Suchergebnisse

Space-weathering as well as shock effects can darken meteorite and asteroid reflectance spectra. We present a detailed comparative study on shock-darkening and space-weathering using different lithologies of the Chelyabinsk LL5 chondrite. Compared to space-weathering, the shock processes do not cause significant spectral slope changes and are more efficient in attenuating the orthopyroxene 2 μm absorption band. This results in a distinct shock vector in the reflectance spectra principal component analysis, moving the shocked silicate-rich Chelyabinsk spectra from the S-complex space into the C/X complex. In contrast to this, the space-weathering vector stays within the S complex, moving from Q type to S type. Moreover, the 2 μm to 1μm band depth ratio (BDR) as well as the 2 μm to 1μm band area ratio (BAR) are not appreciably affected by shock-darkening or shock melting. Space-weathering, however, causes significant shifts in both BDR and BAR toward higher values. Application of the BDR method to the three distinct areas on the asteroid Itokawa reveals that Itokawa is rather uniformly space-weathered and not influenced by regolith roughness or relative albedo changes. © 2020. The Author(s). ; We would like to thank Juan Sanchez for his help with PCA classification, Radoslaw Michallik for his help with the SEM images, and Eric MacLennan for his help with digitizing Figure 9. This research is supported by the Academy of Finland project No. 293975 and the Ministry of Education, Youth and Sports of the Czech Republic grant No. LH12079, NASA SSERVI Center for Asteroid and Lunar Surface Science (CLASS), MINOBRNAUKI project 5.3451.2017/4.6, Minobrnauki project FEUZ-2020-0059, and Act 211 of the Government of the Russian Federation, agreement No. 02. A03.21.0006, and with institutional support RVO 67985831 of the Institute of Geology of the Czech Academy of Sciences. The University of Winnipeg's C-TAPE was established with funding from the Canada Foundation for Innovation, the Manitoba Research Innovation Fund, the ...

Context. Shock-induced changes in ordinary chondrite meteorites related to impacts or planetary collisions are known to be capable of altering their optical properties. Thus, one can hypothesize that a significant portion of the ordinary chondrite material may be hidden within the observed dark C/X asteroid population. Aims. The exact pressure-temperature conditions of the shock-induced darkening are not well constrained. Thus, we experimentally investigate the gradual changes in the chondrite material optical properties as a function of the shock pressure. Methods. A spherical shock experiment with Chelyabinsk LL5 was performed in order to study the changes in its optical properties. The spherical shock experiment geometry allows for a gradual increase of shock pressure from ~15 GPa at a rim toward hundreds of gigapascals in the center. Results. Four distinct zones were observed with an increasing shock load. The optical changes are minimal up to ~50 GPa. In the region of ~50-60 GPa, shock darkening occurs due to the troilite melt infusion into silicates. This process abruptly ceases at pressures of ~60 GPa due to an onset of silicate melting. At pressures higher than ~150 GPa, recrystallization occurs and is associated with a second-stage shock darkening due to fine troilite-metal eutectic grains. The shock darkening affects the ultraviolet, visible, and near-infrared region while changes to the MIR spectrum are minimal. Conclusions. Shock darkening is caused by two distinct mechanisms with characteristic pressure regions, which are separated by an interval where the darkening ceases. This implies a reduced amount of shock-darkened material produced during the asteroid collisions. © T. Kohout et al. 2020. ; Academy of Finland: 293975 ; National Aeronautics and Space Administration, NASA: 0836-2020-0059 ; Russian Foundation for Basic Research, RFBR: 18-38-00598 ; Government Council on Grants, Russian Federation ; AAAA-A19-119071090011-6, RVO 67985831 ; Ministry of Science and Higher Education of the Russian Federation ; cA knolw edgemen. This research is supported by Academy of Finland project no. 293975, NASA SSERVI Center for Asteroid and Lunar Surface Science (CLASS), the Project No. 0836-2020-0059 of the Ministry of Science and Higher Education of the Russian Federation, the Act 211 of the Government of the Russian Federation, agreement no. 02.A03.21.0006, RFBR project no. 18-38-00598, the theme of state assignment of IGG UB RAS number AAAA-A19-119071090011-6, and within institutional support RVO 67985831 of the Institute of Geology of the Czech Academy of Sciences. We would like to thank Andreas Morlok for his constructive review comments on the manuscript.