Directly Characterizing Surficial Hydroxyl/Water on the Moon with the Lunar Trailblazer Mission
Abstract
In 2009, surficial hydroxyl (OH) and potentially water (H2O) were detected on the sunlit lunar surface by near-infrared (NIR) spectrometers on the Chandrayaan-1, EPOXI and Cassini spacecraft. Since that time, the distribution, abundance, and diurnal nature of the hydration has been hotly debated. The Moon Mineralogy Mapper (M3) provided the best spatially-resolved (0.54-2.98 µm; 70-280 m/pixel) coverage of the lunar surface, including a portion of the absorption band attributed to water/OH. Because the majority of minerals in lunar returned samples are anhydrous, and water on the illuminated lunar surface is unstable, M3 was not optimized to rigorously quantify water abundance. The spectral range cut-off just before 3 µm results in ambiguities in the strength, shape, and position of the water/OH absorption band. Attempts to remove the thermal signature from the spectra using different models resulted in conflicting answers about the distribution and possible mobility of water on the lunar surface. In June 2019, NASA selected the SIMPLEx mission Lunar Trailblazer for Phase A/B development, culminating in a Preliminary Design Review in late 2020 and follow-on decision to proceed to flight. Lunar Trailblazer is optimized to make targeted measurements of the infrared properties of the lunar surface. These measurements reveal the form, distribution, abundance and possible temporal change of water/OH together with compositional information to distinguish hydrated materials and thermal information to derive surface temperature. Lunar Trailblazer's objectives are to (1) detect and map water on the lunar surface at key targets to determine its form (OH, H2O, or ice), abundance, and distribution as a function of latitude, soil maturity, and lithology; (2) assess possible time-variation in lunar water on sunlit surfaces; and (3) map the form, abundance, and distribution of water ice in the PSRs, finding any operationally useful deposits of lunar water and locations where it is exposed at the surface for sampling. In all cases, Lunar Trailblazer simultaneously (4) measures surface temperature to quantify the local gradients and search for small cold traps. These measurements advance understanding of volatiles on airless bodies by study of the lunar water cycle and incorporation of water into the lunar crust. Trailblazer will also—particularly if NASA selects a proposed communications system enhancement option—provide reconnaissance for candidate landing sites and provide the highest spatial and spectral resolution shortwave infrared and mid-infrared maps of lunar lithologies across the surface. An enhancement option would also enable near global coverage. Lunar Trailblazer is the first generation of ride-along planetary smallsats, selected under SIMPLEx. NASA directed Trailblazer to baseline a launch opportunity with NASA's Interstellar Mapping and Acceleration Probe (IMAP), launching in 2024. From its separation with the primary, Trailblazer uses its propulsion system to enter a ~100-km polar orbit around the Moon. In pushbroom mode, the spacecraft acquires data with its two science instruments: the High-resolution Volatiles and Minerals Moon Mapper (HVM3), designed to map infrared spectra of hydrated materials, as well as the Lunar Thermal Mapper (LTM), a thermal infrared (TIR) mapping instrument to measure the temperature, composition, and thermophysical properties within each HVM3 pixel. HVM3 is a MatISSE-developed, JPL-built, modernized version of the successful M3 imaging spectrometer and has been optimized to identify and quantify water. LTM is a UK-contributed, University of Oxford/STFC RAL Space-built miniaturized TIR multispectral imager optimized to simultaneously measure temperature, composition, and thermophysical properties. Over Trailblazer's 1-year primary science mission, each instrument will acquire ≥1000 targeted images. Targets include measurements to determine the water content of PSRs using terrain-scattered light, and coverage of targets at multiple latitudes at 3 times of lunar day. Lunar Trailblazer is a PI-led mission at Caltech, managed by JPL with industry partner Ball Aerospace integrating the flight system. Science and mission operations will be led from Caltech. A student collaboration at Caltech and Pasadena City College involves undergraduate students—as well as graduate students and postdocs of the Co-Is—in all aspects of mission design and operations. Acknowledgements: Thanks to the NASA SIMPLEx program and project offices and the entire team and staff contributing to the success of Lunar Trailblazer at Caltech, The Jet Propulsion Laboratory, California Institute of Technology, Ball, Oxford, RAL Space, UKSA, NASA, and other partners. For more information, see trailblazer.caltech.edu. Lunar Trailblazer is funded under NASA contract #80MSFC19C0042.
- Publication:
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43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E.352K