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Applying the Near Earth Asteroid Thermal Model to Lunar Surface Spectra in Search of Hydration Absorption Features

Presentation #302.02 in the session “Moon and Mercury 1”.

Published onOct 26, 2020
Applying the Near Earth Asteroid Thermal Model to Lunar Surface Spectra in Search of Hydration Absorption Features

It has long been believed that the moon is dry, and no water exists on the lunar surface. Samples returned from the Apollo missions contain evidence of water at the ppm level in lunar soils. Recent observations made with three spacecraft have also detected hydroxyl and molecular water absorption features near 3-µm on the lunar surface. The origins of the features are currently theorized to be hydrogen capture from solar wind implantation, the delivery of cometary or asteroidal water, and/or endogenous sources. Although the spacecraft detections are accurate, each space craft is limited. Data from Cassini and Deep Impact are limited in spatial resolution, global coverage and lunar time of day. The Moon Minerology Mapper (M3) that is on board Chandryaan-1 has a high spatial resolution, but only covers the wavelength region up to the 3-µm band. To confirm the detection of OH and/or H2O. The SpeX infrared cross-dispersed spectrograph at the Infrared Telescope Facility (IRTF) was used to obtain lunar data from 1.67-4.2 µm at 1-2 km spatial resolution. After reduction of the spectra, the Near-Earth Asteroid Thermal Model (NEATM) was applied to remove the thermal excess in the spectrum, so analysis of the reflective component can reveal any hydroxyl and molecular water features at the 3-µm band. Data reduction and calibrations were preformed using Spextool software provided by IRTF. To remove atmospheric absorptions from the spectra, observations of a solar-like star at similar atmospheric path lengths as the lunar observations were obtained. Several thermal models have been theorized to accurately remove the thermal component of lunar spectra. However, there is currently debate on the removal of thermal radiation, and analysis of data has led to strikingly different conclusions. The Standard Thermal Model (STM) assumes that no thermal emission arises on the night side of the object, and an idealized nonrotating spherical object at 0° phase angle with a temperature distribution decreasing from the maximum at the subsolar point to zero at the terminator. Being an extension of the STM, the NEATM uses the beaming parameter as a calibration constant, in which it is varied until the best fit to the dataset is found. The solar phase angle is also considered by calculating the thermal flux an observer would detect from the illuminated portion of a smooth sphere visible at a given solar phase angle, instead of using a fixed phase coefficient as done in the standard model. Data were used from three different regions; Montes Alpes, Airy Swirl, and the sub-solar point. The NEATM was applied to each region. Overall these regions are found to be dry, showing no 3-µm absorption features. The result, especially when referencing the sub solar point, is not consistent with the results of Bandfield et al. (2018).

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