New Views of the Moon 2 – Asia 2018 (LPI Contrib. No. 2070) 6023.pdf

IS IT POSSIBLE TO OBSERVE FREE CORE NUTATION OF THE MOON ?. H. Hanada1, M. Ooe2, A. Gusev3 and N. Petrova3, 1National Astronomical Observatory (2-12 Hoshigaoka, Mizusawa, Oshu, Iwate 023-0861, Japan; [email protected]), 2Oshu Space and Astronautical Museum ([email protected]), 3Kazann Federal University ([email protected]; [email protected])

Introduction: The Earth has a fluid core and it can (200×365)=0.03660099-0.00001370=0.03658729 ( the rotate around an axis independently from that of the 27.331 days ) for the FCN period of 20 decades. mantle. If the axis of rotation of the fluid core deviates Fluid Core Resonance: Because there are a lot of from that of the mantle for some reason, two different components of lunar diurnal tides around the 27.3 days, rotations around different axes begin. This phenome- there is possibility that the amplitudes are magnified by non is called the Free Core Nutation (FCN). In case of the resonance of FCN. Not only the tidal variations but the Earth, the period of FCN is about 460 sidereal days, the forced physical librations which are caused by the and when seeing on the rotating coordinate fixed to the same forces must be affected by the resonance. Actual- Earth, the period becomes 2π/Ω(1-1/460), which is ly there are some evidences of resonance in the result very close to 1 sidereal day. There are a lot of compo- of analyses of Lunar ephemeris DE421 expanded to nents of diurnal Earth tides near the period of 1 sidere- over 1000 years [8]. There are, on the other hand, free al day, and the amplitudes of these components are modes such as the precession (about 24 year period), magnified due to the resonant, which is called fluid the Chandler like polar-motion (about 75 years), the core resonance. free librations (about 100 years for latitudinal mode Lunar Core: Whether the Moon has a fluid core and 2.9 years for longitudinal mode) as well as FCN or not is still unclear although it is an important issue [6], and the resonance effects must be complicated. which is related to existence or non-existence of paleo Concluding Remarks: We propose to observe var- magnetic field and thermal history of the Moon. Wil- iation of deformation and gravity on the lunar surface liams et al. [1] suggested energy dissipation inside of in order to detect the effect of resonance of FCN. the Moon from the analysis of Lunar Laser Ranging References: [1] Williams, J. G. et al. (2001) JGR, data, and Harada et al. [2] suggested partial melting 106, 933-968. [2] Harada Y. et al. (2014) Nature Geo, inside of the Moon from the theoretical estimation of 7, 1-4. [3] Garcia, R.F. et al. (2011) Phys. Earth Planet. tidal heating. The results of re-processing the Apollo Int., 188, 96–113. [4] Weber, R. et al. (2011) Science, seismic data also suggest the presence of a liquid core 331, 309–312. [5] Arkani-Hamed, J and D. Boutin and give its size [3,4]. Arkani-Hamed & Boutin [5], on (2017) JGR, 122, 1195–1216. [6] Gusev, A. et al. the other hand, suggested the existence of core dynamo (2015) Rotation, Physical Libration, Interior Structure reversal from analyses of magnetic data. However of the Active and Multi-Layer Moon, pp1-323. [7] there has been no direct observation which shows the Barkin,Y. et al. (2014) Solar Sys. Res. 48, 403–419. existence of the fluid core. [8] Rambaux, N. and J. G. Williams, (2011) Cel. Mech. Free Core Nutation: The period of FCN is esti- Dyn. Astr. 109, 85–100. mated to be from several to 20 decades according to lunar model and the amplitude is less than 16 arc sec- onds [6]. Astronomical observations of FCN might be very difficult because its period is too long. Barkin et al. [7] made a comparison of the semi-empirical series of ephemeris [8] with the analytical theory of librations of the Moon having a liquid core, and estimated the FCN-period be 206 years, and obtained the amplitude and the phase of FCN for the first time. According to Barkin's analyses the amplitude is equal to 0.0395 arc seconds. Observations of deformation or gravity varia- tion affected by resonance of FCN appear on the lunar surface, on the other hand, might be more practical like the Earth. Supposing the mean angular velocity of the

Moon be Ω☾, angular velocity of FCN relative to iner-

tia space be n☾, then FCN is observed on the Moon as

the angular velocity of Ω☾-n☾. It becomes 0.0366-1 /