Simulated Space Weathering of Fe- and Mg-rich Aqueously Altered Minerals Using Pulsed Laser Irradiation

摘要

The aim of this work is to investigate contrasting spectral trends observedin carbonaceous chondrites by simulating space weathering effects on a subsetof minerals found in these meteorites. We use pulsed laser irradiation tosimulate micrometeorite impacts on aqueously altered minerals and observe theirspectral and physical evolution as a function of irradiation time. Irradiationof the mineral lizardite, a Mg-phyllosilicate, produces little reddening anddarkening, but a pronounced reduction in band depths. Irradiation of an Fe-richaqueously altered mineral assemblage composed of cronstedtite, pyrite andsiderite, produces significant darkening and band depth suppression. Thespectral slopes of the Fe-rich assemblage initially redden then become bluerwith increasing irradiation time. Analyses of the Fe-rich assemblage usingscanning and transmission electron microscopy reveal the presence of micronsized carbon-rich particles that contain notable fractions of nitrogen andoxygen. Radiative transfer modeling of the Fe-rich assemblage suggests thatnpFe0 particles result in the initial spectral reddening of the samples, butthe increasing production of micron sized carbon particles results in thesubsequent spectral bluing. The presence of npFe0 and possibly cronstedtitelikely promotes the synthesis of these organic-like compounds. Theseexperiments indicate that space weathering processes may enable organicsynthesis reactions on the surfaces of volatile-rich asteroids. Furthermore,Mg- and Fe-rich aqueously altered minerals are dominant at different phases ofthe alteration process. Thus, the contrasting spectral slope evolution betweenthe Fe- and Mg-rich samples in these experiments may indicate that spaceweathering trends of volatile-rich asteroids have a compositional dependencythat could be used to determine the aqueous histories of asteroid parentbodies.