SPACE WEATHERING OF LUNAR ROCK 76015

The term “space weathering” refers to the cumulative effects of several processes operating at the surface of any solar system body not protected by a thick atmosphere. The products produced by these processes result in substantial changes in the optical and physical properties of planetary surfaces. Space weathering discussions have generally centered on soils, but exposed rocks will also incur the effects of weathering. Rocks have much longer surface lifetimes than an individual soil grain and thus record a longer history of exposure. By studying the weathering products which have built up on a rock surface, we can gain a deeper perspective on the weathering process and better assess the relative importance of various weathering components. Using FIB (focused ion beam) techniques, we have been able to extract several micro-cross-sections through the weathered patina on lunar rock 76015 in which the delicate stratigraphy of the patina coating is beautifully preserved recording roughly 22 million years of exposure to the space environment. Weathering processes, particularly micrometeorite bombardment and sputtering from solar wind particles, can both deposit and erode material from the surface. The patina varies in thickness across our samples from none in some places, to several microns thick in others. It is composed of many discrete melt- and vapor-deposited units of varying sizes and thicknesses, some stretching across much of the section, others only a few hundred nanometers in extent. By volume, there is considerably more melt deposited than vapor or sputtered material. The boundaries between units are generally sharp, suggesting fast cooling rates. The units are compositionally quite heterogeneous, similar to the range seen in lunar agglutinate glasses. Some of the units are rich in nanophase metallic iron, others have little or none. In general, smaller glass units tend to be more enriched in nanophase iron than larger units. Several small grains have been entrained amongst the glass units. The preservation of solar flare particle tracks in the accreted grains and substrate implies cold accretion. The complexity and variability of the stratigraphy within the sections we have studied is providing unique insights into the complex and stochastic nature of space weathering.