Origin of the Moon from the Earth: Some new mechanisms and comparisons

Abstract

Several major criticisms commonly claimed to invalidate theories of lunar origin by fission from the earth are shared by other lunar theories or are answerable by mechanisms associated with fission. Arguments based on the present rate of retreat, which require the moon to be a newcomer to the earth about 1.8 b.y. ago, are in direct conflict with the geologic record; MacDonald's calculations of the amount of tidal energy to be dissipated by lunar capture would require, in middle Precambrian time, surface temperatures of earth approaching those of the sun. One interpretation of his method of many-moon capture to extend the time scale fails by at least two orders of magnitude. The simplest conclusion is that something is wrong with present rate-of-retreat determinations or projections. Discrepancies of present angular momentum of the earth-moon system versus pre-moon values for earth are almost as great for the retrograde capture theory as for the fission theory. The angular momentum at fission can be halved from earlier estimates by using a gravity-stratified ellipsoid plus Chandrasekhar's calculation that all Jacobian ellipsoids are in unstable rotation. The entire remaining angular momentum discrepancy can be accounted for by escape of an incandescent silicate atmosphere volatilized by the huge tidal frictions immediately following fission. By the use of a reasonable set of assumptions, escape of 4% of the mass of the earth using ¾ of the available tidal energy can account for the entire angular momentum discrepancy. Goldreich's calculation of the history of the lunar orbit, with which he argues strongly against fission and against all other lunar theories as well, omits one significant possibility, that earth's equator had been tilted about 10° to the ecliptic prior to the time of fission. If this is true, the inclination of earth's equator and of the lunar orbit to the ecliptic fit nicely into Goldreich's curves. It is concluded that there are fewer outstanding criticisms of the hypothesis of lunar origin by fission than of other hypotheses. The average lunar density and the results of the Surveyor chemical analyses lead us to argue that fission from the earth triggered by the earth core formation is the most probable origin of the moon, considering the data available now.