ALMA measurements of the HNC and HC$_3$N distributions in Titan's atmosphere
| Autor: | Cordiner, M. A., Nixon, C. A., Teanby, N. A., Serigano, J., Charnley, S. B., Milam, S. N., Mumma, M. J., Irwin, P. G. J., Lis, D. C., Villanueva, G., Paganini, L., Kuan, Y. -J., Remijan, A. J. |
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| Rok vydání: | 2014 |
| Předmět: | |
| Druh dokumentu: | Working Paper |
| DOI: | 10.1088/2041-8205/795/2/L30 |
| Popis: | 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. Comment: Accepted for publication in ApJ Letters |
| Databáze: | arXiv |
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