First results of<i>Herschel</i>-PACS observations of Neptune

Abstract

We report on the initial analysis of a Herschel-PACS full range spectrum of Neptune, covering the 51-220 m range with a mean resolving power of 3000, and complemented by a dedicated observation of CH 4 at 120 m. 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.