Lunar Science and Lunar Laser Ranging A white paper submitted to the Panel on Inner Planets – Mercury, Venus, and the Moon of The National Academies Planetary Science Decadal Survey

Lunar Laser Ranging studies the Moon’s internal structure and properties by tracking the variations in the orientation and tidal distortion of the Moon as a function of time. Future missions to the Moon’s surface should include new laser ranging instrumentation capable of improved range accuracy. A Planetary Science Decadal Survey White Paper: “Lunar Science and Lunar Laser Ranging” Page 1 of 7 Fig 1 (a) Lunar laser telescope at McDonald Observatory. (b) Apollo 14 retroreflector array on Moon. 1 Introduction:
Lunar Laser Ranging and Science Range and range rate tracking of spacecraft throughout the solar system has provided a wealth of science information on gravity fields, tides, planetary orientation and spacecraft locations. For distant targets this tracking has been done by radio (Asmar et al., 2009), but for the Moon’s surface the science technique has been laser ranging to corner cube retroreflector arrays. Radio has the advantage of a strong signal from an active transponder, while the passive laser ranging technique has the advantage of excellent accuracy and longer data spans. Lunar Laser Ranging (LLR) data are accurate ranges to retroreflector arrays on the Moon from several stations on the Earth. LLR is designed to obtain scientific information about the Moon, the Earth, the lunar orbit, and connected effects such as the nature of gravity (Dickey et al., 1994). Merkowitz et al. (2009) discuss advanced LLR for precision tests of relativistic gravity. Here we shall concentrate on the lunar science that comes from monitoring variations of lunar orientation and tides. Figure 1 shows a ranging observatory on the Earth and a retroreflector corner cube array on the Moon. Figure 2 shows the locations of the retroreflector arrays on the Moon. The Apollo 11, 14, 15 and Lunokhod 2 arrays are ranged operationally. The Lunokhod 1 array is lost. A broader distribution of LLR sites on the Moon would improve the sensitivity to science parameters determined by LLR. New LLR devices would be designed to reduce the scatter of the individual photons used to make a range normal point thereby reducing the number of photons and the duration necessary to make a very accurate normal point. LLR goals are that new retroreflectors be placed on the Moon and that range data be collected and analyzed. 2 Lunar
Science LLR-determined lunar science depends on monitoring time-varying 3-axis lunar orientation along with solid body tides. The lunar orientation, or three-dimensional rotation, is called physical librations. A recent review of lunar science is in Joliff et al (2006) while recent summaries of LLR lunar science are in Williams et al. (2006, 2009). A summary of important lunar science effects follows. Fluid Core Moment of Inertia: LLR is sensitive to the fluid core moment of inertia, which depends on core density and radius. This is a new LLR lunar science result for the core. The solution for the ratio of fluid moment to total moment gives Cf/C = (12±4)x10, where the subscript f indicates the fluid core (Williams et al., 2009). For a uniform liquid iron core without an inner core this value would correspond to a radius of 390±30 km. Lower fluid densities or presence of A Planetary Science Decadal Survey White Paper: “Lunar Science and Lunar Laser Ranging” Page 2 of 7 Fig 2 Retroreflector arrays on the Moon. an inner core would give larger outer radii for the fluid. Weakly determined at present, an accurate determination of core moment depends on a long time span of high accuracy range data. There is very little information on the core and it is very important to improve this determination. Whole Moon Moment of Inertia: The whole Moon moments of inertia A10 m) which implies active or geologically recent stimulation (Newhall and Williams, 1997; Chapront et al., 1999; Rambaux and Williams, 2009). The 2.9 yr longitude mode with an 11 m amplitude is stimulated, at least in part, by resonance passage (Eckhardt, 1993). The wobble mode, analogous to the Earth’s Chandler wobble, is a large elliptical (28x69 m) 74.6 yr motion of the pole direction. If wobble is stimulated by eddies at the CMB as suggested by Yoder (1981), then ongoing activity might be revealed by future LLR measurements as irregularities in the path of polar wobble. The third mantle mode, a free precession in space, and the liquid core free core nutation are small (<1 m). The former may be detected, but it appears to be sensitive to the interior model. Site Positions: The Moon-centered locations of four retroreflectors are known with submeter accuracy (Williams et al., 1996, 2008). Positions for existing and new LLR sites can be used as control points for lunar cartographic networks, as was done by Davies et al. (1987, 1994, 2000). The four site radii are a valuable check on altimetry from orbit (Fok et al., 2009). Tidal Acceleration and Orbit Evolution: LLR is very sensitive to tidal acceleration of the lunar orbit. Tides on Earth dominate the energy and angular momentum transfer to the orbit and the Moon’s evolution outward. Tidal effects on the Moon are separable from Earth tide effects in the LLR solutions (Chapront et al., 2002; Williams et al., 2009). The total tidal acceleration in orbital mean longitude from Earth and Moon tides is –25.85 arcsec/century, which corresponds to a 3.81 cm/yr recession of the Moon (Williams et al., 2009). Eccentricity rate is also detected. Evolving the lunar orbit backward in time is an important and surprisingly difficult goal. LLR provides numerical values for two sources of dissipation on Earth and two for the Moon. Synergies: LLR is one of several instruments suitable for geophysical exploration of the lunar interior (Neal et al., 2009). Some synergies between the geophysical techniques follow. Seismology: The Apollo seismic network determined the structure of the Moon down to the middle mantle and we anticipate that a broadly distributed future network could identify rays through the lower mantle and core regions. The LLR-determined fluid core moment of inertia could be compared with seismic travel times through the fluid, which depend on radii, fluid density and bulk modulus. Does the low LLR tidal Q come from a partial melt in the lower mantle? A seismic determination of lower mantle seismic dissipation could answer that question. An inner core seismic detection would