Enrichment of heavy nitrogen in Titan's HC3N detected using ALMA

• Main Authors: Cordiner, M., Nixon, C. A., Charnley, S. B., Teanby, N. A., Molter, E. M., Kisiel, Z., Vuitton, V.
• Other Authors: NASA Goddard Space Flight Center (GSFC), School of Earth Sciences Bristol, University of Bristol Bristol, Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales Toulouse (CNES)-Centre National de la Recherche Scientifique (CNRS)-Observatoire des Sciences de l'Univers de Grenoble (OSUG ), Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national des sciences de l'Univers (INSU - CNRS)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB Université de Savoie Université de Chambéry )-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes 2016-2019 (UGA 2016-2019 )-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB Université de Savoie Université de Chambéry )-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes 2016-2019 (UGA 2016-2019 )
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2018
• Subjects: 5210 Planetary atmospheres; clouds; and hazes; PLANETARY SCIENCES: ASTROBIOLOGYDE: 5215 Origin of life; PLANETARY SCIENCES: ASTROBIOLOGYDE: 5749 Origin and evolution; PLANETARY SCIENCES: FLUID PLANETSDE: 6281 Titan; PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS; [PHYS.ASTR.EP]Physics [physics]/Astrophysics [astro-ph]/Earth and Planetary Astrophysics [astro-ph.EP]; North Pole; South Pole; South pole
• Online Access: https://hal.archives-ouvertes.fr/hal-02387142

International audience Trace isotopic ratios contain unique information on the chemical histories of atmospheric gases. From the combined efforts of previous ground and space-based sub-mm and infrared observations, Titan's atmospheric HCN has been found to be substantially enriched in 15N compared with the bulk N2 reservoir. This is theorized to occur as a consequence of isotope-selective photodissociation of N2 in the upper atmosphere, giving rise to an enhanced abundance of atomic 15N that subsequently becomes incorporated into HCN. Here, we present the first maps of cyanoacetylene isotopologues in Titan's atmosphere, including H13CCCN and HCCC15N, detected in the 0.9 mm band using the Atacama Large Millimeter/submillimeter array (ALMA) around the time of Titan's (southern winter) solstice in May 2017 (see Figure 1). The HC3N emission is found to be strongly enhanced over the south pole (by a factor of 5.7 compared to the north pole), consistent with rapid photochemical loss of HC3N from the summer hemisphere combined with production and transport to the winter pole since the previous (2015) ALMA observations. The H13CCCN/HCCC15N flux ratio is consistent with an HC3N/HCCC15N ratio of 67 ± 14, which is significantly enriched in 15N compared with the main molecular nitrogen reservoir (which has a 14N/15N ratio of 167). This confirms the importance of photochemistry in determining the nitrogen isotopic ratio in Titan's organic inventory. In this presentation I will discuss the impact of these results on our understanding of the origin of Titan's N2, the evolution of its atmospheric 14N/15N ratio, and on our knowledge of the detailed photochemical processes occurring in nitrogen-rich planetary atmospheres.