Space weathering on the Moon: Patina on Apollo 17 samples 75075 and 76015

Abstract

Abstract-We studied patinas on lunar rocks 75075 and 76015 from the Apollo collection using a multidisciplinary approach, including scanning electron microscopy (SEM), energy dispersive x-ray spectrometry (EDS), transmission electron microscopy (TEM), wavelength-dispersive x-ray (WDS) mapping, M6ssbauer 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 fiagmental 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 760 15 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 7601 5 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 M6ssbauer (BaMS) spectrum of a patina-covered surface of 76015 is very similar to that of an unweathered surface, indicating that the MGssbauer signal is generated from beneath the patina. Because BaMS "sees" through surface spaceweathering effects to the underlying rock, this technique has great potential for use in robotic missions to other planetary bodies.

An in-depth look at the lunar crater Copernicus: Exposed mineralogy by high-resolution near-infrared spectroscopy

“…McKay et al, 1991);(b) individual grain effects (such as rare gas implantations and glass splashes); (c) and nanometer-scale particles of metallic iron (npFe 0 ) implanted within agglutinates and as spatter on individual soil grains (Baron et al, 1977;Grant, 1975, 1977;Hapke, 1973;Hapke et al, 1975). Nanophase iron is found in two distinct size ranges which affect the regolith's optical properties differently: (1) npFe 0 , present as thin patinas on irradiated grain rims, is on average about 3 nm in diameter (range between 1 to 15 nm; Keller et al, 1998;Keller and Clemett, 2001;Wentworth et al, 1999) and it is linked to the increase in spectral reflectance in the NIR (e.g. Cassidy and Hapke, 1975;Morris, 1980), a phenomenon often referred to as the 'reddening of the continuum slope'; (2) larger nanospheres up to several hundred nanometres (Housley et al, 1973;James et al, 2002;Keller and Clemett, 2001), which are found in agglutinates: their accumulation with time is linked to soil darkening and consequent weakening of spectral features.…”

An improved scheme for modeling the reflectance spectra of space-weathered regoliths

“…Transmission electron microscope (TEM) images have revealed the details of npFe 0 layering along the rims of lunar soil grains (e.g., McKay, 1993, 1997;Wentworth et al, 1999). The simulation of micrometeorite bombardment has been proven to form a vapor-deposited layer containing npFe 0 particles (Sasaki et al, 2001).…”

Space weathering on airless planetary bodies: Clues from the lunar mineral hapkeite

“…Although the production of amorphous rims on lunar soil grains had long been assumed to be a result of solar-wind irradiation damage (23), recent investigations using transmission electron microscopy have revealed that rims of soil grains are not homogeneous and contain a variety of different morphologies and chemical compositions (5). It has been shown that vapor deposition on individual soil grains probably plays a major role compared with the solar-wind sputtering and ion damage in producing the ubiquitous patinas and rims that contain silicarich, amorphous glass with embedded np-Fe 0 (5,14,21). Similarly, a larger proportion of the np-Fe 0 signal in the lunar soil, in particular, in the finest size fractions, is attributed to the surface-correlated, vapor-deposited np-Fe 0 , which increases as the surface area to volume ratio increases (24).…”