Reconstruction of Paleoselenoid using Surface Shapes of Mare Basalts and Flow Directions of Sinuous Rilles

It has been believed that the Moon has kept synchronous rotation throughout the history of the Earth-Moon system. However, the current selenoid (geoid of the Moon, an equipotential surface) has an imbalance in its degree-2 coefficients (Garrick-Bethell et al., 2006). For a synchronously rotating satellite, the ratio of the centrifugal potential to the tidal potential is 1:3, and C20 (=-J2) should be 10/3 of C22 under hydrostatic equilibrium. However, the ratio J2/C22 observed by Kaguya is ~9.1 (Namiki et al. 2009), i.e. J2 is too large relative to C22. In order to study its origin, we try to recover ancient selenoid using the surface shape of lunar maria. We use the results of laser altimeter (LALT) (Araki, et al., 2009) and the terrain camera (TC) of the Japanese lunar explorer Kaguya in order to reconstruct the ancient degree-2 selenoid. Because basaltic lava of the Moon has a relatively low viscosity (Murase and McBirney, 1970), its surface might preserve the fossil selenoid at the time they solidified. In order to evaluate how well the mare surfaces reflect past selenoid, we compare them with present selenoid calculated from the lunar gravity model SGM100h (Matsumoto et al., 2010). Excess masses of the mantle plug and mare basalt make the equipotential surface there convex upward. If the basaltic lava surface preserves the ancient degree-2 selenoid, it should also retain such local bulge. In this study, we compared the topography of five mascon basins; i.e. Imbrium, Serenitatis, Humorum, Crisium and Nectaris with the current selenoid, and found that the three basins Imbrium and Serenitatis, Humorum to retain local bulge consistent with current selenoid. In addition to localized curvatures, these mascon basins were found to have overall tilt relative to the reference surface of the Moon possibly related to ancient degree-2 selenoid. The estimated tilt of the Mare Humorum showed direction of the ideal degree-2 shape (J2 : C22 = 10:3), but the other two did not. In addition to northward tilt due to equatorial bulge, Mare Imbrium showed significant westward tilt although current tidal potential requires eastward tilt. In order to get supporting evidence, we are analyzing the TC topographic data and flow directions of sinuous rille. Sinuous rille is a channel or valley considered to be formed by thermal erosion of basaltic lava flow. R. Nakamura (pers. comm.) found an intriguing fact in one of those, Rimae Plato (northeast of Imbrium), that the lava flew upward along this rille relative to the current selenoid. This might suggest certain fossilized differences between the present and ancient degree-2 selenoid. Lots of sinuous rilles exist around the Mare Imbrium, and we are currently performing systematic investigation of their flow directions.