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[37.08] The Heat of Sublimation of Solid Ethylene and Acetylene

R. N. Nelson (NASA - GSFC / CUA), J. E. Allen, Jr. (NASA - GSFC)

The atmospheres of the outer planets and their satellites include trace, but chemically important, amounts of small hydrocarbon molecules. These molecules also appear in solid form in clouds within the atmospheres and as surface layers on the planet or on haze particles in the atmosphere. The modeling of the composition and photochemistry of these atmospheres depends critically on accurate values of the vapor pressure of these hydrocarbons, which in turn requires accurately measured data. It is important to acquire these data over as much of the temperature and pressure range of interest as possible to reduce the need to extrapolate since this can introduce major errors in vapor pressure. In addition to vapor pressures, study of the behavior of solid-vapor equilibrium for small particles, and especially of surface layers, requires information on the heat of sublimation of the solid phases. Ethylene (C₂H₄) and acetylene (C₂H₂) are present in the atmospheres of Saturn and Titan, so we previously measured the solid phase vapor pressure of the former from 69 to 97 K (Nelson, Allen, and Harris 1997) and of the latter from 84 to 124 K (Nelson and Allen 1998). We have now refined those data and used them to evaluate the heats of sublimation of these molecules over the same temperature ranges.

Nelson, R. N., Allen, J. E., Jr., and Harris, B. C., Jr., 1997, B.A.A.S. \textbf{29}, 1009

Nelson, R. N. and Allen, J. E., Jr., 1997, B.A.A.S. \textbf{30}, 1101

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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.