A TENTATIVE IDENTIFICATION OF HCN ICE ON TRITON

Spectra of Triton between 1.8 and 5.5 mu m, obtained in 2007 May and 2009 November, have been analyzed to determine the global surface composition. The spectra were acquired with the grism and the prism of the Infrared Camera on board AKARI with spectral resolutions of 135 and 22, respectively. The data from 4 to 5 mu m are shown in this Letter and compared to the spectra of N-2, CO, and CO2, i.e., all the known ices on this moon that have distinct bands in this previously unexplored wavelength range. We report the detection of a 4 sigma absorption band at 4.76 mu m (2101 cm(-1)), which we attribute tentatively to the presence of solid HCN. This is the sixth ice to be identified on Triton and an expected component of its surface because it is a precipitating photochemical product of Triton's thin N-2 and CH4 atmosphere. It is also formed directly by irradiation of mixtures of N-2 and CH4 ices. Here we consider only pure HCN, although it might be dissolved in N-2 on the surface of Triton because of the evaporation and recondensation of N-2 over its seasonal cycle. The AKARI spectrum of Triton also covers the wavelengths of the fundamental (1-0) band of beta-phase N-2 ice (4.296 mu m, 2328 cm(-1)), which has never been detected in an astronomical body before, and whose presence is consistent with the overtone (2-0) band previously reported. Fundamental bands of CO and CO2 ices are also present.