Mandt Kathleen E, Mousis Olivier, Lunine Jonathan, Gautier Daniel
Space Science and Engineering Division, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78228, USA.
Université de Franche-Comté, Institut UTINAM, CNRS/INSU, UMR 6213, Observatoire des Sciences de l'Univers de Besancon, France.
Astrophys J Lett. 2014 Jun 20;788(2). doi: 10.1088/2041-8205/788/2/L24. Epub 2014 May 14.
The origin of Titan's nitrogen-rich atmosphere is thought to be ammonia ice, but this has not yet been confirmed. Furthermore, it is uncertain whether the building blocks of Titan formed within the Saturnian subnebula or in the colder protosolar nebula (PSN). Recent measurements of the nitrogen isotope ratio in cometary ammonia, combined with evolutionary constraints on the nitrogen isotopes in Titan's atmosphere provide firm evidence that the nitrogen in Titan's atmosphere must have originated as ammonia ice formed in the PSN under conditions similar to that of cometary formation. This result has important implications for the projected ratio in cometary methane, nitrogen isotopic fractionation in the PSN and the source of nitrogen for Earth's atmosphere.
土卫六富含氮的大气层的起源被认为是氨冰，但这尚未得到证实。此外，土卫六的构成物质是在土星次星云内形成的，还是在较冷的原太阳星云（PSN）中形成的，目前尚不确定。最近对彗星氨中氮同位素比率的测量，结合土卫六大气中氮同位素的演化限制，提供了确凿的证据，表明土卫六大气中的氮必定起源于在类似于彗星形成条件的原太阳星云下形成的氨冰。这一结果对于预测彗星甲烷中的比率、原太阳星云中的氮同位素分馏以及地球大气的氮源具有重要意义。
