Organic Ices in Titan’s Stratosphere

Anderson, C. M.; Samuelson, R. E.; Nna-Mvondo, Delphine

doi:10.1007/s11214-018-0559-5

Cited by 44 publications

(53 citation statements)

References 106 publications

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“…The southern molecular polar enrichment combined with the low stratospheric temperatures were at the origin of the development of a large stratospheric polar cloud first observed in May 2012 by the Cassini Imager SubSystem (ISS; West et al 2016). Several ice signatures were detected inside this cloud at different altitudes: HCN ice (de Kok et al 2014;Le Mouélic et al 2018), C 6 H 6 ice (Vinatier et al 2018), the "haystack" cloud at 220 cm −1 (Jennings et al 2012(Jennings et al , 2015 and the High Altitude South Polar (HASP) cloud possibly containing co-condensed C 6 H 6 :HCN ice (Anderson et al 2018). Seasonal evolution of this cloud was monitored using the Cassini Visible and Infrared Mapping Spectrometer (VIMS) by Le Mouélic et al (2018).…”

Section: Introductionmentioning

confidence: 97%

Temperature and chemical species distributions in the middle atmosphere observed during Titan’s late northern spring to early summer

Vinatier

Mathé

Bézard

et al. 2020

A&A

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We present a study of the seasonal evolution of Titan’s thermal field and distributions of haze, C2H2, C2H4, C2H6, CH3C2H, C3H8, C4H2, C6H6, HCN, and HC3N from March 2015 (Ls = 66°) to September 2017 (Ls = 93°) (i.e., from the last third of northern spring to early summer). We analyzed thermal emission of Titan’s atmosphere acquired by the Cassini Composite Infrared Spectrometer with limb and nadir geometry to retrieve the stratospheric and mesospheric temperature and mixing ratios pole-to-pole meridional cross sections from 5 mbar to 50 μbar (120–650 km). The southern stratopause varied in a complex way and showed a global temperature increase from 2015 to 2017 at high-southern latitudes. Stratospheric southern polar temperatures, which were observed to be as low as 120 K in early 2015 due to the polar night, showed a 30 K increase (at 0.5 mbar) from March 2015 to May 2017 due to adiabatic heating in the subsiding branch of the global overturning circulation. All photochemical compounds were enriched at the south pole by this subsidence. Polar cross sections of these enhanced species, which are good tracers of the global dynamics, highlighted changes in the structure of the southern polar vortex. These high enhancements combined with the unusually low temperatures (<120 K) of the deep stratosphere resulted in condensation at the south pole between 0.1 and 0.03 mbar (240–280 km) of HCN, HC3N, C6H6 and possibly C4H2 in March 2015 (Ls = 66°). These molecules were observed to condense deeper with increasing distance from the south pole. At high-northern latitudes, stratospheric enrichments remaining from the winter were observed below 300 km between 2015 and May 2017 (Ls = 90°) for all chemical compounds and up to September 2017 (Ls = 93°) for C2H2, C2H4, CH3C2H, C3H8, and C4H2. In September 2017, these local enhancements were less pronounced than earlier for C2H2, C4H2, CH3C2H, HC3N, and HCN, and were no longer observed for C2H6 and C6 H6, which suggests a change in the northern polar dynamics near the summer solstice. These enhancements observed during the entire spring may be due to confinement of this enriched air by a small remaining winter circulation cell that persisted in the low stratosphere up to the northern summer solstice, according to predictions of the Institut Pierre Simon Laplace Titan Global Climate Model (IPSL Titan GCM). In the mesosphere we derived a depleted layer in C2H2, HCN, and C2H6 from the north pole to mid-southern latitudes, while C4H2, C3H4, C2H4, and HC3N seem to have been enriched in the same region. In the deep stratosphere, all molecules except C2H4 were depleted due to their condensation sink located deeper than 5 mbar outside the southern polar vortex. HCN, C4H2, and CH3C2H volume mixing ratio cross section contours showed steep slopes near the mid-latitudes or close to the equator, which can be explained by upwelling air in this region. Upwelling is also supported by the cross section of the C2H4 (the only molecule not condensing among those studied here) volume mixing ratio observed in the northern hemisphere. We derived the zonal wind velocity up to mesospheric levels from the retrieved thermal field. We show that zonal winds were faster and more confined around the south pole in 2015 (Ls = 67−72°) than later. In 2016, the polar zonal wind speed decreased while the fastest winds had migrated toward low-southern latitudes.

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Section: Introductionmentioning

confidence: 97%

Temperature and chemical species distributions in the middle atmosphere observed during Titan’s late northern spring to early summer

Vinatier

Mathé

Bézard

et al. 2020

A&A

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“… 9 − 11 For example, both the Voyager Flyby and the Cassini mission to Titan have detected clouds made of HCN-based aerosols that can be expected to contribute to concentrated deposits on the considerably colder surface. 10 Observations and modeling suggest that HCN can react to form various complex organic materials on Titan despite the low temperature. 12 , 13 While there are large uncertainties regarding the nature of early Earth’s atmosphere, HCN is expected to have also been formed there as a product of reactions between nitrogen and methane following solar UV radiation, superflares, shockwaves, and discharge chemistry.…”

Section: Introductionmentioning

confidence: 99%

The Beginning of HCN Polymerization: Iminoacetonitrile Formation and Its Implications in Astrochemical Environments

Sandström

Rahm

2021

ACS Earth Space Chem.

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Hydrogen cyanide (HCN) is known to react with complex organic materials and is a key reagent in the formation of various prebiotic building blocks, including amino acids and nucleobases. Here, we explore the possible first step in several such processes, the dimerization of HCN into iminoacetonitrile. Our study combines steered ab initio molecular dynamics and quantum chemistry to evaluate the kinetics and thermodynamics of base-catalyzed dimerization of HCN in the liquid state. Simulations predict a formation mechanism of iminoacetonitrile that is consistent with experimentally observed time scales for HCN polymerization, suggesting that HCN dimerization may be the rate-determining step in the assembly of more complex reaction products. The predicted kinetics permits for iminoacetonitrile formation in a host of astrochemical environments, including on the early Earth, on periodically heated subsurfaces of comets, and following heating events on colder bodies, such as Saturn’s moon Titan.

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“…Future observations with JWST focusing at IR wavelengths could potentially identify the spectral signatures of the main organic ices in Pluto's haze and help constrain the relative contribution of the Titan-type component. The v4 (3293 cm -1 ) and v8 (664 cm -1 ) absorption bands of solid C4H2 would be good starting points 38 , although co-condensates could demonstrate shifts relative to the anticipated monocondensate spectra 39 . However, given that Pluto's eccentric orbit will modify its atmospheric temperature, this variation could have an impact on the haze load, as well as on the nature of the hazes formed across the orbit.…”

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confidence: 99%

A major ice component in Pluto’s haze

et al. 2020

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Pluto, Titan, and Triton, are all low-temperature environments with a N2/CH4/CO atmospheric composition on which solar radiation drives an intense organic photochemistry. Titan is rich in atmospheric hazes and Cassini-Huygens observations showed their formation initiates with the production of large molecules through ion-neutral reactions. New Horizons revealed that optical hazes are also ubiquitous in Pluto's atmosphere and it is thought that similar haze formation pathways are active in this atmosphere as well. However, we show here that Pluto's hazes may contain a major organic ice component (dominated by C4H2 ice) from the direct condensation of the primary photochemical products in this atmosphere. This contribution may imply that haze has a less important role in controlling Pluto's atmospheric thermal balance compared to Titan. Moreover, we expect that the haze composition of Triton is dominated by C2H4 ice. Pluto's atmosphere is the equivalent of Titan's upper atmosphere above 400 km altitude, with comparable CH4, CO, and N2 density profiles and pressure scale heights 1 . Photochemistry models for these environments demonstrate that the anticipated chemical products are similar 2,3 , therefore Pluto's hazes are thought to be of a similar nature based on molecular growth 4 (see Methods for haze nomenclature). The fraction of the mass flux generated from the photolysis of Titan's main atmospheric composition that ends in haze particles is ~30% 5,6 . Such a yield for Pluto's haze would suggest a mass flux of ~6x10 -15 gcm -2 s -1 (all reported mass fluxes are referred to Pluto's surface). However, the opacity of particles characterizing such a mass flux falls short of the available observations and a twice-higher haze formation efficiency is required to generate enough material to reproduce the UV extinction observations below 200 km 7 . As Pluto's upper atmosphere is much colder than Titan's (~70K compared to ~150K for Titan, see Fig. 1), an increased haze yield for Pluto is surprising. On the other hand, the photochemical gases produced on Pluto may condense at lower pressures than on Titan (Fig. 1). Therefore organic ices could be responsible for, or at least contribute to, the formation of the observed hazes in Pluto's atmosphere. We explore the extent of this contribution using coupled models of atmospheric photochemistry and microphysics, and following the evolution of the organic ice haze particles from their formation in the upper atmosphere to their sedimentation on Pluto's surface. The models are adapted from previous studies of photochemistry and microphysics in Titan's atmosphere, taking into account high-resolution energy

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Organic Ices in Titan’s Stratosphere

Cited by 44 publications

References 106 publications

Temperature and chemical species distributions in the middle atmosphere observed during Titan’s late northern spring to early summer

Temperature and chemical species distributions in the middle atmosphere observed during Titan’s late northern spring to early summer

The Beginning of HCN Polymerization: Iminoacetonitrile Formation and Its Implications in Astrochemical Environments

A major ice component in Pluto’s haze

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