Enhancing LRO Diviner Interpretations of Lunar Regolith Properties Through Laboratory Experiments

ENHANCING LRO DIVINER INTERPRETATIONS OF LUNAR REGOLITH PROPERTIES THROUGH LABORATORY EXPERIMENTS. B. T. Greenhagen1, C. M. Wagoner1, T. M. Powell1, K. L. Donaldson Hanna2, T. Warren2, N. E. Bowles3, M. A. Siegler4, A. Martinez5, E. Jhoti6, and D. A. Paige6, 1Johns Hopkins Applied Physics Laboratory, USA, 2University of Central Florida, USA, 3University of Oxford, UK, 4University of Hawaii at Manoa, USA, 5Southern Methodist University, USA, 6University of California, Los Angeles, USA. (email: [email protected]) Introduction: Diviner is a multispectral radiometer that has observed solar reflectance and TIR emission from the Moon [1]. Over 16 years of operations, the analysis of LRO Diviner Lunar Radiometer observations has been significantly enhanced through groundtruth laboratory experiments as well as numerical models improved with new laboratory measurements of critical parameters. Emission Spectroscopy: The Simulated Airless Body Emission Laboratory (SABEL) at Johns Hopkins Applied Physics Laboratory reproduces relevant surface thermal environments to create appropriate thermal gradients and simulate the wide range of temperatures relevant to spectral measurement of minerals, mineral mixtures, regolith simulant, and Apollo soils (Figure 1) [e.g., 2]. Similar measurements in support of Diviner have been collected by Diviner Co-Is at the University of Oxford / University of Central Florida Planetary Analogue Surface Chamber for Asteroid and Lunar Environments (PASCALE) [e.g., 3], and Stony Brook University Planetary and Asteroid Regolith Spectroscopy Environmental Chamber (PARSEC) [e.g., 4]. The ability to groundtruth Diviner multispectral data with facilities like SABEL, PASCALE, and PARSEC provide significant confidence in compositional interpretations, which are further enhances through the inclusion of additional remote sensing datasets such as the Chandrayaan-1 Moon Mineralogy Mapper and Kaguya Multiband Imager [5]. Figure 1: Schematic of technique used to reproduce a lunar like thermal emission environment in facilities such as SABEL, PASCALE, and PARSEC. Emission Phase Function: Diviner observations extend beyond ‘typical’ nadir pointing (i.e., nearzero emission angle) to include a wide range of illumination and viewing to characterize radiative balance and spectral emission across the full emission phase function (EPF). Thermal models with improved photometric parameters have helped predict the behavior observed by Diviner. Laboratory validation of EPF behavior has been conducted at the University of Oxford Space Environment Goniometer (SEG) [6]. Insights gained from these studies are used to investigate the thermal structure of the lunar regolith and will improve interpretations of TIR datasets of other airless bodies such as BepiColombo MERTIS. Thermal Conductivity: Experimental efforts to determine the thermal conductivity of lunar regolith and simulants at cryogenic temperatures are underway at the Jet Propulsion Laboratory. Using the transient line heat source technique [7], thermal conductivity can be determined over a temperature range of 15–300 K at densities relevant to the lunar surface. Accurate estimates of the thermophysical properties of lunar regolith below ~100 K are important for thermal models of the lunar south pole, particularly within permanently shadowed regions, where surface temperatures can drop as low as 20 K [8]. The ability to predict the stability of cold-trapped volatiles (such as water ice) is, in part, dependent on the constraints of thermal properties of lunar materials at cryogenic temperatures. Preliminary results indicate a decrease in thermal conductivity with temperature, lower than predictions from standard thermal conductivity models for lunar regolith [9]. These findings may have important implications for predicting subsurface temperatures in the cold polar regions of the Moon, where a reduced thermal conductivity at low temperatures produces warmer subsurface temperatures, potentially affecting the thermal stability of cold-trapped volatiles [10]. References: [1] Paige D. A. et al. (2010) SSR, 150, 125-160. [2] Greenhagen B. T. et al. (2020) LPSC 51, Abstract #2171. [3] Shirley K. A. et al. (2025) LPS 56, Abstract #2181. [4] Arnold J. A. et al (2016) JGR: Planets, 121(7), 1342-1361. [5] Lucey P. G. et al. (2021) JGR: Planets, 126(6), e2020JE006777. [6] Curtis R. J. et al. (2021) Rev Sci Inst 92(3):034504. doi: 10.1063/5.0038632. [7] Carslaw H. S. and Jaeger J. C. (1959). Clarendon.[8] Paige D. A. et al. (2010). Science, 330(6003), 479482. [9] Hayne P. O. et al. (2017) JGR: Planets, 122(12), 2371-2400. [10] Martinez A. and Siegler M. A. (2021) JGR: Planets, 126(10), e2021JE006829.