THE CASE FOR TIDAL CAPTURE OF THE EARTH'S MOON: PARADOX AFTER PARADOX

The origin of the Moon remains an unsolved problem. Although work on the ruling paradigm has resulted in impressive simulations (e.g., Canup, 2008, Icarus, 196, p. 518), GIANT IMPACT MODELS (GIMs) are a disappointment when it comes to explaining many of the major features of the Earth and Moon and appear to be incompatible with the COOL EARLY EARTH MODEL. In contrast, GRAVITATIONAL CAPTURE MODELS (GCMs) are, at present, very unpopular in the planetary science community but the version of Malcuit et al. (1992, Proc. Vol., 3^rd Archean Symp., p. 223) is very compatible with the COOL EARLY EARTH MODEL and has explanations for many features of the Earth and Moon.

Two major questions are: (1) Why are GCMs so unpopular? (2) CAN SOME VERSION OF A GCM BE MADE MORE ACCEPTABLE? The approach taken here is to examine some of the PARADOXES associated with GCMs. PARADOX # 1: The encountering body must absorb nearly all of the energy for its own capture. (All previous attempts at capture used the Earth as the major dissipater of energy without success.) PARADOX # 2: Larger planetoids are more capturable than smaller planetoids. (Since tidal capture depends on the deformability of the planetoid to store and subsequently dissipate energy, the smaller bodies simply do not have the physical properties to do it.) PARADOX # 3: “Cool” planetoids are more capturable than “hot” planetoids. [The critical factor here is “Q”, the specific dissipation factor, and 1/Q is the fraction of stored energy that is subsequently dissipated. Cold, rigid planetoids as well has hot, mushy planetoids have high Q values but “cool” planetoids with intermediate viscosity can have very low Q values (Ross and Schubert, 1986, JGR, 91, p. D447)]. All of these statements appear to be counterintuitive but after examination and discussion of the important issues, they all are TRUE. PARADOX # 4: A lunar-like planetoid can be captured from an Earth-like heliocentric orbit but the chemistry of the Moon is not consistent with that place of origin. [However, the chemistry of the lunar body is consistent with an origin near the Sun, as pointed out some time ago by Cameron (1972, Nature, v. 240, p. 299)]. This fourth paradox is still a major hurdle for any GCM but an X-WIND-LIKE model (Wood, 2004, GCA, 68, p. 4007) for the early history of the Sun and Solar System looks promising for generating lunar chemistry.