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R. N. Nelson, J. E. Allen, Jr. (NASA Goddard)
The atmospheres of Saturn and Titan contain trace, but chemically important, abundances of small hydrocarbon molecules in the gas phase. These molecules may also be present as constituents in atmospheric clouds or hazes, and possibly as surface layers in the case of Titan. Modeling the photochemistry and composition of these planetary atmospheres is directly dependent on accurate values for the thermodynamic parameters of their hydrocarbon constituents and this in turn requires accurately measured laboratory data. In order to reduce the need for extrapolation, a process that can introduce significant errors, it is important to acquire these data over as much of the pertinent temperature and pressure ranges as possible. Besides vapor pressures, study of the behavior of solid-vapor equilibrium for small atmospheric particles, and especially for surface layers, requires information about the solid-phase heats of sublimation. We previously measured the vapor pressures of ethane (C2H6) from 73 to 85 K and of propane (C3H8) from 88 to 240 K. We have extended the range of the vapor pressure measurements for C2H6 to include the triple point at 90.3 K and, as presented here, derived the heats of sublimation for these two molecules over these temperature ranges.
Support by NASA's Planetary Atmospheres Program is gratefully acknowledged.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.