Lunar and Planetary Science XXIX 1853.pdf

A RE-EVALUATION OF LUNAR BASALT TYPES THROUGH SPECTRAL ANALYSIS OF FRESH MARE CRATERS Matthew Staid and Carle M. Pieters, Department of Geological Sciences, University, Providence, R.I. 02912 Multispectral data of small have been exam- craters will provide additional information about unit depth ined to characterize the reflectance properties of relatively and regional stratigraphy. immature materials derived from individual mare units. The Red basalts or mare surfaces contaminated by highlands? crystalline deposits associated with fresh mare craters repre- sent emplaced basalt composition more directly than mature A variety of data suggest that extensive regions of the lu- soils whose surfaces may have been affected by non-mare nar surface in the vicinity of mare/highland morphologic contamination and space weathering processes [1,2]. The boundaries may have been affected by the mixing of mafic and goal is to re-evaluate lunar basalt types [3] using the higher more feldspathic materials [e.g. 11,12,13]. These zones may spatial resolution global data from Clementine and more ad- result from small to basin-scale impact events, lateral mixing vanced analyses of the optical properties of lunar soils and across mare-highland contacts, and vertical mixing of cryp- rock powders. tomare and/or mafic intrusions with local highland materials [11, 14, 15]. For several regions of the lunar surface, it is not Approach to immature mare surfaces clear whether the properties of the surface represent the exis- Geochemically distinct mare regions on the surface of the tence of a compositionally distinct unit or the affects of high- have been identified based on measurements of contin- land mixing with adjacent mare regions. uum slope and near-infrared spectral properties [3]. The lunar The relatively red and bright, mare region of Lacus Som- maria exhibit a variety of specific basalt types, the majority of niorum is an example of a mare classification which could which are believed to be unsampled by the or Luna represent either a compositionally unique basalt type ("LBG-" landing missions. Laboratory reflectance studies of lunar soil [3]) or a mixture of highlands and less red Serenitatis basalts and rock powders demonstrate that crystalline lunar materials [11]. Fresh mare craters from eight orbits of Clementine im- exhibit very diagnostic absorption features related to sample agery (153-160) have been used to characterize immature mineralogy [e.g. 4,5,6]. However, since space weathering materials from the Somniorum "LBG-" region and neighbor- processes weaken diagnostic absorption features, it is difficult ing Serenitatis "mISP" and "hDW-" units. Optically immature to characterize the detailed mineralogy of the parent basalt materials from these regions are shown in Figure 2. from mature soils. Though spectral measurements of rela- Material from fresh mare craters in Somniorum less than 3 tively crystalline fresh craters are thought to be the best lunar kms in diameter exhibit spectral properties which are quite analogue to sample rock powders [6,7], the low spatial resolu- distinct from immature crater deposits in northern Serenitatis. tion of previous data has not allowed studies of craters small Though notably redder and brighter, immature Somniorum enough to have sampled individual basalt flows or fresh expo- mare exhibit a strong 1 micron feature characteristic of basalts. sures of basaltic stratigraphy within crater walls. The fact that the immature mare materials exhibit distinguish- High spatial resolution multispectral imagery was ac- ing properties similar to the surrounding soils (redder and quired by the Clementine ultraviolet-visible (UVVIS) camera brighter than soils to the south) indicates the craters and sur- during two months of lunar mapping in 1994 [8]. The five- rounding soil do indeed represent a spectrally distinct basaltic band imagery requires extensive processing for calibration to unit rather than a mixture of mare contaminated by highlands. produce multispectral image mosaics [9,10]. Since the optical Several larger craters under study within Lacus Somniorum properties of immature mare materials are a function of both (greater than 3 km in diameter) are however different from the composition and exposure age, materials from fresh craters small craters and appear to have excavated more feldspathic for a range of crater sizes and presumably maturities were material, indicating that the mare deposits are relatively thin. examined. Optically immature mare craters were selected for Plains Deposits and the dilemma analysis by thresholding 0.75 to 0.95 ratios to identify materi- als exhibiting a strong 1 µm absorption relative to surrounding The Cayley Plains west of Tranquillitatis are intermediate mare materials. Craters were then grouped according to size, in brightness between most mare and highlands. These plains binning approximate diameter ranges of: < 0.5, 0.5-1, 1-1.5, are relatively smooth and topographically similar to inter- 1.5-2, 2-3, 3-5, and 5+ km. A large database was formed from crater mare plains [16,17]. Since pre-Cayley materials may the spectral properties of all craters sampled. Plots produced have been buried or mixed with the plains during their forma- by averaging 5-color spectra for size ranges within composi- tion, the composition and extent of pre-existing materials is tionally distinct units in Tranquillitatis, Serenitatis and Som- unknown. However, the crater Dionysius (19 km) on the edge niorum are shown in Figures 1a and 1b. The number of craters of the Cayley Plains in south-western Tranquillitatis, is of averaged within each size bin is indicated in the figure cap- particular interest to stratigraphic studies of the region because tion. Mare craters show distinct trends which differentiate it exhibits a halo of dark rays which have been interpreted to spatial and geochemical units. Relationships between mature represent the excavation of primary mafic materials [18]. soils and immature deposits can also be readily examined. Five orbits of Clementine imagery over the Cayley Plains Two regions initially evaluated here include sites where region west of Tranquillitatis have been calibrated to examine emplaced basaltic composition may have been obscured by the the spectral properties of these Cayley soils and small craters. effects of lateral mixing or burial. Subsequent study of the As shown in Figure 3, spectral signatures from several loca- optical properties of immature materials associated with larger tions on the interior wall of Dionysius contain properties sug- Lunar and Planetary Science XXIX 1853.pdf

SPECTRA OF FRESH MARE CRATERS: M. Staid and C. M. Pieters

gesting a basaltic composition similar to that of western Tran- roxene), rather than basaltic or gabbroic (high-Ca pyroxene). quillitatis mare. The extensive dark rays around Dionysius, Spectral properties of other relatively large craters, to the north also exhibit a low albedo and blue UVVIS slope similar to (Ariadaeus pair ~10 and 8 km) and west (Cayley 14 km) of Tranquillitatis basalts. Dionysius also suggest a lack of basaltic ejecta from beneath Similar mafic materials, however, were not found in asso- the plains. The lack of mafic deposits associated with many ciation with small and intermediate sized craters in other areas intermediate size craters indicate that the majority of these of the plains to the north and west of Dionysius. Instead, the Cayley Plains are not underlain by a thick or spatially ho- crater materials are much brighter than Tranquillitatis basalts, mogenous region of basalts. Mafic materials excavated by exhibit a redder UVVIS slope and a significantly weaker 1 µm Dionysius may instead result from spatially limited deposits of absorption. The shape of the 1 µm absorption for Cayley pre-Cayley mare underlying the easternmost Cayley plains or Plains craters within the size ranges examined here are gener- impact into a post-Cayley inlet of basalts along the Cayley- ally consistent with a noritic composition (with low-Ca py- Tranquillitatis border.

1b) 415 vs. 750nm crater reflectance for several mare units 1a) Fresh mare craters in compositionally distinct units ( 750nm reflectance increases with crater size bin)

2) Craters in Northern Serenitatis and Lacus Somniorum 3) Craters in the Cayley Plains and SW Tranquillitatis

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