An Indigenous Origin for the South Pole Aitken Basin Thorium Anomaly Ian Garrick-Bethell and Maria T
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GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L13203, doi:10.1029/2005GL023142, 2005 An indigenous origin for the South Pole Aitken basin thorium anomaly Ian Garrick-Bethell and Maria T. Zuber Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA Received 4 April 2005; revised 24 May 2005; accepted 3 June 2005; published 9 July 2005. [1] The northwest portion of the Moon’s South Pole- 1974; Haskin et al., 1996; Haskin, 1998]. Wieczorek and Aitken basin contains an anomalously high abundance of Zuber [2001] proposed on the basis of simple ejecta thorium as determined by Apollo and Lunar Prospector modeling that the Serenitatis basin is a more likely source gamma-ray spectroscopy. The anomaly’s proximity to the of the thorium anomaly than the Imbrium basin. Haskin et antipode of the Imbrium basin has led several investigators al. [2004], however, have alternatively suggested that ejecta to suggest that the anomaly is the result of convergence of from Imbrium or Serenitatis is unlikely to produce the thorium-enriched ejecta from the Imbrium impact. observed pattern. If the above impact-origin hypotheses Examination of this complex region with new higher- are not correct, the SP-A thorium anomaly would have resolution thorium data and several other datasets reveals important implications for the thermal and chemical evolu- that a convergence of ejecta cannot explain the anomaly. tion of SP-A and the farside of the Moon. Here we show Alternatively, we propose an indigenous and likely ancient that an ejecta origin is untenable on the basis of careful source. Citation: Garrick-Bethell, I., and M. T. Zuber (2005), analysis of a variety of data. An indigenous origin for the South Pole Aitken basin thorium anomaly, Geophys. Res. Lett., 32, L13203, doi:10.1029/ 2. Data 2005GL023142. [4] Thorium abundance data comes from an updated version of low altitude (average 30 km) measurements of 1. Introduction the Lunar Prospector gamma-ray spectrometer (LP-GRS) [Lawrence et al., 2003]. Briefly, measured gamma-ray [2] It is generally believed that as the Moon differentiated, counts from 32-second spectra were binned onto a 0.5 Â plagioclase feldspar floated to the top of a magma ocean 0.5 degree/pixel grid, and smoothed to reduce statistical to form the anorthositic crust, while mafic minerals con- scatter with an equal area smoothing algorithm. The full densed to form the mantle. During this process trace width at half maximum (FWHM) of the LP-GRS becomes incompatible elements concentrated in an unincorporated 2 (80 km) under the smoothing operations, with a full magma ocean residue, eventually crystallizing last as the width at full maximum (FWFM) of 240 km (FWFM defined minerals observed in KREEP-rich rocks (i.e. enriched in as where the spatial response function falls to 1%). potassium, rare earth elements and phosphorus). Among the [5] Topography data are from the Clementine laser al- elements found in KREEP, thorium is also abundant and timeter. The 0.25 degree/pixel grid of interpolated elevation serves as an adequate surrogate for detection of incompat- is obtained from filtered along-track range data [Smith et al., ible elements and KREEP. The global distribution of 1997]. Clementine five-band UV-VIS multispectral image thorium is heterogeneous, with the strongest surficial strips are taken from the published CDs and mosaiced expression found on the nearside in the Procellarum KREEP together with the United States Geological Survey Map-a- Terrane (PKT), centered around the Imbrium basin [Jolliff et Planet software at a maximum resolution of 100 meters/ al., 2000]. The farside as a whole has far fewer expressions pixel [Pieters et al., 1994]. In order to obtain qualitative of significant abundances of thorium. The South Pole- mineralogical information from the multispectral images in Aitken (SP-A) basin contains most of the farside thorium an RGB color format, we utilized a ratio technique proposed in an oval shaped region centered at approximately (48°S, by Pieters et al. [2001]. Under this scheme, noritic low-Ca 188°E) [Garrick-Bethell, 2004], with abundances peaking at pyroxene lithologies appear red (‘‘band curvature’’), high- 3.8 mg/g in the northwest anomaly region, Figure 1 Ca pyroxene and/or olivine rich materials are shown in [Lawrence et al., 2003]. Aside from the Compton-Belko- green (‘‘band tilt’’), and blue tones represent anorthositic vich region [Lawrence et al., 2003], the SP-A thorium and/or mature materials (‘‘band strength’’). A shaded anomaly represents the highest concentration of thorium relief basemap from Rosiek and Aeschliman [2001] is also on the farside. utilized. [3] Noting the anomaly’s proximity to the antipode of the Imbrium basin, and its rough association with units of ‘‘grooves and mounds’’, Stuart-Alexander [1978] suggested 3. Results that the anomaly is due to a convergence of thorium- enhanced ejecta from the nearside Imbrium basin. Models 3.1. Anomaly Description of ejecta deposition indicate that significant amounts of [6] Figure 1 shows the thorium enhancement lies in the primary ejecta should land at the antipode [Moore et al., northwest corner of SP-A and ranges in concentration from 2.5 to 3.8 mg/g. The enhanced region can be divided into two northeast-southwest trending regions which we refer to Copyright 2005 by the American Geophysical Union. as North and South Lobes. The lobes each have concen- 0094-8276/05/2005GL023142$05.00 trations over 3.5 mg/g near their centers, with South Lobe L13203 1of5 L13203 GARRICK-BETHELL AND ZUBER: INDIGENOUS THORIUM IN SOUTH POLE AITKEN L13203 walls of Ingenii. However, the thorium signature falls to values below 1.0 mg/g in this region. In South Lobe unit Ig is hardly present except for several instances of crater wall dissection. Instead, Nectarian ‘‘rolling terra’’ constitutes the majority of South Lobe, as is the case for much of the interior of SP-A. 3.3. Relations With Eratosthenian Craters [8] Figure 2b shows that the highest abundance of thorium in South Lobe is associated with an unnamed 50-km-diameter Eratosthenian crater at (40°S, 165°E) [Stuart-Alexander, 1978], herein referred to as South Crater. In North Lobe the high thorium abundances are correlated with the mapped ejecta units of Birkeland (30.2°S, 173.9°E, Figure 1. Map of thorium content in the South Pole- 82.0 km) [Blewett et al., 2000], but the thorium pattern is Aitken basin, in mg/g. not as circular as for South Crater. Focusing on South Crater, we calculate from its diameter a crater depth of 3.4 km [Pike, 1974], and a depth of excavation of 1.1 km being slightly higher, peaking at 3.8 mg/g. Thorium level (from experiments of Stoffler et al. [1975]). Thus if earlier- contours ranging from 2.5 to 3.5 mg/g illustrate this mor- emplaced ejecta from Imbrium is responsible for the thori- phology, and serve hereafter as a convenient quantitative um deposit at South Crater, the ejecta would have to be at means of characterizing the extent of the anomaly, Figure 2. least 1.1 km thick to have remained exposed at the bottom The total anomaly area with regions of thorium above South Crater and its immediate surroundings. Despite the 2.5 mg/g is approximately 202,000 km2. We note that the fact that a purported ejecta blanket at least 1.1-km-thick fell western edge of the two lobes is continuous with the in the vicinity of South Crater, there are no mapped Imbrian western edge of the entire inner SP-A thorium deposit (cf. ejecta units bordering South Crater (Figure 3). Dissected Figure 1). crater walls of purported Imbrian origin are not present in the pre-Nectarian crater underlying South Crater, or in the 3.2. Correlation With Mapped Unit of Grooves larger pre-Nectarian crater Oresme (42.4°S, 169.2°E, [7] Smoothed thorium data shows that correlation with 76.0 km) to the southeast. In the crater Obruchev (38.9°S, the mapped Imbrian unit of grooves, mounds and dissected 162.1°E, 71.0 km) the northern walls bordering Mare crater walls (unit Ig) attributed to the Imbrium impact Ingenii show signs of dissection, but the walls closest to [Stuart-Alexander, 1978], is seen only in North Lobe South Crater do not. If we use the more general numerically- (Figure 2a). To the northwest of North Lobe a large area derived crater scaling laws of Croft [1980] we calculate the greater in size than the FWHM thorium footprint is nearly depth of excavation of South Crater to be 5.0 km. An filled with the mapped unit Ig, including the northwestern Imbrium ejecta blanket of this thickness would have Figure 2. 2.5–3.5 mg/g thorium contours in intervals of 0.5 mg/g, shown in black. (a) Small black diamonds represent mapped units of grooves and mounds from Stuart-Alexander [1978]; (b) small black circles represent units of Eratosthenian craters from Stuart-Alexander [1978], and multi-color contours show basin topography from À3000 to À1000 m, increasing westward in 500 m intervals. 2of5 L13203 GARRICK-BETHELL AND ZUBER: INDIGENOUS THORIUM IN SOUTH POLE AITKEN L13203 3.5. Mare-Thorium Relations [10] The most visually prominent feature of the thorium anomaly is that it is well defined by the eastern border of Mare Ingenii. When combined with the topographic con- tours described in Section 3.3, the two features completely outline the western portion of the anomaly (Figure 2b). We interpret this relationship to be due to mare cross-cutting a thorium-enriched subsurface that may extend approximately halfway into the Ingenii basin from the east.