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arXiv · 1408.6461

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

M. A. Cordiner·C. A. Nixon·N. A. Teanby·J. Serigano·S. B. Charnley·S. N. Milam·M. J. Mumma·P. G. J. Irwin·D. C. Lis·G. Villanueva·L. Paganini·Y. -J. Kuan·A. J. Remijan
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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 ($z\approx70$-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 $z\gtrsim300$ 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 $1.9\times10^{13}$ cm$^{-2}$ and $2.3\times10^{14}$ cm$^{-2}$, respectively.

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M. A. Cordiner, C. A. Nixon, N. A. Teanby, J. Serigano, S. B. Charnley, S. N. Milam, M. J. Mumma, P. G. J. Irwin, D. C. Lis, G. Villanueva, L. Paganini, Y. -J. Kuan, A. J. Remijan. 2014-09-30. ALMA measurements of the HNC and HC$_3$N distributions in Titan's atmosphere. https://doi.org/10.1088/2041-8205%2F795%2F2%2Fl30

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