Moon Mineralogy Mapper Investigation of the Ina Irregular Mare Patch

Introduction: Ina is an irregular mare patch located in the mare unit Lacus Felicitatis near 18° N and 5° E. Irregular mare patches (IMPs) are small (< 5 km across) unusual lunar features that are chacterized by topographically high, smooth deposits (or mounds) that occur with topographically low, blocky or uneven surfaces [1]. Ina (Figure 1) was first identified in Apollo images [2], and has since been hypothesized to be either a volcanic construct [1,2] or a result of sudden degassing [3]. The uneven surface appears to be immature relative to the surrounding area [3-5], and crater counts on the smooth deposits yield model ages <100 Ma [1,3]. The primary hypothesis for a heat source that could generate this late stage volcanism is an enrichment of heat producing elements in a ‘urKREEP’ mantle layer. In this study we use Moon Mineralogy Mapper (M) data to investigate the maturity and mineralogy of Ina and therefore place constraints on its origin. Background: Previous work investigating the maturity of these features used Clementine, Kaguya, and M data [3-5]. These studies proposed that the uneven surface is immature relative to the smooth deposits and the surrounding mare. However, greater maturity does not necessarily imply longer surface exposure [6]. In the case of Ina, the uneven surface may remain immature longer (until the blocks have been broken down) than the smoother mounds and the surrounding mare. Previous Clementine UVVIS multispectral (5 bands) observation based compositional studies suggested that the uneven surface in Ina is primarily composed of freshly exposed high titantium basalt [3, 4]. M spectra have 83 bands and can therefore yield more detailed analyses of the lunar surface [7]. Previous work at Ina with M data was performed with preliminary calibration and the work presented here uses the most recent Level 2 and 3 calibrations available on the Planetary Data System [4]. The goal of this study is to investigate the relationship between the smooth deposits and the uneven surface. If the smooth deposits and the uneven surface have the same composition and maturity levels, they were likely emplaced at the same time. Methods: We used the methods described in Horgan et al. (2014) [8] to remove the continuum from a calibrated M spectral observation using an interactive linear fit with two segments and tie points near 0.7, 1.5, and 2.6 μm. We also computed the optical maturity parameter (OMAT) using methods described in [6]. To verify that we can use the Clementine derived equation and constants with M data, we compared OMAT values from the two datasets at the Aristarchus plateau. The fresh ejecta has Clementine and M OMAT values, respectively, of .306 and .238, while the more mature pyroclastic deposit has Clementine and M OMAT values of .198 and .096. This shows that while the absolute OMAT values of these two datasets are not the same, the relative maturity of two units remains the same. Therefore we can use M derived OMAT values to compare the relative maturity of different units in our study region. Results: Maturity: Our results (Table 1) agree with previous studies in that the uneven surface is less mature than the smooth deposits and the surrounding area. However, our results also show that the smooth deposits are less mature than the surrounding area. A small (< 50 m diameter) fresh crater NW of Ina has a maturity that is between the uneven surface and smooth deposits. Mineralogy: The M spectra (Figure 2) show that the uneven surface, the smooth deposits, and the surrounding mare all exhibit a high calcium pyroxene signature consistent with the basaltic composition found in previous results. Figure 2 also shows that the uneven surface has the deepest absorption bands. Discussion: The M observations show that the uneven surface is less mature than the smooth deposits, and the smooth deposits are less mature than the surrounding area. In addition, the uneven surface, the smooth deposits, and the surrounding mare have similar mineralogies; the main difference between the spectra amongst the three units is the variation in band depth. These observations can be explained by the blockiness of the uneven surface. The uneven surface will remain immature until the blocks are broken down, and the fresh surfaces of the blocks cause the observed strong band depths. Since the smooth deposits will mature more rapidly than the blocky ROI OMAT Uneven Surface .17 ± .05