UNDERSTANDING LUNAR VOLCANIC PROCESSES AND MARE SURFACE AGE-DATING VIA REMOTE SENSING
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2023
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The Moon has a long and complex history of volcanism, which shapes the face that we see from Earth to this day. In this dissertation, we use remote sensing to examine multiple locations on the Moon to understand the regional volcanic processes along with their eruption ages. We begin (chapter 2) with the lunar floor-fractured crater Gassendi and surrounding area, which were examined with high-resolution Lunar Reconnaissance Orbiter Camera imagery and other remote-sensing data to characterize and understand the volcanism in the southwestern region. This region exhibits a variety of volcanic features (e.g., cryptomaria deposits, pyroclastic deposits, maria, lava lakes). We confirm the existence of a previously identified cryptomare deposit, identify an additional cryptomare deposit west of Gassendi crater, and a pyroclastic northeast of Gassendi. Spectral and geochemical anomalies associated with dark-haloed impact craters reveal cryptomaria deposits in the western Gassendi crater floor and previously unmapped mare basalt within northeastern Gassendi. We identified three separate lava lakes on the northeast, northwest, and southwest floor of Gassendi crater based on morphology analogous to terrestrial lava lakes, geochemical signatures, and digital terrain data. Crater count (model) age data suggest the lava lakes were active at ~3.6 Ga (300 Ma after floor emplacement). Criteria used to identify lava lakes in Gassendi were applied globally to locate candidate lava lakes within floor-fractured craters. With the identification of lava lake morphology, both in Gassendi crater and in other floor-fractured craters, the current ascent and eruption models should be revised to allow for at least short-term connection between magma supply at depth and surface lava lakes. Hence, this integration of multiple perspectives afforded by recent remote data sets reveals new views about lunar volcanic processes.
Next (chapter 3), we examine Northeastern Oceanus Procellarum (NE-OP) study area, which is a patchwork of lava flows that range in model age from 1.4 – 3.5 Ga (average age for all count areas is 2.3 Ga), but whose FeO and TiO2 contents deviate little. The intermediate TiO2 content values (4.0–6.8 wt.%) exhibited by the mare in this region represent material that is underrepresented in the current lunar sample collection. The model ages in the study region are bimodal (~2.2 Ga and ~3.0+ Ga), with eruption of lava flows at the Chang‘E-5 landing site occurring at ~3.0 Ga. By comparison, other investigators estimate the model age of the Chang‘E-5 site to be ~1.2 to 1.6 Ga. We find preliminary evidence that differences in measurement methodology may lead to disparate model ages and explain the difference in predicted model age of the Chang‘E-5 site.
We finish (chapter 4) with an examination of three NASA CLPS landing sites in the lunar maria (i.e., Reiner Gamma, Mare Crisium, and Lacus Mortis) and used crater counting techniques to determine the age of the mare (absolute model age). We compare differences in researcher measurement techniques and place the sites in regional context with regards to their lava flow ages. Two researchers performed crater density measurements at the three sites, using identical imagery with the same illumination conditions, and the same software tools. The uniform nature of the analysis environment allowed researchers to use accepted crater counting techniques to determine absolute model ages (AMA), while subsequently allowing the examination of the variations in the personal approaches used by the researchers. Comparisons revealed variations in researcher methodology and resulting AMAs. Landing sites were subdivided into two or more smaller count areas and we determined that all areas have mare basalts that are Imbrian in age. Variations in AMAs between researchers were the result of differences in the number of secondary and degraded craters identified and to a lesser extent crater diameter measurements. Building on the legacy work of the crater counting community, we recommend rigorous secondary crater identification and exclusion, DTM aspect-based diameters to calibrate measurements, high-resolution orbital imagery to improve rimcrest location measurements, and surface imagery to verify rimcrest condition.
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Planetology, Remote sensing, Geology, Cratering, Cryptomare, Impact processes, Moon, Spectral analysis, Volcanism
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