| Abstract: | Abstract— We studied patinas on lunar rocks 75075 and 76015 from the Apollo collection using a multi-disciplinary approach, including scanning electron microscopy (SEM), energy dispersive x-ray spectrometry (EDS), transmission electron microscopy (TEM), wavelength-dispersive x-ray (WDS) mapping, Mössbauer spectroscopy, spectral reflectance, and microspectrophotometry. Based on SEM petrography, we have defined three textural types of patina: glazed, fragmental, and classic (cratered). The presence of classic patina is diagnostic of lunar samples that have been exposed directly to the space weathering environment. It is characterized by the presence of microcraters and glass pancakes and is the patina type studied by earlier workers. Classic patina is found on 76015 but not on 75075. Glazed patina is found on both 76015 and 75075, whereas fragmental patina is found only on 75075. The glazed and fragmental patinas on 75075 were probably formed as a result of relatively large nearby impacts; and although these two types of patina are not strictly the result of direct exposure to the space weathering environment, they are important because they affect the optical properties of the rocks. Field emission gun SEM (FE-SEM) of classic patina on 76015 shows evidence of possible solar wind sputtering erosion. Transmission electron microscope studies of 76015 reveal the presence of impact-generated deposits and solar flare particle tracks which, like microcraters and pancakes, are diagnostic of direct exposure to space weathering processes. The outermost surface of the 76015 patina consists of an amorphous rim very much like the rims found on individual lunar soil grains; this amorphous patina rim probably formed by similar processes of impact-generated vapor condensation and possible sputter deposition. Wavelength-dispersive x-ray element maps of polished thin sections of 75075 and 76015 indicate that patina compositions are poor indicators of the compositions and mineralogies of the rocks underlying them. On average, the reflectance spectra of patinas on both samples are slightly darker than those of their unweathered equivalents. Microreflectance measurements show that a thick patina can dramatically alter the optical properties of the rock on which it forms. The backscatter Mössbauer (BaMS) spectrum of a patina-covered surface of 76015 is very similar to that of an unweathered surface, indicating that the Mössbauer signal is generated from beneath the patina. Because BaMS “sees” through surface space-weathering effects to the underlying rock, this technique has great potential for use in robotic missions to other planetary bodies. |
