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Keywords:

  • Moscoviense basin;
  • impact;
  • Moon

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Structure of the Moscoviense Basin
  5. 3. Double-Impact Formation Hypothesis
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[1] The Moscoviense basin is the most prominent mare basalt filled impact basin on the farside highlands of the Moon. The Moscoviense basin has been studied extensively because of its characteristic geological and mineralogical context and distinct mare volcanism. The success of Kaguya's direct farside gravity recovery and global laser altimetry revealed that the Moscoviense basin shows unique characteristics not only for geological and mineralogical but also for the geophysical contexts. We measured geomorphological features of the Moscoviense basin and the asymmetry suggests an oblique impact formation process. Moho undulation beneath the Moscoviense basin, however, can hardly be explained by a single oblique impact formation process. We propose an alternative hypothesis for the Moscoviense basin formation, which is called “double impact scenario”. This scenario simultaneously accounts for the anomalously large mantle plug, asymmetric surface geo-morphology and excavation of olivine rich material originally located beneath the thickest crust on the Moon. On the basis of a set of Monte-Carlo simulations, we statistically examine the occurrence of a small spatial separation of two nearby basins. The probability of double impact, as seen on the Moscoviense basin where the two impacts occurred ∼80 km apart, is estimated to be about 50%.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Structure of the Moscoviense Basin
  5. 3. Double-Impact Formation Hypothesis
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[2] The Moscoviense basin, which is the most prominent mare basalt filled multi-ring impact basin on the lunar farside highlands, is located at (26°N, 148°E) and has a diameter of the main ring of either 445 km [Wilhelms, 1987] or 420 km [Pike and Spudis, 1987]. Some concentric circular structures (except for the rim) of the Moscoviense basin (140, 220, 300, 630 km in diameter) were reported based on photo data [Pike and Spudis, 1987]. Mare basalt of Mare Moscoviense is divided into four individual basalt flows using nomenclatures, age relationships, and surface composition [e.g., Craddock et al., 1997; Gillis, 1998; Kramer et al., 2008]. Haruyama et al. [2009] reported that mare volcanism at the Moscoviense basin lasted until 2.57 Ga. Morota et al. [2009] determined the thickness and age of individual basalt units of Mare Moscoviense and conclude that the magma production in the upper mantle under Mare Moscoviense is 3–10 times lower than that of the nearside mantle. Pieters et al. [2010] reported spinel-rich rock firstly discovered in the Moscoviense region using newly obtained hyper spectral image data by Moon Mineralogy Mapper (M3) onboard the Chandrayaan-I spacecraft. Moreover, very olivine-rich rocks have been discovered at ring structures of the Moscoviense basin [Yamamoto et al., 2010]. Yamamoto et al. reported that the olivine exposure on the lunar farside is limited to the South Pole-Aitken region and at Moscoviense. They also suggested that the olivine rich rocks are probably upper mantle materials excavated by basin forming impacts.

[3] As stated above, the Moscoviense basin has been studied extensively, but previous studies are mainly concentrated on describing the geological and mineralogical context, and volcanic histories using cratering chronology. This is due to the lack of high resolution farside geophysical and selenodetical data before SELENE. The Japanese lunar explorer SELENE (Kaguya) has finished with great success and it has opened a new era of lunar science. SELENE (Kaguya) 4-way tracking data over the farside improved our knowledge of farside gravity features [Namiki et al., 2009; Matsumoto et al., 2010]. The lunar topography model was also improved by laser altimeter data of Kaguya [Araki et al., 2009]. The Kaguya crustal thickness model [Ishihara et al., 2009] provided detailed views of the crustal thickness variation on the lunar farside. These new selenodetic data enable us to investigate basin structures not only on the lunar nearside, but also on the lunar farside.

[4] In this paper, we first describe characteristics of the Moscoviense basin structures based on Kaguya selenodetic data. Then, we discuss the formation process of the Moscoviense basin.

2. Structure of the Moscoviense Basin

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Structure of the Moscoviense Basin
  5. 3. Double-Impact Formation Hypothesis
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[5] Using the Kaguya 1/16-degree gridded topography model (an updated version of Araki et al. [2009]), we can easily identify two distinct ring structures and one blurred ring structure (Figure 1). We made azimuthally averaged profiles on various reference points, and then we interpreted center and diameter of each ring structure. The innermost-ring structure (about 180 km in diameter) is seen only partially, with a clear rim missing in the northeast part. The middle-ring structure (about 420 km in diameter) is distinct and fully closed. The outermost-ring structure (about 640 km in diameter) is less clear than the middle-ring. These characteristics of the Moscoviense rings are not a common feature of other multi-ring basins. For other multi-ring basins such as the Orientale basin, the most prominent and regular outlined ring is the basin rim-crest and middle-rings have irregular topography. But for the Moscoviense basin, the middle-ring is the most prominent and it has regular topography. These three rings are not concentric but offset from the center from one another. The centers of the three rings are located on a single line (Figure 1 and Table 1). In addition, for the Moho undulation, the shallowest part is offset from the center of the Moho uplift (Figure 1). This offset is not common for basins located at the lunar farside.

image

Figure 1. (top) Surface topographic features, (middle) subsurface structure (Moho discontinuity) map, (bottom) profiles of (left) the Moscoviense basin and (right) the Freundlich-Sharonov basin based on Kaguya selenodetic data. Projections of both maps are orthographic projection centered on (148.5°E, 27.6°N) for the Moscoviense, on (175°E, 18.5°N) for the Freundlich-Sharonov and horizons are 17 degrees from center. Topography and Moho depth are referenced from 1737.4 km radius standard sphere centered on the center of mass of the Moon. Contour intervals of topography and Moho are 1 km and 5 km, respectively. Crosses indicate center locations of each ring structure. Great circles of azimuth of 45° at each reference point were employed as track lines of profiles.

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Table 1. Diameters and Center Locations of Each Ring Structure of the Moscoviense Basin
Ring StructureCenter [Longitude, Latitude]Diameter
1st ring146.4°E, 25.7°N180 km
2nd ring147.4°E, 26.4°N420 km
3rd ring148.5°E, 27.6°N640 km

[6] The offset and linear locations of the ring center and innermost partial ring suggest that the Moscoviense basin formed by an oblique impact. On the other hand, Kaguya gravity and crustal thickness models show that the Moscoviense basin has an extremely large (both in width and height) mantle plug, larger than other basins of the same type such as the Freundlich-Sharonov basin [Ishihara et al., 2009; Namiki et al., 2009; Matsumoto et al., 2010]. In addition, this extremely large mantle plug is not an artifact of mare basalt and crustal intrusion [Ishihara et al., 2009]. The size of the mantle plug of the Moscoviense basin is almost the same as that of the Freundlich-Sharonov basin which has a 600 km diameter main ring. Moreover, the height of the mantle plug of the Moscoviense basin is much higher than that of the Freundlich-Sharonov basin [Ishihara et al., 2009]. For the type I and type II basins [Namiki et al., 2009; Matsumoto et al., 2010], the mantle plug size of the impact basin should be mainly controlled by the size of the excavation cavity at the formation of the crater, and by the pre-impact depth of the Moho discontinuity [Ishihara et al., 2009]. Pre-impact depths of the crust–mantle interface at both basins were almost the same (∼65 km) (Figure 1). So the difference is probably due to the size of excavation. In addition, olivine exposure around the Moscoviense basin supports a very deep excavation of the Moscoviense basin forming impact [Yamamoto et al., 2010]. However, the excavation depth of a single oblique impact is shallower than that of a normal impact [Gault and Wedekind, 1978]. The extremely large mantle plug (i.e. extremely large excavation) is therefore hard to explain by a single oblique impact.

3. Double-Impact Formation Hypothesis

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Structure of the Moscoviense Basin
  5. 3. Double-Impact Formation Hypothesis
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[7] We propose a new hypothesis for the Moscoviense basin formation, which is called “double-impact formation” (Figure 2). The essence of this hypothesis is as follows. The first impact formed the ∼640 km size pre-Moscoviense basin which is similar to the Freundlich-Sharonov basin. After this, the second impact occurred on the floor of the pre-Moscoviense basin and this formed the ∼420 km size multi-ring basin. The partiality of the innermost-ring and the offset between innermost- and middle-ring centers can be interpreted by the influence of the structure of the pre-Moscoviense basin as the pre-existing target structure of the second impact. Furthermore, because of the crustal thinning effect at the first impact, the excavation depth of the second impact could have reached the Moho interface beneath the pre-Moscoviense basin, and mantle materials could have been exposed as ejecta. The resulting structure from two times the dynamic uplift processes should have a huge mantle plug, an offset in rings, and the exposure of olivine rich rock that originally was located at the upper-mantle [Yamamoto et al., 2010]. Ishihara et al. [2009] show that the ratio between pre-impact crustal thickness (Moho depth) and impact scale controls the mantle uplift size for type I and type II basins on the Moon [Namiki et al., 2009; Matsumoto et al., 2010]. The ratio between crustal thickness at the basin center normalized by basin diameter and crustal thickness at the surrounding region normalized by basin diameter shows a linear relationship for all type I and type II basins, while the Moscoviense basin is an outlier. However, when we adopt the double-impact formation hypothesis, each individual impact basin of Moscoviense satisfies the relationship. In addition, the difference between Moscoviense and Freundlich-Sharonov with respect to the amount of mare volcanism can probably be explained by a double impact origin of the Moscoviense basin. If the total amounts of magma production and magma production rate at the two basins are equal, crustal thickness could control the extrusion volume. A thinner and cracked crust resulting from the double impact at the Moscoviense basin is more favorable for magma extrusion than a relatively thicker and rigid crust at the Freundlich-Sharonov basin.

image

Figure 2. Formation history of the Moscoviense basin based on the double impact hypothesis. The first impact creates a pre-Moscoviense basin (∼640 km in diameter), similar to the Freundlich-Sharonov basin, that was probably a multi-ring basin, and the first mantle uplift occurred. The second impact occurred on the floor of the pre-Moscoviense basin and this formed the ∼420 km diameter multi-ring basin and erased the inner rings of the pre-Moscoviense basin.

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[8] The double-impact hypothesis can thus explain both the center offset and the huge mantle plug of the Moscoviense basin. However, are such close double impacts possible statistically? From analysis of current topography (ring structures), the distance between the first and the second impacts is about 80 km. To evaluate the probability of a close double impact, we carried out a simple probability test using Monte-Carlo simulation. We assume that basin forming impacts occur uniformly at the lunar surface. Based on an assumption that basin forming impacts occur uniformly on the whole Moon, we computed locations of spatially random-generated impacts, and then calculated the nearest neighbor distances for all impacts. The minimum value of the nearest neighbor distances was compared with the Moscoviense case (i.e., 80 km). Iterating the procedure 100,000 times, the probability was evaluated. We chose to generate 50 impact locations per set, because present day estimates propose about 50 to 60 impact basins [e.g., Wilhelms, 1987; Pike and Spudis, 1987] (Impact Basin Database compiled by C.A. Wood, available at http://www.lpod.org/cwm/DataStuff/Lunar%Basins.html). In fact, recent results imply many more basins, about 2–3 times as many as previously thought [e.g., Frey, 2008, 2009; Head et al., 2010], which would leave our estimate as conservative. Figure 3 shows the cumulative frequency of the closest distance of impact locations. The possibility of the existence of at least one basin pair with an 80 km separation is about 50%. This probability is not small and thus we infer that a basin-forming double impact with an 80 km offset could have occurred on the Moon.

image

Figure 3. Cumulative frequency of the closest distance of impact locations. The arrow indicates the distance between the centers of the innermost- and outermost-ring features of the Moscoviense basin (80 km).

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4. Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Structure of the Moscoviense Basin
  5. 3. Double-Impact Formation Hypothesis
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[9] We measured centers and diameters of ring structures of the Moscoviense basin using newly obtained Kaguya topographic data. Ring structures are not concentric but show a linear offset. Traditional interpretation of this characteristic is an oblique impact formation of the Moscoviense basin. However, an oblique impact cannot account for the extremely large mantle plug of the Moscoviense basin as estimated from gravity. We thus propose a double impact formation hypothesis for the Moscoviense basin. This hypothesis easily explains the mantle plug size, the exposure of olivine rich material and other features of the Moscoviense basin. The probability of occurrence of double impact basin, as inferred from a Monte Carlo simulation of impact locations, is about 50% for the Moscoviense basin case (impact location distance between first and second impacts of ∼80 km). This probability is not so small as to reject a double impact origin of the Moscoviense basin.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Structure of the Moscoviense Basin
  5. 3. Double-Impact Formation Hypothesis
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[10] We used the Generic Mapping Tools (GMT) software [Wessel and Smith, 1991] for drawing figures and some analysis. This work was supported by the Japan Society for the Promotion of Science under a Grant-in-Aid for JSPS Fellows (20-9211: T.M.) and a Grant-in-Aid for Scientific Research (A) (20244073: S.S.). We thank two anonymous referees for their constructive comments, which helped us to improve the paper.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Structure of the Moscoviense Basin
  5. 3. Double-Impact Formation Hypothesis
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Structure of the Moscoviense Basin
  5. 3. Double-Impact Formation Hypothesis
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information
FilenameFormatSizeDescription
grl27691-sup-0001-t01.txtplain text document0KTab-delimited Table 1.

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