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. 2017 Jul 28;3(7):e1700022.
doi: 10.1126/sciadv.1700022. eCollection 2017 Jul.

ALMA detection and astrobiological potential of vinyl cyanide on Titan

Maureen Y Palmer  1   2   3 , Martin A Cordiner  1   3 , Conor A Nixon  1 , Steven B Charnley  1 , Nicholas A Teanby  4 , Zbigniew Kisiel  5 , Patrick G J Irwin  6 , Michael J Mumma  1
Affiliations

ALMA detection and astrobiological potential of vinyl cyanide on Titan

Maureen Y Palmer et al. Sci Adv. .

Abstract

Recent simulations have indicated that vinyl cyanide is the best candidate molecule for the formation of cell membranes/vesicle structures in Titan's hydrocarbon-rich lakes and seas. Although the existence of vinyl cyanide (C2H3CN) on Titan was previously inferred using Cassini mass spectrometry, a definitive detection has been lacking until now. We report the first spectroscopic detection of vinyl cyanide in Titan's atmosphere, obtained using archival data from the Atacama Large Millimeter/submillimeter Array (ALMA), collected from February to May 2014. We detect the three strongest rotational lines of C2H3CN in the frequency range of 230 to 232 GHz, each with >4σ confidence. Radiative transfer modeling suggests that most of the C2H3CN emission originates at altitudes of ≳200 km, in agreement with recent photochemical models. The vertical column densities implied by our best-fitting models lie in the range of 3.7 × 1013 to 1.4 × 1014 cm-2. The corresponding production rate of vinyl cyanide and its saturation mole fraction imply the availability of sufficient dissolved material to form ~107 cell membranes/cm3 in Titan's sea Ligeia Mare.

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Figures

Fig. 1
Fig. 1. ALMA spectra showing three detected transitions of C2H3CN.
(A) Observed ALMA spectrum of Titan in the vicinity of the CO J = 2–1 line. Detected molecular transitions are labeled; insets show zoomed C2H3CN lines in this region. An absorption feature due to telluric CO is also present, redshifted from Titan’s CO rest frequency. Flux densities are shown in units of Janskys (Jy). (B and C) Zoomed, baseline-subtracted spectra of the regions surrounding the two transitions of C2H3CN detected in (A). Best-fitting NEMESIS models using various vertical abundance profiles are overlaid. (D) Spectral region surrounding the third detected C2H3CN transition, with a best-fitting 300-km step model overlaid (the other model curves are omitted from this panel for clarity). Detected lines of C2H5CN and CH3C15N are labeled. Our detection of CH3C15N may be the first definitive extraterrestrial detection of this acetonitrile isotopolog.
Fig. 2
Fig. 2. Best-fitting C2H3CN vertical abundance profiles derived using NEMESIS (solid lines).
The 300-km step and FSH models provide the best fit to our observations (see Table 2). Profiles from photochemical models are shown for comparison [from Krasnopolsky (14) and Dobrijevic et al. (17); dotted lines], along with the previously inferred ionospheric abundances from Cassini mass spectrometry [from Vuitton et al. (8) and Magee et al. (10)]. Light blue region denotes 1σ error limits on the FSH model.
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References

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