Flows of Impact Melt at Lunar Craters

Lavalike materials that were emplaced in a fluid state occur in and around lunar impact craters whose diameters range from 3 km to more than 200 km and whose ages span a time interval of at least 3.5 billion years. The distribution of the lavalike deposits conforms to asymmetries of other ejecta from the same craters, and the material is concentrated downrange to distances as great as a crater radius. The character and distribution of the lavalike materials support the idea that they formed by impact melting rather than by volcanism. Returned samples indicate that materials with appropriate physical characteristics are generated by partial melting of feldspathic rocks by impact. The geologic evidence at lunar craters suggests that there is more melt rock in and near the craters than is predicted by experiment and theory.