Cat Mountain: A meteoritic sample of an impact-melted asteroid regolith

被引:54
作者
Kring, DA [1 ]
Swindle, TD [1 ]
Britt, DT [1 ]
Grier, JA [1 ]
机构
[1] UNIV ARIZONA, DEPT PLANETARY SCI, TUCSON, AZ 85721 USA
关键词
D O I
10.1029/96JE03139
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
Cat Mountain is a new ordinary chondrite impact melt breccia that contains several shocked chondrule-bearing clasts of L5 material. These clasts are surrounded by a total impact melt of similar composition material which appears to have cooled over a period of a few thousand years, probably within a melt breccia lens in the bottom of a large (>1 km diameter) crater on an L chondrite asteroid. Noble gas isotopes indicate that the sample was involved in at least two different impact events, approximately ssb and 20 Myr ago, following the 4.55 Ga accretion of primitive chondritic material. The 880 Ma event is responsible for the impact breccia texture of the sample, and the 20 Ma event reduced the sample to a meter-sized object. We also infer that another impact occurred between 880 and 20 Ma (possibly the similar to 500 Ma event recorded in many other L chondrites) to jettison the material from the asteroid belt into an orbit that evolved into an Earth-crossing trajectory. The shock-metamorphic processes that occurred at 880 Ma redistributed the opaque phases in the meteorite and altered the crystalline characteristics of silicate phases. This reduced the reflectance of the L5 material and decreased the amplitude of its spectral absorption features. These characteristics are consistent with the spectral characteristics of some C class. asteroids and suggest that some dark asteroids that appear to belong to the C class could be covered with shocked ordinary chondrite material. If one assumes that Cat Mountain came from the same asteroid as other L chondrites with the same cosmic ray exposure age, then the juxtaposition of these different materials suggests asteroids are rubble piles which are heterogeneous on a scale less than 100 m. Furthermore, the structural integrity of Cat Mountain and other L chondrites suggests the strengths of asteroid rubble piles are limited by fractures and contrasting material properties and are thus inherently weak in a ram pressure regime produced when they enter a planetary atmosphere. However, in a regime where the asteroid is the target of impact fragmentation rather than the projectile, the added porosity of a rubble pile structure will compensate for the presence of fractures and absorb a large amount of the impact energy. In this case the structural integrity of the asteroid may appear to be the same as a previously unshocked chondritic material.
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页码:29353 / 29371
页数:19
相关论文
共 89 条
[1]  
ALEXEEV VA, 1995, LUNAR PLANET SCI, V26, P19
[2]  
ANDERS E, 1964, SPACE SCI REV, V3, P583
[3]  
[Anonymous], P 7 LUN SCI C
[4]  
[Anonymous], ROCK COL CHART
[5]  
ASHWORTH JR, 1981, METEORITICS, V16, P291
[6]  
ASPHAUG E, 1993, THESIS U ARIZ TUCSON
[7]   ON THE THERMAL HISTORY OF HEAVILY SHOCKED YANZHUANG H-CHONDRITE [J].
BEGEMANN, F ;
PALME, H ;
SPETTEL, B ;
WEBER, HW .
METEORITICS, 1992, 27 (02) :174-178
[8]   SHOCK INDUCED THERMAL METAMORPHISM AND MECHANICAL DEFORMATIONS IN RAMSDORF CHONDRITE [J].
BEGEMANN, F ;
WLOTZKA, F .
GEOCHIMICA ET COSMOCHIMICA ACTA, 1969, 33 (11) :1351-+
[9]  
BELL JF, 1989, ASTEROIDS II, P921
[10]   INVESTIGATION AND SIMULATION OF METALLIC SPHERULES FROM LUNAR SOILS [J].
BLAU, PJ ;
GOLDSTEIN, JI .
GEOCHIMICA ET COSMOCHIMICA ACTA, 1975, 39 (03) :305-&