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Mineralogical Diversity of Impact Melts on Central Peak of Tycho and Its Vicinity

Introduction: Impact cratering on planetary bodies excavates and re-distributes material from various depths. A sizable amount of energy is commonly partitioned into melting the rocks. The fate of this impact melt in terms of spatial distribution, morphology and mineralogy is varied [e.g. 1-3]. With the availability of high spatial and spectral resolution datasets for the Moon from multiple instruments onboard several missions [e.g. 4-6], a variety of new information is being made available on physical and compositional character of impact melts. We discuss here the spectral variation in impact melts located on the central peaks of crater Tycho and its vicinity and compare them with unmelted rocks on the peaks. Recent LROC observation of Tycho’s peak at Sunrise [7] captured a spectacular scene of a large clast lying over a pool of cracked impact melt providing a context for exploring the relationship between the two entities in space and time. We also attempt to understand this relationship. Crater Tycho and its Peak: Tycho is a ~85 km diameter Copernican age complex crater located on the southern near-side lunar highlands. It’s a bright crater with prominent rays, visible from Earth, even with naked eyes. Impact melt is observed all over the crater: rim, walls, floor and even the peaks, each occurrence displaying a wide diversity of morphologies. Some occur as small smooth melt pools, others have clasts of various sizes. The impact melt also drapes the rocks and is extensively fractured. Mineralogy and morphology from coordinated high resolution datasets: Datasets from Chandrayaan1 Moon Mineralogy Mapper (M), Kaguya Terrain Camera (TC) and LRO Narrow Angle Camera (NAC) were utilized in conjunction with each other to study the impact melts. The spectral diversity of sampled impact melts is illustrated in Figure 1 along with the spectra of unmelted material. The position of absorption bands around 1 and 2 μm and their relative strengths clearly demonstrate differences not only amongst unmelted material and impact melt but also between impact melts. The smooth melt patch located north-east of the central peak, near its base, has a featureless spectra with a possibly weak feature around 1 μm and no absorption at 2 μm. The melt does not seem to contain any significant quantity of clasts based on the high resolution Kaguya TC dataset as shown in figure 2(b). A contrasting spectrum is obtained along the crack in the impact melt, possibly rich in clast fragments, located east of the smooth patch. The spectrum shows strong 1 and 2 μm absorption bands and possibly another absorption around 1.2 μm that could be due the to pyroxene or plagioclase. The immediate surroundings too share the same spectral character indicating the presence of a strong mafic composition rich in pyroxene.