GEOLOGY OF LOWELL CRATER REGION ON THE MOON: ANALYSIS OF REMOTE SENSING DATA

Abstract

Study of geology of the Moon has been the centre stage of planetary science and exploration endeavors. It is the nearest celestial body to the Earth, and possibly, the Moon’s existence is responsible for our own origin and survival. Further, importantly it has recorded the imprints of precious geological events the Earth would have witnessed during its geological history, traces of which are lost now due to the Earth’s dynamic nature. The Moon is imprinted with numerous meteoroid impact craters/basins (microns to several hundred kms in diameter) and dark lava plains known as mare. The impact structures expose subsurface rocks, enabling their study by robotic/manned missions or remote sensing techniques. The mare plains mostly formed between ~3-3.8 Ga, when the Moon was energetic. As the Moon cooled, intensity of volcanism reduced and only minor extrusive activities continued up to the Copernican era ~1 Ga. Interestingly, most of the mare is confined to the basins created by the mega-impact events. It is not clear whether there was any genetic relation of lunar volcanism to the impacts and the subject is debated. The Orientale basin, located on the western limb of the Moon, is the youngest and the most well preserved mega sized multi-ring basin that formed ~3.8 Ga. The basin in only partly filled with basalts therefore it is an ideal location to simultaneously study impact cratering and volcanism and to decipher a relationship between the two, if it exists? Due to these peculiarities, the Orientale basin has been extensively studied providing a broad idea about the general geology and formation of its morphological and compositional facies. It is host to several complex impact craters that provide an opportunity to study the stratigraphic framework of the respective facie they sample. The Lowell crater (centred at ~13oS, 103.4oW; ~66 km in diameter) is one such post-Orientale crater that has formed in the ejecta of Orientale basin between the Outer Rook Ring (ORR) and the Cordillera ring (CR). In this study, topographical, morphological, spectral reflectance, and crater count analysis have been carried out for the Lowell crater region on the northwestern parts of Orientale basin, which covers an area of ~198x198 km2. The Lowell crater is unique since it intersects the ORR and it is the largest and the freshest post-Orientale impact crater in the ii Montes Rook Formation (MRF). Therefore, its geology holds clues to the subsurface geology of the MRF. Though geologically important, this particular crater has largely remained un-explored. High-resolution remote sensing data from state-of-the-art instruments aboard Chandrayaan-1 (2008; ISRO), Kaguya (2007; JAXA) and Lunar Reconnaissance Orbiter (2009; NASA) have been used in this study. Surface topography has been studied using Kaguya TC DTM, morphological analysis has been carried out using images from LRO (WAC and NAC), Kaguya TC and Chandrayaan – 1 M3, and crater counting has been done on Kaguya TC images. For spectral reflectance studies global mode Level 2 M3 data have been used. Surface morphological analysis has revealed that the Lowell crater region is extremely undulating with a relief of ~9.5 km. The highest location lies in the Cordillera scarp (~6 km high) and the deepest areas lie inside the Lowell crater which is ~3-3.5 km deep. Several prominent lineaments possibly related to the formation of the Orientale basin and/or the lunar grid is present in the area. The Lowell crater located amidst uplands is polygonal in shape and it shows N-S asymmetric ejecta distribution. Most of the proximal ejecta are emplaced in the northern side and the exterior impact melt pools are concentrated in the northwestern side. The terraced crater walls on the western side are broader compared to the walls on the eastern side and they show rhombic cross-sections due to cross-cutting of the terraces with pre-existing lineaments. Also, they show comparatively gentle slopes and are largely topographically high compared to the western walls. The central peak is ~1.5 km high (from the crater floor) and it shows melt pond at around the summit, boulders at several places, and prominent slumping and subsidence on the eastern side. Numerous floor irregularities are present on the crater floor, the density being higher on the eastern side. A small (~9 km in diameter) superposed asymmetric rayed crater (referred to as Crater S in the study) is present near to the eastern side rim of the Lowell crater. A pronounced rectilinear resurfacing is also present on the eastern side, which is ~ 3-6 km wide and extends to ~17 km from the floor of Crater S, terminating approximately half way to the central peak of the Lowell crater. The resurfacing show conspicuous scarcity of iii impact craters and exhibits multiple generations of fresh viscous flows confined to a possible graben. A few potential volcanic source regions have been identified in the resurfacing suggesting that the unit could have formed due to volcanism during recent times (?). These fresh flows have superposed parts of comparatively older impact melt flows diverging from the Crater S and descending down the walls of Lowell crater. Spectral diversity in the Lowell crater region has been worked out using representative average spectral reflectance curves, popularly used FCC’s (based on integrated band depth and band ratios), and a Minimum Noise Function (MNF) based color coding. The MNF based FCC effectively captured the spectral variability in the Lowell crater region. The areas surrounding Lowell crater, which are the best approximates for the surface lithology of the Lowell target, largely show presence of massifs of un-shocked anorthosites, shocked anorthosites, Mg-spinel anorthoisites, basalts (of Lacus Veris), and mature soils. The Lowell crater wall is largely composed of norite to anorthositic norite with intermittent anorthosite and Mg-spinel rich areas. The floor of the Lowell crater is largely occupied with gabbroic/basaltic melts and the floor irregularities also show conspicuous clinopyroxene signatures. A part of the proximal ejecta, which is nearest to the Lowell crater in the northwestern and northeastern quadrants also show presence of clinopyroxene. The central peak shows distinct compositional asymmetry. The rocks on the western side of the peak shows gabbroic/basaltic signature, whereas the ones on the eastern side (mostly coinciding with the slumped zone) are largely Mg-rich (low-Ca pyroxene and Mg-spinel bearing rocks). Anomalous spectral signatures showing prominent curved inflexion ~1.25 μm (along with prominent 1 μm and 2 μm absorption feature) have been noticed at several spots on the central peak and at certain sites in the walls. Most of these correspond with the areas showing presence of boulders and could relate to presence of Mg-spinel troctolites or co-association of pyroxene and un-shocked anorthosite, or the 1.25 μm could be due to Fe+2 at the M1 site of pyroxene. iv These observations indicate that the Lowell impact event would have excavated a deep seated pre-existing mafic pluton in addition to the Orientale basin ejecta and pre-existing megaregolith layer of anorthositic to anorthositic norite composition. The pluton would have been emplaced in the megaregolith layer prior to or after the formation of the Orientale basin at a depth of ~6.6 km (or less). The Mg-spinels observed in the central peak would have formed due to melt wall rock reaction in the area of the emplacement of the pluton. Also, Mg-spinel anorthosite exposures have been spotted in other non-mare units of the Orientale; however, a distinct scarcity exists in the IRR, which is supposed to have exhumed rocks from deeper levels in the Orientale region. In most of the observed occurrences in the Orientale basin, pyroxenes (mostly noritic rocks) are present in the adjacent areas. The rock association in the IRR is unshocked plagioclase. The composition of the recently produced resurfacing is distinctively gabbroic/basaltic. However, the older flows (i.e. the impact melts from Crater S) largely show noritic signatures consistent with the composition of the wall of Crater S, emphasizing further that all these flows may not be co-genetic and that the fresh gabbroic/basaltic flows could have an alternate origin possibly volcanic (?). Few signatures of pyroclasts have also been noticed in the area supporting the volcanic preposition. Crater count analysis for selected areas of the Lowell crater region has been performed to resolve the age controversy for Lowell crater (i.e. Imbrian vs Copernican), and to assign a time frame for the formation of the recent resurfacing. It has been estimated that the Lowell crater formed ~374 Ma ago; therefore, it is Copernican in age. Further, count of (plausible) impact craters on the youngest lobes of the resurfacing has indicated that they could be barely ~2-10 Ma old. Finally, the inferences from these studies have been synthesized to assess geological evolution of the Lowell crater region. The highlights of the evolutionary stages include formation of the Lowell crater due to an oblique impact (~30o-45o) from S-SW direction ~374 Ma ago, excavation of a mafic pluton that possibly stalled in the megaregolith beneath the Orientale ejecta, formation of a rayed superposed crater, and apparent indications that the region would have experienced possibly volcanic activity (?) during recent times.