Low-Temperature Vapor Pressures of Ethylene and Propane

R. N. Nelson, J. E. Allen, Jr., B. C. Harris, Sr. (NASA/GSFC)

Mass spectra from the Galileo probe exhibit a cluster of peaks associated with two- and three-carbon hydrocarbons and two have been identified as ethylene and propane (Niemann et al. 1996). These molecules are important in the photochemical cycle of methane and are expected to be present in the atmospheres of the outer planets and Titan. To properly model related physical and chemical processes, e.g., cloud formation, it is important to have accurate thermodynamic data for these and other light hydrocarbons over the appropriate temperature and pressure range.

The apparatus developed to determine the vapor pressures of gases and gas mixtures (Allen, Nelson, and Harris 1996) has been modified to provide a greater temperature range. Using this new system we have measured the vapor pressure of propane which, besides its role as a constituent in outer-planet atmospheres, is also a good calibration source since its vapor pressure is well determined over the temperature range of interest. The vapor pressure of ethylene was then determined. Little data are available for ethylene below its triple point (104 K); however we were able to extend our measurements past that point into the solid-phase region. The results of our vapor pressure measurements for these gases are presented along with comparisons with existing data sets.

Allen, J.E., Jr., Nelson, R.N., Harris, B.C, Sr. 1996, B.A.A.S. tex2html_wrap_inline13 , 1157.

Niemann, H.B. et al. 1996, Science 272, tex2html_wrap_inline15 and P.R. Mahaffy (private communication).