Other
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008dps....40.4204m&link_type=abstract
American Astronomical Society, DPS meeting #40, #42.04; Bulletin of the American Astronomical Society, Vol. 40, p.473
Other
1
Scientific paper
Recent observations of Neptune with the Infrared Spectrometer (IRS) onboard the Spitzer spacecraft reveal newly detected trace species (CH3C2H, C4H2, C6H6) as well as confirm the detection of other hydrocarbon and oxygen species like C2H2, C2H4, C2H6, CH3, and CO2 (Meadows et al., 2008 Icarus, in press; Line et al., this conference). Because of the broad wavelength coverage of the Spitzer/IRS instrument, temperatures can also be constrained from the H2 quadrupole lines and H2 continuum, as well as from the ν4 band of CH4 at 7.7 microns. When temperatures are better constrained in infrared observations, the abundances of emitting species can be better determined. The new determinations of hydrocarbon and CO2 abundances from Spitzer/IRS imply greater abundances of photochemical products than previous measurements from the Voyager time period. We have developed a one-dimensional seasonally variable photochemical model to examine possible reasons for the observed increased abundances. Our results suggest that seasonal variations in hydrocarbon mixing ratios between the Spitzer observations (2005, Ls 270 deg) and the Voyager era (1989, Ls 235 deg) are alone not sufficient to explain the observations. Moreover, the hydrocarbon abundances in our model are relatively insensitive to reasonable changes in the assumed thermal structure, methane abundance, methane homopause level, or adopted chemistry. The observed differences may result from atmospheric dynamics that varies with latitude and/or time, with the Spitzer-era observations suggesting a more stagnant lower stratosphere (i.e., a lower eddy diffusion coefficient or a downwelling region) over a larger portion of the visible globe in 2005 than during the Voyager era. Model results and sensitivity studies will be presented, as will a discussion of the implications for the source of oxygen in Neptune's stratosphere and a comparison with seasonal models of Saturn.
This work was supported by NASA Planetary Atmospheres (NNX08AF05G) and LPI/USRA.
Liang Mao-Chang
Line Michael R.
Mahmud Ayesha
Moses Julianne I.
Orton Glenn S.
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