Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.325b&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 325
Mathematics
Logic
Basin Ejecta, Impacts, Lunar Crust, Moon, Spectral Reflectance
Scientific paper
Basin-forming impacts have played a major role in the geologic history of the Moon, serving to modify and redistribute crustal materials. The Orientale basin has become the model for understanding lunar basin structure [e.g., 1]. Orientale can also provide information on a key aspect of basin formation: the nature of the processes occurring during the emplacement of basin ejecta. In the Schiller-Schickard (SS) region, roughly 1300 km from the basin center, Orientale ejecta has interacted with pre- existing mare basalt [2]. This interaction produced a light plains deposit of the type sometimes referred to as a cryptomare. The amount of mare basalt in the light plains is an expression of the extent of "local mixing," i.e., the degree to which the substrate is disrupted by the impacting ejecta and incorporated into the resulting deposit. The SS region presents an opportunity to test the Oberbeck model of ejecta emplacement [3] because of the presence of a spectrally distinct substrate (mare basalt). Near-infrared (0.6-2.5 micrometer) reflectance spectra for a variety of features in the SS region have been collected with Univ. of Hawaii telescopes and analyzed to extract compositional information from the characteristics of the "1 micrometer" mafic mineral absorption band [4]. Further insight into a multivariate data set such as these spectra can be obtained through application of principal components analysis (PCA) [5]. The mapping of the spectra into PC space aids in the identification of spectral endmembers and reveals mixing trends. Four endmembers identified in the group of SS spectra are (1) mature highlands surfaces, (2) fresh highlands surfaces, (3) mature mare surfaces, and (4) fresh mare surfaces. Spectra for light plains deposits and mare surfaces with highlands contamination fall between these endmembers in PC space. Mixing models based on PCA enable the contribution of each endmember to the mixture spectra to be quantified. Preliminary results indicate that the Schickard light plains contain ~45% mare basalt component. To gain greater spatial coverage, multispectral imaging in the extended visible portion of the spectrum (0.4-1.0 micrometers) is being utilized [6]. Image mixing models using endmembers in the SS region agree with the findings of the spectral study. The Oberbeck model predicts the ratio (r) of local material excavated to the mass of the primary ejecta as a function of the distance from the primary impact, the angle at which ejecta is launched, and the size of the secondary craters produced. For typical Orientale secondaries in the SS area, calculated values of r are ~2, indicating 67% local material. This is somewhat higher than our findings for the fraction of basalt in the light plains. Possible explanations for a lower proportion of local material include a lower cratering efficiency than assumed in the Oberbeck model, or a patchy distribution of pre-Orientale basalt. References: [1] Head J. W. (1974) Moon, 11, 327-356. [2] Bell J. F. and Hawke B. R. (1984) JGR, 89, 6899-6910. [3] Oberbeck V. R. et al. (1975) Moon, 12, 19-54. [4] Blewett D. T. et al. (1992) LPS XXIII, 123-124. [5] Johnson P. E. et al. (1985) Proc. LPSC 15th, in JGR, 90, C805-C810. [6] Blewett D. T. et al. (1993) LPS XXIV, p. 133-134.
Blewett Dave T.
Hawke Bernard Ray
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