The parent bodies of CR chondrites and their secondary history     

Trygve Prestgard  Pierre Beck  Lydie Bonal  Jolantha Eschrig  Jérôme Gattacceca  Corinne Sonzogni  Lisa Krämer Ruggiu 《Meteoritics & planetary science》2023,58(8):1117-1148

Renazzo-type (CR) chondrites are a relatively rare group of carbonaceous chondrites with the vast majority having escaped thermal alteration. This means that CRs are composed of relatively unprocessed material, depending on the extent of aqueous alteration they have experienced. Hydration in CRs ranges from incipient alteration of matrix glass, up to nearly complete replacement of the rock by hydration products. The extent of secondary processes is often difficult to assess in these meteorites, due to their heterogeneity and diversity of alteration products. Yet, this is crucial in order to understand the extent of geological processing that occurred on the primary parent body. Additionally, the parent asteroids of CRs remain a mystery, mainly because terrestrial oxyhydroxide signatures dominate the reflectance spectra of CRs. In this work, we have conducted optical and IR reflectance and transmission spectra of 25 CR chondrites in order to (i) better evaluate the extent of aqueous alteration that occurred on the CR parent body, and (ii) find possible parent body candidates. Terrestrial oxyhydroxides were removed from 12 samples, as these tend to interfere with the optical-IR spectra of CRs. Our results suggest, among other, that (i) aqueous alteration in most of our CRs was limited to the matrix and (ii) most CRs may stem from a continuum of X-to-C complex asteroids, depending on their extent of aqueous alteration. More specifically, the endmembers being Xk/Xn types and Cgh/Ch types. This has strong implication in regard to what we can expect from the Psyche mission.  相似文献   

Erratum: Depolarization and Faraday effects in galaxies     

D. D. Sokoloff  A. A. Bykov  A. Shukurov  E. M. Berkhuijsen  R. Beck  A. D. Poezd 《Monthly notices of the Royal Astronomical Society》1999,303(1):207-208

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POLIS: A spectropolarimeter for the VTT and for GREGOR     

W. Schmidt  C. Beck  T. Kentischer  D. Elmore  B. Lites 《Astronomische Nachrichten》2003,324(4):300-301

The polarimetric Littrow Spectrograph POLIS is designed for vector polarimetry at high angular and spectral resolution. It measures the magnetic field simultaneously in the photosphere and the chromosphere of the sun. Both branches of the polarimetry unit are dual beam systems with a single rotating modulator for both wavelengths and polarizing beam splitters in front of each CCD camera. POLIS has been installed at the VTT on Tenerife and has seen First Light on 17 May 2002. A modified version of POLIS will be developed for the balloon mission Sunrise . That version will have UV capabilities down to 200 nm.  相似文献   

Impact melt rocks from the Late Paleocene Hiawatha impact structure,northwest Greenland     

William R. Hyde  Adam A. Garde  Nynke Keulen  Sebastian N. Malkki  Steven J. Jaret  Tod Waight  Pierre Beck  Iain McDonald  Nicolaj K. Larsen 《Meteoritics & planetary science》2023,58(6):789-814

Impact melt rocks formed during hypervelocity impact events are ideal for studying impact structures. Here, we describe impact melt rock samples collected proximal to the 31 km wide 58 Ma Hiawatha impact structure, northwest Greenland, which is completely covered by the Greenland Ice Sheet. The melt rocks contain diagnostic shock indicators (e.g., planar deformation features [PDF] in quartz and shocked zircon) and form three groups based on melt textures and chemistry: (i) hypocrystalline, (ii) glassy, and (iii) carbonate-based melt rocks. The exposed foreland directly in front of the structure consists of metasedimentary successions and igneous plutons; however, the carbonate-based impactites indicate a mixed target sequence with a significant carbonate-rich component. Well-preserved organic material in some melt rocks indicates that North Greenland at the time of impact was host to abundant organic material, likely a dense high-latitude temperate forest. Geochemical signatures of platinum-group elements in selected samples indicate an extraterrestrial component and support previous identification of a highly fractionated iron impactor in glaciofluvial sand. Our results illustrate the possibility to study impact structures hidden beneath a thick ice sheet based on transported samples and this opens a new avenue for identifying other potential impact craters in Greenland and Antarctica.  相似文献