Photochemical activity of HCN-C4H2 ices in Titan's lower atmosphere

David Dubois 1, 2 Murthy Gudipati 2 Nathalie Carrasco 1
1 IMPEC - LATMOS
LATMOS - Laboratoire Atmosphères, Milieux, Observations Spatiales
Abstract : Revealed by the Cassini and Voyager Missions, a plethora of volatile species are formed in Titan's upper atmosphere from the initial N2/CH4 (~98/2%) composition. As they precipitate, most of the volatiles condense in the colder lower atmosphere where they can form icy clouds (e.g. C4N2 , HCN, C2H6) which have been detected above the poles. HCN is the most abundant nitrile in Titan's atmosphere and suspected to condensate in Titan's lower stratosphere (typically <100 km). Micron-sized HCN ice particles were also observed above the south pole at high altitudes (300 km). This cloud is thought to have been formed in the post-equinox winter polar vortex. In the north polar regions, HCN is also a likely prominent contributor to the haystack spectral signature seen at 221 cm-1. These stratospheric ices may contribute as condensation nuclei for ices deeper down in the troposphere. Furthermore, C4H2 , a simple alkyne formed by the chemistry and relatively abundant in the stratosphere, condenses near 75 km, lower than HCN. C4H2 can also absorb the lesser energetic photos at these low altitudes, forming the radical C4H which then reacts with CH4 , causing the consumption of methane. Consequently, the loss mechanism for C4H2 is photo-chemistry. As soon as C4H2 condenses, small molecule and complex accretion with HCN may occur. The reactive state that these ices may undergo with long-UV radiation after they form is still largely unknown. We explore these conditions by studying HCN-C4H2 ice mixtures in the laboratory, by using the Titan Organic Aerosol Spectroscopy and chemisTry (TOAST) setup at JPL's Ice Spectroscopy Laboratory (ISL). These ices are then irradiated at long-UV wavelengths pertaining to these low-altitude regions at low-controlled temperatures. The residue is analyzed using long-IR absorption. Our results show a solid-state HCN consumption due to irradiation, to which C4H2 acts as a catalyst, indicating HCN ice particle ageing in the troposphere may be facilitated by more complex condensed nuclei.
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https://hal.archives-ouvertes.fr/hal-01796628
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Submitted on : Tuesday, June 5, 2018 - 3:14:57 PM
Last modification on : Wednesday, May 15, 2019 - 4:11:41 AM

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  • HAL Id : hal-01796628, version 1

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David Dubois, Murthy Gudipati, Nathalie Carrasco. Photochemical activity of HCN-C4H2 ices in Titan's lower atmosphere. Cassini Science Symposium 2018, Aug 2018, Boulder, United States. ⟨hal-01796628⟩

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