot showing model thicknesses of South Ray ejects relative to distance from the crater rim ………………………. 172 TABLES Page TABLE 1. Ejects distribution models for South Ray crater …………………………………………………………………………………………. 171 INTRODUCTION localities sampled lie within rays of ejects from South South Ray is a fresh, blocky crater, 680 m across and Ray crater and to what extent or depth these localities 135 m deep, in the southern part of the Apollo 16 landing may have been covered by ejects is to estimate the site. Its bright rays extend northward radially across the apparent volume of the crater and then to distribute this traverse area. Sampling of ejects from South Ray was a volume as ejects using several models. In one set of prime objective of the mission because of its location on models, ejects are confined within observable ray the Cayley plains, 6.2 km south of the Lunar Module patterns; in a second set, ejects are not confined. The (LM). Direct sampling and photographing of its rim and purpose of this chapter is to determine a reasonable flanks, where less equivocal provenances could have model for the areal distribution and variation in thickness been established, were prohibited by the distance from of ejects material within rays as a function of distance the LM, the limited time available for traversing, and the from South Ray crater. A stratigraphic interpretation of anticipated roughness of the terrain. Therefore additional materials ejected from South Ray is treated elsewhere evidence and interpretation are required to relate certain (AFGIT, 1973; Ulrich and Reed, this volume). samples collected to South Ray. The approach taken here in determining which of the DEFINITION OF SOUTH RAY EJECTA Part of the problem in defining South Ray ejects is to 160 EJECTA DISTRIBUTION MODEL, SOUTH RAY CRATER 161 establish those properties that characterize the ejecta South Ray, the height of the rim (fig. 5), and the thick on the lunar surface and correlate them with reflected ness of ejecta at the rim. brightness and topography using the best available orbital photography. Block concentrations, surfaces disturbed by the impact of ballistic debris, lineations MEASUREMENT OF EJECTA- AND RAY-COVERED AREAS produced by deposition of ejecta, and individual secondary craters were observed on a local scale by The area covered by mappable rays was measured (at a the astronauts and can be seen in photographs taken scale of 1:50,000) with a planimeter, in concentric annuli by them. For a crater the size of South Ray, these (or bands) one crater diameter (680 m) wide, expanding features, are not resolvable on orbital photographs outward from the crater rim, as illustrated by figure 6. A except at a few places. The properties of crater ejecta plot of the area of ejecta measured within each annulus, seen on orbital photography are reflected brightness figure 7, shows the ejecta-covered areas to be clearly and irregularities in topographic expression. Bright asymmetric in their distribution around South Ray. In areas around young lunar craters photographed under each of the third through seventh annuli, however, they remain nearly constant at 7-9 km2 while the total annulus high sun-elevation angles are produced by a 2 combination of effects: (1) concentrations of blocks area increases by 2.0 km per ring. Beyond the seventh annulus, the ray-covered areas decrease at a nearly and rock fragments, (2) steep surface slopes, and (3) 2 composition of material. Brightness contrasts in constant rate of 0.9 to 1.0 km per annulus. Another surface materials of a crater and its ejecta decrease means of viewing these data is to plot the percentage of with the age of the crater. Topographic expression of total area within each annulus covered by ray material as ejecta is most evident near the crater, and its a function of distance from South Ray rim (fig. 8). The definition is a function of photographic resolution. histogram shows that only the six inner annuli are more For South Ray, contrasts in reflected brightness were than 50 percent covered and that all of the Apollo 16 to delineate the ejecta on orbital photography, because samples come from annuli where less than 57 percent of the high sun-elevation angle of available photographs the area is covered by mappable rays. proved to be a sensitive indicator of ejecta distribution (figs. 1 and 2). The distribution of bright regions, MEASUREMENT OF RIM HEIGHT AND CRATER VOLUME including South Ray, its flanks, and rays or filaments extending radially from it, attest to the crater's youth. Using the topographic map of South Ray (fig. 4), a Bright areas beyond the rim are inferred to be covered precrater surface was estimated by extrapolating contours partly to completely by ejecta from the crater. from outside the hummocky rim across the existing crater. Intersections of this surface with the crater wall MAPPING TECHNIQUES determine the elevation of the original ground surface. Previous mapping of ejecta distribution around Differences in elevation of points on the rim crest and the South Ray consisted of compilation by visual projected original ground surface beneath the points inspection Apollo 14 photographs (Hodges, 1972a; represent the rim height. Values obtained in this way Elston others, 1972b; Muehlberger and others, 1972; range between 19 and 26 m; the average is 22 m. fig. 1, pl.2 this volume). Here, digital processing of Another method of calculating these values is to Apollo panoramic camera photographs taken when determine the average difference in elevation between the sun-elevation angle was 60° was used to delineate the rim crest and the outer margin of hummocky ejecta, the distribution of ejecta from South Ray beyond the shown in the sketch map (fig. 5) along with elevation crater flanks. An unaltered photograph of the landing differences between the rim crest and this edge. The site, figure 2, was digitized and, by means of range in values for rim height by this method is 13 to 28 computer filtering techniques, regenerated to enhance m, the average about 20 m. From this, we consider the reflected brightness variations at three different levels average rim height to be near 20-22 m. (fig. 3). These images, together with figure 1 and The rim height includes two components, the amount selected premission photographs, were the basis for that the original ground surface was uplifted and the compilation of a ray map of South Ray ejecta (see thickness of the ejecta deposited on the uplifted surface Reed, fig. 4, this volume). The procedure required a (fig. 9). For terrestrial explosive craters, the percentage minimum of arbitrary judgment in drawing the of the total rim height resulting from upwarping of boundaries of ray-covered areas. The units mapped ground surface ranges from 17 to 71 percent (Carlson were designated as continuous thin to discontinuous, and Jones, 1965, table 1); for six craters in alluvium 31 to and discontinuous ejecta. 366 m across, an average of 45 percent of the rim height A topographic map of South Ray crater (fig. 4) is the result of upwarp of the original ground surface. enables us to estimate the amount of material ejected Recent drilling at Meteor Crater, Ariz., reveals that 35 to from 60 percent of the present 162 GEOLOGY OFTHEAPOLLO16AREA,CENTRAL,LUNARHIGHLANDS FIGURE 1.- Premission photomosaic map of Apollo 16landing site (Apollo 14-70mm photographs 9515, 9520, 9522, 9530, and 9605, Mosaicking and rectification by G. M. Nakata, aiir brush retouching by P.M. Bridges, USGS Planetary Cartography Group, Flagstaff, Ariz.) 164 GEOLOGY OFTHEAPOLLO16AREA,CENTRAL LUNAR HIGHLANDS A B C FIGURE 3. – Digitally enhanced regenerations of photograph in figure 2.A, B, and C are light, medium, and heavy “stretches”