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Calculate surface gravity and atmospheric pressure on half-diameter Earth
Ample evidence is now available (see refs ,  in attached file) that 200 million years ago, the Earth had approximately half its current diameter. Calculations are wanted for gravity and atmospheric pressure at the surface of this half-diameter Earth. Assume that the mass of the atmosphere and the mass of the Earth then was the same as now. Demi-Earth surface gravity will be higher because the surface-centre distance is halved, and surface atmospheric pressure will be higher because the same amount of atmosphere overlaid one-quarter of the area. (ZBL#159). More details are given in the attached file ZBL159X.pdf.
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The zomb response as in the attached file shows that on a half-diameter Earth with the present mass and atmosphere, the surface gravity and the surface atmospheric pressure would both be 4 times the current values. The depth of the atmosphere would be one-quarter of the present value, based on the "atmospheric scale height", which is the height which you must go for the atmosphere pressure to decrease by a factor of "e"=2.74.
These values have some perhaps surprising, if logical, implications. With a denser atmosphere, movement goes a little towards what we regard as swimming, rather than flying, with 'upthrust' of four times the current norm. Hence the existence of pterosaurs in the past with wingspans much greater than the current atmospheric pressure would support. With a four times higher escape velocity, 'Demi-Earth' would be expected to lose atmosphere to space at a much reduced rate compared to the present.
Surface-dwelling plants and animals would thus have been living at a pressure equivalent to that found today under 30 metres of water. We know of current life which exists in the deep oceans at much more than a hundred times this pressure.
(Calculations by dragozzine).
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