Structural disturbances of the lunar surface caused by spacecraft

From the lunar surface survey performed with a narrow-angle camera of the Lunar Reconnaissance Orbiter (LRO) spacecraft, the distributions of the phase ratios of the Apollo 11 and 12 landing sites and the Ranger 9 impact site were mapped. In the acquired images, the traces of the structural disturbances of the lunar regolith layer caused by the jet flows are seen. In the Ranger 9 impact site, one can see the crater of about 15 m across with a ray system, which is hardly noticeable in the brightness picture, but has a high contract in the phase ratio picture. The character of the photometric anomaly of the rays of this crater shows that they are formed by the ejected stones composing the rugged relief, which induces a strong shadow effect. At the same time, the influence of jet flows from the rocket engines smooths the relief and leads to the photometric anomaly of the opposite sign. The estimate of the maturity degree of the lunar regolith in the Apollo 11 and 12 landing sites obtained from the SELENE spectral survey suggests that the depth of the influence of the rocket engines on the soil is small, and the surface of the impact crater formed by the Ranger 9 spacecraft contains a large amount of the immature soil.     

Removal of topographic effects from lunar images using Kaguya (LALT) and Earth-based observations

Lunar images acquired at non-zero phase angles show brightness variations caused by both albedo heterogeneities and local topographic slopes of the surface. To distinguish between these two factors, altimetry measurements or photoclinometry data can be used. The distinction is especially important for imagery of phase-function parameters of the Moon. The imagery is a new tool that can be used to study structural anomalies of the lunar surface. To illustrate the removal of the topographic effects from photometric images, we used Earth-based telescopic observations, altimetry measurements carried out with the Kaguya (JAXA) LALT instrument, and a new photoclinometry technique that includes analysis of several images of the same scenes acquired at different phase angles. Using this technique we have mapped the longitudinal component of lunar topography slopes (the component measured along the lines of constant latitude). We have found good correlations when comparing our map with the corresponding data from Kaguya altimetry. The removal of the topographic surface properties allows for the study of the phase-function parameters of the lunar surface, not only for flat mare regions, but for highlands as well.     

Probable swirls detected as photometric anomalies in Oceanus Procellarum

Images of the lunar nearside obtained by telescopes of Maidanak Observatory (Uzbekistan) and Simeiz Observatory (Crimea, Ukraine) equipped with Canon CMOS cameras and Sony CCD LineScan camera were used to study photometric properties of the lunar nearside in several spectral bands. A wide range of lunar phase angles was covered, and the method of phase ratios to assess the steepness of the phase function at different phase angles is applied. We found several areas with photometric anomalies in the south-west portion of the lunar disk that we refer to as Oceanus Procellarum anomalies. The areas being unique on the lunar nearside do not obey the inverse correlation between albedo and phase-curve slope, demonstrating high phase-curve slopes at intermediate albedo. Low-Sun images acquired with Lunar Orbiter IV and Apollo-16 cameras do not reveal anomalous topography of the regions, at least for scales larger than several tens of meters. The areas also do not have any thermal inertia, radar (70 and 3.8 cm), magnetic, or chemical/mineral peculiarities. On the other hand they exhibit a polarimetric signature that we interpret to be due to the presence of a porous regolith upper layer consisting of dust particles. The anomalies may be interpreted as regions of very fresh shallow regolith disturbances caused by impacts of meteoroid swarms consisting of rather small impactors. This origin is similar to one of the hypotheses for the origin of lunar swirls like the Reiner-γ formation. The photometric difference between the shallow and pervasive (Reiner-γ class) swirls is that the latter appear to have a significant amount of immature soils in the upper surface layers.     

The negative polarization parameters of the light scattered by the lunar surface: Mapping

The images of the southwestern part of the lunar disk showing the distributions of the negative polarization parameters of the light scattered by the lunar surface are presented. The distributions of the negative polarization minimum P _min, the inversion angle α_inv, and the polarization slope at the inversion point h significantly differ from the albedo image. This testifies to the fact that polarimetry yields independent information on the structure and optical properties of the lunar regolith.     

The phase ratios of the color index: Mapping of two regions of the near side of the Moon

From the ground-based colorimetry performed for two surface regions of the near side of the Moon, images of the phase ratio of the color index C(600 nm/470 nm) have been built for the phase angles between 2° and 95°. It has been found that for phase angles smaller than α ∼ 40°–50°, the color index of the highlands grows with the phase quicker than that of the mare regions. For larger phase angles, α > 50°, a reverse situation is observed. The laboratory data on the spectrophotometry of the lunar samples confirm the peculiarities found in the phase dependence of color. The influence of multiple scattering on the phase dependence of the color of the mare and highland regions of the Moon are discussed.     

Photometric anomalies of the lunar surface studied with SMART-1 AMIE data

We present new results from the mapping of lunar photometric function parameters using images acquired by the spacecraft SMART-1 (European Space Agency). The source data for selected lunar areas imaged by the AMIE camera of SMART-1 and the data processing are described. We interpret the behavior of photometric function in terms of lunar regolith properties. Our study reveals photometric anomalies on both small (sub-kilometer) and large (tens of kilometers) scales. We found the regolith mesoscale roughness of lunar swirls to be similar in Mare Marginis, Mare Ingenii, and the surrounding terrains. Unique photometric properties related to peculiarities of the millimeter-scale regolith structure for the Reiner Gamma swirl are confirmed. We identified several impact craters of subkilometer sizes as the source of photometric anomalies created by an increase in mesoscale roughness within the proximal crater ejecta zones. The extended ray systems reveal differences in the photometric properties between proximal and distant ejecta blankets. Basaltic lava flows within Mare Imbrium and Oceanus Procellarum indicate higher regolith porosity for the redder soils due to differences in the chemical composition of lavas.     

Lunar Clinopyroxene and Plagioclase: Surface Distribution and Composition

The Clementine UVVIS images and the spectral and chemical (mineral) characteristics of lunar soil samples previously measured by the Lunar Samples Characterization Consortium were used to map the plagioclase and clinopyroxene abundance in the lunar surface material. An excess of plagioclase was found in young highland craters (e.g., in the crater Tycho) and in their ray systems. For clinopyroxenes, analogous behavior was observed in mare craters (e.g., in the crater Aristarchus). The maps for the FeO and Al₂O₃ bulk contents and the contents of these oxides in plagioclase and clinopyroxene were estimated by the same technique. These maps were compared to each other and to the predicted distribution of the lunar regolith maturity. The regolith of highland ray systems (e.g., the Tycho crater system) is characterized not only by low maturity but also by peculiar iron and aluminum contents: the lower the soil maturity degree, the smaller the iron content and the greater the aluminum content. This is confirmed by the data for the lunar soil samples from the Apollo 16 landing site. A cluster analysis of the “clinopyroxene content-maturity” and “plagioclase content-maturity” correlation diagrams allowed the mineral mapping of the lunar surface to be performed.__________Translated from Astronomicheskii Vestnik, Vol. 39, No. 4, 2005, pp. 291–303.Original Russian Text Copyright © 2005 by Shkuratov, Kaydash, Pieters.     

Structural disturbances of the lunar surface near the Lunokhod-1 spacecraft landing site

The American Lunar Reconnaissance Orbiter spacecraft acquired high-resolution images of the landing sites of the Apollo manned spaceships and the Luna automatic space probes. In the images taken with the LROC Narrow-Angle Camera, the traces of anthropogenic influence on the lunar surface are seen in these places. However, such traces are not always noticeable sufficiently well, since they are masked by inhomogeneities in the brightness of the examined surface region caused by its topographic features and albedo variations. To increase the potential of identifying the disturbances of the initial structure of the lunar surface, the data should be analyzed with so-called phase-ratio imaging. Its essence is that the ratio of two coinciding images of the same surface region obtained at different phase angles is calculated. This method was applied to the analysis of the landing site of the Soviet Luna-17 space probe that transported the Lunokhod-1 rover to the lunar surface. The structural disturbance caused by the impact of jet flows from the probe’s engines and the tracks of the Lunokhod-1 wheels, which are faintly discernible in the usual images, has been detected.     

The lunar crater Giordano Bruno as seen with optical roughness imagery

Images acquired by the LRO NAC allow 3D phase-ratio imagery of several areas in the crater Giordano Bruno. This is a new optical, remote-sensing technique that allows a determination of the optical roughness of the lunar surface. Our study confirms complicated impact melt movement on the flanks and the floor of the crater. In many cases, however, flow structures seen on the inner wall can be attributed to regolith/debris taluses rather than impact melt flows. It was found that the whirlpool-like formation seen near the western side of the crater Giordano Bruno has a small central depression that can be interpreted as either a vortex cavity or a feature which resembles such a vortex formed by viscous flows coming from the crater flanks. We discuss several features that confirm the young age of the crater, concluding, however, that it was not formed within human history; its age is, likely, on the order of 1 My.     

Optical measurements of the Moon as a tool to study its surface

This survey is a general overview of modern optical studies of the Moon and their diagnostic meaning. It includes three united parts: phase photometry, spectrophotometry, and polarimetry. The first one is devoted to the progress in the photometry of the Moon, which includes absolute albedo determination to refine the albedo scale (e.g., to connect lunar observations and the data of lunar sample measurements) and mapping the parameters of a lunar photometric function (e.g., the phase-angle ratios method) with the aim of making qualitative estimates of regolith structure variations. This part also includes observations of the lunar opposition effect as well as photogrammetry and photoclinometry techniques. In particular, available data show that because of the low albedo of the lunar surface, the coherent backscattering enhancement hardly influences the lunar opposition spike, with the exception of the brightest lunar areas measured in the NIR. The second part is devoted to chemical/mineral mapping of the Moon's surface using spectrophotometric measurements. This section also includes analyses related to the detection of water ice or hydroxyl, prognoses of maturity, and helium-3 abundance mapping. In particular, we examine the relationship between superficial OH/H₂O compounds spectrally detected recently and bulk “water ice” found earlier by the Lunar Prospector GRS and LRO LEND, assuming that the compounds are delivered to cold traps (permanently shadowed regions) with electrostatically levitated dust saturated by solar wind hydrogen. Significant problems arise with the determination of TiO₂ content, as the correlation between this parameter and the color ratio C(750/415 nm) is very non-linear and not universal for different composition types of the lunar surface; a promising way to resolve this problem is to use color ratios in the UV spectral range. The third part is devoted to mapping of polarization parameters of the lunar surface, which enable estimates of the average size of regolith particles and their optical inhomogeneity. This includes considerations of the Umov effect and results of spectropolarimetry, negative polarization imagery, and measurements of other polarimetric parameters, including the third Stokes parameter. Although these three research divisions have not been developed equally and the numbers of proper references are very different, we try to keep a balance between them, depicting a uniform picture. It should be emphasized that many results presented in this review can be applied to other atmosphereless celestial bodies as well.