Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..127b&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Mathematics
Logic
Scientific paper
Introduction Radar-dark diffuse features (DDFs) associated with craters on Venus surface are observed in the Magellan radar images for the majority of impact craters on the planet (~ 65% of craters with diameter larger than 30 km have associated DDFs [1]). DDFs have been interpreted as deposits (mantles) of loose material disintegrated, ejected, and lifted by the impact. This material represents distal crater ejecta excavated from the deepest subsurface layers reached by the impact process. Thus, the study of DDFs can potentially give information about properties of subsurface material on Venus. The population of craters on Venus's surface is rather small (~ 1000) due to shielding by the dense atmosphere and geologically young surface ages. Sometimes, the distal deposits of craters partly overlap, but the majority of DDFs are rather isolated. This allows the study of properties of individual DDF. I studied 94 DDFs located in the 40°S - 60°N latitude zone using data obtained by Magellan mission to Venus. 84 DDFs under study are associated with craters with diameters of 25 - 80 km, and 10 DDFs are dark parabolas of smaller craters (10 - 22 km). Aging of DDF material DDFs are characterized by different shapes and sizes. Geological studies showed relationship between shapes of DDFs and crater ages. Craters with parabolas are thought to be the youngest on the planet [2]. Morphological sequence of DDFs from dark parabola (DP) to dark halo (DH) to faint dark halo (FH), and to no dark deposits (NH) has been interpreted as a degradation sequence of DDFs, which reflects an age progression of craters from young to old [1, 3]. Presently, the particular mechanism of parabola degradation is not well understood; however, movement of loose material by winds seems to be the process responsible for degradation and removal of DDFs [4]. The parabola aging processes are accompanied by the decrease of dielectric permittivity of the mantle material, which can be caused by chemical weathering and/or the decrease of mantle density. Average relative (dimensionless) dielectric permittivity <ɛ> and Hagfors' roughness parameter <ξ> of radar dark surfaces near craters in comparison with the adjacent "typical" surface are shown in Table 1. The dielectric permittivity was derived from the Fresnel reflectivity measurements in Magellan radar altimeter experiment [5]. Radar-dark and "typical" surfaces were defined according to their radar cross-sections in the Magellan SAR mosaics: lower than the planetary average and in the range of 0 - +2 dB from the planetary average, respectively. 90 crates under study exhibit higher average permittivity for dark surface in comparison with typical one. The difference is not high and varies from 0.01 up to 0.75. Four crates show the opposite signature; the difference does not exceed 0.2. These cases include three FH craters and one NH crater. Dark surfaces are also characterized by lower roughness in comparison to "typical" ones (Table 1). Dielectric permittivity of dark material near NH craters (as shown in Table 1) is lower than the permittivity of DP craters, however, the difference does not exceed 0.32 (~7%). Properties of Deep Subsurface Rocks In general, deposited material at some distance from the source crater is related to some excavation depth [6]. On Venus the process of fine ejecta deposition is strongly affected by atmosphere [2]. Fine ejecta are shifted westward by the atmospheric superrotation. DDFs seem to be composed by material excavated from different depths. These depths are limited by the maximal excavation depth for every crater. The maximal excavation depth during crater formation is roughly estimated to be about 1/10 of the transitional crater diameter [6]. I used the estimates of the mean dielectric permittivity of radar-dark surface near craters and corresponding maximal excavation depths to look for signatures of regional subsurface layers with distinctive electromagnetic properties. All craters under study have been used. To account for the possible aging of parabola material, for CH, FH, and NH craters I considered a range of the dielectric permittivity values from the actual mean value to a value 7% higher. This analysis showed some difference in dielectric properties for subsurface rocks over the Venus surface listed in Table 2. I assumed 23% porosity of the DDF mantle (equal to the porosity of densely packed spheres of the same radius [7]). Under this assumption, the mantle with dielectric permittivity of 4.25 - 4.4 is made of rock fragments with the permittivity of 7.3 - 7.7, and the mantle permittivity of 4.7 - 5.3 corresponds to rock permittivity of 8.7 - 10.9. (Rayleigh mixing formula was used for these calculations [8].) Dielectric permittivity equal to 5 - 7 is typical for solid granite and values of ~9 are observed for solid basalt [8]. This illustrates, that the observed difference in the dielectric permittivity may correspond to significant variations of rock composition. Thus, my observations indicate the presence of noticeable regional variations of composition of the subsurface material on Venus. References [1] Basilevsky, A. and Head, J. (2002) JGR, 107, doi:10.1029/2001JE001584. [2] Campbell, D. et al. (1992) JGR, 97, 16249 - 16277. [3] Izenberg, N. et al. (1994) Geophys. Res. Lett., 21, 289-292. [4] Bondarenko, N. and Head, J. (2006) LPSC XXXVII, A1494. [5] Ford, P. and Pettengill, G. (1992) JGR 97, 13103- 13114. [6] Melosh, H. (1989), Impact cratering: A geologic process. [7] Carrier, W. et al. (1991) Lunar Sourcebook, 475- 594. [8] Campbell, M. and Ulrichs, J. (1969), JGR, 74, 5867-5881.
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