Exploring Lunar Volcanism At Two Unique Mare Basalt Sites: Ina Caldera And The Marius Hills Volcanic Complex
Rationale for Mare Sites Mare basaltic volcanism represents the bulk of the Moon’s secondary crust, reflecting the composition and evolution of the Moon’s interior over time. Understanding volcanism and the evolution of the Moon and Solar System are key science objectives in NASA’s 2014 strategic plan, the Decadal survey, and the NRC 2007 report on the Scientific Context for Exploration of the Moon. Two key sites of mare volcanism on the lunar surface provide two different strategies for investigating the range, flux, and diversity of basaltic volcanism. Focused surface mission at Ina D The Ina caldera (Fig. 1) is a unique and well-preserved exposure  that may be as young as 18 Ma . Although its youthful interpretation has been recently questioned , Ina represents a relatively localized and unusual lunar volcanic process that is not yet understood. Investigations of the small area (~7 km2) and small scale of the surface materials (sharp contacts between deposits and many outcrops less than 10 m across) require surface exploration. A focused mission on the lunar surface can address specific scientific objectives, including: 1) Are sub-meter-scale fractures typical of young volcanic deposits present? 2) Are sub-meter-scale collapse features or pitting present? 3) What are grain sizes, cohesiveness, and extents of regolith formation on the surface? 4) What is the composition and mineralogy? 5) What is the age of the surface? Fortunately, the Lunar Reconnaissance Orbiter (LRO), particularly the LRO Camera (LROC), has already collected data providing 2-m scale topography and better than 0.5 m/pix imaging of Ina for landing site characterization. These data are being used to assess and plan surface operations at this site for a small lander concept, IMPEL (Irregular Mare Patch Exploration Lander), led by a team at NASA JSC. This mission would answer key science questions through targeted surface analyses ranging from high-resolution in-situ imaging, spectroscopy, and geo-mechanical testing. While mobility and sample return could enhance science return by providing details of surface ages, eruption ages, and compositions, the most pressing questions can be addressed through a short-duration surface mission. Long-duration surface mission at the Marius Hills The Marius Hills Volcanic Complex is also unique, relatively well-preserved, and includes numerous volcanic expressions: cinder-spatter cones, lava flows, lava tubes, parasitic shield volcanoes, and a broad regional volcanic shield [e.g., 4-6]. A southern portion of the complex is crossed by the Reiner Gamma albedo feature, which could also be explored. The bulk of the Marius Hills volcanic deposits were emplaced around 3.5 Ga, but small areas could be much younger (~1.2 Ga) based on crater size–frequency distributions, with eruptions continuing over several billion years . A long-duration surface mission to explore this region, lasting through several nights and traversing many kilometers, would provide insight into a broad timespan of lunar volcanic history (from peak eruptions to waning, late-stage sputters). Key questions to be addressed at the Marius Hills include: 1) What is the span of ages of eruptions across the Marius Hills? 2) What are the compositions of the eruptions and did the composition evolve over time? 3) How did the volatile content and eruption rates evolve over time? 4) What are the implications for the evolution of lunar volcanism and interior? While single-point or localized analyses of part of the Marius Hills would certainly provide key insight into its volcanic history, a long-duration and highly mobile surface mission (e.g., ) would permit investigation of a range of volcanic eruptions across the shield. The LRO mission has collected a vast suite of data from this area that can be applied to landing site and traverseability characterizations. Preliminary work using 2-m scale digital elevation models has already demonstrated that many key targets for analysis in the Marius Hills are accessible by a rover that can navigate slopes up to 15° (Fig. 2) . References  Schultz et al. (2006) Nature, 10.1038/nature05303.  Braden et al. (2014) Nature Geosci, 10.1038/ngeo2252.  Qiao et al. (2017) Geology, 10.1130/G38594.1.  Greeley (1971) Moon 3: 289-314.  Lawrence et al. (2013) JGR-P, 10.1002/jgre.20060.  Haruyama et al. (2009) GRL, 10.1029/2009GL040635.  Hiesinger et al. (2016) LPSC, Abs#1877.  Robinson et al. (2011) LEAG Meeting, Abs#2042.  Stopar et al. (2016) LEAG Meeting, Abs#5074.