The Moon as a piece of Mercury

The giant impact on Earth proposed by some to generate the Moon could not have happened because the amount of carbonate rock and organic carbon present on Earth closely approximates the amount of CO₂ predicted for the Earth's early atmosphere by comparison with the present atmosphere of Venus. It is suggested that the Moon may have fissioned from Mercury more than 4.4 billion years ago and may have been captured by the Earth no later than 3.2 billion years ago.A model for the origin of the Earth-Moon system that has been gaining favor recently visualizes a Marssize body impacting Earth and excavating enough material from the Earth to form the Moon (Newsome &Taylor, 1989). This giant impact not only would have sterilized the planet (Maher &Stevenson, 1988), obliterating any form of life that might have existed at the time, but also would have blown off the Earth's atmosphere (Kaula, 1990). As the story goes, Venus suffered no giant impact and conserved its CO₂ atmosphere, whereas the loss of her CO₂ atmosphere cooled the Earth's surface allowing the oceans to rain out (Kaula, 1990). It is widely believed that the different histories in the early evolution of their atmospheres set markedly different courses for the subsequent evolution of the two planets. I agree with this conclusion but disagree with the cause for the difference.If the giant impact really happened, it must have taken place not only after the Earth's accumulation was practically completed but also after a substantial core had formed (to account for the lesser density of the Moon).Venus has approximately 89 kg/cm² of CO₂ for a total of 4.110²⁰kg. The Earth with a mass 1.226 greater than Venus, should have had 5.010²⁰ kg of CO₂. If entirely precipitated as carbonate, the Earth should now have 1.110²¹ kg of carbonate. A recent estimate (Hay, 1985) gives the total volume of carbonate rock on Earth as 276.110⁶ km³ which, for an average density of 2.83, corresponds to a mass of 7.810²⁰ kg. This figure and the ratio limestone/shale/sandstone of 15/74/11 (Garrels &Mackenzie, 1971) leads to the carbonate inventory shown in Table 1.