ALMA measurements of the HNC and HC${}_3$N distributions in Titan's atmosphere

Abstract

We present spectrally and spatially-resolved maps of HNC and HC${}_3$N emission from Titan's atmosphere, obtained using the Atacama Large Millimeter/submillimeter Array (ALMA) on 2013 November 17. These maps show anisotropic spatial distributions for both molecules, with resolved emission peaks in Titan's northern and southern hemispheres. The HC${}_3$N maps indicate enhanced concentrations of this molecule over the poles, consistent with previous studies of Titan's photochemistry and atmospheric circulation. Differences between the spectrally-integrated flux distributions of HNC and HC${}_3$N show that these species are not co-spatial. The observed spectral line shapes are consistent with HNC being concentrated predominantly in the mesosphere and above (at altitudes $z\gtrsim 400$ km), whereas HC${}_3$N is abundant at a broader range of altitudes ($z\approx 70$-600 km). From spatial variations in the HC${}_3$N line profile, the locations of the HC${}_3$N emission peaks are shown to be variable as a function of altitude. The peaks in the integrated emission from HNC and the line core (upper-atmosphere) component of HC${}_3$N (at $z\gtrsim 300$ km) are found to be asymmetric with respect to Titan's polar axis, indicating that the mesosphere may be more longitudinally-variable than previously thought. The spatially-integrated HNC and HC${}_3$N spectra are modeled using the NEMESIS planetary atmosphere code and the resulting best-fitting disk-averaged vertical mixing ratio (VMR) profiles are found to be in reasonable agreement with previous measurements for these species. Vertical column densities of the best-fitting gradient models for HNC and HC${}_3$N are $1.9\times 10^{13}$ cm${}^{-2}$ and $2.3\times 10^{14}$ cm${}^{-2}$, respectively.

arXiv commentAccepted for publication in ApJ Letters

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