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 CH4at 120 μm. Numerous spectral features due to HD (R(0) and R(1)), H2O, CH4, 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 Voyagerradio-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 CH4mixing ratio in the mid stratosphere is (1.5 ±0.2) × 10-3, and CH4appears to decrease in the lower stratosphere at a rate consistent with local saturation, in agreement with the scenario of CH4stratospheric injection from Neptune's warm south polar region; (iv) the H2O stratospheric column is (2.1 ±0.5) × 1014cm-2but its vertical distribution is still to be determined, so the H2O 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.