He in permanently shadowed lunar polar surfaces

Because of their cryogenic temperatures, analysis indicates that permanently shadowed polar lunar craters may have substantially higher levels of ³He than sunlit lunar surfaces and are conservatively estimated to contain as much as 50 ppb or more.     

A Search for Water Ice at the Lunar Poles with Clementine Images

Optical and near-IR signatures of water ice on the Moon's surface were sought in the permanently shadowed regions near its poles. Significant amounts of multiply-scattered radiation partly illuminate primary shadows cast by craters and other features. If there is water ice in the permanently shadowed regions of the Moon's surface, its spectral signature should appear in this multiply-scattered light. This investigation can be done most effectively with observations obtained by spacecraft, because most selenocentric positions occupied by the Earth will also be occupied by the Sun at some point in time, and because the lunar poles are seen only obliquely to a terrestrial observer. Images obtained by Clementine are particularly well-suited to this task, because the spacecraft's polar orbit allowed images of the poles to be acquired on nearly every orbit, resulting in literally thousands of images taken within a few degrees of each pole, and because the filters on the ultraviolet-visual camera (UVVIS) and the near infrared camera (NIR) occur at major absorption bands or within important continuum features of water ice. Approximately 5800 images obtained by the UVVIS camera and 1800 images obtained by the NIR camera were calibrated and combined into coadded mosaics to create multispectral maps of the lunar poles with the highest possible signal-to-noise. Unfortunately, analysis of our UVVIS mosaics indicates that any possible signal from multiply-scattered light in primary shadows was overwhelmed by instrumental stray light. For the NIR camera, we were able to determine the normalized reflectance of several regions that were identified by Margot et al. (1999, Science284, 1658-1660) as permanent shadows. We have identified one permanently shadowed crater with a 1.5-μm band spectral signature indicative of between 2.5 and 21% fractional coverage of H₂O frost. However, the same region shows a 2.0 μm spectral signature that is inconsistent with the presence of any water.     

Lofted charged dust distribution above the Moon surface

We developed kinetic theory for the charging processes of small dust grains near the lunar surface due to interaction with the anisotropic solar wind plasma. Once charged, these dust grains, which are exposed to the electric field in the sheath region near the lunar surface, could loft and distribute around such heights off the surface where they reach equilibrium with the local gravitational force. Analytical solutions were derived for the charging time, grain floating potential, and grain charge, characterizing the charging processes of small dust grains in a two-component and in a multi-component solar wind plasma, and further highlighting the unique features presented by the high streaming plasma velocity. We have also formulated a novel kinetic theory of sheath formation around an absorbing planar surface immersed in the anisotropic solar wind plasma in the case of a negligible photoelectric effect and presented solutions for the sheath structure. In this study we combined the results from these analyses and provided estimates for the size distribution function of dust that is expected to be lofted in regions dominated by the solar wind plasma, such as near the terminator and in nearby shadowed craters. Corresponding to the two dominant streaming velocity peaks of 300 and 800 km/s, mean dust diameters of 500 and 350 nm, respectively, are expected to be found at equilibrium at heights of relevance to exploration operations, e.g., around 1.5 m height off the lunar surface. In shadowed craters near the terminator region, where isotropic plasma should be dominating, we estimate mean lofted dust diameter of 800 nm around the same 1.5 m height off the lunar surface. The generally applicable solutions could be used to readily calculate the expected lofted size distribution near the lunar surface as a function of plasma parameters, dust grain composition, and other parameters of interest.     

The advanced Moon micro-imager experiment (AMIE) on SMART-1: Scientific goals and expected results

The advanced Moon micro-imager experiment (AMIE) is the imaging system on board ESA mission to the Moon SMART-1; it makes use of a miniaturised detector and micro-processor electronics developed by SPACE X in the frame of the ESA technical programme. The AMIE micro-imager will provide high resolution CCD images of selected lunar areas and it will perform colour imaging through three filters at 750, 915 and 960 nm with a maximum resolution of 46 m/pixel at the perilune of 500 km. Specific scientific objectives will include (1) imaging of high latitude regions in the southern hemisphere, in particular the South Pole Aitken basin (SPA) and the permanently shadowed regions close to the South Pole, (2) determination of the photometric properties of the lunar surface from observations at different phase angles (physical properties of the regolith), (3) multi-band imaging for constraining the chemical and mineral composition of the surface, (4) detection and characterisation of lunar non-mare volcanic units, (5) study of lithological variations from impact craters and implications for crustal heterogeneity. The AMIE micro-imager will also support a Laser-link experiment to Earth, an On Board Autonomous Navigation investigation and a Lunar libration experiment coordinated with radio science measurements.     

Estimation of the Area of the Perpetually Shaded Lunar Surface

We present the results of computer simulation of the shadowing effect for three types of surface: (1) cratered, (2) formed by a random profile with Gaussian statistical height and slope distributions, and (3) a two-scale surface representing a cratered area that is complicated by small-scale random relief. The calculations are based on data on the distribution of lunar craters derived from the diameter/depth ratio and on the assumption of the equilibrium distribution of the crater population in the circumpolar areas of the lunar surface. We determined the characteristics of perpetually shaded areas of the lunar surface: the probability of the constant shadowing of an arbitrary surface point, the fraction of the perpetually shaded area as a function of selenographic latitude, the latitudinal dependence of the perpetually shaded area, and the total area of the perpetually shaded surface. The calculations showed that the presence of structural features of different scale on the lunar surface can considerably increase the estimate of the fraction of the perpetually shaded area compared to existing estimates.     

Dust transport near electron beam impact and shadow boundaries

When the moon enters the plasma sheet of the earth, high energy electron fluxes are incident upon the lunar surface. Some regions are in the shadow of these fluxes due to topographic features. Large electric fields were found at similar shadow boundaries created by the electron beams incident upon an obstacle in the laboratory. Potentials on the beam-illuminated surface follow beam energies and were negative relative to potentials on the shadowed surface. Charged dust particles in the beam-illuminated region were observed to move into the shadow due to these electric fields. The oblique incidence of the electron fluxes upon craters can lead to a portion of the crater surface in the beam-illumination and another portion in the shadow. Dust particles on the slopes of the craters can thus experience large electric fields and transport downhill to fill the bottom of the craters. This mechanism may contribute to the formation of dust ponds observed by the NEAR-Shoemaker spacecraft at Eros, and might be at work on the lunar surface as well. In the laboratory, we used electron fluxes with energies up to 90 eV to bombard an insulating half-pipe. An angle of incidence was chosen so that the impact occurred on farside of the slope and left the bottom and the nearside slope in the shadow. Dust particles on the beam-illuminated slope moved down along the surface toward the bottom of the half-pipe and hopped to the bottom as well, while particles on the shadowed slope remained at rest.     

Regolith properties in the south polar region of the Moon from 70-cm radar polarimetry

The south polar region of the Moon contains areas permanently shadowed from solar illumination, which may provide cold traps for volatiles such as water ice. Previous radar studies have emphasized the search for diagnostic polarization signatures of thick ice in areas close to the pole, but near-surface regolith properties and regional geology are also important to upcoming orbital studies of the shadowed terrain. To study regional regolith variations, we collected 70-cm wavelength, 450-m resolution, dual-circular polarization radar data for latitudes 60-90° S using the Arecibo and Greenbank telescopes. The circular polarization ratio, μc, is sensitive to differences in rock abundance at the surface and up to tens of m below the surface, depending upon the regolith loss tangent. We observe significant variations in μc, attributed to changes in the surface and subsurface rock population, across the south polar highlands. Concentric haloes of low polarization ratio surrounding Hausen, Moretus, and other young craters represent rock-poor ejecta layers. Values of μc up to ∼1 occur in the floors and near-rim deposits of Eratosthenian and Copernican craters, consistent with abundant rocky ejecta and/or fractured impact melt. Enhanced μc values also correspond to areas mapped as Orientale-derived, plains-forming material [Wilhelms, D.E., Howard, K.A., Wilshire, H.G., 1979. USGS Map I-1162], and similar polarization properties characterize the permanently shadowed floors of craters Faustini and Shoemaker. Small areas of very high (>1.5) circular polarization ratio occur on shadowed and seasonally sunlit terrain, and appear to be associated with small craters. We suggest that regolith in low-lying areas near the south pole is characterized by a significant impact melt component from Orientale, which provides a source for excavation of the block-rich ejecta around small craters observed in this and earlier radar studies. The lower portion of the interior wall of Shackleton crater, permanently shadowed from the sun but visible from Earth, is not significantly different in 70-cm scattering properties from diurnally/seasonally sunlit areas of craters with similar morphology.     

LunarEX—a proposal to cosmic vision

While the surface missions to the Moon of the 1970s achieved a great deal, scientifically much was also left unresolved. The recent plethora of lunar missions (flown or proposed) reflects a resurgence in interest in the Moon, not only in its own right, but also as a record of the early solar system including the formation of the Earth. Results from recent orbiter missions have shown evidence of ice or at least hydrogen within shadowed craters at the lunar poles.     

A mechanism for the production of lunar crater rays

Observations of high resolution photographs of part of one of the prominent rays of the lunar crater Copernicus show that there is a concentration of small bright rayed and haloed craters within the ray. These craters contribute to the overall ray brightness; they have been measured and their surface distribution has been mapped. Sixty-two percent of the bright craters can be identified from study of high resolution photographs as concentric impact craters. These craters contain in their ejecta blankets, rocks from the lunar substrate that are brighter than the adjacent mare surface. It is concluded that the brightness of the large ray from the crater Copernicus is due to the composite effect of many small concentric impact craters with rocky ejecta blankets. If this is the dominant mechanism for the production of other rays from Copernicus and other large lunar craters, then rays may not contain significant amounts of ejecta from the central crater or from large secondary craters. They may in fact only reflect local excavation of mare substrate material by myriads of small secondary or tertiary impact craters.     

INSTRUMENTATION FOR GEOLOGICAL FIELD WORK ON THE MOON

A human return to the Moon will require that astronauts are well equipped with instrumentation to aid their investigations during geological field work. Two instruments are described in detail. The first is a portable X-ray Spectrometer, which can provide rapid geochemical analyses of rocks and soils, identify lunar resources and aid selection of samples for return to Earth. The second instrument is the Geological and Radiation environment package (GEORAD). This is an instrument package, mounted on a rover, to perform in-situ measurements on the lunar surface. It can be used for bulk geochemical measurements of rocks and soils (particularly identifying KREEP-enriched rocks), prospect for ice in shadowed areas of craters at the poles and characterise the lunar radiation environment.     

The Nature of Anomalous Formations in the Polar Regions of Mercury and the Moon

Craters located in the polar regions of Mercury and the Moon are studied. The areas of permanently shadowed zones in the polar regions of both celestial bodies are computed. In the case of the Moon, variations of the position of its rotation pole with respect to the ecliptic pole during the 18.6-year period were taken into account. In the case of Mercury, the computations were performed for a period equal to one Mercurial solar day. The variations of temperature are computed for craters coinciding with the areas of high hydrogen content for the Moon and areas with anomalous reflective properties for Mercury, including craters with anomalous areas discovered with the upgraded radio telescope of the Arecibo observatory (Harmon and Perillat, 2001). Craters that may contain deposits of water ice or other volatile compounds are identified in the polar regions of both celestial bodies.     

Constraints on the source of lunar cataclysm impactors

Multiple impact basins formed on the Moon about 3.8 Gyr ago in what is known as the lunar cataclysm or Late Heavy Bombardment. Many workers currently interpret the lunar cataclysm as an impact spike primarily caused by main-belt asteroids destabilized by delayed planetary migration. We show that morphologically fresh (class 1) craters on the lunar highlands were mostly formed during the brief tail of the cataclysm, as they have absolute crater number density similar to that of the Orientale basin and ejecta blanket. The connection between class 1 craters and the cataclysm is supported by the similarity of their size-frequency distribution to that of stratigraphically-identified Imbrian craters. Majority of lunar craters younger than the Imbrium basin (including class 1 craters) thus record the size-frequency distribution of the lunar cataclysm impactors. This distribution is much steeper than that of main-belt asteroids. We argue that the projectiles bombarding the Moon at the time of the cataclysm could not have been main-belt asteroids ejected by purely gravitational means.     

Crater frequencies on lava-covered areas related to the moon's thermal history

Making use of Orbiter and Apollo photographs, frequency counts of craters down to 2 km diam as indicators of the relative ages of lunar features, have been made on 264 areas, including 15 terrae, 27 recognized maria, 174 flat-floored craters and 48 lava-covered areas with indefinite boundaries designated as ‘marets’. Analysis of frequency counts on flat-floored craters on the basis of this data and re-assessment of former results, combined with the relatively restricted age range of lunar samples, make it unlikely that the present observations are able to reach back in time to impacts on an assumed primordial floating crust. The range of crater frequencies on the marets, together with their wide distribution over the lunar surface, suggest lava migrations to the surface within autonomous domains each with its own chronology, covering an extensive period of lunar history. The close association of marets with flat-floored craters provides a reasonable origin for the floor material of these latter objects. The lava migrations associated with the marets suggest that internal heating may be a more important factor in the origin of lunar surface features than had formerly been supposed. Kopal's views on the origin of the moon's multiple moments of intertia (1972) are considered to support the concept of autonomous domains. It is considered that the time sequence of separate lava flows represented by the marets may be a reflection of physical processes within the moon responsible for the successive lava flows associated with the larger maria.     

Morphometric Characterization and Reconstruction Effect Among Lunar Impact Craters

Impact craters on the lunar surface have a variety of morphometric characteristics that are very useful in understanding the evolutionary history of lunar landscape morphologies. Based on digital elevation model data and photographs from China’s Chang’E-1 lunar orbiter, we develop morphologic parameters and quantitative methods for presenting the morphometric characteristics of impact craters, analyzing their relational distribution, and estimating the relative order of their formation. We also analyze features in profile where craters show signs of having formed on the edge of previously existing craters to show that superimposed impacts affect morphologic reconstructions. As a result, impact craters have significant effects on the reconstruction of ancient topography and the estimation of relative formation ages.     

A study of lunar impact crater size-distributions

Discrepancies in published crater frequency data prompted this study of lunar crater distributions. Effects modifying production size distributions of impact craters such as surface lava flows, blanketing by ejecta, superposition, infilling, and abrasion of craters, mass wasting, and the contribution of secondary and volcanic craters are discussed. The resulting criteria have been applied in the determination of the size distributions of unmodified impact crater populations in selected lunar regions of different ages. The measured cumulative crater frequencies are used to obtain a general calibration size distribution curve by a normalization procedure. It is found that the lunar impact crater size distribution is largely constant in the size range 0.3 km ?D ? 20 km for regions with formation ages between ≈ 3 × 10⁹ yr and ? 4 × 10⁹ yr. A polynomial of 4th degree, valid in the size range 0.8 km ?D ? 20 km, and a polynomial of 7th degree, valid in the size range 0.3 km ?D ? ? 20 km, have been approximated to the logarithm of the cumulative crater frequencyN as a function of the logarithm of crater diameterD. The resulting relationship can be expressed asN ～D ^α(D) where α is a function depending onD. This relationship allows the comparison of crater frequencies in different size ranges. Exponential relationships with constant α, commonly used in the literature, are shown to inadequately approximate the lunar impact crater size distribution. Deviations of measured size distributions from the calibration distribution are strongly suggestive of the existence of processes having modified the primary impact crater population.     

A photometric investigation of the lunar crater rays

This investigation deals with accurate photometric data concerning a number of rays of Tycho, Copernicus, Kepler, and Aristarchus. They have been derived from plates taken at the Yerkes Observatory in a night of a total lunar eclipse near phase angle 0°. By comparing the normal albedo with that of the surroundings of the rays we found that they can be interpreted as samples of telescopically unresolved bright patches. The fractional areak covered by these patches varies along the ray and shows that they are composed of a number of separate ray elements. The observed value ofk is in accordance with counts on a Ranger photograph.The distribution of the brightness along the rays has also been compared with the mass distribution of the ejecta in the rays around terrestrial explosion craters. The mean length of the lunar rays is in full accordance with its extrapolated terrestrial value. We cannot assume, however, that the rays are regions covered with a homogeneous layer of white powder, because the comparison with the terrestrial explosion craters gives an unprobable value for the height of the layer of the ejecta. The same results follow now from the photometric properties of the rays.From a comparison with the difference in albedo at the Surveyor's footprints follows the suggestion that the lunar rays are composed of bright patches, where the surface material came into a state of lower porosity, while it has a higher porosity in the dark halos around the craters. A suspected dark halo around Tycho has photometrically been measured and the results prove that it really exists. Kepler also shows a very weak halo.     

Lunar Beagle and Lunar Astrobiology

The study of the elements and molecules of astrobiological interest on the Moon can be made with the Gas Analysis Package (GAP) and associated instruments developed for the Beagle 2 Mars Express Payload. The permanently shadowed polar regions of the Moon may offer a unique location for the “cold-trapping” of the light elements (i.e. H, C, N, O, etc.) and their simple compounds. Studies of the returned lunar samples have shown that lunar materials have undergone irradiation with the solar wind and adsorb volatiles from possible cometary and micrometeoroid impacts. The Beagle 2’s analytical instrument package including the sample processing facility and the GAP mass spectrometer can provide vital isotopic information that can distinguish whether the lunar volatiles are indigenous to the moon, solar wind derived, cometary in origin or from meteoroids impacting on the Moon. As future Lunar Landers are being considered, the suite of instruments developed for the Mars Beagle 2 lander can be consider as the baseline for any lunar volatile or resource instrument package.     

Atlas of lunar radar maps at 70-cm wavelength

The intensity distribution of lunar radar echoes has been mapped for two-thirds of the earth-visible lunar surface at a wavelength of 70 cm. The depolarizing effects of the lunar surface were observed by simultaneously receiving the radar echoes in opposite polarizations. These echoes were mapped with areal resolutions of 25–100 km². Mappings with this resolution confirmed that the young craters have enhanced returns. A few craters were found to have enhanced echoes only from their rims. Backscattering differences were also observed between various areas within a mare, between different highland areas, and between maria and adjacent highlands. These scattering differences were interpreted with a simple model, which assumed that the surface backscattered with varying amounts of quasi-specular and diffuse power. Only an increase in the diffuse power was needed to give the numerical values of the enhancements.     

Dust transport in photoelectron layers and the formation of dust ponds on Eros

We investigate the electrostatic transport of charged dust in the photoelectron layer over the dayside surface of an asteroid. Micron-sized dust particles may be levitated above the surface in the photoelectron layer. Horizontal transport within the layer can then lead to net deposition of dust into shadowed regions where the electric field due to the photoelectron layer disappears. We apply a 2D numerical model simulating charged dust dynamics in the near-surface daytime plasma environment of Asteroid 433 Eros to the formation of dust deposits in craters. We find that dust tends to collect in craters and regions of shadow. This electrostatic dust transport mechanism may contribute to the formation of smooth dust ponds observed by the NEAR-Shoemaker spacecraft at Eros. The size distribution of transported dust depends on the particle density and work function, and the work function of the surface and solar wind electron temperature and density. With reasonable values for these parameters, μm-sized and smaller particles are levitated at Eros. Micrometeoroid bombardment is not a sufficient source mechanism for electrostatic transport to create the Eros dust ponds. Laboratory measurements of dust in a plasma sheath show that dust launched off the surface by direct electrostatic levitation can provide a sufficient source for transport to produce the observed Eros ponds.