Photopolymerization of HCN/HC3N mixed ice to produce tholins that incorporate HCN
Abstract
HCN and HC3 N are both major components of Titan's atmosphere, and are thought to participate in the formation of the condensed ices, aerosols, and photopolymerized aggregates that comprise Titan's haze layers. Only 10% of incident sunlight reaches Titan's surface, and shorter wavelengths are attenuated in the upper atmosphere. While HC3 N absorbs ultraviolet (UV) photons even at longer wavelengths of 300 nm, HCN has an absorption threshold of 230 nm. HC3 N ice polymerizes under UV irradiation and, when warmed to room temperature, a non-volatile residue that is a laboratory analogue of Titan's haze ("tholin") remains. HCN and HC3 N both condense at very similar temperatures. Amino acids and nucleobases have been derived from tholins when in contact with oxygen-rich species. We have conducted experiments with co-condensed HCN and HC3 N, as well as HCN ice film condensed on top of a HC3 N photopolymer film, to determine the formation mechanisms of tholins containing HCN. The work presented here considers a co-deposition of HCN and HC3 N ice at 100 K that is then irradiated with a defocused laser at 250 nm. The rates of consumption of the two molecules are followed using FTIR spectroscopy, and volatile species that desorb upon warming are monitored with quadrupole mass spectrometry. For a HCN:HC3 N ratio of 1:2, excited HC3 N preferentially reacted with HCN over HC3 N by a factor of ≥2 to produce a darkly colored co-polymer. This demonstrates that HCN can polymerize even if it is not directly excited (but indirectly photoactivated), which extends the altitude range at which HCN can polymerize. This significantly increases the amount of polymer that can form and the amount of pre-biotic molecules that may form once the haze particles reach Titan's surface and interact with oxygen-rich species such as water-ice.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.P23D3541M
- Keywords:
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- 5210 Planetary atmospheres;
- clouds;
- and hazes;
- PLANETARY SCIENCES: ASTROBIOLOGY;
- 6207 Comparative planetology;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 6281 Titan;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 5405 Atmospheres;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS