Remote sensing surveys of the Moon and Mars show evidence of lava tubes, which are potential safe havens for human crews and their equipment. Ground penetrating radar (GPR) can be used to map tubes because the void/rock interface at tube ceilings and floors strongly reflects radar pulses. We have tested the capacity of GPR to sense lava tube geometry at Lava Beds National Monument in California, USA. GPR and detailed light detection and ranging (LiDAR) data are presented for two tubes: Skull Cave, with a few meters of overburden, diameter~10-20 m, and a rubbly floor; and Valentine Cave, with similarly thin overburden, diameter~1-3 m, and a flatter smoother floor. On both caves GPR clearly resolves the ceiling and permits good estimates of the cave width as validated with LiDAR data. Where GPR fails, the primary cause is inferred to be strong out-of-plane effects due to complex 3-D geometries. Recovery of the floor position requires migrating the GPR data with a 2-D velocity model, as signal velocity is faster in void space. We find that floor position is recoverable in caves whose voids are taller than the radar wavelength (~3 m in this study). Forward modeling assuming planetary parameters suggests that GPR should be similarly successful on the Moon or Mars.
We have the technological capability-today-to map and explore mines and caves on Earth. The purpose of this white paper is to urge NASA's support for development of technology needed to enter a planetary cave with a scientific payload for life detection. In the next pages, we tabulate the current state of technology & review the main challenges associated with: 1) remote characterization of volcanic caves, 2) subsurface exploration vehicles with advanced subsurface autonomy + communications + operations capabilities, 3) sensor systems developed for life and biosignature identification. Let's go to volcanic caves on Mars! MOTIVATIONHumans have looked for extraterrestrial biosignatures on the surfaces of other planets and moons. These surfaces are often exposed to conditions and processes that exceed the physical limits of life, e.g., intense cosmic radiation, impact events, and large thermal extremes, that would render difficult the preservation of biosignatures over geologic time.Planetary caves might provide protection from cosmic radiation, small-scale impact events, and have relatively stable thermal environments. These characteristics may well permit preservation of biosignatures over long periods of time and make them a prospective astrobiology target for biosignatures beyond Earth (Boston et al., 2001;Léveillé & Datta, 2010;Martins et al., 2017). A cave with natural openings offers direct access to the subsurface without drilling and deeper penetration into subsurface materials than could be obtained from a rover, landed platform, or penetrator launched from orbit. However, current technological and mechanical limitations associated with ingress and navigation, and communication and/or data retrieval make their exploration challenging.Caves form through a number of processes, but those on the Moon and Mars identified using satellite data are thought to be lava tubes. On Earth, most lava tubes are associated with basaltic lava, a material predicted to be ubiquitous on all rocky planets. On the Moon and Mars, hundreds of vertical collapse pits have been identified using a number of remote sensing approaches (Greeley, 1971;Cushing et al., 2015;2007;Haruyama et al., 2009;; many of these may be skylights providing direct access to intact caves, some of which may be be substantially larger than those found on Earth due to the combination of lower gravity and higher eruption rates on these smaller planetary bodies (e.g., Blair et al., 2017). Future planetary astrobiology missions would be well-served to include lava tubes as a high-priority target for investigation.NASA's SMD currently supports several Earth-based planetary cave analog investigations through its PSTAR (Planetary Science and Technology through Astrobiology Research) program; a table summary of these efforts is also included. We conclude with mention of current and ongoing technology developments both internal and external to NASA that could advance planetary cave identification, access, and exploration.
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