Research ArticleASTROBIOLOGY

ALMA detection and astrobiological potential of vinyl cyanide on Titan

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Science Advances  28 Jul 2017:
Vol. 3, no. 7, e1700022
DOI: 10.1126/sciadv.1700022
  • 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 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.

  • Table 1 Detected C2H3CN transitions and line fluxes.

    Molecular line frequencies are from Kisiel et al. (11). Line fluxes and 1σ errors were obtained from the continuum-subtracted ALMA spectra.

    LineFrequency (MHz)TransitionEu (K)Flux (Jy·kHz)
    C2H3CN (1)230488241,23–231,2214124 ± 4
    C2H3CN (2)230739250,25–240,2414628 ± 4
    C2H3CN (3)231952242,22–232,2114725 ± 6
  • Table 2 Best-fitting C2H3CN model abundances.
    ModelAbundance
    (parts per billion)
    zr (km)N (cm−2)χ2P
    Step (100 km)0.26 ± 0.041004.9 × 10141.250.001
    Step (200 km)0.74 ± 0.072001.0 × 10141.070.19
    Step (300 km)2.83 ± 0.243005.3 × 10131.010.44
    Step (400 km)12.6 ± 1.14003.7 × 10131.020.40
    FSH model0.36 ± 0.222971.4 × 10141.020.37

Supplementary Materials

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Integrated flux contour map for C2H3CN, summed over the three detected transitions.
    • table S1. Observational parameters.

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