Engineering topics

Donahue, Thomas M.

Publications and source records attributed to Donahue, Thomas M..

3 records

NASA NTRS1993

Venus methane and water

Data collected by the Pioneer Venus Large Probe Neutral Mass Spectrometer are presented and discussed. Results indicate the presence of a large amount of methane in the Venus atmosphere from 60 km to the surface. Deuterium transfer from atmospheric HDO to poorly deuterated methane may account for the puzzling apparent gradient in the water vapor mixing ration below 10 km. Deuterium transfer within the mass spectrometer may cause reduction in the apparent ratio of HDO to H2O. Accounting for the deuterium atoms leads to a revised water vapor mixing ratio of 28 ppm. Arguments against the methane detected being purely atmospheric are overwhelming. The methane may have been generated by a reaction between a highly deuterated atmospheric constituent and a poorly deuterated instrumental contaminant.

Donahue, Thomas M.↗

NASA NTRS1992

Solar cycle variations in H(+) and D(+) densities in the Venus ionosphere - Implications for escape

The hydrogen ion concentrations recently observed on Venus, near solar minimum, by the Ion Mass Spectrometer on the Pioneer Venus Orbiter in the anti-solar sector of the ionosphere are more than an order of magnitude less than those previously observed at solar maximum. This strong solar cycle variation has a profound effect on the escape of hydrogen (and deuterium) from Venus; almost all escape occurs during solar maximum. After adjustment for solar cycle variation, a planet-averaged hydrogen escape flux of 0.6-1.4 x 10 exp 7/sq cm per sec is obtained along with a large deuterium fractionation factor of 0.1-0.14. These results suggest at least two plausible scenarios for the evolution of water on Venus: (1) water vapor on Venus may be approaching a steady state if the escape flux is balanced by endogenous sources of water or (2) the present day D/H ratio of 0.024 could be established by Rayleigh fractionation of an early low D/H water reservoir if the escape flux was sufficiently large in earlier times. An early water endowment at least 340 times today's abundance would be needed.

Donahue, Thomas M.↗

NASA NTRS1991

Planetary Sciences: American and Soviet Research

Papers presented at the US-USSR Workshop on Planetary Sciences are compiled. The purpose of the workshop was to examine the current state of theoretical understanding of how the planets were formed and how they evolved to their present state. The workshop assessed the types of observations and experiments that are needed to advance understanding of the formation and evolution of the solar system based on the current theoretical framework.

Donahue, Thomas M.↗