The global albedo of the Moon at 1064 nm from LOLA

Abstract

The Lunar Orbiter Laser Altimeter (LOLA) measures the backscattered energy of the returning altimetric laser pulse at its wavelength of 1064 nm, and these data are used to map the reflectivity of the Moon at zero-phase angle with a photometrically uniform data set. Global maps have been produced at 4 pixels per degree (about 8 km at the equator) and 2 km resolution within 20°latitude of each pole. The zero-phase geometry is insensitive to lunar topography, so these data enable characterization of subtle variations in lunar albedo, even at high latitudes where such measurements are not possible with the Sun as the illumination source. The geometric albedo of the Moon at 1064 nm was estimated from these data with absolute calibration derived from the Kaguya Multiband Imager and extrapolated to visual wavelengths. The LOLA estimates are within 2σ of historical measurements of geometric albedo. No consistent latitude-dependent variations in reflectance are observed, suggesting that solar wind does not dominate space weathering processes that modify lunar reflectance. The average normal albedo of the Moon is found to be much higher than that of Mercury consistent with prior measurements, but the normal albedo of the lunar maria is similar to that of Mercury suggesting a similar abundance of space weathering products. Regions within permanent shadow in the polar regions are found to be more reflective than polar surfaces that are sometimes illuminated. Limiting analysis to data with slopes less than 10°eliminates variations in reflectance due to mass wasting and shows a similar increased reflectivity within permanent polar shadow. Steep slopes within permanent shadow are also more reflective than similar slopes that experience at least some illumination. Water frost and a reduction in effectiveness of space weathering are offered as possible explanations for the increased reflectivity of permanent shadow; porosity is largely ruled out as the sole explanation. The south polar crater Shackleton is found to be among the most reflective craters in its size range globally but is not the most reflective, so mass wasting cannot be ruled out as a cause for the crater's anomalous reflectance. Models of the abundance of ice needed to account for the reflectance anomaly range from 3 to 14% by weight or area depending on assumptions regarding the effects of porosity on reflectance and whether ice is present as patches or is well mixed in the regolith. If differences in nanophase iron abundances are responsible for the anomaly, the permanently shadowed regions have between 50 and 80% the abundance of nanophase iron in mature lunar soil.

Evidence for the sequestration of hydrogen-bearing volatiles towards the Moon’s southern pole-facing slopes

“…Results from LRO's Lunar Observing Laser Altimeter albedo (LOLA) were used to suggest deposits of surface water frost on the walls of Shackleton crater using 1064 nm albedo measurements (Smith et al, 2010;Zuber et al, 2012). Lucey et al (2014) found LOLA albedo differences that are consistent with an interpretation of water frost deposition in PSR vs. non-PSR and on pole-facing slopes (PFS) vs. equator-facing slopes (EFS). Ground truth evidence of the existence of hydrogenbearing materials within a south polar PSR was established by the NIR and ultra-violet (UV) detection of water and molecular hydrogen within the excavated plume made by the exhausted rocket motor of the Lunar CRater Observing and Sensing Satellite (LCROSS) as it impacted Cabeus crater (Colaprete et al, 2010;Hurley et al, 2012).…”

“…Another potential explanation for the epithermal contrasts is that EFS and PFS slopes are diurnally exposed to very different temperature distributions that may lead to asymmetric space weathering, optical maturity or mass wasting effects that could influence epithermal rates (Lucey et al, 2014;Lawrence et al, 2014). Such a geochemical or geophysical alteration of the regolith identified here specific to PFS, would then have to at least partially cause the relative suppression of epithermal flux on PFS and also be latitude dependent, and maximized at the poles thus emulating the effects of locally enhanced hydrogen concentrations towards PFS.…”

Bistatic radar observations of the Moon using Mini-RF on LRO and the Arecibo Observatory

“…Recent measurements using neutron spectroscopy [Feldman et al 1998[Feldman et al , 2001Lawrence et al, 2006;Elphic et al, 2007;Mitrofanov et al, 2010], nearinfrared reflectance spectroscopy [Pieters et al 2009], thermal infrared radiometry [Paige et al 2010], far UV imaging and spectroscopy Hayne et al, 2015], and LIDAR reflectance [Zuber et al 2012;Lucey et al 2014], and in situ measurements [Colaprete et al, 2010] all point to the presence of water on the Moon. The Lunar Crater Observation and Sensing Satellite (LCROSS) impact into a permanently shadowed portion of Cabeus crater has provided the most direct evidence for the presence of water in the form of ice [Colaprete et al, 2010].…”

Bulk hydrogen abundances in the lunar highlands: Measurements from orbital neutron data

“…This new understanding has resulted from multiple new types of measurements from orbital remote sensing and lunar sample analysis. Recent spacecraft observations and data analyses of the lunar poles have revealed new information about both the species, and spatial and depth-dependent distribution of volatiles within permanently shaded regions (Colaprete et al, 2010;Spudis et al, 2010;Miller et al, 2014;Lucey et al, 2014). Away from the poles, orbital spectral reflectance measurements have identified surficial enhancements of H 2 O/OH (Clark, 2009;Pieters et al, 2009;Sunshine et al, 2009;Klima et al, 2013).…”