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

Transcription

1 Uefeorrfrcs B Planefary Science 34, (1999) 0 Meteontical Society, 1999 F'nnted in USA Space weathering on the Moon: Patina on Apollo 17 samples and SUSAN J. WENTWORTHI*, LINDSAY P. KELLER2, DAVID S. McKAY3 AND RICHARD V. MORRIS3 :Lockheed Martin, C23,2400 NASA Road 1, Houston, Texas 77058, USA *MVA, Inc., 5500/200 Oakbrook Parkway, Norcross, Georgia 30093, USA 3NASA Johnson Space Center, Houston, Texas 77058, USA *Correspondence author's address: susan.j.wen~orth@lsc.nasa.gov (Received 1998 June I; accepted in revised form 1999 March 4) Abstract-We studied patinas on lunar rocks and 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 but not on Glazed patina is found on both and 75075, whereas fragmental patina is found only on The glazed and fiagmental patinas on 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 shows evidence of possible solar wind sputtering erosion. Transmission electron microscope studies of 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 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 and 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 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. INTRODUCTION Space weathering is a term used for the modification of materials exposed to space at the surfaces of planets or small solar system bodies that lack protective atmospheres. It results from a number of processes including micrometeorite (and larger) impacts, solar wind, solar flares, and cosmic rays. An understanding of space weathering is of key importance to lunar and planetary science. Space weathering on the moon has created a record of the extralunar environment through geologic time (e.g., recording changes in flux and composition of the solar wind and meteoroids). Processes of space weathering are responsible for the formation and maturation of the lunar regolith as a whole. The optical, chemical, and geological properties of space-weathered material may be profoundly different from those of unweathered equivalents; therefore, it is critical to understand the processes and effects of space weathering in order to interpret data obtained by future remote sensing and landing missions. The surfaces of many rocks and soil grains in the Apollo collection have macroscopically distinct patinas, which are coatings or discolorations caused by exposure to the space weathering environment. Patinas have been studied in the past (e.g., Morrison et al., 1973; Blanford et al., 1974; Hartung et al., 1978; Zook, 1978), but detailed characteristics of patinas and mechanisms of patina formation are still not well understood. Space-weathering processes create both erosional and accretionary features. Characteristic features of direct exposure to the space-weathering environment include hypervelocity microcraters, small circular glass splashes (pancakes), implanted rare gases, nanophase iron (formed by reduction from FeO), and solar flare tracks (e.g., Morrison et al., 1973; Hartung, 1980; Walker, 1980; Allen et al., 1996). Other common features, such as larger glass splashes, vapor deposits, accreted soil particles, and galactic cosmic-ray tracks, also can be found on or in lunar rocks; these features indicate near-surface exposure but they are not proof of direct, face-up exposure to space. Many of the earlier patina studies emphasized the determination of surface exposure age and micrometeorite flux based on microcrater and glass pancake populations. Blanford et al. (1974) found textures on a chip of (one of the rocks of our present study) that suggested solarwind sputtering erosion, a space-weathering effect which had been postulated but never proven. However, their evidence was limited by the resolution of their scanning electron microscope. Morrison and Clanton (1979) obtained the highest resolution images possible at the time using a JEOL JEM-IOOC TEMSCAN with a point-topoint resolution of 30 A (3 nm) in the SEM mode. The smallest craters they observed were -200 A (20 nm) in diameter; smaller 593

2 594 Wentworth et al, features could not be interpreted. They found no evidence of solarwind sputtering erosion, but they suspected that the surfaces of their samples had been affected during sample preparation. Because technological advances now permit higher resolution imaging and more detailed chemical analysis, we are reanalyzing some of these samples that were studied previously. We began this study of lunar patinas as a part of our long-term studies of space weathering and regolith-formation processes in order to answer these questions: What are the characteristic compositions and petrographic features of patinas? Are there different types of patinas? If so, how did the patinas form? How strongly do the characteristics of a patina depend on the nature of the host rock? Why do some light-colored rocks have dark patinas? Are patinas on big rocks and boulders similar to patinas on individual small soil grains or are they fundamentally different? What is the relationship of patina to the TEM-scale amorphous coatings found on soil grains (Keller and McKay, 1993, 1997)? Can a rock be characterized by its patina characteristics? TECHNIQUES AND SAMPLES We surveyed available lunar sample catalogs and tabulated the samples that were documented as having patinas, coatings, or zap pits (small craters). For the present study, we chose chips from patinabearing surfaces from a variety of rocks, including mare basalt 75075, crystalline matrix breccia 76015, and dilithologic (anorthosite and impact melt) breccia (Neal and Taylor, 1993; Meyer, 1994; Ryder and Norman, 1980). Our work thus far has been focused on the two Apollo 17 samples, which were subdivided into several pieces. Patina-bearing and unweathered surfaces of untreated chips were used for bulk reflectance spectroscopy and backscatter MBssbauer spectrometry (BaMS) as described by Shelfer et al. (1995). An untreated 3 x 3 mm-sized fragment of 76015, 186, FIG. I Macroscooic Dhotoeraohs and low-maenification SEM images of ADOIIO 17 samoles and showing well-develoned Datina surfaces. (a) Macroscopic view of hi&-t'i mare basalt surface T (NASA-photo Sj ), whch was face down onihe Moo: when colleckd; block at bottom of photo is 1 cm across. Patina (P) is visible as dark coating; areas of fresh surface (F) can also be seen. Samples for present study were from about the center of the face shown in this photo. (b) Macroscopic view of vesicular crystalline matrix breccia surfaces B and W, with reproduced lunar lighting and orientation (NASA photo S ); block at bottom of photo is I cm across. Most of surface visible in photo is space-weathered. Typical surface darkening found on patina surface (P) is evident. Some dark spots in photo are vesicles but others, such as (M), are microcraters, which are haloed by bright spall zones of freshly-exposed rock (F); samples for the present study came from surface W of 76015, which was face up on the Moon when it was collected. (c) Scanning electron microscope (SEM) image of 75075,166. Patina (P) is coherent and in some places is up to 50 pn thick; texture is distinctly different from that of the fresh underlying rock (F). No microcraters are present. (d) Scanning electron microscope image of 76015,186 patina surface showing typical glass-lined pit (M) and spall zone (F), in which fresh rock surface has been exposed; spall zone overlaps spall zone (0) of another microcrater (not shown). Patina on is discontinuous and thin (<20 pm).

3 Space weathering on the Moon 595 with a very dark surface discoloration, was selected for detailed analysis using a microscope photometer (Zeiss MPM 400) to obtain reflectance spectra from submillimeter-sized regions of the surface coating as well as from freshly fractured surfaces of the rock chip (Keller et al., 1996). This technique was used previously to measure the visible reflectance of lunar pyroclastic glass (Allen et al., 1994); complete details of the microspectrophotometry technique are given in Bradley et al. (1996). Following the reflectance measurements, a small fragment of the patina on was embedded in low-viscosity epoxy. Transmission electron microscope (TEM) specimens were prepared using an ultramicrotome to cut thin sections nm thick, with the sample oriented so that multiple crosssections of the patina-rock interface were obtained. The microtome thin sections were analyzed in a JEOL 2010 (200 kv) TEM equipped with a Noran thin window energy dispersive x-ray (EDS) spectrometer. Quantitative EDS analyses were obtained using the Cliff-Lorimer thin-film analysis method with experimental k-factors obtained from a variety of synthetic and mineral standards, including NIST-SRM-2063 thin film standard, troilite, and Ni2Si04 (details given in Bradley, 1994). The EDS spectra were collected such that the errors based upon counting statistics were <3% for major elements. Feldspars were analyzed with low beam currents in order to avoid beam damage that can cause element diffision away from the analysis volume. Larger patina-bearing chips of 75075,166 and 76015,186 were carbon coated for scanning electron microscopy (SEM) and qualitative EDS using a JEOL 35CF SEM equipped with a PGT System IV energy dispersive detector. Chips of 76015,186 patina were also used for high-resolution SEM imaging with a Philips XL-40 field emission gun SEM (FE-SEM). Subsamples of both and were used to make polished petrographic thin sections oriented normal to the patina surfaces. The polished thin sections (75075,183; 75075,184; 75075,185; 75075,186; 76015,196; 76015,197) were used for SEMEDS and backscattered electron imaging (BE1 or BSE) with the JEOL 35CF SEM, and also for wavelength-dispersive x-ray (WDS) element mapping with a Cameca 1 OOSX scanning electron microprobe. Apollo 17 sample is a coarse-grained high-ti mare basalt from Station 5 near the southwest rim of Camelot Crater, the largest crater (650 m across) visited by the Apollo I7 crew. Basalt consists mostly of pyroxene,, and ilmenite, with minor olivine and small amounts of armalcolite, interstitial silica, troilite, and Fe-Ni metal (Neal and Taylor, 1993). Because was found as a loose fragment on a boulder, and because its composition is different from the other Station 5 basalts (75015, 75035, and 75055), which are thought to represent Camelot subfloor material, it is thought that was emplaced at Station 5 by a post-camelot impact event (Wolfe et al., 1981). Macroscopically (Fig. la), the patina consists mostly of a dark gray to black coating. In some places, the dark patina is replaced by or covered with a thin layer of reddish-brown material, which is concentrated in depressions. Fresh basalt, with a distinct coarse-grained texture and lighter gray color, is exposed in a few areas where the patina has been chipped away (either on the Moon or during laboratory processing). Nearsurface (within -1 m of the surface) Kr and Ar exposure ages for range from 119 to 143 Ma (Horz et al., 1975). Although has a distinct patina, hypervelocity impact craters and solar flare tracks, which would indicate direct exposure to space, have not been documented for any surface of The samples of for the present study came from surface T (Fig. la). Catalog descriptions (Lunar Receiving Laboratory, 1973; Neal and Taylor, 1993) indicate that surface T was the only side of with a significant patina. This surface was face down on the Moon at the time the sample was collected. Apollo 17 sample (Fig. Ib) is a poikilitic crystalline matrix breccia with a nonmare composition, consisting largely of and low-ca pyroxene, with minor augite, olivine, ilmenite, armalcolite, and Fe-Ni metal (Meyer, 1994). It was collected from block 5 of the Station 6 boulder on the south slope of the North Massif. Based on cosmic-ray tracks, Kr isotopes, and noble gas data for and other Station 6 samples, its exposure age is -22 Ma (Crozaz et al., 1974). Solar-flare track data indicate that block 5 broke off the Station 6 boulder -1 Ma ago (Crozaz et al., 1974). The patina previously was studied by Blanford et al. (1974) who performed SEM analyses along with track studies. Figure Ib is a macroscopic view of with reproduced lunar lighting and orientation. The sample used in this study (76015,186) was taken from the W1 surface, which was face up on the Moon at the time the sample was collected. Macroscopically, the patina on (Fig. 1 b) consists of a fine-grained, brownish gray discoloration that is darker in some areas than others. White to light gray spots marking relatively fresh microcraters with spall zones (craters ranging -5- IOOOpm in diameter) are common. RESULTS AND DISCUSSION Scanning Electron Microscope Petrography and Patina Classification At low magnifications in the SEM (Fig. lc), the patina on basalt appears to be coherent. As noted above, the patina is chipped in some places, exposing fresh basalt. The thickness of the patina is variable and ranges up to 50 pm thick. The patina layer obscures the mineralogy of the underlying basalt, but it conforms to the shape of the underlying minerals and preexisting fractures in such a way that a hint of the texture of the underlying rock is still evident (Fig. 1 c). Like previous workers (e.g., Lunar Receiving Laboratory, 1973), we found no microcraters or glass pancakes on 75075, which would have indicated that the patina surface had been exposed directly to space. At higher magnifications in the SEM (Fig. 2a,b), it is clear that has two distinct types of patina. One type consists of a thin layer or layers of heterogeneous glassy material (Fig. 2a). It conforms to the underlying rock very closely but causes rounding of sharp edges. We define this smooth, glassy patina as glazed patina; note that we have changed this term from our previously defined accretionary coalesced (AC) term (McKay et al., 1995), which was less descriptive and more cumbersome. The other distinct type of patina on consists of fine-grained fragmental material welded to the surface of the rock (Fig. 2b), which we term fragmental patina; we previously termed this type as accretionary welded fragmental (AWF) patina (McKay et al., 1995). Most of the individual grains in the fragmental patina are rounded to subspherical, and they seem to be fairly well sorted, with most grains in the 1-5 pm diameter range. Fragmental patina is concentrated in depressions, whereas glazed patina is most prominent on protrusions. We infer that the dark gray to black patina seen macroscopically is glazed patina and that the macroscopic reddish-brown material is fragmental patina. Multiple layers of both types of patina are present in some areas. In contrast to the patina, there is clear evidence of direct exposure to space on breccia 76015, even at low magnifications in SEM (Fig. Id). Microcraters with spall zones (pits pm across) are common. The spall zones overlap, indicating that the

4 596 Wentworth et al. FIG. 2. Scanning electron microscope images of patina on and 76015: (a) glazed patina on 75075,166; (b) fragmental patina on 75075, 166; (c) classic patina on 76015,186; microcraters and glass pancakes (disks) indicate that the surface has been exposed directly to the space weathering environment. Substrate is. (d) Example of well-developed patina on pyroxene in 76015,186; surface has abundant microcraters, glass pancakes, and spherules; substrate (flat, angular areas) is still identifiable as pyroxene patina surface is saturated with microcraters and has reached a steady state, as previously noted by Blanford et al. (1974). The surface of has a wide range of apparent degree of space weathering over very small distances; some areas are highly weathered whereas others appear to be fresh. The patina is discontinuous and generally very thin (120 pm thick). As a result, the mineralogy of the underlying rock is still apparent for most of the surface, although the original texture of the rock has been obscured to some extent by impact fracturing. Most crater pit linings and spall zones on are themselves weathered, containing glass pancakes along with smaller (15 pm) microcraters, which are too small to have spall zones (Fig. 2c). Other features commonly found on 76015, such as glass spherules, stringers, larger glass splashes, and sintered mineral fragments, do not necessarily indicate direct exposure to the space weathering environment (as noted above). Because microcraters and glass pancakes are characteristic of surfaces that have been exposed directly to space, we use them to define a third type of patinaclassic patina-so named because it is the patina type studied by earlier workers (e.g., Blanford et al., 1974); we previously called this type accretionary microcratered pancake-bearing (AMP) patina (Wentworth et al., 1996). Classic patina is the most common type of patina on 76015; some areas have a smooth patina very similar to the glazed patina defined for 75075, except that microcraters and glass pancakes are found on the version of glazed patina, whereas they are completely absent from Sample does not seem to have any patina equivalent to the fragmental patina. The accumulation of classic patina appears to proceed in the same way on different minerals and glasses. For example, the surface shown in Fig. 2c is moderately weathered, and the surface shown in Fig. 2d (at the same scale) is more heavily weathered pyroxene. Both surfaces show the same types of features. Differences in the degree of weathering, like those seen in Fig. 2c,d, are common on the patina surface. The wide range in types and extent of weathering effects in different areas of indicates that space-exposure histories are highly variable at the micron scale. High-resolution Scanning Electron Microscopy of We have begun high-resolution SEM imaging of patina-covered chips of ,186. The recent development of field-emission gun scanning electron microscopy permits imaging of surfaces at a much higher resolution than has ever been possible. The FE-SEM reveals that the surface of is extremely heterogeneous even at a submicron scale. For example, Fig. 3a and b are medium-resolution

5 Space weathering on the Moon 597 FE-SEM images of typical patina. The scale of these images is about the same as those of Fig. 2c,d. Microcraters and glass pancakes can be discerned easily. The general surface texture shown in Fig. 3b is more smooth than that in Fig. 3% but both figures contain hints of very fine-grained textural heterogeneity. High-resolution FE-SEM images (Fig. 3c,d) demonstrate that dramatic features are indeed present at a very fine scale. Solar-wind sputtering erosion is a likely cause of the etched texture of the surface shown in Fig. 3c (area not shown at lower magnification). Etching appears to have been both crystallographically controlled and directional; further evidence of directional etching is seen in the glassy rim of the microcrater at the center of the image, which is more eroded on one side than the other. Possible solar-wind etching can also be seen in Fig. 3d (a higher resolution view of the Fig. 3a area); in this case, directional etching is not obvious. The etched surface has been partly covered by the subsequent deposition of either splash glass or vapor-deposited material, or a combination of both. The presence of vapor deposits on the surfaces of lunar rocks and soil grains was postulated by Hapke et al. (1975) but was not seen in SEM or TEM until recently (Keller and McKay, 1993, 1997) during TEM studies of fine-grained lunar soils. We cannot positively identify the deposits in Fig. 3d as vapor deposits because FE-SEM/EDS at this scale cannot resolve the compositional characteristics that would distinguish vapor deposits from splash glass. However, TEMEDS can discern compositions at these scales, as discussed below and in Keller and McKay (1997). Backscatter Electron Imaging and Element Maps Backscatter electron imaging of a cross section through the patina shows that it is characterized by dramatic layering (Fig. 4a). The layers are heterogeneously distributed and discontinuous. The layers of rounded grains are fragmental patina like that shown in Fig. 2b. Most of the rounded grains are slightly irregular (i.e., it does not appear that they were originally spherules but instead that they might have been rounded by abrasion). The thin glass layers in Fig. 4a are glazed patina layers equivalent to that shown in Fig. 2a. The number of layers of glazed and fragmental patina varies from place to place; in Fig. 4a, there are several interbedded layers of glazed and fragmental patina, all superimposed on a layer of angular rubble at the rock-patina interface. The WDS element maps (Fig. 4b) of the same area show that the average composition of the patina is different from that of the underlying rock, at least on a micron scale. The element maps and qualitative SEMEDS analyses indicate that the bulk of the glass layers and rounded grains are composed of Fe-,Ti-rich glass. This glass is also enriched in Al and possibly Ca relative to the underlying rock, which is mostly low-ca pyroxene at the interface. The WDS element maps (Fig. 4b) also indicate that there is a general depletion of Mg in the patina except for isolated Mg-rich patches, which are augite clasts, as indicated by their corresponding Ca enrichments. Very high-ti areas in the patina correlate with Fe enrichments, which is consistent with ilmenite. Clasts of augite,, and ilmenite are mixed in minor amounts with FIG. 3. Field emission SEM images of 76015,186 patina; substrate is mostly. (a) Typical patina surface on 76015,186, showing abundant accretionary spherules. (b) Another typical area of patina on 76015,186; arrows indicate small spherules buried by thin layers of glass. (c) Eroded microcrater (center of image) in 76015,186 ; rough, etched-looking surface is evidence of probable solar-wind sputter erosion; etching seems to be directional. (d) Closeup of same area as shown in 3a; more than one generation of space weathering is evident because etched surface (E) in some areas is coated by splash glass or vapor deposits (V), upon which spherules have been deposited.

6 598 Wentworth et al. FIG. 4. Backscattered electron image and WDS element maps of typical patina cross section, polished thin section 75075,185. (a) Backscattered electron image; distinct layers of fragmental patina are interspersed with thin, discontinuous layers of glass (glazed patina), which are indicated by arrows; brightness of each glass or mineral is proportional to average atomic number; black areas are voids that were filled with epoxy when the thin section was made. (b) Wavelength-dispersive x-ray element maps of same area as in (a); both the glazed and the fragmental patina layers in this sample consist mostly of heterogeneous glass, which is enriched in Ti, Al, and possibly Ca, and depleted in Mg, relative to underlying rock. Host rock at contact is mostly low-ca pyroxene, with augite (high-ca pyroxene) at far right of image and below contact, and some olivine (low-ca, high-fe, Mg) further below surface; angular rubble at rocwpatina interface is mostly ilmenite (very high-ti, Fe), (high-ca, Al), and augite (high-ca, Mg); these mineral fragments are most likely fragments from host rock. glass fragments and interspersed layers of glazed patina. A modal analysis of the patina in thin section was done using qualitative SEMEDS. A grid pattern was used, and voids were excluded. Data were collected from several areas with assorted patina characteristics. The combined data (200 points total) show that the patina consists of 89.0% Fe-, Ti-glass; 5.0% high-ca pyroxene; 3.0% ; and 3.0% ilmenite. The Fe-, Ti-rich glass occurs as rounded grains and as irregular grains; it also forms the thin layers of glazed patina (Fig. 4a). Mineral fragments are generally concentrated in the patches of rubble found between the host rock and patina, as shown in Fig. 4a. Grains in rubble patches were not counted in the modal analysis because, although they are commonly found in depressions in the rock, they are not actually part of the patina. The rubble layer in Fig. 4a consists of angular fragments of ilmenite, augite, and, which were trapped by the deposition of the glazed and fragmental patinas. The patina is thin and discontinuous in cross section (Fig. 5a), as expected from SEM imaging of surface chips (Figs. 2

7 Space weathering on the Moon 599 FIG. 5. Backscattered electron image and WDS element maps of typical patina cross section, polished thin section 76015,197. (a) Backscattered electron image: patina is thin and heterogeneous; minerals at rock boundary are (dark) and IowCa pyroxene (small brighter grain); elongated grain in rock below patina contact (and below low-ca pyroxene) is augite; black areas are epoxy-filled voids. (b) Wavelength-dispersive x-ray element maps of same area as in (a). Patina is enriched in Ti, Fe, and Mg and seems slightly depleted in A1 and Ca relative to in host rock. Apparent slight enrichments in 0 and S are characteristic of patina; these apparent enrichments are not present on unweathered edges of thin sections. and 3). The WDS element maps (Fig. 5b) show that the patina is compositionally heterogeneous. In the area shown in Fig. 5a,b, the host rock consists of and low-ca pyroxene, with an augite grain just below the interface between the rock and the patina. The patina appears to be enriched in 0 and S relative to the underlying rock. The patina is also enriched in Ti, Fe, and Mg relative to. Some of these enrichments are concentrated in small areas and are probably due to the presence of ilmenite, spinel, and pyroxene clasts in the patina. Aluminum and Ca are depleted relative to the underlying because of the addition of material from other parts of the rock, and possibly from the local soil. Transmission Electron Microscope Petrography Transmission electron microscopy of the ultra-microtomed thin sections of 76015,186 shows that much of the patina consists of a complex mixture of melt glass, mineral grains, and other accreted materials (Fig. 6). The melt glass ranges from composition to silica-rich with abundant, submicron-sized Fe-metal inclusions (Table 1); some of the glass is also vesicular on a submicron scale. The crystalline grains within the patina are predominantly with minor ilmenite. Transmission electron microscope data indicate that the inclusions in the patina contain high solar-flare track densities, ranging from -1 x 10'1 cm-2 near the 76015

8 600 Wentworth eta/ FIG. 6. Transmission electron microscope images of ultramicrotome thin sections of 76015,186. (a) Dark-field TEM image showing a high density of solarflare tracks (-10 cm-2) in within the patina (dark linear features are the tracks, some of which are indicated with arrows; symbol in the upper left of the figure indicates the orientation relative to the patina surface; tracks show an apparent preferred orientation perpendicular to the surface of the rock. (b) Bright-field TEM image of small portion of the outermost surface of patina showing chromite grain that has partly melted to produce CrFe metal grains in melt glass; superimposed on the chromite is a vapor-deposited rim of amorphous silica-rich material with abundant nanometer-sized grains of Fe metal (see text). (c) High-magnification TEM image of vapor-deposited amorphous coating on melt glass (note strong similarity in microstructure to rim shown in (b)); layering of Fe metal grains within rim indicates multiple episodes of vapor deposition. (d) Bright-field TEM image of submicroscopic glass droplet (HASP) adhering to outermost patina surface; note vapor-deposited rim between droplet and substrate. weathering surface to values in the low 10*O cm2 range at a depth of a few microns. Several grains in the patina show a preferred orientation of tracks normal to the patina surface (Fig. 6a). This preferred orientation suggests that these grains acquired tracks differently from typical lunar soil grains. Tracks in lunar soil grains normally have a random orientation because regolith gardening processes constantly rotate the grains (Borg et a/., 1980, 1983). If, as we believe, the tracks in the grains in the patina were acquired after the grains were in place, then there are a number of interesting implications. For example, the track densities place constraints on the age of the patina in this particular region of the surface (-lo4- los years, e.g., Sandford, 1986). The presence of tracks also indicates that the accreted grains have not been heated above the track annealing temperature since they acquired their tracks. With pulse-heating, the damage trail left by a solar flare particle is annealed at temperatures of 60&800 C (Sandford and Bradley, 1989). Thus, these track-rich grains escaped the thermal effects from subsequent nearby micrometeorite impacts. Not all grains in the patina were so fortunate, however. Figure 6b shows a chromite grain at the patina surface that has been partly melted to form CrFe metal inclusions in a silica-rich matrix. The outermost surface of the patina is covered by an amorphous rim of silica-rich glass -50 to 80 nm thick with numerous inclusions of fine-grained (1-5 nm diameter) Fe metal (Fig. 6b,c). Highmagnification images show that the Fe metal grains occur in distinct layers within the amorphous rim (Fig. 6c). The amorphous rim is relatively uniform in composition and thickness, and coats, ilmenite, melt glass, and chromite (Table 1). Analyses by TEMEDS were collected at various locations along the lateral extent of the rim. The microstructural and chemical characteristics of this rim are similar to the inclusion-rich rims on individual soil grains described by Keller and McKay (l997), and so we infer that the formation process was also the same, that is, that the rim represents material that was deposited on the rock surface, largely by condensation of impact-derived vapors, with a contribution from sputter deposition. Adhering to the surface of the amorphous rim are submicrometer glass spheres with a wide range of compositions (Fig. 6d). Quantitative TEWEDS compositions of six of these droplets are given in Table 2. Of the six analyzed droplets, none are monomineralic melts. Three have Al abundances that exceed the values for Si and are classified as HASP (high-al, Si-poor) glasses, and the other three are silica-rich. The HASP glasses are believed to represent evaporative residues of melt droplets produced in impacts (Naney et a/., 1976) and are :specially common in the finest size fraction of

9 Space weathering on the Moon 60 1 TABLE 1. Quantitative TEMEDS compositions (atom%) of amorphous rim material on the uppermost surface of patina on fragments of crystalline matrix breccia 76015,186.* No. 0 Mg Al Si Ca Ti Cr Fe Substrate ilmenite chromite glass Avg Sd *Listed in order of decreasing Si abundance lunar soils (Keller and McKay, 1992). These droplets are intimately associated with microcraters and zap pits on the patina surface, and so we believe that most, if not all, of the droplets are of local origin (i.e., from itself). Spectral Reflectance and Microspectrophotometry Reflectance spectra of bulk surfaces of 75075,166 and 76015,186 (Fig. 7qb) show that, for both samples, the patinas are slightly darker than equivalent unweathered rock surfaces, a result expected from reflectance studies of lunar soils, which show a distinctive darkening with increasing maturity thought to be caused by the impact reduction of FeO to nanophase Fe (e.g., Adams and McCord, 1973). In addition to bulk spectral reflectance, microspectrophotometry (reflectance spectra from multiple regions -0.1 mm in diameter using a microscope photometer) was done for the chip of 76015,186 that was later ultra-microtomed for TEM. Typical spectra over the wavelength range of 380 to 1040 nm from both the patina and the underlying are given in Fig. 8qb. The patina has a much lower albedo than the (by a factor of -4) and has a slightly redder slope over the visible wavelengths. It should be noted that the microspectrophotometry spectra in Fig. 8qb represent the extreme of optical effects observed from the patina-coated surface that was analyzed. For this particular sample, the patina coverage varies considerably at the millimeter scale. Use of the microscope photometer, however, allows reflectance data to be obtained from the same surfaces that are subsequently analyzed by SEM and TEM. Backscatter Mossbauer Spectroscopy Shelfer et al. (1995) have developed a backscatter Mossbauer spectrometer instrument with the objective of developing instrumen- TABLE 2. Quantitative TEMlEDS compositions (atom%) of typical submicroscopic glass droplets on the outermost surface of patina on fragments of crystalline matrix breccia 76015,186.* No *vg Sd Al Si *Listed in order of decreasing Si abundance. Ca Ti Cr tation for future robotic missions to the Moon and other planetary bodies. Mbssbauer spectroscopy, a nuclear resonance technique, provides detailed information about the local electromagnetic environment of individual 57Fe nuclei. It yields information about Fe a Patina 0'----I----'----'----' Wavelength (nm) Interior I -ly Bulk 920 nm %f Wavelength (nm) FIG. 7. Reflectance spectra of patinas, fresh unweathered rock surfaces (interiors), and bulk powdered samples; total reflectance includes significant contribution (factor of 2-3) from white sample holder. (a) Reflectance spectra for 75075,166 and (b) reflectance spectra for 76015,186; patinas on both and are darker than unweathered samples. Upper lines are spectra separated to show shapes (not to scale).

10 602 Wentworth et al. a 60 v 9) u ) I patina O1 W P X, Cl(2+) Transmission, powdered bulk sample 4 L Backscatter, b A E60~ 8 H c substrate 01 I Wavelength (nm) FIG. 8. Typical microspectrophotometer reflectance spectra from : (a) thick patina and (b) freshly fractured surface of underlying ; spectra were obtained from regions -0.1 mm in diameter. the distribution of Fe among the minerals in a rock and about the oxidation state of Fe. Transmission Mossbauer spectroscopy, with the source and detector on opposite sides of the sample, is commonly used in the laboratory; but backscatter geometry is preferable for robotic missions to other planetary bodies because the source and detectors are on the same side of the sample, and also because no sample preparation is needed. Using the BaMS instrument developed by Shelfer et al. (1 995), we determined the backscatter Mossbauer spectrum of the patinacovered side of (Fig. 9). Its signature is very similar to that of the equivalent unweathered rock surface, and also to that of crushed rock powder determined by transmission Mossbauer spectroscopy (Fig. 9). The similarities between spectra from weathered and fresh material indicate either that the proportions of Fe in ilmenite, pyroxene, and olivine are the same in the patina as they are in the fresh rock, or that most of the backscatter Mossbauer signal came from below the patina. The latter possibility seems likely, because theoretical calculations suggest that the BaMS signal could come from as much as 100 pm below the sample surface. Therefore, Mossbauer may be a very useful instrument because it does not seem to be affected by the patina, whereas data obtained by other methods are clearly influenced by patinas on surfaces of rocks. SCENARIO A possible scenario for the formation of classic patina, as found on , is as follows: (1) Fresh rock is exposed at the lunar surface by a large (>I000 pm) impact. Some patina material may be deposited on the rock as a result of this initial impact.... I.- I Velocity (mmls) FIG. 9. MOssbauer spectra of patina and equivalent unweathered surface of 76015,186. Spectra from patina surface seems to be indistinguishable from that of the unweathered host rock. (2) Accretionary patina accumulates by deposition of vapor and sputter products and accretion of melts and fine fragments. (3) Along with accretion, minor erosion occurs by fine-scale (4 pm) microcratering and sputter erosion. (4) Rock surfaces are renewed occasionally by pm-sized craters with spall zones. Spalling occurs frequently enough that the patina buildup is not very thick (e.g., a maximum of -20 pm for 76015). (5) Patina accretion begins again on renewed surfaces. (6) The patina surface may reach a steady state. (7) Eventually, the rock is worn away by spallation from microcraters. (8) Alternatively, the rock may be destroyed by catastrophic disruption by a large impact. (9) Instead of (7) or (8), the rock may be buried and preserved. This could happen at any stage of the sequence. Possible scenarios for the formation of glazed and fragmental patinas like those found on are somewhat different, and possibly simpler, than that for classic patina. Basalt probably underwent (I) (above), and it is possible that a single impact event exposed the rock and deposited all of its patina at the same time. It is also possible that patina accumulated on from a few to several subsequent nearby impacts, with intervals of accumulation of rock and soil fragments between the impacts. Further work may reveal whether the surface was directly exposed to space during any of the intervals. In essence, though, scenarios for glazed and fragmental patina formation include (I), (8), and (9), given above. SUMMARY We have determined a classification scheme for different textural types of patina on Apollo 17 samples and Mare basalt has two distinct patina types: fragmental and glazed. The patina on breccia is classic patina, which is characterized by the presence of microcraters and glass pancakes, which are direct evidence that the surface has been exposed directly to space. Sample also contains some areas of glazed patina similar to that defined for The lack of classic patina on indicates that its patina surface did not form as a result of

11 Space weathering on the Moon 603 direct, face-up exposure to space. Using FE-SEM, we have also documented evidence of probable sputtering erosion on the spaceweathered surface of In TEM, much of the patina consists of a complex mixture of impact glass, mineral grains, and other accreted materials. The composition of the glass ranges from -like to silica-rich; Fe-metal inclusions are abundant. Mineral clasts in the patina are predominantly with minor ilmenite. Solar-flare track measurements in grains found in the patina constrain the patina age at years in the region studied. Because of the wide range in degree of weathering in different regions on the surface, it is probable that solar-flare track ages would also have a wide range. The outermost surface of the patina has a thin ( nm) amorphous rim of silica-rich glass, which contains abundant inclusions of fine (1-5 nm diameter) Fe metal grains in distinct layers. The microstructural and chemical characteristics of the amorphous rim on the patina are very similar to those of rims on individual lunar soil grains (Keller and McKay, 1997). These similarities strongly indicate that the rims on the soil grains and the patina formed by the same processes (ie., by condensation of impact-generated vapors with a contribution from sputter deposition). Submicron-sized glass spheres, which adhere to the surface of the amorphous rim, have HASP or silica-rich compositions. They are very similar to spheres found in the finest fractions of lunar soils and probably formed by similar impact volatilization processes. Wavelength dispersive x-ray elemental mapping of and indicates that patina compositions are not a good indicator of compositions of minerals in the underlying rocks. Reflectance spectra of and patinas are darker than those of unweathered equivalent surfaces, as expected from studies of lunar soils. 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