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For lunar complex impact craters the size of Olcott (81 km), the morphology typically includes a rim, terraced walls, and an uplifted coherent central peak on the crater floor (Chapter 1, section 1.5.2). The morphologies of central peaks can be simple with a single peak, or complex with a cluster of peaks (Hale and Head, 1979).

The central uplift at Olcott crater is a cluster of peaks with variable heights (Fig. 3.2, 3.6B), and can be classified as a complex central uplift, with a ring-like central peak (Baker et al., 2011). This could indicate a transitional state of the crater, with an initial central peak collapsing to form a ring feature due to gravitational effects. Hydrocode simulations suggest that the formation of peak rings involves two mechanisms – the collapse of the crater rim inwards and the collapse of the central uplift outwards (Collins et al., 2002). However, the crater diameter size of Olcott at 81 km is far less than the crater diameter where the transition from a central peak to a peak ring occurs (diameters > 200 km; Baker et al., 2011) therefore this is unlikely to be the case. An alternative explanation is that the target surface where the Olcott event occurred is not homogeneous highland terrain, but instead a heterogeneous target. Modeling results and studies of terrestrial craters indicate that much of the crater collapse during the final modification stages depends on the strength of the target materials (Melosh, 1989). Parameters including the extent of ejecta deposits, depth of the crater floor, and heights of uplifted

Figure 3.8: 3-D views of the central peak hills within Olcott crater. The black dashed rectangle refers to the location of the central peak hill discussed in the text,

and featured in Fig. 3.8C. The lettered legend refers to the location of spectral profiles in Fig. 3.8D. A) This is a LRO-WAC context view of central uplift area discussed in 3.8C, D (10x vertical exaggeration). Image width is ~55 km. White

arrow refers to location of spectral sampling in Fig. 3.8C. B) M3 derived IBD

parameter map of the area in Fig. 3.8A (10x vertical exaggeration). Image width is ~55 km. [Image credit: ISRO/NASA/GSFC/Arizona State University]

Figure 3.8 (cont’d): Image and spectral characterizations of a central peak hill at Olcott crater. C) LRO NAC (product # M108460950RC) of the central peak hill discussed in text and highlighted in Fig. 3.8A, B. The surface texture is similar to that characterized in Figure 3.7B. Image width is 2.5 km across. The letters refer to

sample spectral profiles in Fig. 3.8D. D) Sample spectral profiles of spots identified in Fig. 3.8C. There are variable absorption strengths near 1 µm, 1.3 µm and 2 µm suggesting the presence of high-Ca pyroxene, Mg-spinel and plagioclase feldspar at

sections are dependent on the pre-impact target layering (Collins et al., 2008). Crater morphologies with ring-like central peaks have been observed for craters between 50 and 205 km, however in these cases, there are also coherent peaks on the crater floor thereby classifying the features as protobasins (Baker et al., 2011). However, it is unknown at this point, if the target is heterogeneous at those crater sites. By fusing the spectral detail from M3 IBD parameter map over topographic LOLA data, we observe that many areas within the Olcott central uplift have multiple colour variations indicating the possibility of multiple spectral trends (Fig. 3.3D, 3.8). From the sample spectral profiles using M3 data (Fig. 3.5D, 3.8D), we note the presence of both pyroxene-rich and pyroxene poor

materials in the central uplifts. Image data of the uplifts at Olcott crater display a mixture of rough, blocky materials (boulders) and thin impact melt deposits draped along the surface (Fig. 3.8C). This variability in surface texture may explain the variation in spectral properties. This observation is further investigated, using one of the peaks as an example, to assess both the spatial and spectral information and determine the connection if any. The spectral parameter composite map (Fig. 3.3D, 3.8B) indicates that the

mineralogical content on one of the uplifted peaks, a 1 km feature (black dashed box in Fig. 3.8), has variable intensities of the individual parameters, indicating a potential variation in spectral characteristics and types of absorption features found. Spectral profiles sampled along the peak surface (Fig. 3.8D) indicate in general the weak occurrence of a 1 µm absorption, and strong 2 µm abundance. At sample sites “A” and “B”, there is a lack of an absorption feature at 1 µm and a broad absorption feature observed at 2 µm, suggesting the presence of Mg-spinel (Pieters et al., 2011). At sample spot “B”, absorption features of both plagioclase feldspar and Mg-spinel can be observed (Fig. 3.8D). The presence of an absorption feature at 1.3 µm is typical of Fe-bearing plagioclase feldspar. This occurrence of Mg-spinel on the central hill is similar to observations made on the central uplift region of Theophilus crater, 100 km complex crater on the lunar nearside (Dhingra et al., 2011). It is interesting to note that while this central uplift feature discussed here shows signs of the occurrence of Mg-spinel, other areas, spectrally sampled, within the central uplifts do not show a similar spectral signature (Fig. 3.5D). A more detailed investigation and thorough spectral sampling of

areas within Olcott crater.

The identification of Mg-spinel within the central uplifts at Olcott crater is a significant observation. Mg-spinel has only been observed at two other locations on the Moon – Theophilus crater on the lunar nearside, and Moscovience basin on the lunar farside (Pieters et al., 2011; Dhingra et al., 2011). These studies propose that Mg-spinel is present within the lunar crust and is excavated from great depth. Olcott crater is ~735 km south west of the western edge of Mare Moscoviense – a known volcanic mare patch on the lunar farside. This new observation of Mg-spinel at these distances provides new estimates on the lateral and vertical extent of the Mg-spinel rock type within the lunar crust. An addition of a new “data point” of Olcott crater to the global dataset of Mg- spinel bearing complex craters will help better constrain the origin for this rock type.