Spitzer IRS Analysis of Neptune

Recent (2005) Spitzer Infrared Spectrometer (IRS) data of Neptune between 5 and 20 um contain a wealth of information about the chemical composition and temperature structure of its cold atmospheres. Emission features in its spectrum arise from many hydrocarbons, including ethane, acetylene and methane, and they appear to be superimposed on top of a collision-induced H2 continuum. As the derivation of all other parameters depends on the assumed temperature profile, special efforts were taken to ensure that the stratospheric profile between 1 bar and 0.3 mbar matched the H2 continuum and the H2 S(1) quadrupole feature at 17 um. Additionally we matched the methane v4 feature at 7.7 um in order to constrain the stratospheric temperature profile above the 0.3-mbar level as well as the methane stratospheric volume mixing ratios (VMRs). After the determination of the temperature profile and methane VMRs, the VMRs for several species were then determined through fitting their corresponding features in the spectrum, initially by scaling existing photochemical models (see Mahmud et al., this conference). These species include methylacetylene, diacetylene, benzene, acetylene, ethane, methyl radical, ethylene and carbon dioxide which are all derived from methane photochemistry.

I would like to acknowledge the NASA USRP program for supporting this work.