Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.543m&link_type=abstract
Meteoritics, vol. 30, no. 5, page 543
Other
2
Astroblemes, Craters, Amguid, Aurounga, Bp, Heubury, Impact, Meteorite, Oasis, Roter Kamm, Spider, Wolfe Creek, Zhamanshin, Impacts, Radar Mapping
Scientific paper
The Spaceborne Radar Laboratory (SRL) orbited Earth in April and October of 1994 operating two imaging radars: X-SAR, an X-band (3 cm lambda) instrument, and the polarimetric SIR-C, a combination L-band/C-band (24 cm and 5.6 cm lambda). More than 150 terrestrial meteorite craters and astroblemes are presently known. Three of these, Wolfe Creek in Australia; Roter Kamm in Namibia; and Zhamanshin in Kazakhstan, were planned targets and were imaged successfully with multiple passes and look directions. Several other impact sites were fortuitously imaged while radar data were being collected for other purposes. These sites include B.P. and Oasis structures in Libya, Aourounga multi-ring feature in Chad, Amguid crater in Algeria, and the Spider astrobleme and Henbury crater field in Australia. Wolfe Creek (19 degrees 10'S; 127 degrees 47'E; 875 m dia) Both the elevated rim and the inner floor of this crater appear as radar bright features. Strong radar returns are due to blocky rubble textures in the rim and desert vegetation within the central bowl. Associated linear sand dunes show differential penetration properties in the various radar wavelengths and polarization. Roter Kamm (27 degrees 46'S; 016 degrees 18'E; 2.5 km dia) This bowl-shaped crater is mostly buried by wind-blown sands. Comparison of differential radar penetration patterns due to changes in wavelength and look direction reveal concealed target rocks and a buried possible ejecta unit. Zhamanshin (48 degrees 24'N; 060 degrees 48'E; 14 km dia) This unusual impact structure, first detected by the presence of glassy impact melt products [1], has very little topographic relief and is nearly invisible on survey-quality radar imagery. Fully processed images, however, enhance subtle vegetation patterns which highlight regional streams. These drainage patterns are now being analyzed in detail to better delineate boundaries and internal structure of this feature. B.P. Structure (25 degrees 19'N; 024 degrees 20'E; 2.8 km dia) Wind-blown sands which cover much of this relatively small feature make it difficult to distinguish from numerous dark sandstone outcrops using only optical images. Radar, however, penetrates the shallow sand mantle to reveal a nearly complete radar-bright bullseye pattern typical of central-uplift style impact structure. Oasis Structure (24 degrees 35'N; 24 degrees 24'E; >11.5 km dia) Oasis astrobleme was originally described as an elevated ring of sandstone some 5.1 km wide in desert sands. Examination of optical satellite images detected subtle concentric patterns more than 11 km across [2]. SIR-C images reveal strong arcuate reflectors buried beneath the sand at an even larger diameter of greater than 17 km. Aurounga (19 degrees 06'N; 019 degrees 15'E; 12.6 km dia) Although this highly circular depression has been noticed in numerous remote sensing studies, eg.[3], it usually has been associated with a large volcanic field and attributed to endogenic forces. Recent reports of shatter cones [4] and microscopic shock metamorphic effects [5] now demonstrate an impact origin. The radar-dark ring is a sand-filled trough which interupts a regional pattern of yardangs, wind-cut parallel ridges and grooves, developed in surrounding sandstones. Amguid (26 degrees 05'N; 004 degrees 23'E; 450 m dia) Situated in elevated rocky highlands [6], the small Amguid crater is nearly overprinted by surrounding radar backscatter. A dry central bowl is partially filled with smoothly surfaced fine-grained playa deposits which absorb radar energy and/or reflect it away from the spacecraft. The result is a distinct radar-dark disk within a bright regional ground clutter. Spider (16 degrees 44'S; 126 degrees 05'E; 13 km dia) Named for a radially splayed fault system in its center, Spider is the exposed root structure of a central-uplift impact feature [7]. Radar slope effects on processed data clearly delineate its size and internal complexity. Henbury craters (24 degrees 35'S; 133 degrees 09'E; largest ca.150 m dia) Although quite small, Henbury crater field [8] appears distinctly radar bright on survey -qualilty imagery. Strong radar backscatter may be due to a combination of impact-disrupted sedimentary horizons and of soil dielectrical properties altered by a significant meteoritic iron content [9]. References: [1] Garvin J. B. and Schnetzler C. C. (1994) GSA Spec. Pap. 293, 249-257. [2] Dietz R. S. and McHone J. F. (1979) Apollo Soyuz Test Proj. Summary Sci. Rept. (2) NASA SP-412, 183-192. [3] Roland N. W. (1976) Geol. Jahrb., Reihe A, 33, 117-131. [4] Becq-Giraudon J. F. et al. (1992) Comptes Rendus de l'Academ. des Sciences, Ser.2, 315, 83-88. [5] Grieve R. A. F. and Therriault A. M. (1995) LPS XXVI, 515-516. [6] Lambert P. et al. (1980) Meteoritics, 15, 157-159. [7] Harms et al. (1980) Nature, 286, 704-706. [8] Milton D. J. (1968) Geol. Surv. Prof. Pap. 599-C, C1-C16. [9] Hodge P. W. and Wright F. W. (1971) JGR, 76, 3880-3895.
Blumberg Dan G.
Greeley Ronald
McHone John F. Jr.
Underwood James R. Jr.
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