| Summary: | The discovery of water on the Moon raises the possibility that lava-water, or phreatomagmatic, interactions have occurred on the lunar surface in the past. Such interactions may have formed pseudocraters, crater-like landforms that result from steam explosions that occur when lava flows come in contact with surface or near-surface water or ice. We present a study of Mare Frigoris, a volcanic plain just north of the Mare Imbrium impact basin on the Moon. Clusters of irregular, circular features on the basaltic lava flows in this area resemble pseudocrater fields in Iceland, and they are located in a region with inferred high hydrogen and hydroxyl content. In order to determine if the candidate pseudocraters are indeed phreatomagmatic in origin, remote sensing analyses and a numerical model were used. Narrow-angle camera (NAC) and wide-angle camera (WAC) images from the Lunar Reconnaissance Orbiter Camera (LROC) were used to map the morphologies of the candidate pseudocraters and to compare them to the morphology of phreatomagmatic pseudocraters on Earth. Lunar Orbiter Laser Altimeter (LOLA) topographic data were used to make measurements of the morphology and dimensions of the candidate pseudocraters in Mare Frigoris to evaluate their similarity to the pseudocraters in Iceland. A numerical model for the explosion dynamics of pseudocraters was then used to simulate lava-ground ice interactions on the surface of the Moon. Using parameters appropriate for the surface conditions on the Moon, the model results estimate that 48-79 ppm water is required to produce pseudocraters of similar size and shape to those observed at Mare Frigoris. Constraints on the amount of water present on the Moon is important for determining the best landing sites for future exploration missions, since in situ water resources can be used for drinking, life support, and the production of rocket fuel. The presence and quantity of water on the Moon also has important implications for models of lunar formation and evolution.
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