Letter to the Editor

First detection of hydrogen isocyanide (HNC) in Titan’s atmosphere

¹ LESIA – Observatoire de Paris, CNRS, Université Paris 06, Université Paris–Diderot, 5 place Jules Janssen, 92195 Meudon, France
e-mail: raphael.moreno@obspm.fr
² Instituto de Astrofísica de Andalucía (CSIC), Granada, Spain
³ Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany
⁴ Space Research Centre of Polish Academy of Sciences, Warsaw, Poland

Received: 30 September 2011
Accepted: 22 November 2011

Abstract

We report on the first identification of hydrogen isocyanide (HNC) in Titan’s atmosphere, from observations using the HIFI instrument on the Herschel^⋆ Space Observatory. An emission line from the HNC J = 6 → 5 rotational transition at 543.897 GHz was measured in Titan on June 14 and December 31, 2010. Radiative transfer modeling indicates that the bulk of HNC is located above 400 km, with a column density in the range (0.6−1.5) × 10¹³ cm^-2, but the observations cannot establish its vertical profile. In particular HNC could be restricted to the upper thermosphere (~1000 km), in which case its local abundance relative to HCN could be as high as  ~0.3. HNC is probably formed mostly at ionospheric levels (950–1150 km) from dissociative recombination of HCNH⁺ and possibly other heavier nitrile ions. Ionospheric loss of HNC occurs by protonation with XH⁺ ions. Additional formation (e.g. from N(⁴S) +³CH₂) and loss routes (e.g. from isomerization to HCN) in the neutral atmosphere remain to be investigated.