South Pole-Aitken Research Articles

Structure of the South Pole-Aitken Basin from KAGUYA Selenodesy Data

The South Pole-Aitken basin (SPA) is the largest (2500km in diameter), deepest and presumably oldest impact basin in the solar system. SPA was characterized by an ellipse with axes 2400 by 2050 km, but more precise topography and gravity data are necessary to discuss the structure of large basin like SPA. We use localized representation of gravity potential where Slepian functions were used to estimate the gravity field over certain areas of the Moon and a spherical cap area with a radius of 40 degree from the SPA center. The direction of an ellipse denoting the depression is similar to the previous result. The region with the thinnest crust is offset southward from the center of SPA. We analyzed interior structure of small basins in and around SPA. We interpret that a positive gravity anomaly corresponds to a Moho uplift. Just around the rim of SPA, obscure circular structure Amundsen-Ganswindt has a distinct Moho uplift, suggesting a buried impact structure.

The South Pole-Aitken basin thorium anomaly and its enrichment characteristics and mechanisms

遥感探测表明月表Th 元素分布存在全球不均衡性, 形成分别以雨海-风暴洋和南极-艾肯(SPA) 盆地为中心的Th 异常区. 研究认为雨海-风暴洋的Th 主要来源于对其下覆KREEP 物质的开掘, 但对 SPA 盆地的Th 来源有各种不同的认识. 本文通过对SPA 盆地的地形、盆地构造、撞击坑分布、火山作用和地球化学特征的分析, 以及对Th 富集机理的探讨, 认为SPA 盆地内Th 更可能来源于撞击作用对富Th 下月壳物质的开掘；Th 的弧形富集特征与盆地构造有很大相关性, 并可能受到Apollo 盆地形成的影响.

Olivine-rich exposures in the South Pole-Aitken Basin

The distribution and the geological context of the olivine-rich exposures in the South Pole-Aitken (SPA) Basin on the Moon were investigated based on the spectral data obtained from the Spectral Profiler (SP) and Multiband Imager (MI) onboard the Japanese lunar explorer Kaguya/SELENE. The olivine-rich exposures are found only in the peak rings or central peaks of the Schrödinger basin and Zeeman crater, which are located in the outer region of the SPA Basin and not in the center region. On a localized scale, the olivine-rich materials are exposed on landslide features on the crater walls or sloped wall of the central peaks or the peak rings. Another observational finding is the co-existence of olivine-rich and plagioclase-rich materials on a kilometer scale spanning most of the olivine-rich sites in the Schrödinger basin. Pyroxene-rich materials are found in fresh craters outside the peak rings or the central peaks with olivine-rich materials. Based on these results, the following scenario are proposed: (1) the impact to form the SPA Basin melted a large amount of the lunar upper mantle and crust, and distributed the melted materials to the outer region; (2) local differentiation of melted materials hid the olivine-rich materials in the center region of the SPA Basin; (3) later impacts that formed the Schrödinger and Zeeman craters excavated and exposed the olivine-rich materials to the surface again; and (4) space weathering and regolith gardening obscured the olivine-rich spectra at the exposure sites, but recent, small scale impacts or landslides on the sloped wall exposed fresh olivine-rich materials, allowing the identification of the olivine-rich exposures by spectral remote-sensing. This suggests that several, different scale events play an important role in forming the surface distributions of originally deep-seated materials on the Moon, as well as on other planetary bodies.

Farside explorer: unique science from a mission to the farside of the moon

Farside Explorer is a proposed Cosmic Vision medium-size mission to the farside of the Moon consisting of two landers and an instrumented relay satellite. The farside of the Moon is a unique scientific platform in that it is shielded from terrestrial radio-frequency interference, it recorded the primary differentiation and evolution of the Moon, it can be continuously monitored from the Earth–Moon L2 Lagrange point, and there is a complete lack of reflected solar illumination from the Earth. Farside Explorer will exploit these properties and make the first radio-astronomy measurements from the most radio-quiet region of near-Earth space, determine the internal structure and thermal evolution of the Moon, from crust to core, and quantify impact hazards in near-Earth space by the measurement of flashes generated by impact events. The Farside Explorer flight system includes two identical solar-powered landers and a science/telecommunications relay satellite to be placed in a halo orbit about the Earth–Moon L2 Lagrange point. One lander would explore the largest and oldest recognized impact basin in the Solar System— the South Pole–Aitken basin—and the other would investigate the primordial highlands crust. Radio astronomy, geophysical, and geochemical instruments would be deployed on the surface, and the relay satellite would continuously monitor the surface for impact events.

Lunar potassium distribution: Results from Chang’E-1 gamma ray spectrometer

The global map of potassium is represented in this paper from Chang’E-1 (CE-1) Gamma-ray Spectrometer (CGRS) for its one-year mission. Assuming a linear relationship between net count rate and its abundance, the average potassium abundance of individual landing sites is used as ground-truth for the calibration to derive the global map of absolute concentration. Although CGRS spectra have a lower signal-to-noise ratio, the translated map still keeps relative variations. As calculated from Apollo, Lunar Prospector, and Kaguya, global potassium map from CGRS shows high concentrations on the lunar nearside and secondary concentrations located in the South Pole-Aitken (SPA) basin on the farside. The comparison with Lunar Prospector potassium map shows a good correlation, though abundances on the highlands of the farside are much lower than that of Lunar Prospector. Since the footprint of CGRS measurements is larger than the sampling radius of each landing site, the calibrated map shows a larger variation range of the scale than that of Lunar Prospector, which was derived using theoretical calculation; namely, the calibrated map has higher values in the areas with high concentration while having lower values for the areas with lower concentration. However, the derived potassium map is more consistent with the lunar sample data than that of Lunar Prospector.

Surface mineralogy and stratigraphy of the lunar South Pole-Aitken basin determined from Clementine UV/VIS and NIR data

The South Pole-Aitken (SPA) basin, located on the lunar far side, is one of the oldest and largest recognised impact structures in the solar system. The basin is a proposed site for future sample return missions and human bases due to the unique geological environment and its potential for preservation of water ice in areas of near-permanent shadow. Here, we report surface mineralogy maps of the central and northern parts of the SPA basin, based on Clementine UV/VIS and NIR spectral data. Clementine LIDAR data and SMART-1 AMIE images provide additional geomorphological and stratigraphic information. A noritic mineralogical composition is identified as the deepest stratigraphic unit exposed on the basin floor. Norite is found in nearly all central peaks and in large topographical structures that have punched through an upper, often basaltic or gabbroic layer, including the Leibnitz and Apollo sub-basins. The thin layer of gabbroic/basaltic composition is distributed over large parts of the SPA basin floor and presumably overlays the noritic basement of apparent lower-crustal origin. Our data do not confirm the presence of olivine-rich material in the SPA basin, including at Olivine Hill, suggesting the mantle material was not excavated during the basin-forming impact.

Thorium abundances of basalt ponds in South Pole-Aitken basin: Insights into the composition and evolution of the far side lunar mantle

[1] Imbrian-aged basalt ponds, located on the floor of South Pole-Aitken (SPA) basin, are used to provide constraints on the composition and evolution of the far side lunar mantle. We use forward modeling of the Lunar Prospector Gamma Ray Spectrometer thorium data, to suggest that at least five different and distinct portions of the far side lunar mantle contain little or no thorium as of the Imbrian Period. We also use spatial correlations between local thorium enhancements and nonmare material on top of the basalt ponds to support previous assertions that lower crustal materials exposed in SPA basin have elevated thorium abundances, consistent with noritic to gabbronoritic lithologies. We suggest that the lower crust on the far side of the Moon experienced multiple intrusions of thorium-rich basaltic magmas, prior to the formation of SPA basin. The fact that many of the ponds on the lunar far side have elevated titanium abundances indicates that the far side of the Moon experienced extensive fractional crystallization that likely led to the formation of a KREEP-like component. However, because the Imbrian-aged basalts contain no signs of elevated thorium, we propose that the SPA impact event triggered the transport of a KREEP-like component from the lunar far side and concentrated it on the nearside of the Moon. Because of the correlation between basaltic ponds and basins within SPA, we suggest that Imbrian-aged basaltic volcanism on the far side of the Moon was driven by basin-induced decompressional melting.

New insights into lunar petrology: Distribution and composition of prominent low-Ca pyroxene exposures as observed by the Moon Mineralogy Mapper (M3)

[1] Lunar geochemical groups such as Mg suite, ferroan anorthosite, and alkali suite rocks are difficult to distinguish from orbit because they are defined by both modal mineralogy and elemental composition of their constituent minerals. While modal mineralogy can be modeled, only specific minerals or elements can be directly detected. At near-infrared (NIR) wavelengths, pyroxenes are among the most spectrally distinctive minerals, and their absorption bands are sensitive to structure and composition. Pyroxenes thus provide important clues to distinguish these geochemical groups and to understand lunar crustal evolution. Using Moon Mineralogy Mapper data, we search for lithologies dominated by strong low-calcium pyroxene (LCP) signatures. We compare the NIR absorptions of 20 LCPs to a suite of synthetic pyroxenes to determine which lunar pyroxenes appear magnesian enough to be candidate Mg suite norites. We detail three prominent regions of LCP (1) in South Pole–Aitken Basin (SPA), (2) south of Mare Frigoris, and (3) north of Mare Frigoris. The absorption band positions suggest that the LCPs north of Mare Frigoris and those in SPA are compositionally similar to one another and of ∼Mg50–75, implying that the mafic material excavated by the SPA impact was relatively iron-rich. Modified Gaussian modeling results suggest that the Apollo basin may have tapped different composition material than is exposed in much of SPA. The LCPs located in the highlands south of Mare Frigoris exhibit absorption bands at short wavelengths consistent with Mg > ∼80. The coincidence of these Mg-rich LCPs with the thorium measured by Lunar Prospector make them good candidates for KREEP-related Mg suite pyroxenes.

Directional wavelet analysis on the sphere: Application to gravity and topography of the terrestrial planets

[1] The spectral relations (admittance and correlation) between gravity and topography are often used to obtain information on the density structure, flexural support, and heat flow of planetary lithospheres. Mapping spatial variations in these quantities requires spatiospectral analysis techniques. Here we describe the application of a directional, continuous spherical wavelet transform using a wavelet basis constructed from the superposition of azimuthally adjacent complex Morlet wavelets, in a manner similar to the “fan” wavelet developed in the plane. The method is applied to gravity and topography of the Earth, Venus, Mars, and the Moon. The wavelet coefficients are used to compute isotropic and directional wavelet autospectra and cross spectra, which are then combined to form the admittance and correlation functions. The resulting maps offer insights into lithospheric structure of the terrestrial planets. In particular we show that the Earth and Venus have uniformly low positive admittance and high correlation, whereas Mars and the Moon display hemispherical contrasts with large negative and anisotropic coefficients coinciding with lowlands. As has long been known, the two largest impact basins in the inner solar system, the South Pole–Aitken basin on the Moon and the Hellas basin on Mars, display low positive admittance and high correlation, indicating isostatic compensation. In contrast, most other impact basins, particularly the Martian and lunar mascons, show negative coefficients at low wavelet degrees suggesting flexural support by a strong lithosphere. These results imply that, although simple isotropic flexural models can account for most observations, future models may need to incorporate anisotropy as an additional parameter.

Timing and duration of mare volcanism in the central region of the northern farside of the Moon

Age determinations of lunar mare basalts are essential for understanding the thermal evolution of the Moon. In this study, we performed new crater size-frequency measurements in mare deposits in the central region of the northern farside, consisting of Lacus Luxuriae, Buys-Ballot, Campbell, and Kohlschutter, using high-resolution images obtained by SELENE (Kaguya) Terrain Camera. The estimated model ages of the mare deposits range from 2.7 to 3.5 Ga. On the basis of model ages for all investigated mare deposits in the central part of the northern farside, considering the results of previous studies, we concluded that mare volcanism in this region began at least as early as 3.9 Ga and continued until ∼2.6 Ga. From a comparison of model ages in the region and the South Pole-Aitken basin, we found that mare volcanism in these regions ended at the same time, suggesting that the South Pole-Aitken basin formation impact had a minor effect on mare volcanism in the region.

Testing Mars-inspired operational strategies for semi-autonomous rovers on the Moon: The GeoHeuristic Operational Strategies Test in New Mexico.

BackgroundWe tested the science operational strategy used for the Mars Exploration Rover (MER) mission on Mars to determine its suitability for conducting remote geology on the Moon by conducting a field test at Cerro de Santa Clara, New Mexico. This region contains volcanic and sedimentary products from a variety of provenances, mimicking the variety that might be found at a lunar site such as South Pole-Aitken Basin.MethodAt each site a Science Team broke down observational “days” into a sequence of observations of features and targets of interest. The number, timing, and sequence of observations was chosen to mimic those used by the MERs when traversing. Images simulating high-resolution stereo and hand lens-scale images were taken using a professional SLR digital camera; multispectral and XRD data were acquired from samples to mimic the availability of geochemical data. A separate Tiger Team followed the Science Team and examined each site using traditional terrestrial field methods, facilitating comparison between what was revealed by human versus rover-inspired methods.Lessons LearnedWe conclude from this field test that MER-inspired methodology is not conducive to utilizing all acquired data in a timely manner for the case of any lunar architecture that involves the acquisition of rover data in near real-time. We additionally conclude that a methodology similar to that used for MER can be adapted for use on the Moon if mission goals are focused on reconnaissance. If the goal is to locate and identify a specific feature or material, such as water ice, a different methodology will likely be needed.

Laser altimetry data of Chang’E-1 and the global lunar DEM model

The Laser AltiMeter (LAM), as one of the main payloads of Chang’E-1 probe, is used to measure the topography of the lunar surface. It performed the first measurement at 02:22 on November 28th, 2007. Up to December 4th 2008, the total number of measurements was approximately 9.12 million, covering the whole surface of the Moon. Using the LAM data, we constructed a global lunar Digtal Elevation Model (DEM) with 3 km spatial resolution. The model shows pronounced morphological characteristics, legible and vivid details of the lunar surface. The plane positioning accuracy of the DEM is 445 m (1σ), and the vertical accuracy is 60 m (1σ). From this DEM model, we measured the full range of the altitude difference on the lunar surface, which is about 19.807 km. The highest point is 10.629 km high, on a peak between crater Korolev and crater Dirichlet-Jackson at (158.656°W, 5.441°N) and the lowest point is −9.178 km in height, inside crater Antoniadi (172.413°W, 70.368°S) in the South Pole-Aitken Basin. By comparison, the DEM model of Chang’E-1 is better than the USA ULCN2005 in accuracy and resolution and is probably identical to the DEM of Japan SELENE, but the DEM of Chang’E-1 reveals a new lowest point, clearly lower than that of SELENE.

Chang’E-1 gamma ray spectrometer and preliminary radioactive results on the lunar surface

The Chang’E-1(CE-1) spacecraft took a gamma-ray spectrometer (hereafter, CGRS) to detect the element distributions on the lunar surface in a circular, 200 km altitude, polar orbit with approximately 2 h periodicity. CGRS consists of two large CsI(Tl) crystals as the main and anticoincidence detectors. The large CsI crystal of CGRS has a higher detector effective area than other lunar gamma ray spectrometers. For its 1-year mission, gamma ray spectra including many peaks of major elements and trace elements on the lunar surface have been measured by CGRS. Global measurement within 0.55–0.75 MeV is given here to describe the distribution of radioactive composition (e.g., uranium and thorium) on the lunar surface. Although CGRS has a lower energy resolution that cannot separate the uranium peak from others in this energy region, 609 keV uranium gamma ray line dominates the shape of the spectrum in this energy region. Therefore, the radioactive map can indirectly describe the uranium distribution on the lunar surface. The radioactive map shows that the higher radiation is concentrated in the Procellarum KREEP Terrene (PKT) on the nearside with an oval shape. The secondary high-radiation is located in South Pole-Aitken (SPA) basin. Lunar highlands have lower concentration. The relationship between radiation and topography displays different linear correlations for lunar highlands and SPA basin, which imply the different processes for these two regions.

Minerals mapping of the lunar surface with Clementine UVVIS/NIR data based on spectra unmixing method and Hapke model

The distribution of minerals on the lunar surface is information which could contribute to studying lunar origin and evolution. In this paper, the distribution of clinopyroxene, orthopyroxene, olivine, ilmenite, and plagioclase on the lunar surface has been mapped based on Hapke radiative transfer model and linear unmixing of spectra with Clementine UVVIS/NIR data. The results have been validated on the basis of minerals modal abundance data of the Apollo samples, and problems in the minerals abundance mapping have been analyzed. The validation based on analysis data of Apollo samples indicates that plagioclase mapped in this paper represents the total abundance of plagioclase and agglutinitic glass. The minerals mapping results show that the lunar surface is mainly composed of pyroxene, plagioclase, agglutinitic glass, and ilmenite. Basalt in the lunar mare is mainly composed of clinopyroxene and ilmenite, and lunar highland is mainly composed of plagioclase and agglutinitic glass. Orthopyroxene is mainly distributed on the north of Mare Imbrium, on the south of Maria and Aitken Basin. According to our results, there is probably no large area of olivine distribution on the lunar surface which is different from earlier published results. Therefore, emphasis should be put on the olivine distribution in the minerals mapping using hyperspectral data such as M 3 of Chandrayaan-1 and IIM of ChangE-1.

Overview of Elemental Distributions on the Moon Observed by SELENE GRS

The high precision gamma-ray spectrometer (GRS) is carried on the first Japan's large-scaled lunar explorer, SELENE (KAGUYA) circling a polar orbit. The GRS consists of a large Ge crystal as a main detector and massive bismuth germanate crystals and a plastic scintillator as anticoincidence detectors. Since the successful launch on Sep. 14, 2007, radiation damage in the Ge detector has been induced from the incidence of energetic particles from space, which degraded its energy resolution. Then the Ge detector was annealed for 2 days at 85±5 K and the resolution is recovered to the level at the initial phase of the early observation. The special operations were conducted in December, 2008 in order to measure backgrounds gamma rays from the materials of the spacecraft body and the GRS instrument itself. The GRS data from the 100 km altitude reveal the global distribution of trace elements of Th and K on the Moon which delineates the distributions of KREEP component of lunar materials. Th and K-rich materials are concentrated around the Imbrium basin in Procellarum KREEP Terrane (PKT) and intermediately concentrated in South Pole–Aitken basin area on the farside. Results from special operations and observation are described and discussed.

A PRIMARY STUDY OF THE RELATIONSHIP BETWEEN LUNAR SURFACE TOPOGRAPHY AND PHYSIOGNOMY AND GEOLOGICAL INFORMATION COUPLING

According to DEM on the lunar surface based on Chang'E-1 LAM data and maps of major minerals and some elements on the lunar surface based on Clementine multi-spectral data,this paper systematically analyzed the coupling relationship between topography and physiognomy and rock distribution on the lunar surface.It is tentatively considered that two deep ruptures trending respectively southwest and southeast are distributed on the lunar surface.Under the action of an internal momentum,the tensile force and the external force,there are distributed on the lunar surface the U-landscape pattern,the plagioclase on the lunar highland,the mares basalt and the high FeO abnormal basic rocks in Aitken Basin.

Bouguer gravity anomaly of the Moon from CE-1 topography data: Implications for the impact basin evolution

In this study, the terrain correction for lunar free-air gravity anomaly (FAGA) is calculated in spherical coordinates based on the global topography data detected by the laser altimeter on Chang’E-1 (CE-1). The obtained lunar Bouguer gravity anomaly (BGA) reveals density irregularities of the interior mass. BGA is important in characterizing the mascon basins. According to the BGA of the Moon, the South Pole-Aitken (SPA) basin is considered the largest mascon basin on the Moon, and the feature of BGA in the basin implies the impacting direction. Further, the mascon basins seem to be classified into two types, Type Highland and Type Plain. For the mascon basins of Type Highland the dense materials mainly come from the shallow crust, which are associated with the basalt deposits. The other type, Type Plain, includes mascon basins whose major dense materials may be located deep at the lithosphere, corresponding to the uplifted mantle.

Ultramafic impact melt sheet beneath the South Pole–Aitken basin on the Moon

The Spectral Profiler onboard the Japanese lunar explorer Kaguya conducted the first hyperspectral survey around the South Pole‐Aitken basin (SPA) on the Moon that is one of the largest impact structures in the Solar System. The radiative transfer modeling revealed that fresh crater central peaks inside SPA are commonly composed of an ultramafic assemblage dominated by Magnesium rich orthopyroxene, suggesting a homogeneous layer buried under SPA. The extensive ultramafic layer could be a relic of a gigantic impact melt pool produced by the SPA‐forming impact. The preponderance of orthopyroxene could have resulted from significant fractional crystallization of the lunar magma ocean prior to the onset of plagioclase floatation.

Multiring-forming large bolide impacts and evolution of planetary surfaces

In the past few years, meteoritic and cometary impacts have emerged as a major geological agent in the construction and evolution of planetary surfaces. Formation of complex central ring, peak ring and multiring craters involves excavation and melting of large volumes of crustal material. High-resolution geophysical mapping measuring gravity, magnetics, and topography of the Moon and Mars have recently provided information on the subsurface structure of large basins and aided in identifying buried giant craters. The terrestrial crater record has been significantly erased by tectonic, magmatic, and erosion processes and only a small proportion of impact structures remain. Record of multiring craters is limited to three examples: Vredefort, Sudbury and Chicxulub. Deep geophysical surveys and geochemical and isotopic studies of those craters provide means to evaluate the influence of large impacts on the lithospheric and crustal evolution by providing estimates of excavation depth and volume, amounts of material fragmented, ejected, vaporized and melted, and effects on the crustal stratigraphy and crustal thickness. Analyses on the melt from Vredefort, Sudbury, and Chicxulub indicate andesitic composition derived from lower-crustal material. The melt formed inside the lower transient cavity from lower crustal material that was then redistributed and emplaced in upper-crustal levels, resulting in crustal redistribution. Crystalline basement clasts fragmented and incorporated into the breccias show varying degrees of alteration but no significant thermal effects. Ejecta were deposited locally within the crater region and ballistic material and fine ejecta are globally distributed on the planetary surface. Impacts influence the crust–mantle boundary, with Moho uplift. Material from the mantle was not incorporated into the melt and impact breccias, indicating that the excavation cavities were confined to the lower crust. This is also apparently the case for the giant basins on the Moon, including the 2500 km diameter South Pole-Aitken Basin. Considering the numbers of large multiring basins, possible flux of large impacts, and effects on target surfaces, crustal scale redistribution of material during those large impacts has played a major role in the evolution of planetary surfaces.

Rugged crater ejecta as a guide to megaregolith thickness in the southern nearside of the Moon

The southern highlands of the Moon comprise superposed ejecta layers, individually as thick as a few kilometers, from the major basins. Smaller (1–16-km-diameter) impact craters that penetrate this layered megaregolith and excavate material from depth have radar properties that provide insight into the variability of megaregolith thickness above a postulated basement of large crustal blocks. We observe a significant difference in the population of radar-bright craters, 1–16 km and larger in diameter, between regions of the southeastern near-side highlands north and south of ~lat 48°S. There are about one-third more radar-bright craters north of this line than to the south, broadly coincident with the mapped boundary between southern deposits mapped as pre-Nectarian age and those of Nectarian–Imbrian age to the north. The radar-bright crater population is consistent with a megaregolith thickness of ~1.5 km in the north and ~2.5 km in the south, a difference we attribute to South Pole–Aitken basin ejecta.