Water ice may be allowed to accumulate in permanently shaded regions on airless bodies in the inner solar system such as Mercury, the Moon, and Ceres Watson K, et al. (1961) J Geophys Res ...66:3033–3045. Unlike Mercury and Ceres, direct evidence for water ice exposed at the lunar surface has remained elusive. We utilize indirect lighting in regions of permanent shadow to report the detection of diagnostic near-infrared absorption features of water ice in reflectance spectra acquired by the Moon Mineralogy Mapper M (3) instrument. Several thousand M (3) pixels (∼280 × 280 m) with signatures of water ice at the optical surface (depth of less than a few millimeters) are identified within 20° latitude of both poles, including locations where independent measurements have suggested that water ice may be present. Most ice locations detected in M (3) data also exhibit lunar orbiter laser altimeter reflectance values and Lyman Alpha Mapping Project instrument UV ratio values consistent with the presence of water ice and also exhibit annual maximum temperatures below 110 K. However, only ∼3.5% of cold traps exhibit ice exposures. Spectral modeling shows that some ice-bearing pixels may contain ∼30 wt % ice that is intimately mixed with dry regolith. The patchy distribution and low abundance of lunar surface-exposed water ice might be associated with the true polar wander and impact gardening. The observation of spectral features of H₂O confirms that water ice is trapped and accumulates in permanently shadowed regions of the Moon, and in some locations, it is exposed at the modern optical surface.
•We produced three space weathering maps based upon Kaguya MI data.•Lunar swirls show reduced abundances of nanophase iron particles.•Space weathering is less efficient at higher latitudes.•Earth's ...magnetotail reduces nanophase iron production on the lunar nearside.•Relative abundance of microphase to nanophase Fe is lower on the Moon than Mercury.
Space weathering is a continuous process operating on the surface of the Moon producing nanophase and microphase iron on the rims of mineral grains and within agglutinates. The spectral effects due to these nanophase and microphase iron particles in the lunar regolith are well characterized in the visible and near-infrared wavelengths. In this work, we used the Hapke radiative transfer technique to model multispectral data (415–1550 nm) from the Kaguya Multiband Imager to map the nanophase and microphase iron abundances across the lunar surface. In order to produce these maps, we developed a reflectance model for silicate host particles as a function of FeO content using magnetic and Lunar Soil Characterization Consortium (LSCC) data. We tested the accuracy of our radiative transfer technique and found that the model can predict the nanophase and microphase iron abundances within 0.1 wt% Fe and 0.6 wt% Fe, respectively. By using a FeO and ilmenite map and our relationship between host particle reflectance and FeO abundance, we produced a nanophase, microphase, and submicroscopic iron abundance map. We found the approximate saturation limit for nanophase and microphase iron with respect to FeO. In general, the nanophase and microphase iron saturation limit increases with FeO content, except in soils that have FeO content of 6–15 wt%, the nanophase iron saturation is nearly constant. We also observed that the microphase iron abundance saturates before the nanophase iron abundance. These maps could be used to determine relative age and maturity, but with limitations. Our analyses of these maps showed that (1) swirls contain low abundances of nanophase iron, but no anomalous microphase iron abundances, (2) nanophase and microphase iron abundances are lower at higher latitudes, which suggests lower solar wind and micrometeoroid impact flux at these latitudes, (3) decreased nanophase iron abundances in the nearside suggest a lower solar wind flux due to Earth's magnetotail, and (4) in contrast to Mercury, the microphase to nanophase iron abundance ratio is lower on the Moon due to lower surface temperatures and less intense space weathering.
•UV spectra and temperature indicate water ice is exposed near the lunar south pole.•Temperature controls water ice abundance on the Moon.•Water ice abundance increases with decreasing temperature ...<110K.•Water ice abundances of 0.1–1% are observed within the major cold traps.•Two populations of cold traps suggest a parity between water sources and losses.
We utilize surface temperature measurements and ultraviolet albedo spectra from the Lunar Reconnaissance Orbiter to test the hypothesis that exposed water frost exists within the Moon’s shadowed polar craters, and that temperature controls its concentration and spatial distribution. For locations with annual maximum temperatures Tmax greater than the H2O sublimation temperature of ∼110K, we find no evidence for exposed water frost, based on the LAMP UV spectra. However, we observe a strong change in spectral behavior at locations perennially below ∼110K, consistent with cold-trapped ice on the surface. In addition to the temperature association, spectral evidence for water frost comes from the following spectral features: (a) decreasing Lyman-α albedo, (b) decreasing “on-band” (129.57–155.57nm) albedo, and (c) increasing “off-band” (155.57–189.57nm) albedo. All of these features are consistent with the UV spectrum of water ice, and are expected for water ice layers >∼100nm in thickness. High regolith porosity, which would darken the surface at all wavelengths, cannot alone explain the observed spectral changes at low temperatures. Given the observed LAMP off-band/on-band albedo ratios at a spatial scale of 250m, the range of water ice concentrations within the cold traps with Tmax<110K is ∼0.1–2.0% by mass, if the ice is intimately mixed with dry regolith. If pure water ice is exposed instead, then up to ∼10% of the surface area on the 250-m scale of the measurements may be ice-covered. The observed distribution of exposed water ice is highly heterogeneous, with some cold traps <110K having little to no apparent water frost, and others with a significant amount of water frost. As noted by Gladstone et al. (Gladstone, G.R. et al. 2012. J. Geophys. Res.: Planets 117(E12)), this heterogeneity may be a consequence of the fact that the net supply rate of H2O molecules to the lunar poles is very similar to the net destruction rate within the cold traps. However, an observed increase in apparent H2O abundance with decreasing temperature from ∼110K to 65K suggests that destruction of surface frosts by impact gardening and space weathering is spatially heterogeneous. We find a loosely bimodal distribution of apparent ice concentrations with temperature, possibly due to competition between vertical mixing by impact gardening and resupply of H2O by vapor diffusion at sites ∼110K. Finally, we cannot rule out the possibility that the colder population of ice deposits is in fact primarily carbon dioxide ice, although peak temperatures of ∼65K are slightly higher than the usual CO2 sublimation temperature of ∼60K.
•This analytic model describes the depth overturned by impact gardening as a function of time.•Secondary impacts are the dominant drivers of mixing in the top meter of lunar regolith.•Model results ...are consistent with the mixing rate calculated from Apollo cores and inferred from impact related surface features.
In this work we update the regolith mixing model presented by Gault et al. (1974), including new input values and reworking key parameters. Much as Gault et al. did, we present a way to calculate the rate at which lunar regolith is overturned at depth. The model describes a mixing front that proceeds downward from the surface following a power-law function of time. Our most important update is the inclusion of secondary impacts. Our calculations show that secondaries are necessary to produce the reworking rate inferred from the depth distribution of surface-correlated material in Apollo cores (Fruchter et al., 1977; Morris, 1978; Blanford, 1980), from the rate at which splotches rework the top 3 cm of regolith (Speyerer et al., 2016), and from the rate at which Diviner cold spots (Bandfield et al., 2014) and crater rays (Pieters et al., 1985; Hawke et al., 2004; Werner and Medvedev, 2010) are reworked into background regolith. Overturn calculations that only consider the impact of primaries fail to describe observed reworking rates at all depths and timescales. We conclude that secondary impacts dominate mixing in the top meter of lunar regolith.
•The lunar South Pole exhibits enhanced reflectance at maximum temperatures below 110K that may indicate the presence of widespread surface water ice.•Anomalously bright locations are found at both ...the North and South poles in regions of permanent shadow that may represent local concentrations of water frost.•Reflectance excursions near 200K and 300K may indicate the presence of volatiles more refractory than water ice.•There is a general correlation of temperature and reflectance that is attributed to the effect of space weathering.
We find that the reflectance of the lunar surface within 5° of latitude of the South Pole increases rapidly with decreasing temperature, near ∼110K, behavior consistent with the presence of surface water ice. The North polar region does not show this behavior, nor do South polar surfaces at latitudes more than 5° from the pole. This South pole reflectance anomaly persists when analysis is limited to surfaces with slopes less than 10° to eliminate false detection due to the brightening effect of mass wasting, and also when the very bright south polar crater Shackleton is excluded from the analysis. We also find that south polar regions of permanent shadow that have been reported to be generally brighter at 1064nm do not show anomalous reflectance when their annual maximum surface temperatures are too high to preserve water ice. This distinction is not observed at the North Pole. The reflectance excursion on surfaces with maximum temperatures below 110K is superimposed on a general trend of increasing reflectance with decreasing maximum temperature that is present throughout the polar regions in the north and south; we attribute this trend to a temperature or illumination-dependent space weathering effect (e.g. Hemingway et al., 2015). We also find a sudden increase in reflectance with decreasing temperature superimposed on the general trend at 200K and possibly at 300K. This may indicate the presence of other volatiles such as sulfur or organics. We identified and mapped surfaces with reflectances so high as to be unlikely to be part of an ice-free population. In this south we find a similar distribution found by Hayne et al. (2015) based on UV properties. In the north a cluster of pixels near that pole may represent a limited frost exposure.
Mineral maps of the Moon Lucey, Paul G.
Geophysical research letters,
April 2004, Letnik:
31, Številka:
8
Journal Article
Recenzirano
Odprti dostop
Global maps of the distribution of plagioclase, orthopyroxene, clinopyroxene and olivine on the Moon were derived from radiative transfer analysis of 400,000 Clementine UVVIS spectra. Plagioclase ...inversely correlates with iron while clinopyroxene positively correlates with iron showing these are the major carriers of aluminum and iron respectively. The distribution of olivine in the maria agrees with previous studies; in the highlands the abundance of olivine is low but ubiquitous at a few percent, except within the South Pole‐Aitken basin where it is only present in very small exposures. In the very anorthositic farside highlands, olivine is often the sole mafic mineral. The abundance of orthopyroxene is generally low, excepting elevated abundances in the nearside highlands and in areas near and within South Pole‐Aitken basin. Mare units with elevated abundances of orthopyroxene are found in some mare and cryptomare deposits distant from the sample return sites.
We used infrared data from the Lunar Reconnaissance Orbiter (LRO) Diviner Lunar Radiometer Experiment to globally map thermophysical properties of the Moon's regolith fines layer. Thermal ...conductivity varies from 7.4 × 10−4 W m−1 K−1 at the surface to 3.4 × 10−3 W m−1 K−1 at depths of ~1 m, given density values of 1,100 kg m−3 at the surface to 1,800 kg m−3 at 1 m depth. On average, the scale height of these profiles is ~7 cm, corresponding to a thermal inertia of 55 ± 2 J m−2 K−1 s−1/2 at 273 K, relevant to the diurnally active near‐surface layer, ~4–7 cm. The temperature dependence of thermal conductivity and heat capacity leads to an ~2 times diurnal variation in thermal inertia at the equator. On global scales, the regolith fines are remarkably uniform, implying rapid homogenization by impact gardening of this layer on timescales <1 Gyr. Regional‐ and local‐scale variations show prominent impact features <1 Gyr old, including higher thermal inertia (> 100 J m−2 K−1 s−1/2) in the interiors and ejecta of Copernican‐aged impact craters and lower thermal inertia (< 50 J m−2 K−1 s−1/2) within the lunar cold spots identified by Bandfield et al. (2014). Observed trends in ejecta thermal inertia provide a potential tool for age dating craters of previously unknown age, complementary to the approach suggested by Ghent et al. (2014). Several anomalous regions are identified in the global 128 pixels per degree maps presented here, including a high‐thermal inertia deposit near the antipode of Tycho crater.
Plain Language Summary
We measured the Moon's temperature cycles with the Lunar Reconnaissance Orbiter's Diviner instrument to make the first global maps of important physical properties of the dusty surface layer. These maps reveal a rich new view of the last billion years of impact processes and volcanism on the Moon. Impacts by meteorites cause the breakdown of rocks and accumulation of regolith—the granular surface materials. Our results show that regolith formation is a rapid process, which homogenizes and redistributes fine particles over large distances. These new observations provide a wealth of data for future study and also suggest a new technique for determining the ages of craters on the Moon and other planetary surfaces, using temperatures to infer the depth of accumulated regolith.
Key Points
We present global maps of regolith thermophysical properties
The Moon's upper ~4–7 cm of regolith has a globally averaged thermal inertia of ~55 J m−2 K−1 s−1/2 at a reference temperature of 273 K
The upper lunar regolith is remarkably uniform, with the upper ~10 cm homogenized on >1 Gyr timescales
As of June 19, 2010, the Lunar Orbiter Laser Altimeter, an instrument on the Lunar Reconnaissance Orbiter, has collected over 2.0 × 109 measurements of elevation that collectively represent the ...highest resolution global model of lunar topography yet produced. These altimetric observations have been used to improve the lunar geodetic grid to ∼10 m radial and ∼100 m spatial accuracy with respect to the Moon's center of mass. LOLA has also provided the highest resolution global maps yet produced of slopes, roughness and the 1064‐nm reflectance of the lunar surface. Regional topography of the lunar polar regions allows precise characterization of present and past illumination conditions. LOLA's initial global data sets as well as the first high‐resolution digital elevation models (DEMs) of polar topography are described herein.
Numerical modeling of the peak‐ring basin formation showed that the peak‐ring forms from the material that is part of the central uplift outwardly thrust over the inwardly collapsing transient crater ...rim. Simulations of the lunar basin formation showed that the peak or inner ring in peak ring or multiring basins, respectively, is composed of the overturned crust and deep‐seated material, possibly from the upper mantle. Numerical impact simulations were used to trace the depth of origin of material exposed within the peak (or inner) ring. We estimate the scaling trends between basin size and the depth of origin of material exposed within the ring. We also report on the likely crust, mantle, and projectile abundances exposed within the ring. Quantifying the excavation depths during the formation of the peak or inner ring provides a step toward understanding the lunar crust and mantle stratigraphy.
Key Points
Impact simulations were used to determine depth of origin of material exposed within the peak ring in lunar basins
Basin size‐dependent trends show depth of origin for a range of lunar impact basins, up to the upper mantle
Basin size‐dependent relationships help determine lunar composition and stratigraphy
Ryder and Wood (1977) suggested that the lunar crust becomes more mafic with depth because the impact melts associated with the large Imbrium and Serenitatis basins are more mafic than the surface ...composition of the Moon. In this study, we reexamine the hypothesis that the crust becomes more mafic with depth; we analyze the composition of crater central peaks by using recent remote sensing data and combining the best practices of previous studies. We compute the mineralogy for 34 central peaks using (1) nine‐band visible and near‐infrared data from the Kaguya Multiband Imager, (2) an improved version of Hapke's radiative transfer model validated with spectra of lunar soils with well‐known modal mineralogy, and (3) new crustal thickness models from the Gravity Recovery and Interior Laboratory data to examine the variation in composition with depth. We find that there is no increase in mafic mineral abundances with proximity to the crust/mantle boundary or with depth from the current lunar surface and, therefore, that the crust does not become more mafic with depth. We find that anorthosite with very low mafic abundance (“purest anorthosite” or PAN) is a minority constituent in these peaks, and there is no clear evidence of a distinct PAN‐rich layer in the middle crust as previously proposed. The composition of most of the central peaks we analyze is more mafic than classically defined anorthosites with an average noritic anorthosite composition similar to that of the lunar surface.
Key Points
The lunar crust does not become more mafic with depth
Very pure anorthosite is a minor constituent of central peaks
Orthopyroxene is the dominant mafic mineral throughout the crust
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