Spatially-resolved millimeter-wavelength maps of Neptune

We present maps of Neptune in and near the CO (2-1) rotation line at 230.538 GHz. These data, taken with the Combined Array for Research in Millimeter-wave Astronomy (CARMA) represent the first published spatially-resolved maps in the millimeter. At large (∼5 GHz) offsets from the CO line center, the majority of the emission originates from depths of 1.1-4.7 bar. We observe a latitudinal gradient in the brightness temperature at these frequencies, increasing by 2-3 K from 40°N to the south pole. This corresponds to a decrease in the gas opacity of about 30% near the south pole at altitudes below 1 bar, or a decrease of order a factor of 50 in the gas opacity at pressures greater than 4 bar. We look at three potential causes of the observed gradient: variations in the tropospheric methane abundance, variations in the H2S abundance, and deviations from equilibrium in the ortho/para ratio of hydrogen. At smaller offsets (0-213 MHz) from the center of the CO line, lower atmospheric pressures are probed, with contributions from mbar levels down to several bars. We find evidence of latitudinal variations at the 2-3% level in the CO line, which are consistent with the variations in zonal-mean temperature near the tropopause found by Conrath et al. (Conrath, B.J., Gierasch, P.J., Ustinov, E.A. [1998]. Icarus 135, 501-517) and Orton et al. (Orton, G.S., Encrenaz, T., Leyrat, C., Puetter, R., Friedson, A.J. [2007]. Astron. Astrophys. 473, L5-L8).