The Mare Imbrium Paradox and the Origin of Vallis Schröteri
Figure 1. (a) Lava flow lobes in Mare Imbrium. Dashed box outlines the approximate field of view for Figure 1b. Apollo 15 Metric photograph AS15-M-1557. (b) Close up view of thick flow margins in Mare Imbrium. Flow margins are marked by dashed, white lines. Apollo 17 70-mm Hasselblad photograph AS17-155-23714.
The Lunar Reconnaissance Orbiter (LRO) satellite signifies the revitalization of scientific interest in our moon. Using two instruments, the Lunar Reconnaissance Orbiter Camera (LROC) and Lunar Orbiter Laser Altimeter (LOLA), on the satellite, we plan to investigate the paradox of the Mare Imbrium lava flow field and the origin of the largest sinuous rille on the moon, Vallis Schröteri.
The main aspect of our research involves resolving the origin of channeled lava flows and rilles on the Moon. For the purpose of our studies, we have defined channeled lava flows as having a well-defined central conduit bound by thick flow margins, called levees. Rilles refer to channel features that appear to be carved into the surface with little or no expression of a flow margin or levee.
Figure 2. (a) A natural color image of the ~170 km long Vallis Schröteri, where the white boxes outline field of view for 1b, 1c, and 1d. (b) The mound surrounding the crater at the source area of Vallis Schröteri, known as the cobra head. Apollo 15 photograph AS15-0332. (c) Outcrops are exposed in the upper portion of Vallis Schröteri's southern wall. Apollo photograph AS15-95-12968. (d) The inner rille mysteriously crosscuts the wall of the primary rille. Apollo photograph AS15-92-12509.1
The best-developed channeled lava flows on the lunar surface occur in Mare Imbrium where the flow field extends over 1600 km from the source region and the flow margins are especially thick. This contrasts lunar sinuous rilles, like Vallis Schröteri, where there is a deep, meandering channel, but no thick flow margins. Apollo 15 astronauts observed layers of lava flows in outcrops along the upper wall of Hadley Rille and laboratory experiments show that lunar lavas have a low viscosity, which suggest lunar lava flows are typically very thin (<10 m). Over time, the flow margins could be difficult to detect as they are buried by regolith, degraded by impacts, or coalesce with other flows. The observed absence of thick lava flows, except for Mare Imbrium, is consistent with this theory. So, 'Why is the Mare Imbrium flow field very thick?' and 'How come lunar rilles are more common than channeled lava flows on the lunar surface?' These are two guiding questions for our research.
There are three central objectives to our research. First, we want to analyze the morphology of the Mare Imbrium lava flows including measuring flow thickness and channel dimensions so we can qualitatively and quantitatively describe the eruption process necessary to form the extensive flow field. Second, we plan to reassess the geologic processes involved in the formation and evolution of Vallis Schröteri through analysis of the topography and small-scale morphology of the rille. Third, we will compare the morphology and emplacement processes of lunar channeled lava flows and lunar rilles to terrestrial, Martian, and Venusian analogues. Learn about other CEPS lunar research.
1. Garry et al. (2007), "Analysis of the Morphology and Emplacement of Volcanic Features on the Moon with the Lunar Reconnaissance Orbiter" proposal.
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