First results of Herschel-PACS observations of Neptune

We report on the initial analysis of a Herschel-PACS full range spectrum of Neptune, covering the 51–220 μm range with a mean resolving power of ~3000, and complemented by a dedicated observation of CH_4 at 120 μm. Numerous spectral features due to HD (R(0) and R(1)), H_(2)O, CH_4, and CO are present, but so far no new species have been found. Our results indicate that (i) Neptune's mean thermal profile is warmer by ~3 K than inferred from the Voyager radio-occultation; (ii) the D/H mixing ratio is (4.5 ± 1) × 10^(-5), confirming the enrichment of Neptune in deuterium over the protosolar value (~2.1 × 10^(-5)); (iii) the CH_4 mixing ratio in the mid stratosphere is (1.5 ± 0.2) × 10^(-3), and CH_4 appears to decrease in the lower stratosphere at a rate consistent with local saturation, in agreement with the scenario of CH_4 stratospheric injection from Neptune's warm south polar region; (iv) the H_(2)O stratospheric column is (2.1 ± 0.5) × 10^(14) cm^(-2) but its vertical distribution is still to be determined, so the H_(2)O external flux remains uncertain by over an order of magnitude; and (v) the CO stratospheric abundance is about twice the tropospheric value, confirming the dual origin of CO suspected from ground-based millimeter/submillimeter observations.