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0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System the CRATERS of MARE IMBRIUM James Whitford-Stark

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0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System the CRATERS of MARE IMBRIUM James Whitford-Stark THE CRATERS OF MARE IMBRIUM, James Whitford-Stark, Department of Geological Sciences, Brown University, Providence, RI 02912. A total of 372 craters larger than 1.25 km in rim crest diameter occurring within Mare Imbrium have been studied to characterize their physical parameters, relative age, and the properties of the mare material which they excavated. Measure- ments were made on Lunar Orbiter IV high resolution photographs and included the rim crest diameters and continuous ejecta blan- ket diameters. Where the crater did not have a visible ejecta blanket, the rim width was measured. Several authors (112) have used the difference in rim crest height of fresh and flood- ed craters to estimate the depth of lava flooding. The topogra- phic resolution of the majority of the craters within Imbrium is insufficient for direct measurement of rim height so it was necessary to devise an indirect method, It was therefore as- sumed that the measured craters had the rim heights of fresh craters which follow the relationships A) R = 0.036 D~-~~~ for rim crest diameters L 17 km, B) and R = 0.236~~~~~~for rim crest diameters 717 km where R is the rim height and D is the crater diameter in kilometers (3). Further expressions have been calculated defining the rim width and ejecta blanket radius as functions of crater diameter and radius as C) W = 0.257~~~~'~ for rim crest diameters < 17 km, D) W = 0.467~~0~~~for rim crest diameters > 17 km, E) Re = 2.248~~~~~~~where w is the flank width, D is the crater diameter(31, Rc is the crater ra- dius and Re is the radius of the continuous ejecta measured from the crater center (4) , By substitution of C, D, and E into A and B it is possible to define the rim height as a function of ejecta width or flank width, whichever is applicable, For exam- ple, R~ = 0.036 1.011 which defines the rim height (~2)as a function of flank width (W) for craters 4 17 km diameter and which defines the rim height (~3)as a function of ejecta radius (Re). If a crater has been flooded then R2 or R3 should be less than R. The same applies if the crater is old since the ejecta blanket will become less readily definable on photographs, The difference between R and R2 or R3 is thus a function of depth of flooding of the crater and/or its age. Inaccuracies arise from the actual measurements on the craters and the scatter in expres- sions A through E defined in (3) . Comparison of the calculated 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System THE CRATERS OF MARE IMBRIUM James Whitford-Stark rim height differences and photographs indicate that craters with differences in excess of 20 m have been partly flooded, Of the craters measured, 26.61% appear to be unflooded (fig. 1) while the greatest height differences were measured at Wallace and Archimedes K (300-350 m). There were four structures which did not have well defined flanks; the ring Lambert R, a ring to the immediate north of Aristillus, a degraded crater to the west of Mt. Huygens, and Sinus Iridum. Assuming these structures to be completely flooded fresh craters, minimum lava thicknesses of 1.2, 0.9, 0.85 and 2.1 km respectively are calculated on the ba- sis of expression A. Excepting Iridum, these structures are be- tween the second and third rings of the Imbrium basin. These structures, plus possibly Archimedes and Cassini, are the only visible craters within Imbrium which could predate the eruption of mare basalt. This is supported by the spectral vidicon imag- ery- data which indicates that the majority of the craters have spectral responses of mare rather than highland craters. Excep- tions are the craters Piko K, Tobias Mayer GA, Eratosthenes A and B, and Wallace B which have responses more typical of high- land material. They are all located near the mare edge or the inferred sub-basalt extension of an Imbrium ring. Their calcu- lated excavation depths are 640, 920, 1200, 1080 and 750 meters, assuming them to be fresh craters and employing the expression E = 0.196~1.010 where E is the depth and D the rim crest diame- ter (3). It is therefore concluded that the lack of large ( > 35 km diameter) partly buried craters within Imbrium indi- cates that the mare basalts are everywhere in excess of 950 m in thickness except near the basin edges and over the submerged rings. Some large craters (Autolycus, Archimedes, Timocharis and ~ambert)appear to have penetrated the mare basalt fill and to have excavated KREEP-rich highland basalts (5). If such high thorium concentrations are found associated with the smaller craters, such as Euler and Diophant~~, it would be possible to determine more accurately the minimum depth to which KREEP ba- salts extend than the 3.7 km presently estimated(5) for the Lam- bert excavation. 1. DeHon, R., 1975, -Proc. --Lunar Sci. Conf. m, p. 2553-2561. 2. Hbrz, F., 1978, Proc. Lunar Planet. &. Conf. 9th, p. 3311- 3331. 3. Pike, R. J., 1977, In Impact and Explosion Craterinq (eds. D. J. Roddy et g.),Pergammon, p. 489-509. 4. Moore, H. J. et a.,1974, Proc. Lunar Sci. Conf. 5, p. 71- 100. 5. Metzger, A. E. et &., 1979, Proc. Lunar Planet. Sci. Conf. loth, in press. 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System Number of craters Depth of flooding in meters Fiquse 1. Hir 'lo ,-??.;.; c'-.cwing .the 2epth 05 ,''.r~(linq nf 372 cr>"-.r?::s :.~ithinMare I:.iu. The numbers above the blocks indicate the percentage of the total population which fall within the specified depth of flood- ing range., The inset diagram indicates the parameters employed in equations A through E of the text. .
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