DIRECT DETECTIONS OF SURFACE EXPOSED WATER ICE IN THE LUNAR POLAR REGIONS. Shuai Li1, Paul G. Lucey1, Ralph E. Milliken2, Paul O. Hayne3, Elizabeth Fisher2, Jean P. Williams4, Dana M. Hurley5 and Richard C. Elphic6. 1University of Hawaii; 2Brown University; 3University of Colorado Boulder; 4University of California, Los Angeles; 5Johns Hopkins University Applied Physics Laboratory; 6Ames Research Center. shuaili@hawaii.edu Introduction: The low temperature of permanently shaded regions (i.e. < 100 K) on the Moon may allow cold trapping of water that was sourced from solar wind implantation, impact delivery, and the lunar interior [1], and such deposits may preserve a record of the evolution of water in the inner solar system. Knowing the distribution of water ice on the Moon is critical for understanding its origin, stability, processes of deposition, and its viability for in situ resource utilization.

There are a number of strong indications of the presence of water ice in similar cold traps at the lunar poles [2-4], but none are unambiguously diagnostic of surface exposed water ice and inferred locations of water ice from different methods are not always correlated. Epithermal neutron counts, for instance, can be used to estimate hydrogen in the upper tens of centimeters of the lunar regolith, but such data cannot discriminate between H2O, OH or H [4]. Ratios of reflected ultraviolet (UV) radiation measured by the Lyman Alpha Mapping Project (LAMP) instrument onboard the Lunar Reconnaissance Orbiter (LRO) have been interpreted to indicate the presence of H2O near the lunar south pole [3], but the observed signatures may not be uniquely attributable to water ice because OH may exhibit similar characteristics at UV wavelengths [5]. High reflectance values at 1064 nm wavelength have also been observed near the lunar poles by the Lunar Orbiter Laser Altimeter (LOLA) and may be consistent with water ice, but fine particles and lunar regolith with lower degrees of space weathering may also give rise to higher reflectivity at this wavelength, making this interpretation non-unique [2, 6].

The Moon Mineralogy Mapper (M3) instrument on the Chandrayaan-1 spacecraft acquired the highest spatial and spectral resolution NIR data currently available at a global scale, including the polar regions. The wavelength range of M3 (0.46 – 2.98 µm) is too limited to properly discriminate OH/H2O species using fundamental vibration modes in the 3 µm region [7, 8], and in this study we focus on the detection of diagnostic overtone and combination mode vibrations for H2O ice that occur near 1.3, 1.5 and 2.0 µm. Numerical modeling results

suggest NIR spectra representing as little as 5 wt.% (intimate mixing) or 2 vol. % (linear areal mixing) water ice are expected to exhibit all three of these absorptions [9]. Methods: We obtained all M3 images and mosaicked them from 75-90° N/S using a stereographic projection. Only data between 1 µm and 2.5 µm (band 22 – 73 of M3 data) were applied in this study considering their higher SNR relative to longer wavelengths [10] and all strong ice absorptions are located in this wavelength range [11].

The positions of absorption shoulders and centers of pure water ice reported by Clark [11] were applied as criteria for identifying ice in the lunar polar regions (Table 1). Pixels were marked as ice-bearing if their spectra exhibited three such absorptions centered near 1.3 µm, 1.5 µm, and 2.0 µm, matching all conditions in Table 1. Spectral angles between the spectra of potential ice deposits and the spectrum of pure ice shown in Fig. 1 were also calculated, and a spectral angle less than 30° was empirically applied to further constrain the possible ice deposits.

Results and Discussion: Several thousand M3 pixels consistent with the presence of water ice were identified within 15° of the poles. The spectra of all ice bearing pixels were averaged and plotted for the northern and the southern polar regions (Fig. 1). The 1.5 µm absorption appears broader and more asymmetric in the average spectrum for the south compared with that from the north (Fig. 1), which is suggestive of larger grain sizes in the former [11]. Spectral mixing model results suggest that the weak absorption near 1.1 µm and the blue slope of the M3 spectra may be indicative of 30 wt.% ice or higher if it is mixed intimately with regolith, or over 20 vol.% if ice occurs as patches within otherwise ice-free regolith [9].

Almost all M3-derived ice locations also exhibited extreme LOLA reflectance and UV

Table 1. Ice absorptions at 1.0-2.5µm [11] Absorption

(µm) Shoulder (µm) Center (µm) Min Max Min Max

1.3 1.130 1.350 1.242 1.323 1.5 1.420 1.740 1.503 1.659 2.0 1.820 2.200 1.945 2.056

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ratio values consistent with the presence of water ice, as well as annual maximum surface temperatures below 110K [9]. The small number of candidate locations with slightly higher surface temperatures (110 – 160 K) or lower LOLA reflectance values may be false detections due to noise in M3 data. The agreement between these four data sets constitutes a robust detection of water ice at the optical surface in these locations.

The distribution of surface exposed water ice exhibits strong spatial coherence with temperatures less than 110K (Fig. 2), suggesting that temperature is one of the major controlling factors. However, not all regions less than 110K (cold traps) show ice exposures, such as cold traps in craters Amundsen, Hedervari, Idel’son L, and Wiechert near the south pole; the cold trap of Bosch crater near the north pole. The ice stability depth at these locations is coincidentally greater than zero when the Moon is hypothesized on its paleo-axis [13], which indicates that surface ice may only be retained at long timescale cold traps associated with the polar wander, similar to Ceres [14].

The patchy distribution and low abundance of surface exposed water ice in lunar cold traps may reflect a low rate of water supply and a fast rate of ice destruction (i.e. impact gardening) [9].

The absorption minima near 1.5 µm of the M3 spectra of ice-bearing pixels appear to be shifted ~0.05 µm to longer wavelengths compared with those of pure ice frost (Fig. 1). Such band shifts may reflect the increase of hydrogen bond strength due to bound water [15]. Alternatively, the 0.05 µm shift of the spectra of ice-bearing pixels may suggest that the ice was

condensed from vapor phase either due to impacts or migrations through the lunar exosphere [9]. It is also possible that such a shift may reflect higher D/H ratios [9]. However, we cannot rule out the possibility that the 0.05 µm shift of the 1.5 µm absorption is due to the low SNR of the M3 data. Conclusions: Unique IR absorptions of ice near 1.3, 1.5, and 2.0 µm were detected using the M3 data near the lunar polar regions. The detected ice-bearing pixels also exhibited high LOLA albedo and UV ratios as well as annual maximum surface temperatures below 110K. The distribution of ice exposures is dominantly controlled by the temperature, and may also be affected by the water supply rate, ice destruction rate, and thermal environment. The shift of the 1.5 µm absorption may indicate the low-density ice condensed from water vapor that might be associated with impacts and water migrations through the lunar exosphere. References: [1]. K. Watson et al., JGR, (1961). [2]. E. A. Fisher et al., ICARUS, (2017). [3]. P. O. Hayne et al., ICARUS, (2015). [4]. D. Lawrence et al., JGR, (2011). [5]. C. A. Hibbitts et al., in DPS. (Provo, Utah, 2017). [6]. M. T. Zuber et al., Nature, (2012). [7]. C. M. Pieters et al., Science, (2009). [8]. S. Li, R. E. Milliken, Sci. Adv, (2017). [9]. S. Li et al., PNAS, (Revision Submitted). [10]. R. Green et al., JGR, (2011). [11]. R. N. Clark, JGR, (1981). [12]. J. P. Williams et al., ICARUS, (2017). [13]. M. Siegler et al., Nature, (2016). [14]. N. Schorghofer et al., Astrophys. J, (2017). [15]. E. Libowitzky, Chem. Mon., (1999).

Fig. 1. Averaged M3 spectra of ice-bearing pixels in the north and south pole, respectively, compared with the spectrum of pure ice frost.

Fig. 2. Locations of ice-bearing pixels seen by the M3 data and further constrained by the LOLA, Diviner, and LAMP UV band ratio data near the north (a) and south (b) pole, respectively [9]. Each dot represents a M3 pixel and was enlarged for visualization.

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