Research Note

New determination of the HCN profile in the stratosphere of Neptune from millimeter-wave spectroscopy

¹ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: rezac@mps.mpg.de
² Department of Astrophysical Sciences, Princeton University, Princeton NJ 08544, USA
³ Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac, France
⁴ CNRS, LAB, UMR 5804, 32270 Floirac, France

Received: 20 December 2013
Accepted: 31 January 2014

Abstract

Context. Periodic monitoring of the atmospheric composition is the cornerstone of planetary atmospheric science. It reveals temporal and/or spatial variations. Ground-based observations of rotational lines from the (sub-)millimeter wavelength range is a suitable method to obtain the mean HCN profile in Neptune’s startosphere.

Aims. We aimed at deriving new constraints on the disk-averaged HCN stratospheric profile and abundance. The 14-year gap between the last published observations and ours of HCN in Neptune can be used to constrain any possible time variation of this main nitrogen-bearing molecule at the probed altitudes. This temporal variation could additionally reveal, albeit indirectly, the dominant process responsible for the origin of the nitrogen compoundsin the stratosphere of Neptune.

Methods. Spectra of the HCN (J = 3–2) line at 265.886 GHz were obtained with the 1.3 mm receiver of the Submillimeter Telescope (SMT) at the Arizona Radio Observatory (ARO) using several backends simultaneously. The spectral resolution of the analyzed datasets was 1 MHz and 250 kHz, providing a signal-to-noise ratio of 20 and 11, respectively. Pre-processing of the spectra involved baseline removal and de-noising using the empirical mode decomposition technique. The spectra were then inverted using a line-by-line radiative transfer model to obtain the vertical profile of HCN between 2 mbar to 10 μbar and derive the column density.

Results. The retrieved mean stratospheric HCN mole fraction is (1.3 ± 0.6) × 10^-9 above 0.5 millibar, corresponding to a column density of 2.2 × 10¹⁴ molecules cm^-2. The data are consistent with a pronounced HCN decrease below the 0.6 mbar level, which agrees with previous findings.