How does the Fission Theory plausibly motivate the lack of volatiles in the Moon and the Earth?
The Fission Theory was proposed by George Darwin in 1879 as a naturalistic explanation of the origin of the moon. The philosophical movement in science during the late 1800s was to find a naturalistic explanation for everything in science. George Darwin being greatly influenced by the work of his father in attempting to find a naturalistic explanation for all of life on earth attempted to find a naturalistic explanation for the formation of the moon.
Darwin's theory was that the rapidly spinning proto planet that was the earth threw out a large piece of the earth creating the pacific ocean and the moon. This theory explained the lack of volatile substances on the earth, as well as the origin of the pacific ocean and the moon. The volatile materials on the earth would have been thrown out into space at the same time and event that created the moon.
The fission theory is almost completely abandoned by scientists today. The exploration of the moon brought back rocks from the moon. The moon rocks were thought to be older than the rocks at the bottom of the Pacific Ocean. The predictions of the age of the Rocks by the Fission Theory turned out to be false. Also modern plate tectonics gives a better explanation of the origin of the Pacific Ocean.