From: Thu 1:23 PM Subject: {MPML} Digest Number 1301To: mpml@yahoogroups.com There are 3 messages in this issue. Topics in this digest: 1. 732 Oaxaca From: Jim Roe 2. Discovery of a Double Impact Crater in Libya From: Ron Baalke 3. Re: 732 Oaxaca From: "P. Clay Sherrod" ________________________________________________________________________ ________________________________________________________________________ Message: 1 Date: Thu, 18 Dec 2003 09:35:19 -0600 From: Jim Roe Subject: 732 Oaxaca It is with mixed emotions that I report the de-commissioning of observing station 732 Oaxaca. The telescope is removed to St. Louis, Missouri. The observatory is converted to a storage building. Several thousands of asteroid position measurements were made and reported during its brief history and it has a record of 100 nearly-certain-to-be-numbered discoveries (41 currently numbered, 36 principal designations, and 23 orbital arcs greater than 30 days). Also to its credit is the first new discovery of the new millenium - 2000AA. To my knowledge, these are the only asteroids ever discovered from the territory of the Republic of Mexico. While one phase is ended, another will arise. Jim Roe ________________________________________________________________________ ________________________________________________________________________ Message: 2 Date: Thu, 18 Dec 2003 08:51:51 -0800 (PST) From: Ron Baalke Subject: Discovery of a Double Impact Crater in Libya Observatoire aquitain des sciences de l'Univers Floirac, France For further information, please contact: Philippe Paillou UMR 5804, OASU (Observatoire aquitain des sciences de l'Univers) paillou@obs.u-bordeaux1.fr (33) (0)557 776 126 16 December 2003 Discovery of a double impact crater in Libya 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, we realized 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). We observed quantities of shatter cone structures on the site, all located close to the inner ridge of the NE crater. Large outcrops of allochthonous impact breccia could also be observed in both craters. We could find several quartz grains presenting planar fractures (PFs) in these breccia. Such planar microstructures are diagnostic shock effects in a pressure range from 5 to 20GPa. We can assert 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, we 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. ________________________________________________________________________ ________________________________________________________________________ Message: 3 Date: Thu, 18 Dec 2003 11:51:26 -0600 From: "P. Clay Sherrod" Subject: Re: 732 Oaxaca Jim... this is horribly sad and sudden news; I hope that all is well with you and indeed Oaxaca will be in the books forever. I am sure that everyone here echoes such a feeling of loss from this. Best of luck in your continuing pursuits. Clay -------------------- Dr. P. Clay Sherrod Arkansas Sky Observatory Harvard MPC H41 (Petit Jean Mountain) Harvard MPC H43 (Conway) Harvard MPC H44 (Cascade Mt.) http://www.arksky.org/ ----- Original Message ----- From: "Jim Roe" To: Sent: Thursday, December 18, 2003 9:35 AM Subject: {MPML} 732 Oaxaca > It is with mixed emotions that I report the de-commissioning of > observing station 732 Oaxaca. The telescope is removed to St. Louis, > Missouri. The observatory is converted to a storage building. > > Several thousands of asteroid position measurements were made and > reported during its brief history and it has a record of 100 > nearly-certain-to-be-numbered discoveries (41 currently numbered, 36 > principal designations, and 23 orbital arcs greater than 30 days). Also > to its credit is the first new discovery of the new millenium - 2000AA. > To my knowledge, these are the only asteroids ever discovered from the > territory of the Republic of Mexico. > > While one phase is ended, another will arise. > > Jim Roe > > > > > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > MPML is supported in part via the 2002 Shoemaker NEO Grant Program of The Planetary Society (http://www.planetary.org) > > NOTICE: Material quoted or re-posted from the Minor Planet Mailing List should be proceeded > by the following attribution: > > FROM THE MINOR PLANET MAILING LIST [date]. For the full text or to subscribe, please visit: > MPML Home page: http://www.bitnik.com/mp > MPML FAQ: http://www.bitnik.com/mp/MPML-FAQ.html > MPML's Yahoogroups page: http://www.yahoogroups.com/group/mpml > > To unsubscribe from this group, send an email to: > mpml-unsubscribe@yahoogroups.com > > > > > Yahoo! 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