46th Lunar and Planetary Science Conference (2015) 1316.pdf

HIGH PERMITTIVITY REGIONS IN PROCELLURAM AND MARE IMBRIUM FOUND BY (KAGUYA). A. Kumamoto1, K. Ishiyama1, S. Oshigami2, J. Haruyama3, and Y. Goto4, 1Tohoku Univer- sity (Aoba, Aramaki, Aoba, Sendai 980-8578, Japan. E-mail: [email protected]), 2National Astro- nomical Observatory of Japan, 3Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 4Kanazawa University.

Introduction: The determination of the effective surface consist of various components such as off- permittivity of the lunar surface material is useful for nadir surface echoes, volume scatters from the subsur- discussion of their composition and porosity. Assuming face layers, and echoes from the subsurface reflectors. the Maxwell-Garnett mixing relation and parameters In this study, we assumed that most of them was off- based on samples [1], the bulk density bulk of nadir surface echoes. The median of off-nadir echo the lunar surface materials can be derived from their intensities were derived in 360 x 180 areas of 1 (lon- effective permittivity r by using the following equa- gitude) x 1 (latitude). tion: In addition, we have derived the global distribution 1  1 of the surface roughness parameters. The RMS height r  0.217 . (1) 3 , or Allan deviation of the surface height, can be ob- bulk g/m  r  2 Bulk density of the lunar surface material depends on tained by 2 2 the abundances of voids and heavy components such as  x  zx  x zx , (2) ilmenite. The dataset obtained by Lunar Radar Sounder where z(x) is height of the surface derived from the (LRS) onboard SELENE (Kaguya) [2] enables us to SELENE TC/DTM, x is baseline length, and <> de- perform global high-resolution mapping of the lunar notes the average. If we assume the self-affine surface surface permittivity because (a) the observation was model, the roughness parameters H and s can be ob- performed from the polar orbiter at an altitude of about tained by the least square fitting of the RMS heights to 100 km, and (b) the operation frequency was 4 - 6   x  sxH , (3) MHz in which thermal emissions is negligible. We should note that the echo powers from the lunar surface in a baseline length range from 30 m to 3 km. The depends not only on the permittivity but also on the roughness parameters were derived in 360 x 180 areas roughness of the lunar surface. As for the roughness, of 1 (longitude) x 1 (latitude). we can use SELENE Digital Terrain Model (DTM) The off-nadir surface echo power can be calculated based on Terrain Camera (TC) observation [3]. We can based on the radar equation. Assuming Kirchhoff Ap- therefore calculate expected echo powers by applying proximation (KA), the backscattering coefficient in the Kirchhoff Approximation (KA), and compare them radar equation can be obtained from the roughness with observed echo powers in order to determine the parameters H, s, and the permittivity [6, 7, 8]. Using effective permittivity. Also in another analysis [4], we the roughness parameters H and s obtained by have determined the effective permittivity of the lunar SELENE TC/DTM and assumed permittivity, we can uppermost basalt layers in several regions by using calculate the expected off-nadir surface echo powers delay time of the subsurface echoes and actual depth of and compare them with observed off-nadir surface the subsurface boundary estimated by multiband imag- echo power. Based on the comparison, we can deter- es [5] around the craters excavating the uppermost bas- mine the most plausible effective permittivity. alt layers. The derived dielectric constant was less than Results: The global distributions of roughness pa- that of the Apollo samples, which suggested that there rameters, H and s, were obtained based on SELENE were macro cracks made by the meteorite impacts. TC/DTM. The Hurst exponent H is <0.5 in the maria, (1-H) Analysis method used in this study can be applicable in and >0.7 in the highlands. The parameter s is >1 m (1-H) determination of the effective permittivity in wider area in the maria, and <0.3 m in the highlands. The on the moon because it does not need craters. global distribution of H is similar with that based on Analyses Method: The global distributions of the LRO [9]. By applying the analysis method mentioned echo powers in a frequency range of 4 - 6 MHz were above, we could obtain the observed and calculated derived from the SELENE/LRS dataset. In this study, off-nadir surface echo powers. Based on them, we we have used the intensity of off-nadir surface echoes could estimate the effective permittivity of the lunar in an incident angle from 10 to 20, which are meas- surface materials in 360 x 180 areas of 1 (longitude) x ured after the arrival of the nadir surface echo. We 1 (latitude) as shown in Figure 1. The estimated effec- should note that the echoes measured after the nadir tive permittivity is 2 - 3 in the highland, 3 - 4 in the 46th Lunar and Planetary Science Conference (2015) 1316.pdf

maria. In addition, it was found that there areas whose book: A user’s guide to the Moon, edited by G. H. effective permittivity reaching ~ 5 in the eastern part of Heiken, G. H. et al., 475-594, Cambridge Univ. Press, Oceanus Procellarum and the western part of Mare New York. [11] Wieczorek, M, et al. (2013), Science, Imbrium. 339 (6120), 671-675, doi:10.1126/science.1231530. Discussion: By applying Equation (1) to the esti- [12] Carrier, W. D. III et al. (1991) Lunar source book: mated effective permittivity of the lunar surface, we A user’s guide to the Moon, 475-594. can derive the bulk density of the lunar surface materi- als. The derived bulk density is 1.2 - 1.8 g cm–3 in the highlands, 1.8 - 2.3 g cm–3 in the maria, and approxi- mately 2.6 g cm–3 in the high-permittivity areas in Oce- anus Procellarum and Mare Imbrium. Some previous studies have reported the bulk densities of lunar surface materials. The average density of the Moon is 3.34 g cm–3 [10]. We can expect that the bulk density of the surface material is lower than the average density. Based on gravity-field observations performed by the GRAIL satellites, the bulk density up to a depth of sev- eral km in the highlands was estimated to be 2.55 g cm– 3 [11]. Based on measurements of Apollo drill-core samples, the bulk density to a depth of 3 m was esti- mated to be 1.3 - 1.9 g cm–3 [12]. The reason why the bulk densities estimated in the previous studies are different is probably because the bulk density of the soil and rocks increases with depth, and the effective Figure 1: Roughness parameter H (Hurst exponent) depth range for estimation of the bulk density depends map in the nearside of the Moon derived from on the methods. SELENE TC/DTM. The areas of high permittivity and the high bulk density in the eastern part of Oceanus Procellarum and western part of Mare Imbrium coincide with young lava flow units in Procellarum KREEP Terrain (PKT) region. We can consider two possible reasons: (i) The regolith layer is thinner than other mare regions due to short exposure to the meteorite impacts. (ii) The bulk density is higher than other mare regions due to high abundance of the ilmenite. Acknowledgements: The authors wish to express their sincere thanks to all of the SELENE (Kaguya) team member. This work was partially supported by a JSPS KAKENHI grant (number 25420402). References: [1] Fa, W. and Wieczorek, M. A. (2012), , 218, 771-787, doi: 10.1016/j.icarus.2012.01.010. [2] Ono, T. et al. (2010) SSR, 154, 145-192, doi:10.1007/s11214-010-9673-8.

[3] Haruyama, J. et al. (2008) EPS, 60, 243-255. [4] Figure 2: Effective permittivity map in the nearside of Ishiyama, K. et al. (2013), JGRE, 118, 1453-1467, the Moon derived from the observed and calculated doi:10.1002/ jgre.20102. [5] Ohtake, M. et al. (2008), off-nadir surface echoes based on SELENE/LRS data EPS, 60, 257–264. [6] Franceschetti, G. et al. (1999) and SELENE TC/DTM. IEEE Trans. Antennas Propagat., 47, 9, 1405-1415. [7] Shepard, M. K., and Campbell, G. A. (1999) Icarus, 141, 156-171. [8] Bruzzone, L. et al. (2011) Proc. IEEE, 99, 5, 837-857, doi: 10.1109/JPROC.2011.2108990. [9] Rosenburg, M. A. et al. (2011) JGRE, 116, E02001, doi:10.1029/ 2010JE003716. [10] Vaniman, D. (1991), Lunar source