Detection of CO in Triton's atmosphere and the nature of surface-atmosphere interactions

Context. Triton possesses a thin atmosphere, primarily composed of nitrogen, sustained by the sublimation of surface ices. Aims. We aim at determining the composition of Triton's atmosphere to constrain the nature of surface-atmosphere interactions. Methods. We perform high-resolution spectroscopic observations in the 2.32-2.37 mu m range, using CRIRES at the VLT. Results. From this first spectroscopic detection of Triton's atmosphere in the infrared, we report (i) the first observation of gaseous methane since its discovery in the ultraviolet by Voyager in 1989; and (ii) the first ever detection of gaseous CO in the satellite. The CO atmospheric abundance is remarkably similar to its surface abundance, and appears to be controlled by a thin, CO-enriched, surface veneer resulting from seasonal transport and/or atmospheric escape. The CH4 partial pressure is several times higher than inferred by Voyager. This confirms that Triton's atmosphere is seasonally variable and is best interpreted by the warming of CH4-rich icy grains as Triton passed southern summer solstice in 2000. The presence of CO in Triton's atmosphere also affects its temperature, photochemistry, and ionospheric composition. An improved upper limit on CO in Pluto's atmosphere is also reported.