Distribution of the subsurface reflectors of the western nearside maria observed from Kaguya with Lunar Radar Sounder

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L18202, doi: /2009gl039835, 2009 Distribution of the subsurface reflectors of the western nearside maria observed from Kaguya with Lunar Radar Sounder Shoko Oshigami, 1 Yasushi Yamaguchi, 1 Atsushi Yamaji, 2 Takayuki Ono, 3 Atsushi Kumamoto, 3 Takao Kobayashi, 4 and Hiromu Nakagawa 3 Received 1 July 2009; revised 5 August 2009; accepted 7 August 2009; published 17 September [1] The Lunar Radar Sounder (LRS) performed global soundings and detected possible subsurface echoes at the nearside maria. This paper examines the relationship between the distribution of the detected subsurface echoes and the surface ages in the western nearside maria. Continuous subsurface reflectors are clearly detected only in a few areas consisting of about 10% of the western nearside maria at apparent depths from hundreds to more than a thousand meters. These reflectors are not generally the basements of the mare but are the interface between different basaltic rock facies. Comparison between the distribution of the detected subsurface boundaries and the surface ages suggests that most of the detected subsurface reflectors were formed more than 3.4 billion years ago. Based on the strong connection between the accumulation rate of regolith and the absolute age, the detected subsurface boundaries appear to be relatively thick regolith layers accumulated during the depositional hiatuses of basaltic lavas. Citation: Oshigami, S., Y. Yamaguchi, A. Yamaji, T. Ono, A. Kumamoto, T. Kobayashi, and H. Nakagawa (2009), Distribution of the subsurface reflectors of the western nearside maria observed from Kaguya with Lunar Radar Sounder, Geophys. Res. Lett., 36, L18202, doi: /2009gl Introduction [2] The Lunar Radar Sounder (LRS) was one of the fifteen scientific instruments onboard Kaguya (SELENE). The LRS system transmits a radar pulse whose frequency is linearly modulated from 4 MHz to 6 MHz in 200 ms with a peak power of 800 W [Ono and Oya, 2000]. The range resolution is 75 m in vacuum [Ono and Oya, 2000], whereas the sampling interval in the flight direction is about 75 m when the spacecraft altitude is 100 km. The LRS detects echoes reflected from subsurface discontinuities where the dielectric constants of the rocks change abruptly. The primary objective of the LRS is to contribute to an understanding of the evolution of the Moon through an investigation of the stratigraphic and tectonic lunar subsurface features [Yamaji et al., 1998]. [3] The first scanning of the lunar subsurface structures was done by the Apollo Lunar Sounder Experiment (ALSE) onboard the Apollo 17 Command and Service Module. At 1 Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan. 2 Graduate School of Science, Kyoto University, Kyoto, Japan. 3 Graduate School of Science, Tohoku University, Sendai, Japan. 4 Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon, South Korea. Copyright 2009 by the American Geophysical Union /09/2009GL different orbits, the ALSE observed part of the Mare Serenitatis and Mare Crisium and revealed subsurface layering in these maria with HF band (5 MHz) radar [Phillips et al., 1973; Maxwell, 1977; Sharpton and Head, 1982]. The LRS performed global soundings and also detected many subsurface echoes in Mare Serenitatis, Mare Crisium, and in other nearside maria at an apparent depth of several hundred meters [Ono et al., 2009]. [4] Ono et al. [2009] concluded that the prominent echoes under Mare Serenitatis are due to buried regolith layers accumulated during the depositional hiatus of mare basalts. This conclusion was reached by comparing the subsurface features with the surface geology. The subsurface horizons observed in the ALSE profiles also have been interpreted as continuous, lateral zones of a high-porosity material such as buried regolith [May et al., 1976; Maxwell, 1977; Peeples et al., 1978; Phillips and Maxwell, 1978]. However, Ono et al. [2009] do not reveal the spatial distribution of the reflectors under the nearside maria. This paper considers the distribution and depth of the continuous subsurface reflectors under the western nearside maria clearly observed in the LRS data. The relationship between the distribution and the surface age will be considered in order to identify the subsurface reflectors. The characteristics of the surface echo intensities observed with the LRS are to be discussed by H. Nakagawa et al. (Global mapping of HF radar surface echo power of the Moon derived from Kaguya/LRS, manuscript in preparation, 2009). 2. Basic Principles of LRS Data Analysis [5] The time series data of the A-scope, which is a plot of the received echo power as a function of range, are called the B-scan [Kobayashi et al., 2002]. Figures 1a and 1b show a sample B-scan image of the LRS data along the Kaguya orbit. The vertical axis indicates the relative apparent depth. In this paper, the apparent depth is defined as the depth obtained assuming that a radar pulse propagates in the lunar subsurface at the speed of light in vacuum. The actual depth of the subsurface reflector, d, is calculated from the apparent depth, D, and the relative permittivity of the layer through which the radar is propagated, e r, using p d ¼ D= ffiffiffiffi e r The dielectric constant of the materials found in the lunar surface has been measured using the Apollo samples [see, e.g., Carrier et al., 1991]. Also, Ono et al. [2009] estimated the permittivity of the materials under lunar maria through model calculation using the LRS data. The estimated permittivity is from 5.8 to 11.9 [Ono et al., 2009]. Since the ð1þ L of6

2 Figure 1. B-scan images of the LRS data for (a) Mare Imbrium and (b) Oceanus Procellarum. The vertical axes indicate the apparent range with respect to the lunar surface. The lower figures indicate prominent subsurface reflectors. permittivity of each site cannot be accurately determined, it is not possible to discuss in detail the actual depths of the subsurface reflectors. [6] The echo signals in the A-scope data can be classified into the following categories: (1) surface nadir echo, (2) surface off-nadir backscattering echo, and (3) subsurface echo [Kobayashi et al., 2002]. The most intense signals usually come from the nadir point. Subsurface echoes are sometimes masked by the numerous echoes from the surface craters that appear as obvious hyperbolic patterns in the B-scan images. Also, Figures 1a and 1b suggest that reflectors with an apparent depth of about 200 m or less are masked because they cannot be distinguished from the strong surface echoes. In addition, the spot reflectors are difficult to detect in the LRS B-scan data. Instead, the authors focus on laterally continuous subsurface reflectors in this paper. [7] To distinguish surface and subsurface echoes in the LRS data, at least two data sets observed from adjacent orbits are used. It is assumed that the subsurface reflectors are locally flat. Ranges from the satellite to a surface off-nadir reflector differ between two or more adjacent orbits, while apparent ranges to a subsurface reflector are the same with each adjacent orbit. Therefore, if the echo signals observed from two or more adjacent orbits have the same apparent range, the echo signals will be classified as subsurface echoes. The averaged longitudinal interval between the adjacent orbits is about 0.13 degrees (4 km at 0 N) for the sounding observations by the LRS. Peeples et al. [1978] employed this technique to identify subsurface echoes from the ALSE data. [8] The maximum depth of the radar sounding observations below the lunar surface depends on the loss tangent value of the lunar materials and is estimated using the simplified radar equation [Phillips et al., 1973]. The expected loss tangent, tan d, is from to for the most mare basalt of which the total content of TiO 2 and FeO is from 13 to 33wt%. The range of the value of the TiO 2 and FeO contents for mare basalts is reported by Haskin and Warren [1991] and Hiesinger et al. [2001], for example. The calculated loss tangent values are based on the following regression equation [see, e.g., Carrier et al., 1991]: log 10 ðtan dþ ¼ 0:045ðTiO 2 þ FeOÞ 2:754 ð2þ To estimate the maximum depth of sounding, the threelayered model is assumed: free space (1st propagation medium), basalt layer (2nd propagation medium), and regolith layer (subsurface reflector). Plausible relative permittivities of basalt and regolith are 8.0 and 4.0, respectively. An observable depth for each mare is 2.9 km under the assumption that the loss tangent value of the 2nd propagation medium is homogeneously Onthe other hand, an observable depth of 0.3 km is obtained if the 2of6

3 Table 1. Maximum Apparent Depths of the Reflecting Interface Under the Western Nearside Maria Estimated From the LRS Data and the Previous Estimates of the Mare Thickness Region Maximum Apparent Depth of Subsurface Reflectors Estimated Mare Thickness [De Hon, 1979] Estimated Mare Thickness [Williams and Zuber, 1998] Mare Humorum 20 S to 28 S, 315 E to 318 E 700 m 2.25 km 3.6 km Mare Imbrium 44 N to48 N, 324 E to 332 E 600 m 1.0 km 5.2 km (Sinus Iridum) 30 N to46 N, 343 E to 355 E 1,600 m 2.25 km 5.2 km 17 N to21 N, 333 E to 336 E 600 m 0.50 km 5.2 km Oceanus Procellarum 29 N to52 N, 285 E to 302 E 1,200 m 0.75 km N/A 29 N to35 N, 317 E to 318 E 500 m 0.50 km N/A 14 N to17 N, 322 E to 324 E 400 m 0.75 km N/A loss tangent value is Other parameters used in these calculations such as the transmission power, the antenna gain, the transmitted wavelength, the spacecraft altitude, and galactic noise level depend on Ono and Oya [2000]. Therefore, theoretically, it is possible to detect subsurface echoes from an apparent depth of from 0.8 km to 5.1 km. 3. Depth of Possible Subsurface Reflectors [9] The basalt thickness of most lunar maria has been estimated using crater morphology [see, e.g., De Hon, 1979; Williams and Zuber, 1998]. De Hon [1979] estimated the thickness of mare basalt by assuming that partially flooded craters which formed prior to mare filling follow the relationship between the diameter and the rim height of fresh lunar craters examined by Pike [1974]. Williams and Zuber [1998] determined the depth/diameter relationship for unflooded impact basins and estimated mare basalt thicknesses as a difference between the predicted depth-to-diameter trend and the observed basin depth. The partially-buried crater method by De Hon [1979] has some uncertainties [Hörz, 1978; Wieczorek et al., 2006]. The measurements by Williams and Zuber [1998] seem to be more reliable in terms of refining these uncertainties. Table 1 shows a comparison of the basalt thicknesses and the estimated depths of the possible subsurface echoes in the LRS data. For example, De Hon [1979] estimated that the maximum mare thickness at the site in Figure 1a (the northeastern part of Mare Imbrium) is more than 2 km. On the other hand, the apparent depth of the deepest subsurface reflector found in Figure 1a is about 1.1 km. This gives an expected actual depth of about 400 m, assuming that the relative permittivity of the mare deposits above the boundary is uniformly 8.0. Given the uncertainty in the dielectric constant of the subsurface materials and in the partially flooded crater method, Table 1 suggests that the estimated depths of the deepest subsurface discontinuities within each mare are mostly shallower than previous estimates of total mare basalt thickness. In addition, the detected subsurface reflectors are nearly parallel to the local lunar surface. These results indicate that the reflectors are not generally the basements of the mare but an interface between different basaltic rock facies, as suggested by Ono et al. [2009]. Therefore the estimated depths of the deepest reflectors in each mare do not represent the total basalt thickness. As discussed in section 2, it is possible that radar pulses do not reach basin bottoms except for the marginal areas because of radar attenuation within the propagation medium. [10] In the northern part of Oceanus Procellarum, the maximum depth of the possible subsurface echoes is larger than previous estimates. The multiple reflectors with different depth are detected in this region (Figure 1b), as well as in the northeast part of Mare Imbrium (Figure 1a). The apparent depth of the deepest subsurface reflector is 1200 m (Table 1), while the expected actual depth is 400 m, assuming that the relative permittivity of the mare deposits over the deepest reflector is uniformly 8.0. Therefore mare thickness in this site seems to be greater than 400 m. This estimate is larger Figure 2. A topographic map of the western nearside provided by USGS. The domains bounded by black lines indicate detectable areas of possible subsurface echoes. The white lines represent the locations of Figure 1. 3of6

4 Figure 3. Histograms of the surface age of the areas where possible subsurface echoes are detected in (a) Mare Humorum, (b) Mare Imbrium, and (c) Oceanus Procellarum, and that of the whole area of each mare. The surface ages of Mare Humorum (Figure 3a) and Mare Imbrium (Figure 3b) are based on Hiesinger et al. [2000], and those of Oceanus Procellarum (Figure 3c) are based on Hiesinger et al. [2003]. than the previous one by De Hon [1979] of less than 250 m in this area. 4. Distribution of the Detected Subsurface Echoes and Surface Ages [11] Clear continuous subsurface reflectors are detected only in a few areas from the B-scan data: the western nearside maria which include parts of Mare Humorum, Mare Imbrium, and Oceanus Procellarum (Figure 2). Those regions cover 10% of the western nearside maria. [12] The spatial distribution of the possible subsurface reflectors under the western nearside maria observed in the LRS data is compared with a map of the model ages for the spectrally defined units [Hiesinger et al., 2000, 2003]. These researchers subdivided the lunar maria into spectrally homogeneous units and determined the age of each unit based on crater counting. Figure 3 shows histograms of the surface age of the areas where subsurface echoes are detected and that of the whole area of each mare in Table 1 and Figure 2. Most of the possible subsurface echoes are detected in the areas where the surface age is older than 3.4 billion years (Ga). This result suggests that most of the detected subsurface reflectors were formed more than 3.4 billion years ago. This supports the interpretation that the detected subsurface boundaries are relatively thick regolith layers that accumulated by meteoroid bombardments and pyroclastic eruptions. [13] The rate of regolith accumulation is not constant with time and was probably higher for the period more than 3.5 billion years ago in response to the changing rate of impact crater production [Shkuratov and Bondarenko, 2001]. Regolith layers are thickened during the depositional hiatuses of basaltic lavas. Hiesinger et al. [2000, 2003] determined the surface ages of the maria. Those studies are useful to infer the ages of subsurface reflectors if they crop out at the surface [Ono et al., 2009]. However, most of the reflectors found in this study are obliterated by younger basalts. If the detected subsurface boundaries are compositional boundaries, then the distribution of the possible subsurface echoes implies that the composition of erupted basaltic lavas dramatically changed more than 3.4 billion years ago. Hiesinger et al. [2001] reported that mare basalts older than about 3.3 Ga have a wider variety in iron abundances than younger basalts and older basalts show the full range of iron concentrations (i.e., 3 19wt%). However, a contrast of iron content does not necessarily imply the difference of relative permittivity because the relative permittivity is independent of chemical composition, while it strongly depends on density, for the lunar sampled materials [Carrier et al., 1991]. Thus, it can be concluded that the detected subsurface interfaces are relatively thick regolith layers. Peeples et al. [1978] indicated that bandwidth limitations of the ALSE HF-1 (5 MHz) system require regolith layers to be greater than 2 m thick to act as reflecting horizons. It is possible that the distribution map of subsurface reflectors (Figure 2) might be coincident 4of6

5 with that of buried regolith horizons thicker than such a lower boundary. That is, the detectability of subsurface reflectors might depend on their thickness. In addition, the rare occurrence of continuous subsurface reflectors suggests that regolith layers were often washed away by the flooding of basaltic lavas. The surface ages indicate the minimum ages of their substrata: areas with older surface are underlain by basalts older than the surface. Thus, the buried basalts under older areas were subject to the heavy bombardment more probably than those under younger areas when they lay at the surface. This difference accounts for the result that continuous subsurface reflectors were found under older areas more often than under younger areas. [14] The relationships between the spatial distribution of the subsurface echoes and the surface ages for each area shown in Table 1 and Figure 2 will be discussed. For Mare Humorum, a single horizontal reflector is clearly detected (Table 1 and Figure 2). Figure 3a shows histogram of the surface age of the detected areas in Mare Humorum and the whole mare based on Hiesinger et al [2000]. The surface ages of the detected area are 3.45 Ga (unit H7) and 3.53 Ga (unit H5) [Hiesinger et al., 2000]. [15] Next Mare Imbrium will be considered. A clear single reflector is detected in a part of Sinus Iridum, at the southern edge and in the northeastern part of Mare Imbrium (Table 1 and Figure 2). Figure 3b is a histogram of the surface age of the detected areas in Mare Imbrium and the whole mare based on Hiesinger et al [2000]. The surface ages of the detected area in Sinus Iridum are 3.01 Ga (unit I21) and 3.26 Ga (unit I17), while those in the southern edge and the northeastern part of Mare Imbrium are 3.10 Ga (unit I19) and from 3.11 Ga (unit I18) to 3.52 Ga (unit I5) [Hiesinger et al., 2000]. [16] Finally, Oceanus Procellarum will be considered. Possible subsurface echoes are detected at three areas: the northern part of the mare, the northeastern and the southeastern parts of the middle area of the mare (Table 1 and Figure 2). Figure 3c shows histogram of the surface age of the detected areas in Oceanus Procellarum and the whole mare based on Hiesinger et al. [2003]. The surface units of the detected areas are remarkably older than the other parts of Oceanus Procellarum. The surface ages of the detected area in the northern part of Oceanus Procellarum are 3.44 Ga (unit P10) and 3.47 Ga (unit P9), while the surface ages of the northeastern and the southeastern parts of the middle Oceanus Procellarum are 3.48 Ga (units P7 and P5, respectively) [Hiesinger et al., 2003]. 5. Conclusions [17] Continuous subsurface reflectors are clearly detected at an apparent depth of several hundreds to over a thousand meters in limited areas consisting of about 10% of the western nearside maria. In most areas, the estimated depth is much smaller than previously estimated. A comparison of the subsurface echo distribution with the surface ages reveals that the detected subsurface boundaries are distributed under the relatively old units. The surface age of the detected areas in Oceanus Procellarum is remarkably limited and older than the other parts of the mare. The older the surface unit is, the older the basalt unit beneath the surface unit is. Therefore, the regolith layer accumulated on a basalt unit under on older surface might be relatively thicker due to a higher rate of regolith accumulation assuming that the growth period is constant. Thus it can be concluded that the detected subsurface interfaces are relatively thick regolith layers and that detectability depends on their thickness. [18] Acknowledgments. The Kaguya mission was conducted by the Japan Aerospace Exploration Agency (JAXA). The authors would like to thank Y. Takizawa (project manager), S. Sasaki (project scientist), and M. Kato (science manager) for their efforts in accomplishing the Kaguya mission, and K. Tanaka, Y. Iijima, S. Nakazawa, H. Ohtake, S. Sobue, H. Hoshino, H. Okumura, Y. Yamamoto, and J. Kimura (team members of the Kaguya mission) for their helpful discussions related to creating the highly reliable mission instruments. The authors also thank Yuriy Shkuratov and an anonymous reviewer for constructive reviews of the manuscript. References Carrier, W. D., III, G. R. Olhoeft, and W. Mendell (1991), Physical properties of the lunar surface, in Lunar Sourcebook: A User s Guide to the Moon, edited by G. H. Heiken, D. T. Vaniman, and B. M. French, pp , Cambridge Univ. Press, New York. De Hon, R. A. (1979), Thickness of the western mare basalts, Proc. Lunar Planet. Sci. Conf., 10th, Haskin, L., and P. Warren (1991), Lunar chemistry, in Lunar Sourcebook: A User s Guide to the Moon, edited by G. H. Heiken, D. T. Vaniman, and B. M. French, pp , Cambridge Univ. 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6 Yamaji, A., S. Sasaki, Y. Yamaguchi, T. Ono, J. Haruyama, and T. Okada (1998), Lunar tectonics and its implications for the origin and evolution of the Moon, Mem. Geol. Soc. Jpn., 50, T. Kobayashi, Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources, 30 Gajeong-dong, Daejeon , South Korea. A. Kumamoto, H. Nakagawa, and T. Ono, Graduate School of Science, Tohoku University, 6-3, Aramaki Aoba, Aoba-ku, Sendai, Miyagi , Japan. S. Oshigami and Y. Yamaguchi, Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya , Japan. A. Yamaji, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto , Japan. 6of6