The Titan <sup>14sup>N/<sup>15sup>N and <sup>12sup>C/<sup>13sup>C isotopic ratios in HCN from Cassini/CIRS
- 作者：S ; rine Vinatier ; Bruno Bé ; zard ; Conor A. Nixon
- 关键词：Titan ; Abundances ; atmosphere ; Infrared observations
- 刊名：Icarus
- 出版年：2007
- 期刊代码：90_00191035
- 类别：ph
- 出版时间：15 November 2007
- 卷：191
- 期：2
- 页码：712-721
- 文件大小：909 K
摘要
We report the detection of H<sup>13sup>CN and HC<sup>15sup>N in mid-infrared spectra recorded by the Composite Infrared Spectrometer (CIRS) aboard Cassini, along with the determination of the <sup>12sup>C/<sup>13sup>C and <sup>14sup>N/<sup>15sup>N isotopic ratios. We analyzed two sets of limb spectra recorded near 13–15° S (Tb flyby) and 83° N (T4 flyby) at 0.5 cm<sup>−1sup> resolution. The spectral range 1210–1310 cm<sup>−1sup> was used to retrieve the temperature profile in the range 145–490 km at 13° S and 165–300 km at 83° N. These two temperature profiles were then incorporated in the atmospheric model to retrieve the abundance profile of H<sup>12sup>C<sup>14sup>N, H<sup>13sup>CN and HC<sup>15sup>N from their bands at 713, 706 and 711 cm<sup>−1sup>, respectively. The HCN abundance profile was retrieved in the range 90–460 km at 15° S and 165–305 km at 83° N. There is no evidence for vertical variations of the isotopic ratios. Constraining the isotopic abundance profiles to be proportional to the HCN one, we find f="/science?_ob=MathURL&_method=retrieve&_udi=B6WGF-4P429DR-1&_mathId=mml5&_user=10&_cdi=6821&_rdoc=23&_acct=C000050221&_version=1&_userid=10&md5=97c6a847e23296e18bf9275dce1c56b6">
at 15° S, and f="/science?_ob=MathURL&_method=retrieve&_udi=B6WGF-4P429DR-1&_mathId=mml6&_user=10&_cdi=6821&_rdoc=23&_acct=C000050221&_version=1&_userid=10&md5=08db34edc0d8f55a2dc8dd57995d6e84">
at 83° N, two values that are statistically consistent. A combination of these results yields a <sup>12sup>C/<sup>13sup>C value equal to f="/science?_ob=MathURL&_method=retrieve&_udi=B6WGF-4P429DR-1&_mathId=mml7&_user=10&_cdi=6821&_rdoc=23&_acct=C000050221&_version=1&_userid=10&md5=d7bf0ef3260409b837578cba2c794a41" title="Click to view the MathML source">75±12. This global result, as well as the 15° S one, envelop the value in Titan's methane (f="/science?_ob=MathURL&_method=retrieve&_udi=B6WGF-4P429DR-1&_mathId=mml8&_user=10&_cdi=6821&_rdoc=23&_acct=C000050221&_version=1&_userid=10&md5=04c56f5776be2ef7064ebdaecd43b6af" title="Click to view the MathML source">82.3±1) [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779–784] measured at 10° S and is slightly lower than the terrestrial inorganic standard value (89). The <sup>14sup>N/<sup>15sup>N isotopic ratio is found equal to f="/science?_ob=MathURL&_method=retrieve&_udi=B6WGF-4P429DR-1&_mathId=mml9&_user=10&_cdi=6821&_rdoc=23&_acct=C000050221&_version=1&_userid=10&md5=d5ed7d534656b456f810ddd73e568fc7">
at 15° S and f="/science?_ob=MathURL&_method=retrieve&_udi=B6WGF-4P429DR-1&_mathId=mml10&_user=10&_cdi=6821&_rdoc=23&_acct=C000050221&_version=1&_userid=10&md5=f3f343583540fcefb7de8b213a7b2523">
at 83° N. Combining the two values yields <sup>14sup>N/<sup>15sup>N = 56 ± 8, which corresponds to an enrichment in <sup>15sup>N of about 4.9 compared with the terrestrial ratio. These results agree with the values obtained from previous ground-based millimeter observations [Hidayat, T., Marten, A., Bézard, B., Gautier, D., Owen, T., Matthews, H.E., Paubert, G., 1997. Icarus 126, 170–182; Marten, A., Hidayat, T., Biraud, Y., Moreno, R., 2002. Icarus 158, 532–544]. The <sup>15sup>N/<sup>14sup>N ratio found in HCN is 
f" alt="not, vert, similar" border=0>3 times higher than in N<sub>2sub> [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779–784], which implies a large fractionation process in the HCN photochemistry.