Exploring Pit Craters To Understand The Lunar Maria
Science / Exploration Questions to be Addressed at the Site: Images returned by Kaguya and LRO have revealed the presence of deep mare pits containing tens of meters of layered bedrock stratigraphy exposed in their walls ([1-3], Fig. 1). A mission to a mare pit would address numerous top priority lunar science goals laid out in community reviews , the Decadal Survey , and the Lunar Exploration Roadmap  in the following ways: (1) Flood basalts are common in the Solar System, and the lunar maria are a type example for presumed high flow-rate, low viscosity, sheet-like emplacement. However, field work on terrestrial flood basalts has revealed that many of these units were emplaced as inflated pahoehoe flows instead [e.g. 7]. Whether flood basalts were mostly emplaced turbulently or incrementally has important implications for the physics of their movement through the crust , as well as their effects on planetary atmospheres, from the creation of an ephemeral atmosphere on the Moon  to dramatic climate shifts on Mars [e.g., 10]. Documenting the thickness, flow morphologies, and petrologic textures of lava layers in a sequence would address this ambiguity. (2) Analysis of Apollo mare basalt samples has revealed significant compositional variety and implied source regions, even among basalts taken from a single landing site . Characterizing chemical and mineralogical trends along a stratigraphic section of the maria would help reveal how partial melting processes and mantle source regions evolved through time. Analysis of an in-place stratigraphy would shed light on whether the lavas came from a long-lived magma source that evolved through time, or from unique magma sources. Characterizing chemical and mineralogical changes within a single lava flow would provide necessary context for identifying samples representative of parental melts. (3) Many of the mare pits provide access not only to bedrock stratigraphy but also to a cross-section through the lunar regolith. The deepest Apollo core was nearly 3 m long , but some of the near-vertical regolith funnels surrounding the mare pits are more than 10 m deep . The exposure offered in the lunar pits would also allow access to the regolith/bedrock interface for the first time, where the extent of fracturing of the original bedrock to create the regolith could be assessed. In addition, paleoregolith layers, potentially preserved between basalt layers, would yield insight about the intermittency of mare eruptions and the timescales for regolith formation . (4) In some cases, lunar mare pits may open into subsurface void spaces or lava tubes [1-3,13]. Characterizing these tubes would yield information about lava flux rates and the distances that insulated lava could flow from the vent. Information about lava tubes and caves is also highly sought after for the purposes of human exploration and habitation . Human settlements located in lava tubes would benefit from a stable, benign temperature, and would be protected from cosmic rays and micrometeorites. Landing Site Assessment: The mare pits are located in generally flat regions, with the only major hazard being the pit itself. The Marius Hills pit is located at the bottom of a wide sinuous rille among some broad volcanic cones. The Marius Hills, Tranquillitatis, and Fecunditatis pits are located in the equatorial regions of the nearside of the Moon. The other large pits, located in Ingenii (farside) and Lacus Mortis (nearside), are located in the midlatitudes. Of the large pits, the Marius Hills pit currently has the most evidence to suggest the presence of a subsurface tube , although the subsurface space at both Tranquillitatis and Ingenii extends for at least 20 m . Mobility Requirement: Mobility is required to access the science and exploration targets at this site. A platform that could reach the edge of the pit could view the macro-morphology of the lavas. A robot that could survive a fall into the pit could potentially confirm the existence of a cave. In order to fully explore these targets, however, a climbing or rappelling rover (or astronaut) is required, with sufficient payload capacity to measure the mineralogy and elemental chemistry of the lava layers, and to look closely at their morphologies, and flow textures. All of the critical measurements described above can be accomplished in-situ, with simple, high heritage instruments. References:  Haruyama, J., et al. (2009) GRL 36, L21206  Robinson, M.S. et al. 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