Comparison of carbon, nitrogen and water budgets on Venus and the Earth

Carbon, nitrogen and water abundances at the surfaces of Earth and Venus ar e compared in order to investigate the distributions of these elements betw een fluid and solid envelopes in both planets. The atmosphere of Venus cont ains about twice more carbon (1.25 x 10(20) kg of C) and nitrogen (4.8 x 10 (18) kg of N) than the atmosphere, hydrosphere and sediments of the Earth ( 5.4 x 10(19) kg of C and 3 x 10(18) kg of N). After scaling to the planetar y masses, surface excesses of 1 x 10(20) kg of C and 2.7 x 10(18) kg of N a re calculated for Venus relatively to the Earth. The simplest proposed expl anation is the storage of 25 ppm of C and 0.7 ppm of N in the Earth mantle due to lithospheric subduction, a mechanism not operating on Venus. Followi ng the same line of arguments, the Earth has an excess surface water of abo ut 1.2X10(21) kg of H2O, when compared to Venus. Under the hypothesis of a primordial sizable water mass on Venus, thermodynamic modeling of mineral s tabilities reveals that hydrous minerals are stable at the P-T conditions o f both the surface and the deep crust of Venus, and thus could be the carri ers of the missing Venusian water. A combination of crustal hydration and h ydrogen escape processes may explain the present-day low amount and high de uterium/hydrogen (D/H) ratio (2.5 X 10(-2)) of water in the Venusian atmosp here relatively to the Earth's hydrosphere (1.55X10(-4)). Alternatively, if the high D/H ratio of Venus only resulted from hydrogen escape, the reserv oir of remaining oxygen must have been involved in the oxidation of a rock layer of about 50 km in depth. (C) 2000 Elsevier Science B.V. All rights re served.