Primitive Terrestrial Atmospheres (Hydrogen Escape, Photochemistry, Tides, Precambrian, Hydrocyanic Acid).

Abstract

A 23.3 year periodicity preserved in a 2500 million year old b and ed iron-formation is interpreted as reflecting the climatic influence of the lunar nodal tide, the signature of which has been detected in the modern climate. The lunar distance is deduced to have been 52 Earth radii. The influence of the lunar nodal tide is also detected in varves dating to 680 million years B.P. The implied history of Precambrian tidal friction is in excellent agreement with both more recent paleontological evidence and the long-term stability of the lunar orbit. The solar semidiurnal thermal tide was resonant with the natural period of the atmosphere when the day was (TURN)21.3 hours. This took place at the end of the Precambrian. The resonant atmospheric tide would have been large enough (.01 bar at the surface) to have influenced the weather. In contrast to lunar oceanic tides, the gravitational torque on the thermal tide accelerates the Earth's rotation rate; near resonance the opposing torques were comparable, so that the day may have been stabilized near 21.3 hours for much of the Precambrian. A sustained resonance does not conflict with the available evidence. Methane photochemistry in the primitive terrestrial atmosphere is studied using a detailed numerical model. Methane is oxidized cleanly and efficiently provided CO(,2) is more abundant than CH(,4). If CH(,4) and CO(,2) abundances are comparable, a large fraction of the methane present is polymerized, forming alkanes in the troposphere and polyacetylenes and nitriles in the upper atmosphere. Production of HCN from CH(,4) and N(,2) in the anaerobic atmosphere and its subsequent removal in rainwater could have been efficient; net production varying from .01% to 10% of the methane consumed. In the absence of a magnetic field, high ancient solar EUV and X-ray fluxes would have permitted an ocean of hydrogen to escape as a transsonic wind from a primordial accretionary greenhouse atmosphere in as little as 25 million years. The terrestrial magnetic field would have been strong enough to have prevented a freely flowing wind, reducing escape by one or two orders of magnitude with respect to an otherwise identical Venus.