Impact cratering is now recognized as a major geological process on Earth. In particular, giant impacts had a fundamental influence on the geological and biological evolution of our planet with possible climatic effects. There are more than 160 confirmed impact craters on Earth, among which 17 are located in Africa, but it is estimated that only 10% of impact craters larger than 10km and younger than 100Ma are known.
The Sahara is a particularly favorable region to host young impact craters, but according to cratering rate estimates, most of them still remain to be discovered, hidden under dry sandy sediments. Only four confirmed impact craters are currently known in eastern Sahara.
Two are located in eastern Libya: B.P. (British Petroleum) structure and Oasis crater, and two are located in northern Chad: Aorounga and Gweni-Fada craters. While optical sensors can only image the desert's surface, it was shown twenty years ago that orbital Synthetic Aperture Radar (SAR) could retrieve subsurface information hidden under a few meters of dry sand.
Within an international project -- dubbed SAHARASAR -- that aims at mapping the near subsurface of the Sahara and Arabian regions using satellite-borne radar, the team produced a regional-scale radar mosaic at 100m resolution over the eastern Sahara from existing JERS-1 archives (a Japaneese satellite operated from 1990 to 1998).
This unique data set allowed us to discover a double circular structure in the southeastern part of the Libyan desert. Fieldwork confirmed that this formation is an unknown double impact crater with a diameter around 10 km.
The newly discovered structures are located 110km west of Djebel Arkenu and 250km south of Kufra oasis in Libya, at coordinates N22 deg 04', E23 deg 45'. It is a flat and hyperarid area, presenting a Cretaceous sandstone formation covered by active aeolian deposits and Quaternary soils, located tens of kilometers away from any track, in a hazardous zone due to the proximity of Second World War minefields.
The optical Landsat-7 image shows a sandy region with large sand dunes trending SW-NE, while the corresponding L-band radar image extracted from the JERS-1 radar mosaic reveals two circular structures partially hidden by Quaternary deposits.
The radar scene then clearly reveals a double circular structure composed of a southwestern crater 10.3km in diameter and a northeastern crater of diameter 6.8km. The NE crater is composed of concentric inner and outer rings separated by a depression filled with sediments, also observed in the optical scene.
Its morphology is very similar to the Aorounga crater in Chad, corresponding to a typical complex crater. The SW crater also presents a circular shape with possibly three concentric annular ridges. The host rock of the double circular structure is a cross-bedded coarse-grained to conglomeratic sandstone of Lower Cretaceous age containing plant fossils and thin shale interbeds, leading to an estimated impact age of less than 140Ma.
A field survey was carried out during April 2003 in order to obtain definitive proof of the impact origin of the observed structures (i.e. shatter cones, high shock pressure metamorphism, planar microstructures in quartz grains, high pressure polymorphs such as coesite and stishovite, Iridium enrichment). Significant quantities of shatter cone structures on the site, all located close to the inner ridge of the NE crater were observed.
Large outcrops of allochthonous impact breccia could also be observed in both craters. Several quartz grains presenting planar fractures (PFs) in these breccia were also found. Such planar microstructures are diagnostic shock effects in a pressure range from 5 to 20GPa.
It can be asserted from these observations that the newly discovered circular structures were produced by the impact of a 500m diameter pair of asteroids. Because of the proximity of Djebel Arkenu, the team has proposed to name the two new impact craters as follows: "Arkenu 1" for the NE crater and "Arkenu 2" for the SW crater.
Peer reviewed publication and references: Ph. Paillou, A. Rosenqvist, J.-M. Malézieux, B. Reynard, T. Farr, E. Heggy, "Discovery of a double impact crater in Libya: the astrobleme of Arkenu", Acad. Sci. Paris, C.R. Geoscience, 335 (2003), 1059-1069.Related Links
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NASA Releases Near-Earth Object Search Report
Pasadena - Sep 11, 2003
NASA has released a technical report on potential future search efforts for near-Earth objects after a year of analysis by scientists working on this issue. This Science Definition Team was chartered to study what should be done to find near-Earth objects less than 1 kilometer in size.
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