LPI Contribution No. 958 31

near Taruntius Crater appears to be relatively young (~1 Ga) [14]. Of TABLE 1. A comparison of Fe concentration between mare surfaces >80 floor-fractured craters mapped by Wilhelms ( Plate 5 in [15]), and crater ejecta; crater ejecta tend to have higher FeO content ~15 have pyroclastic deposits, suggesting that these features may relative to the mare surface they are superposed on. have provided an enhanced environment for explosive volcanic eruption. Location of Mare Unit Surface FeO wt% Ejecta FeO wt% The spatial association of volcanic eruption sites and impact Mare Orientale 6–12 12–16 craters and basins on the Moon is believed to be related to crustal Lacus Veris 8–10 12–14 thinning beneath impact sites. Small pyroclastic deposits are ob- Mare Marginis 10–14 14³16 served in regions with crustal thicknesses ranging from 30 to 80 km Mare Smythii 12–14 8–10 [2], with the majority near 50 km. On the nearside, these crustal Mare Australe 8–12 12–16 thicknesses are typical of the margins of the major maria; on the farside, thinner crust is observed in the South Pole/Aitken Basin and the Moscoviense Basin, sites where pyroclastic deposits are ob- Discussion: Maps of Fe abundance for farside mare surfaces served. We are currently evaluating these data to investigate impli- show low concentrations of Fe (8–14 wt% FeO) when compared to cations for modes of eruptions of lunar pyroclastic deposits. soil samples from the Apollo landing sites (11–16 wt% FeO). Are References: [1] Clementine Basemap Mosaic, 15 volumes, farside basalts inherently lower in Fe, or are they more highly con- prepared by USGS for NASA, available via the Planetary Data taminated by impact mixing between highland and mare lithologies? System Imaging Node. [2] Zuber et al. (1994) Science, 266, 1839– To answer this question we have looked at the composition of 1843. [3] Gaddis et al. (1985) Icarus, 61, 461–488. [4] McEwen et small crater ejecta within the maria. Craters serve as bore holes al. (1994) Science, 266, 1858–1861. [5] Robinson et al. (1996) LPS penetrating the surface of the mare unit to expose fresh, less-contami- XXVII, 1087–1088. [6] Head and Wilson (1979) Proc. LPSC 10th, nated mare material. Maps of Fe concentration show that mature 2861–2897. [7] Hawke et al. (1989) Proc. LPSC 19th, 255–268. mare surfaces and ejecta from craters superposed on the uppermost [8] Shoemaker et al. (1994) Science, 266, 1851–1854. [9] Gaddis et regolith layer have different FeO percentages (Table 1). Crater ejecta al. (1997) LPS XXVII, 389–390; Gaddis et al. (1998) LPS XXIX; are found with lower and higher concentrations of Fe relative to the Rosanova et al. (1998) LPS XXIX. [10] Head and Wilson (1992) surrounding mare surface. GCA, 56, 2155–2175. [11] Coombs and Hawke (1992) Proc. LPS, Craters in Figs. 1b and 2b exhibit concentric rings of increasing Vol. 22, 303–312. [12] Weitz et al. (1997) LPS XXVIII, 1533. Fe toward their center. This is an indication that the subsurface is [13] Schultz (1976) Moon, 241; Wichman and Schultz (1995) JGR, more mafic than the surface composition. Craters have excavated 100, 21201. [14] Spudis (1989) NASA TM 4210, 406–407. fresh basaltic material that is less contaminated with highland mate- [15] Wilhelms (1987) USGS Prof. Paper 1348. rial than the surrounding mare surface. Mare basalt units with low Fe content surfaces are associated with areally small deposits, which are prevalent on the farside, and where craters proximal to mare units DIFFERENCES OBSERVED IN IRON CONTENT BE- have deposited highland material over the basalt (e.g., Maunder and TWEEN CRATER EJECTA AND SURROUNDING MARE Mare Orientale). BASALT SURFACES: IMPLICATIONS FOR SAMPLE In the second case, the Fe content for crater ejecta is lower than REMOTE SENSING INTEGRATION. J. J. Gillis1,2 and P. D. the surrounding mare material (Table 1, Mare Smythii). The low-Fe Spudis1, 1Lunar and Planetary Institute, Houston TX 77058, USA, ejecta is produced when craters are large enough to excavate high- 2Previously at Rice University, Houston TX, USA; now at Washington land material from beneath the mare unit. This scenario allows an University, St. Louis MO 63130, USA ([email protected]). estimate of mare basalt thickness [9]. The thickness of the mare unit is calculated by bracketing the diameter at which craters have exca- Introduction: Remote sensing techniques [e.g., 1,2] are uti- vated low-Fe highland material and diameter at which they have not. lized to extend sample data to regional and global scales. Equally The thickness of the basalt is calculated using the relation of crater important is knowledge of rock types not represented in the current diameter to depth of excavation [10]. This technique of calculating sample collection [3]. Before either of these questions can be ad- mare deposit thickness, when combined with previous techniques dressed, one question must first be answered: How does remote [11,12], will improve the resolution at which mare units are mapped. analysis of a planet’s surface relate to the uncompromised composi- Conclusions: It is important to observe not just the surface tion of bedrock? The paucity of exposed rocks on the lunar surface composition of the mare unit but also the ejecta from fresh craters. means that remote chemical and mineralogical analysis (e.g., The assessed surface composition represents hybridized rock types Clementine [4]) records direct information about the soils. In this that are the product of impact mixing processes. Higher Fe compo- work we examine ways to evaluate how and if the composition of the sition for crater ejecta relative to the exposed mare surface signifies surface material is representative of the bedrock material below. contamination with highland material. The difference in Fe abun- Methods: Processing of Clementine images was performed dance relates to the amount of surface contamination. Such contami- using ISIS software developed by the USGS, in Flagstaff [5]. Images nation must be corrected before attempting to understand lunar basalt were converted to absolute reflectance using photometric equations compositions. This reflects the depth, size, and age of the mare basalt [6]. Maps displaying the distribution of Fe were constructed using unit. Craters that have low-Fe concentrations may be used to calcu- Clementine 750- and 950-nm images [7]. The technique for calculat- late the thickness of the mare unit. ing Fe abundance is shown [8] to have eliminated the effects of References: [1] Alder et al. (1973) Proc. LSC 4th, 2783–2791. surface brightness and albedo, thus yielding accurate Fe concentra- [2] McCord T. B. et al. (1976) Icarus, 29, 1–34. [3] Pieters C. (1978) tions for the Apollo and Luna landing sites. Proc. LPSC 9th, 2825–2849. [4] Nozette and the Clementine Sci- 32 Workshop on New Views of the Moon

ence Team (1994) Science, 266, 1835–1839. [5] Gaddis L. (1996) (1974) LS V, 200–202. [12] De Hon R. A. and Waskom J. D. (1976) GSA Abstr. with Progr., 28(7), A386. [6] McEwen A. S. (1996) LPS Proc. LSC 7th, 2729–2746. XXVII, 841–842. [7] Lucey P. G. et al. (1995) Science, 268, 1150– 1153. [8] Blewett D. T. et al. (1997) JGR, 102, 16319–16325. [9] Gillis J. J. et al. (1997) LPS XXVIII, 419–420. [10] Croft S. K. (1980) Proc. LPSC 11th, 2347–2378. [11] Eggleton R. E. et al.

Fig. 2. (a) Clementine 750-nm albedo image of the mare-filled Jenner Fig. 1. (a) Clementine 750-nm image of the mare-filled crater Buys- (J) crater and surrounding mare deposits. Jenner is located in central Mare Ballot (175°E, 21°N). It is located near the center of the Freundlich- Australe. (b) Iron concentrations for mare basalt units surrounding the Sharonov Basin. (b) Iron map of mare basalt within Buys-Ballot. The two crater Jenner. The surface of the basalt units is low in FeO (scale same high-Fe spots in the upper right of the mare deposit are craters that have as Fig. 1b), while crater ejecta on the surface of the basalt units have exposed subsurface mafic material. consistently higher FeO contents.