Search for complex nitriles in Titan’s stratosphere

1) Introduction:

The atmosphere of Titan is known to host a complex organic chemistry [1,2,3]. From the Voyager missions, and later the Cassini-Huygens mission, several hydrocarbons and nitriles have been detected and their seasonal variations have been monitored during a period of one Titan season (30 years). Photochemical models that have also predicted the presence of other minor species, among which some have infrared transitions in the 5-25 mm spectral range. We have observed Titan with IRTF/TEXES in September 2022, searching for two complex nitriles C₄H₃N and C₄H₇N. We published an analysis of the data with the support of updated spectroscopic databases (like in HITRAN and GEISA) and new data from recent laboratory work [4].

2) Photochemistry:

• Cyanopropyne (CH₃C₃N) was detected in the mm range with Alma [5], in band 6 (∼230 – 272 GHz). We have selected to search for cyanopropyne (C₄H₃N) because it has a strong band (n₁₀) with a Q-branch at 499 cm^-1 [6]. We have used a photochemical model previously applied to Titan, Pluto and Triton [7a] to simulate the profile of CH₃C₃ Dominant formation is through reaction of C₂N with ethylene (C₂H₄), reactions of CN with methylacetylene (CH₃C₂H) and allene (CH₂CCH₂), and the recombination of CH₃C₃NH⁺, while below 900 km the dominant CH₃C₃N loss mechanism is photolysis. The measured cross sections allow for decreased uncertainties in the photolysis of CH₃C₃N [7b]. The simulations suggest a C₄H₃N abundance of a few 10^-10 in the stratosphere.
• Isobutyronitrile (C₃H₇CN) has not yet been detected, but has a band at 538 cm^-1; production is dominated by the C₂H₄CN + CH₃ and CN + C₃H₈ reactions and loss is driven by photodissociation. Its present upper limit is ~10^-7 in the thermosphere and ~10^-11 in the stratosphere.

3) Observations

We observed Titan in September 2022 using the TEXES thermal infrared imaging spectrometer at the Infrared Telescope Facility (Mauna Kea, Hawaii) to search for C₄H₃N and C₄H₇N in the 20-micron region and to monitor the infrared signatures of hydrogen cyanide (HCN) and cyanoacetylene (HC₃N), along with acetylene (C₂H₂ and C₂HD).

The TEXES data were also used for a study of the variations of HCN and HC₃N and for a retrieval of D/H from C₂HD/C₂H₂.

4) Spectroscopic data:

Absorption coefficients and absorption cross-sections have been obtained for the two noncyclic cyanopropyne (CH3C3N) and the isobutyronitrile (i-C₃H₇CN) organic molecules at room temperature. The gas phase spectra of the nitriles were recorded between 160 and 3500 cm^-1 using an infrared Fourier transform spectrometer. The spectral resolution was 0.056 cm^-1. For the 18-20 μm spectral region an additional resolution of 0.01 cm^-1 was used. Among the various absorption bands observed, some, as the ν₁₀ band of cyanopropyne around 500 cm^-1, are particularly interesting for detecting and quantifying these molecules in astrophysical objects other than Titan.

The retrieved absorption cross-sections were used in radiative transfer simulations of the observations using the PSG radiative transfer code by [9] and a radiative code for Titan used in analyzing CIRS data. We published the results and perspectives [4].

5) TEXES analysis results

• 499 cm^-1 range: with current TEXES data, we derive an upper limit for cyanopropyne of about 3×10^-9 (Fig. 1). The C₂HD line intensity is in reasonable agreement with the nominal value of C₂H₂ at the equator (2.5×10^-6), with D/H = 5×10^-4. The HC₃N band is not visible because the value at the equator (3×10^-10) is too low (detection limit: around 10^-9; [1,2]).
• 538 cm^-1 range: The upper limit of isobutyronitrile is around 3×10^-7. The band is weak and there is no apparent structure. Even with a resolution of 0.01 cm^-1, the instrumental spectrum is not resolved. The C₂HD is consistent with the nominal value C₂H₂ = 2.5×10^-6 with D/H = 5×10^-4.
• 746 and 1247 cm^-1 ranges: TEXES data appear to be in good agreement with CIRS measurements and the nominal model for the continuum and the band wings. In the future, we plan to investigate the Cassini/CIRS large averages in order to search for the nitriles in the FP1 and FP3 spectral ranges.

6) Conclusions and future prospects

The spectroscopic data covering both FIR and MIR are available and should allow future quantitative detection of these two molecules. They will be included in the 2024 update of HITRAN. Measurements at lower temperature and pressure should also be performed since they will improve the quality of the detection in astrophysical object such as the stratosphere of Titan. Although no detection was achieved with these observations, we plan to use the new laboratory data in conjunction with larger telescopes in the future, like the 8-m telescope Gemini, to improve the limit of detection. Such observations should provide significant insights in our understanding of the Titan nitrile chemistry, in particular for C₄H_x species.

References

• [1] Coustenis, A., 2021. In Read, P. (Ed.), Oxford Research Encyclopedia of Planetary Science. Oxford University Press.
• [2] Nixon, C., 2024. ACS Earth and Space Chemistry 8 (3), 406-456.
• [3] Waite et al., 2007. Science 316, 870.
• [4] Jacquemart et al., 2025. JQSRT 2025, 109466.
• [5] Thelen et al., 2020. https://arxiv.org/pdf/2010.08654.pdf
• [6] Cerceau, F., et al., 1985. Icarus 62, pp. 207–220.
• [7] a. Lavvas et al. 2021 Astr. 5, 289-297; b. Lammarre et al. 2016, JQSRT 182, 286-295
• [8] Coustenis et al., 1993. Icarus, 102, 240−260.
• [9] Villanueva et al., 2018. https://arxiv.org/abs/1803.02008

 

 

Figure 1: simulations and results for cyanopropyne in the region around 500 cm^-1.

 

Figure 2: simulations and results for isobutyronitrile in the region around 538 cm^-1.