Workshop on New Views of the Moon 6045.pdf

INTEGRATED MULTISPECTRAL AND GEOPHYSICAL DATASETS: A GLOBAL VIEW OF LUNAR PYROCLASTIC DEPOSITS. L. Gaddis1, C. Rosanova1, B.R. Hawke2, C. Coombs3, M. Robinson4, and J. Sable5 (1U.S.G.S. Astrogeology, 2255 N. Gemini Drive, Flagstaff AZ 86001, USA ([email protected]), 2PGD/SOEST, 2525 Correa Road, University of Hawaii, Honolulu HI 96822, 3Department of Geology, College of Charleston, Charleston SC 29424, 4Northwestern University, Department of Geological Sciences, 1847 Sheridan Road, Evanston IL 60208m USA, 5Amherst Col- lege, Amherst MA, USA.

Introduction: We are integrating multispectral Clementine rock and glass-rich juvenile material with small amounts of UVVIS data [1] with crustal thickness data [e.g., 2] to ex- basaltic caprock material. Examples of Group 1 deposits amine the composition and distribution of lunar pyroclastic are found on the floors of Atlas Crater (45°N, 45°E), Frank- deposits. Examples are the large deposits of Apollo lin Crater (29°N, 48°E), and near Grimaldi Crater (1°S, 17/Taurus Littrow and Aristarchus [e.g., 3,4], and the small 64°W). Group 2 mafic bands are centered near 0.96 mm; deposits (or endogenic “dark-halo” craters) located along Group 2 spectra are similar to those of mature mare depos- fractures in the floors of Alphonsus [e.g., 5; 6], Atlas [7], its, and they are dominated by cpx. Small pyroclastic de- and Schrodinger [8] craters. Our early efforts focus on the posits in Group 2 appear to consist largely of basaltic mate- small pyroclastic deposits because of their relative youth rial. Examples are located east of Aristoteles Crater (50°N, (~1 b.y. in some cases), their broad global distribution, and 21°E and 28°E). Group 3 mafic bands are broad and cen- the fact that their small sizes may have inhibitied early tered near 1.0 mm; Group 3 deposits are dominated by oli- Earth-based (~500 m spectral spot size at best) spectral vine and opx; the olivine is almost certainly associated with analyses. We are now studying a variety of small deposits, juvenile material, and the opx is likely to have been wall including those of the Atlas Crater, Franklin Crater, Eastern rock [11]. Examples of Group 3 small pyroclastic deposits Frigoris highlands, Oppenheimer Crater, Lavoisier Crater, are those of J. Herschel Crater (62°N, 42°W), Alphonsus and Orientale Crater regions. Our goals are (1) to under- Crater (13°S, 4°W), and south of Cruger Crater (17.5°S, stand the full extent of interdeposit compositional variations 67°W). among small lunar pyroclastic deposits; (2) to evaluate the possible effects of soil maturation and lateral mixing on the Compositional Analyses: We used the USGS ISIS soft- “true” compositions of these deposits; (3) to determine the ware to create and examine Clementine UVVIS multispec- prevalence and nature of intradeposit compositional varia- tral mosaics (~100 m/pixel) of areas representative of the 3 tions; (4) to identify and characterize the juvenile compo- major compositional classes of small lunar pyroclastic de- nents of these deposits; and (5) to understand the implica- posits. Compositional analyses of these deposits are based tions of these results for studying lunar eruption mecha- on color-ratio comparisons for each area. The ratios exam- nisms. ined are the 450nm/750nm or uv/vis ratio, suggestive of relative titanium content, and the 950nm/750nm ratio, a Previous Work: More than 90 lunar pyroclastic deposits measure of the 1.0-micron band strength and suggestive of have been recognized [e.g., 9]. Lunar pyroclastic deposits relative mafic content (low=strong or deep 1.0-micron band; have been split into "regional" and "localized" deposits on high=weak or shallow band). Although the systematics the basis of size, morphology, and occurrence. Regional have not been resolved for application to lunar pyroclastic deposits can be up to several 1000 km2 in size, while local- deposits, these ratio values can also be interpreted in terms ized or small pyroclastic deposits are typically 200 to 500 of relative soil maturity: a mature soil is red and has a rela- km2 in size [3]. Regional deposits are thought to have been tively shallow 1.0-micron band, while a more immature soil emplaced as products of continuous or Strombolian-style is blue, with a deeper 1.0-micron band. eruptions, with wide dispersion of well-sorted pyroclasts [e.g., 10]. Analyses of Apollo samples and Earth-based Figure 1 shows color ratio values extracted from each of the spectral reflectance studies have identified a significant three classes (11 sites, including Orientale, Oppenheimer, component of Fe2+-bearing volcanic glass beads in many of Nernst, Lavoisier, and Compton), with spectra obtained in the regional pyroclastic deposits (e.g., [3]). the vicinity of the probable vent area. All of the deposits shown have ~shallow mafic band depths (all >1), and they Intermittent or Vulcanian-style eruptions are likely to have are relatively blue (uv/vis ratios > 0.54, comparable to many produced the small pyroclastic deposits, with explosive lunar highlands deposits). Regarding interdeposit composi- removal of a plug of lava within a conduit and forming an tional variations, we see three major clusters. The larger endogenic vent [10, 11]. The small pyroclastic deposits cluster (upper right) has a wider range of mafic band depths have been further subdivided into three compositional and uv/vis values, the mafic bands are shallower (ratio val- classes on the basis of their "1.0-mm" or mafic absorption ues are higher) than those of the smaller clusters in the bands in Earth-based spectra (e.g., [11]). Group 1 mafic center and lower left, and the uv/vis values are generally bands are centered near 0.94 mm; spectra resemble those of higher (bluer). The larger cluster includes several different typical highlands and are indicative of the presence of feld- small pyroclastic units, including those of Atlas/south and spar-bearing mafic assemblages dominated by opx. Group 1 Franklin--these are the Group 1 deposits (similar to high- deposits appear to be mixtures of highlands-rich country lands)--and they are joined by the Orientale deposits and our Workshop on New Views of the Moon 6045.pdf

SMALL PYROCLASTICS: L. Gaddis et al.

“new” deposits at Oppenheimer and Lavoisier. The central larum and near Mare Nubium. Also, several small deposits cluster has deposits such as Atlas/north, E Frigoris W and E are found in the floors of floor-fractured craters, where they and the deposit in the crater floor located northeast of La- are associated with endogenic craters on fractures [13]. voisier--these can be classified as Group 2 deposits (similar Many of the floor-fractured craters are Imbrian or Pre- to mare deposits). The smaller cluster to the lower left, Imbrian in age (>3.2 b.y.), although a pyroclastic deposit includes only the Group 3, J.Herschel Crater deposit. near Taruntius crater appears to be relatively young (~1 b.y.) [14]. Of >80 floor-fractured craters mapped by Several interesting aspects of these data must be noted. Wilhelms [15; Plate 5], ~15 have pyroclastic deposits, sug- First, we have corroborated the Earth-based spectral classi- gesting that these features may have provided an enhanced fication scheme: we generally see 3 distinct spectral environment for explosive volcanic eruption. classes. The mafic band depths show the expected trend of increasing mafic band depth from Group 1>Group 2>Group The spatial association of volcanic eruption sites and impact 3 deposits. Although mixed spectral signatures cannot be craters and basins on the Moon is believed to be related to ruled out, it appears that the coarser spatial resolution of the crustal thinning beneath impact sites. Small pyroclastic Earth-based spectral data has successfully characterized the deposits are observed in regions with crustal thicknesses small pyroclastic deposits. Second, we have classified the ranging from 30 to 80 km [2], with the majority near 50 km. Orientale spectra as a small pyroclastic deposit belonging to On the near side, these crustal thicknesses are typical of the Group 1--this deposit has been described and modeled as margins of the major maria; on the far side, thinner crust is strombolian by Weitz et al. [12]. The unusual annular observed in the South Pole/Aitken basin and the Moscovi- structure is suggestive of a vulcanian eruption mechanism, ense basin, sites where pyroclastic deposits are observed. but the large size of the Orientale pyroclastic deposit is We are currently evaluating these data to investigate impli- compatible with a strombolian style of eruption. In our cations for modes of eruptions of lunar pyroclastic deposits. observations, the Orientale pyroclastic deposit is bluer than most Group 1 small pyroclastics, but has shallow mafic References: [1] Clementine Basemap Mosaic, 15 volumes, bands similar to those of the Group 1/Franklin Crater de- prepared by USGS for NASA, available via the Planetary posits. Finally, the Group 3 spots of J. Herschel form an Data System Imaging Node; [2] Zuber et al. (1994) Science, entire small cluster; with the the deepest mafic bands and 266, 1839–1843. [3] Gaddis et al. (1985) Icarus, 61, 461– the reddest of the small pyroclastic deposits, these deposits 488. [4] McEwen et al. (1994) Science, 266, 1858–1861. are so far the most unequivocal representatives of juvenile [5] Robinson et al. (1996) LPS XXVII, 1087–1088. [6] Head mafic materials observed among the small pyroclastic de- and. Wilson (1979) Proc. LPSC 10th, 2861–2897. [7] posits. Hawke et al. (1989) Proc. LPSC 19th, 255–268. [8] Shoe- maker et al. (1994) Science, 266, 1851–1854. [9] Gaddis et Distribution And Occurrence: Small pyroclastic deposits al. (1997) LPS XXVII, 389–390; Gaddis et al. (1998) LPS are widely distributed across the lunar near side, and with XXIX; Rosanova et al. (1998) LPS XXIX. [10] Head and the Clementine global dataset we now realize that far side Wilson (1992) GCA, 56, 2155–2175. [11] Coombs and and polar regions of the Moon also serve as host to several Hawke (1992) Proc. LPSC 22, 303–312. [12] Weitz et al. small pyroclastic deposits. Most small pyroclastic deposits (1997) LPS XXVIII, 1533. [13] Schultz (1976) Moon, 241; are observed as relatively isolated deposits in highlands Wichman and Schultz (1995) JGR, E10, 21201. [14] Spudis near the margins of major mare deposits on the near side. (1989) NASA TM 4210, 406–407. [15] Wilhelms (1987) However, two clusters of small pyroclastic deposits are ob- USGS Prof. Pap. 1348. served along the northwestern margin of Oceanus Procel-

415/750 atl dmds 415/750 or 415/750 atl dmdn 415/750 lav dmd Fig. 1. Clementine UVVIS data: Color ratio data for 415/750 fr dmds 415/750 lav dmdsse 415/750 fe dmd 415/750 lav dmdn three types of small pyroclastic deposits on the Moon. 415/750 fw dmd 415/750 opp s 25pix 415/750 jh dmd 0.62

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0.54 Small Pyroclastic Deposits

0.52 1 1.02 1.04 1.06 1.08 1.1 1.12 950/750