ALMA Measurements of the HNC and HC_3N Distributions in Titan's Atmosphere

Abstract

We present spectrally and spatially resolved maps of HNC and HC_3N emission from Titan's atmosphere, obtained using the Atacama Large Millimeter/submillimeter Array 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_3N 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_3N 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 ≳ 400 km), whereas HC_3N is abundant at a broader range of altitudes (z ≈ 70-600 km). From spatial variations in the HC_3N line profile, the locations of the HC_3N 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_3N (at z ≳ 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_3N spectra are modeled using the NEMESIS planetary atmosphere code and the resulting best-fitting disk-averaged vertical mixing ratio 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_3N are 1.9 × 10^(13) cm^(–2) and 2.3 × 10^(14) cm^(–2), respectively.

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