Lunar and Planetary Science XXIX 1339.pdf

A SPECTRAL STUDY OF THE HADLEY-APENNINE REGION OF THE MOON. D. T. Blewett*, B. R. Hawke, and P. G. Lucey, Planetary Geosciences, Hawaii Institute of Geophysics and Planetology, 2525 Correa Road, Honolulu, HI 96822. *present address: Science & Technology International, 733 Bishop St., Honolulu, HI 96813. ([email protected])

Introduction. The Hadley-Apennine region of the tered at 26.1° N, 3.7° E covering a latitude range of ~19.7- Moon, defined here as the area from roughly 19-36° N and 35.7° N and longitude range of 2.4° W-6.4° E, at 125 2° W-6° E, lies mostly within the Apennine Mountain ring m/pixel spatial resolution. The processing was done with of the Imbrium basin. The formation of the Imbrium basin the U.S. Geological Survey's ISIS system. The processing was a major event in lunar history, and the basin's deposits sequence and variable settings (e.g., for the photometric form a key stratigraphic marker [e.g., 1, 2]. The Hadley- correction) are described by [8]. This calibrated image cube Apennine (HA) region contains a variety of interesting geo- served as the basis for the production of a number of addi- logic features, and a study of these features can supply in- tional data products, including FeO and TiO2 maps pre- formation on important lunar geologic processes. These pared using the algorithms of [8], a maturity image [9, 12], features include the landing site, Hadley and 415 nm/750 nm and 950 nm/750 nm ratio images. and the mare basalts of Palus Putredinis, pyroclastic depos- Near-infrared reflectance spectra. Twenty-three spectra its [3], exposures of the Apennine Bench Formation [4, 5], of features in the HA region were obtained with the Univer- two rayed craters (Aristillus and ) which may have sity of Hawaii 2.2 m telescope at Mauna Kea Observatory delivered material to Apollo 15 [6], and highland material using the Planetary Geosciences InSb circular-variable-filter deposited by the Imbrium basin-forming event. This study spectrometer. Standard observing techniques and data re- provides the opportunity to employ recently-developed algo- duction procedures were employed [10]. The interpretation rithms for the determination of FeO and TiO2 in the lunar of lunar near-IR spectra has been summarized by [13, 14]. surface from Clementine UVVIS camera and Galileo SSI In order to extract mineralogical information, an analysis imaging [7, 8, 9]. The goals of this report are to use the focusing on the mafic mineral absorption band near 1 µm spacecraft data in concert with Earth-based telescopic re- was conducted [15, 16]. The analysis derives four key flectance spectra to investigate the composition of HA mare spectral parameters: near-IR continuum slope, wavelength basalts, the composition and distribution of pyroclastic de- position of the "1 µm" band minimum, band depth, and posits, the composition of the Apennine Bench Formation, band full-width at half-maximum depth. Although spectral the nature of material exposed and emplaced by the Aristil- parameters for some of the spectra used here have been lus impact, and the composition of highland units (Imbrium previously published [2, 13], all spectra were re-analyzed ejecta deposits). for this study to insure consistency. Only minor departures from the previous values were found, and none were consid- Data and Methods of Analysis. For this study we have ered to be enough to change the interpretation. three major data sets at our disposal: near-infrared (near-IR) As an aid to the understanding of compositional and reflectance spectra collected with Earth-based telescopes, mixing relationships among the spectra, a principal compo- and images from the Clementine UVVIS camera and Gali- nents analysis (PCA) was performed. The use of PCA for leo SSI camera. These data sets are highly complementary. lunar spectra was developed by [17], and has been applied The Clementine images provide high spatial resolution and by [16, 18-20]. In addition to the group of spectra for fea- illustrate local and regional spatial relationships, but are of tures in the HA region, eighteen reference spectra repre- low spectral resolution (five channels between 0.4 and 1.0 senting the broad diversity of known lunar spectral types µm). The Galileo images have spectral coverage similar to were also included in the PCA. This provides the opportu- Clementine, but are of lower spatial resolution. The tele- nity to examine the Hadley-Apennine spectra within a scopic reflectance spectra are hyperspectral in nature (~120 global context. The reference spectra were analyzed in the channels between 0.6 and 2.5 µm) and so permit minera- same manner as the Hadley-Apennine spectra. logical information to be extracted from an analysis of the shape of the ferrous iron absorption band near 1 µm. The Findings spatial coverage of the reflectance spectra is determined by 1. Apennine Mountains/Imbrium Ejecta. We have ex- the telescope configuration and the size of the spectrometer amined a number of highland units in the region to provide aperture. The apertures used in the observations reported information on the composition of Imbrium basin materials. here isolated spots on the lunar surface ~2-20 km in diame- The two major facies, the Apenninus material and the Alpes ter. The telescopic observation and data reduction proce- Formation, are compositionally diverse. It appears that the dures are such that absolute brightness information is not Alpes is higher in Fe and Ti than the Apenninus, though the preserved, and the resulting spectra are inherently scaled to interpretation is complicated by the possible presence of a value of 1.0 at a reference wavelength [10]. This contrasts pyroclastic material in the regolith developed in these units. with the Clementine and Galileo data, which can be cali- Results for two small deposits believed to be Imbrium im- brated to absolute reflectance [e.g., 11, 8]. pact melt indicate a composition closer to that of the Alpes Clementine UVVIS Images. Approximately 1050 indi- Formation. vidual Clementine UVVIS camera frames were assembled into a five-band image cube in orthographic projection cen- Lunar and Planetary Science XXIX 1339.pdf

HADLEY-APENNINE REGION: D. T. Blewett et al.

2. Apennine Bench Formation. The Apennine Bench crater may be an area of mare basalt uncontaminated by rays Formation is of great interest as a probable large exposure rather than a deposit of dark material related to the streak. of non-mare volcanic materials. Previous studies [2, 4, 5] The results of analysis of near-IR spectra [this work, 2, 22] have presented strong evidence that the Bench is a light and Clementine UVVIS spectra [23] indicate that highland plains unit with a surface composition dominated by vol- lithologies of approximately anorthositic norite composition canic KREEP basalt. Near-IR spectra for small craters and are exposed in the crater's wall and central peak. The rays mature areas of the Bench are consistent with the presence and continuous ejecta of Aristillus are revealed by the of assemblages spectrally dominated by low-Ca pyroxene, in Clementine and Galileo compositional maps to be composed addition to some high-Ca pyroxene in some instances. The of material lower in FeO than the surrounding mare, and FeO content appears to be ~12 wt.%, based on examination higher in TiO2 than either the nearby mare, highlands or of four well-exposed areas of Bench. The TiO2 content is Apennine Bench. Synthesis of the FeO, TiO2 and near-IR more problematic, with values generally <1 wt.% in both spectral results produced a model for Aristillus that is con- the Clementine and Galileo maps. This is about 1-2 wt.% sistent with the stratigraphy of the Hadley-Apennine region lower than predictions based on Apollo orbital geochemical proposed by [4]. The deepest layer in the Aristillus target sensor data and the composition of returned samples hy- site was of highland composition, relatively low in FeO and pothesized to be fragments of the Apennine Bench Forma- TiO2, probably similar to the Apenninus material. This tion. Future work will try to reconcile these differences in material is represented in the crater's peak mountains, and terms of the following: smearing of signal over the large possibly as a component of some wall and floor slumps pixel sizes of the gamma-ray detector, problems related to where it may represent deep material displaced by the cra- the photometric correction of Clementine images, the cali- tering process but lacking sufficient energy to be ejected bration of the TiO2 algorithm, and possible mineralogical from the crater cavity. Above the basement highland rocks reasons for the breakdown of the Ti-mapping assumptions. was a thickness of extrusive KREEP volcanics equivalent to the surface exposures of the Apennine Bench Formation 3. Mare Deposits. Surface mare basalts in the bulk of visible today. The KREEP volcanics are intermediate in Palus Putredinis and portions of visible in FeO and low to intermediate in TiO2. Covering the Apen- our image range from 15-17 wt.% FeO and have ~1 wt.% nine Bench material at this location was higher-Ti mare TiO2, though contamination from crater rays is common and basalt, such as found in Lacus Mozart, and the low-Ti mare makes certain determinations difficult. From materials basalt presently on the surface of the Palus and nearby Mare exposed in Hadley Rille and in a number of dark-halo im- Imbrium. The mixing of these components in varying pro- pact craters, there appear to be basalts higher in Fe and/or portions can account reasonably well for the composition of Ti below the surface in some locations. Lacus Mozart is the the materials emplaced and exposed by Aristillus. A clump only major deposit of higher-Ti (~2 wt.% TiO2) found in of Aristillus ejecta was apparently emplaced near Rimae our image. III and IV, accounting for an Fe and Ti anomaly seen in this otherwise normal-looking section of Apennine 4. Dark Mantling Deposits. Related to mare basalts are Bench Formation. the many dark mantling deposits of probable pyroclastic origin found in the Hadley-Apennine region. The most im- References: [1] Spudis, Geology of Multi-Ring Impact portant deposit is near Rima Fresnel. This deposit has near- Basins, Cambridge U.P., 1993. [2] Spudis et al., Proc. IR spectral characteristics and Fe and Ti contents very LPSC 18th, 155, 1988. [3] Hawke et al., Proc. LPSC 10th, similar to the mare basalts in the region. These findings 2295, 1979. [4] Hawke and Head, Proc. LPSC 9th, 3285, support the hypothesis [21] that the Rima Fresnel pyroclas- 1978. [5] Spudis, Proc. LPSC 9th, 3379, 1978. [6] Ryder et tics originated in a vulcanian eruption which deposited ma- al., Geology 19, 143, 1991. [7] Lucey et al., Science 268, terial dominated by mare basalt plug rock. The principal 1150-1153,1995. [8] Blewett et al., J. Geophys. Res. 102, components analysis conducted on the near-IR spectra 16319, 1997. [9] Lucey et al., J. Geophys. Res., in press, shows evidence of mixing of the mare-rich pyroclastics with 1998. [10] McCord et al., J. Geophys. Res. 86, 10883, the surrounding Apennine Bench materials. The dark man- 1981. [11] Pieters et al., Science 266, 1844, 1994. [12] tling deposit in a valley near Lacus Mozart also appears to Lucey et al., Bull. Am. Astron. Soc. 27, 1108, 1995. [13] consist mainly of mare material. Clementine data indicates Pieters, Rev. Geophys. 24, 557, 1986. [14] Pieters, in Re- that the pyroclastic deposit in the valley has higher Ti than mote Geochem. Analysis, Cambridge U.P., p. 309, 1993. Rima Fresnel, and hence it may have been emplaced in [15] Lucey et al., Proc. LPSC 16th, D344, 1986. [16] Ble- association with the higher-Ti basalts in Lacus Mozart. wett et al., J. Geophys. Res. 100, 16959, 1995. [17] Smith et al., Proc. LPSC 15th, C797, 1985. [18] Johnson et al., 5. Aristillus Crater. The crater Aristillus (55 km-diam.) Proc. LPSC 15th, C805, 1985. [19] Pieters et al., J. Geo- is a major feature in the study area, and complicated compo- phys. Res. 90, 12393, 1985. [20] Blewett et al., Geophys. sitional relationships are exhibited in its deposits. Near-IR Res. Lett. 22, 3059, 1995. [21] Hawke et al., Proc. LPSC spectra and spacecraft compositional maps indicate that the 19th, 255, 1989. [22] Smrekar and Pieters, Icarus 63, 442, dark streak on the northeastern wall is a splash of impact 1985. [23] S. Tompkins, Ph.D. dissert., Brown Univ., 1997. melt. The streak has an FeO content roughly 2-4 wt.% higher than the nearby crater wall material, so higher Fe contributes to its low albedo. A dark patch close to the