Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.568r&link_type=abstract
Meteoritics, vol. 30, no. 5, page 568
Computer Science
Breccias, Regolith, Chondrites, H, Cosmic Rays, Solar, Nuclear Tracks
Scientific paper
Introduction: Recently taken photographs of the asteroids Ida and Gaspra show cratered surfaces similar to the moon. Weak outlined craters indicate the existence of regolith layers. Certain meteorites, the regolith breccias, reflect asteroidal regoliths and contain informations about irradiation conditions and gardening processes on asteroidal surfaces. Olivine grain separates of four H-chondritic regolith breccias, Acfer 111, 153, 192 and Bremervorde, were etched for four hours in WN solution [1] to reveal nuclear tracks in olivines. This procedure was done to determine the track "background" produced by galactic cosmic rays (GCR) during transit from the parent body to earth. Additionally whole thin sections of the investigated meteorites were etched for less than thirty minutes to reveal high nuclear track densities which were induced by solar cosmic rays (SCR) on the parent body surface. The short etching time protects extremely high irradiated grains from destruction during the etching procedure. Etched thin sections allows the determination of the spatial distribution of preirradiated components in a given meteorite and a view on the irradiation features of single components within their natural surrounding. Therefore a comparison with the more intensively investigated lunar regolith is possible. Results: I. One of the investigated samples (Acfer 153) shows a layering of preirradiated grains as it is observed in lunar regolith drill cores. Obviously the meteorite samples reflect on a small scale a part of the parent body's stratigraphy [2]. II. The content of preirradiated components varies within the investigated meteorites between <1% and 50% whereby, the lunar mean value is in the order of 70% [e.g., 3]. III. The highest observed track densities are around 10^10 tracks/cm^2. In lunar breccias this value exceeds 10^11 tracks/cm2 [e.g., 4]. IV. Almost every preirradiated component in the H chondrites shows a clearly visible track density gradient. In contrast only around 10% of these components from lunar breccias show this significant feature [e.g., 5]. Conclusions: The observed differences could be explained according to the model by [6]. The lower gravity of the asteroids and a higher impactor flux in the orbit of the asteroid belt compared to the moon causes more ejected and more widespread material by impact of external bodies [6, 7]. Therefore asteroidal surfaces are more frequently covered with new unirradiated material. That implies that asteroidal surfaces are exposed for a shorter time and the reworking by microimpacts of the top layer is less intensive than on the moon. This could explain the lower content of preirradiated components and lower maximum track densities within the investigated H-chondrites. Furthermore the higher rate of reworking on the moon causes turnover of grains at the very surface and repeated irradiation in different geometries by solar cosmic rays. This blurs the previously track density gradient and lowers the amount of grains with a clearly visible track density gradient in lunar regolith samples. In comparison the nuclear track irradiation features in silicate grains from lunar and chondritic regolith breccias differ only in quantity but not in quality. Therefore, the H- chondritic parent body seems to be covered by a regolith layer similar to the lunar regolith. References: [1] Krishnaswami et al. (1971) Science, 174, 287-291. [2] Romstedt J. and Metzler K. (1994) LPS XXV, 1157-1158. [3] Goswami J. N. and Lal D. (1977) Proc. LSC 8th, 813-824. [4] Dran J. C. et al. (1970) EPSL, 9, 391-400. [5] Crozaz G. and Dust S. (1977) Proc. LSC 8th, 3001-3016. [6] Housen K. R. et al. (1979) Icarus, 39, 317-351. [7] Gault D. E. and Wedekind J. A. (1977) Proc. Symp. Planetary Cratering Mechanics, Pergamon.
Metzler Knut
Romstedt Jens
No associations
LandOfFree
Preirradiated Grains in H-Chondritic Regolith Breccias: an In Situ Investigation does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Preirradiated Grains in H-Chondritic Regolith Breccias: an In Situ Investigation, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Preirradiated Grains in H-Chondritic Regolith Breccias: an In Situ Investigation will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-830917