The Search for Allene in Titan's Stratosphere using Mid-IR Spectra from the TEXES Instrument on IRTF

Allene (CH₂CCH₂), the isomer of propyne (CH₃CCH), has long been theorized to be present in Titan's stratosphere. Previous studies with the Composite Infrared Spectrometer (CIRS) instrument on Cassini have reported upper limits on the order of 1 ppbv. These studies have also reported the challenges in making a positive detection of allene. The v10 band (centered at 845 cm^-1) of allene is located directly under the very strong v9 band of ethane (C₂H₆) (800 cm^-1 - 880 cm^-1), requiring observations of a high enough spectral resolution to resolve individual lines for each molecule. The TEXES instrument at the NASA InfraRed Telescope Facility (IRTF) is capable of reaching a resolving power of R 40,000-100,000, depending on the wavelength of observation. A search for allene using this instrument was done in Roe, et al. [1], however at the time of the study accurate spectral line data was not available, so while the observations suggested the presence of allene, it was unable to be confirmed.

Recently, a new linelist for allene has been made available [2]. Using this newly calculated linelist and the NEMESIS radiative transfer algorithm, we model high resolution mid-IR spectra collected by IRTF at TEXES in July 2017 to continue the search for allene in Titan's atmosphere. A positive detection and measurement of allene on Titan will help to constrain photochemical models of Saturn's most enigmatic moon, and may shed light on other aspects of Titan's atmosphere, including H-atom abundance [3]. Additionally, the double-bond present in allene is readily polymerizable, thus a definite detection and measurement of the gas could aid in understanding haze production mechanisms.

References

[1] Roe, et al., Update on the TEXES Titan Mid-Infrared Spectral Survey EPSC-DPS Joint Meeting 2011, 2011, Vol 6, 1398

[2] Jolly, et al., Search for evidence of Allene on Titan with new spectroscopic data EPSC Abstracts, 2015, Vol 10, EPSC2015-171

[3] Li, et al., Vertical Distribution of C3 Hydrocarbons in Titan's Atmosphere ApJL, 2015, Vol 803, L19-L26