Time-variability and north pole enhancement of Titan’s atmospheric water abundance

Introduction

Oxygen is the universes third most abundant element (Bergman et al. 2021) and is extremely rich in the Saturnian system (Feuchtgruber et al. 1997). Studies have found oxygen-bearing molecules and ions in Saturn’s atmosphere (Esposito et al. 2005) , in its plasma environment (Wilson et al. 2016) and on its moon Enceladus (Thomas et al. 2016). In contrast, Titan is scare in oxygen species, boasting a rich atmosphere of hydrocarbons and nitriles (Vuitton et al.2024) that interact uniquely, often consuming free oxygen or locking it away as water ice. This results in an anoxic, organic and diverse environment with little oxygen to terminate reactions (Nixon et al. 2024).

To date, only three oxygen-bearing molecules have been detected in Titan’s atmosphere: CO (Lutz et al. 1983), CO₂ (Sameulson et al 1983) and H₂O (Coustenis et al. 1997). These molecules form from externally delivered OH, O⁺  and/or H₂O being photodissociated by solar UV and energetic particles in the upper atmosphere, before recombining and being transported downwards via atmospheric mixing (Vuitton et al. 2019).  Of the detected species, the least well-understood is water vapour. CO and CO₂ have been studied extensively and exhibit little variation in latitude, time or altitude (Teanby et al. 2019). Yet, investigations into H₂O have been limited to single measurements or large averages (Vuitton et al 2007,  Cui et al 2009, Cottini et al. 2012, Bauduin et al 2018). Its weak infrared emission lines and low atmospheric abundances make it difficult to model. Water plays a vital role in Titan’s atmosphere, distributing oxygen and acting as a tracer for atmospheric dynamics. Its chemical pathways may produce species important for astrobiology like formaldehyde (Nixon 2024).

 

Figure 1: Schematic showing the potential pathways of H2O and its transport through the atmosphere until condensation near the tropopause. Not all reactions have been included.. The chemistry shown is based on Vuitton et al. 2019 and Nixon 2024. Molecules in green denote those predicted by photochemical models but not yet been detected.

Method

Here, we present the first reported latitudinal and temporal variability of H₂O in Titan’s atmosphere. Using the NEMESIS radiative transfer code (Irwin et al. 2008) with temperature a priori profiles from Teanby et al. 2019 and a vertical water a priori profile from Vuitton et al.2019, we retrieve water abundance in Titan’s stratosphere from 157 far-infrared high-resolution Cassini CIRS FIRNADCMP observations (Flasar et al. 2004) across the entire mission and moon. To improve the fit and account for variations in the baseline caused by aerosols, we fit scaled gaussian basis function (see associated EPSC 2026 poster for more information). Due to low signal-to-noise in some spectra and low abundances, 51 observations were averaged in 13 bins. Our derived column abundances are consistent with all previous measurements of water in Titan’s middle atmosphere, and the retrieved profiles show consistency with upper atmosphere upper limits derived from INMS (Vuitton et al. 2007, Cui et al 2009). These results provide important constraints for future photochemical models and GCM's, particularly those focused on oxygen chemistry and organic molecule formation. 

Results

At the poles, lower column abundances are observed due to reduced temperatures, which decrease the saturation vapour pressure. We also find high retrieved scale factors (applied to the a priori) at the north pole indicating that water is mildly enhanced by a factor of 3 relative to mid-latitudes similar to trace gases like HCN (Teanby et al. 2012), although much weaker. This is likely caused by water-rich air subsiding into the polar vortex, concentrating and adiabatically heating within the confines of the polar mixing boundary (Teanby et al. 2017). We find no evidence of seasonality, however we do find statistically significant time-variability at low-to-mid latitudes. Analysis of atmospheric residence times and comparison with the TAM GCM (Lombardo et al. 2023) shows the variability is not explained by photochemistry or atmospheric dynamics and may indicate a time-varying source. This idea was also previously suggested by Moreno et al. 2012 and Bauduin et al. 2018. We explore potential drivers of time-variability and conclude that short-term month-scale variations in Enceladus’ neutral torus, and Saturn’s dynamic magnetospheric environment could be responsible. The derived incoming OH flux needed to explain our results matches the calculated OH flux from the neutral torus, implying Enceladus could be the dominant source of Titan's water. 

 

Figure 2:  Step-by-step schematic showing the potential path of H2O (OH) molecules from Enceladus to Titan, highlighting consistency or variability at each step. Each band represents a different type of neutral torus and the dotted lines originating from Saturn represent the magnetic field.

References

Bauduin. A. et al. 2018. Icarus 301, 136–151. doi: 10.1016/j.icarus.2017.09.039. 

Bergman. M.  et al. 2021. Monthly Notices of the Royal Astronomical Society 508 (2). Doi: 10.1093/mnras/stab2160

Cottini. V. et al. 2012. Icarus 220 (2), 855–862. doi: 10.1016/j.icarus.2012.06.009. 

Coustenis. A. et al. 1998. A&A 336, 85-89.

Cui. J. et al. 2009. Icarus 200, 581–615. doi: 10.1016/j.icarus.2008.11.005. 

Esposity. L. et al. 2005. Science, 307 (5713). Doi: 10.1126/science.1105606

Feuchtgruber. H. et al. 1997. Nature 389, 159-162. Doi: 10.1038/38236

Flasar. F. M. et al. 2004. Space Science Reviews 115 (1–4), 169–297. doi: 10.1007/s11214-004-1454-9. 

Irwin, P. G. J. et al. 2008. Journal of Quantitative Spectroscopy and Radiative Transfer 109 (6), 1136–50. Doi: 10.1016/j.jqsrt.2007.11.006.

Lombardo. N.A. et al. 2023. JGR: Planets, 123. Doi: 10.1029/2023JE008061.

Lutz. B.L. et al. 1983. Science, 220 (4604), 1374-1375. Doi:10.1126/science.220.4604.1374.

Moreno. R. et al. 2012. Icarus 221, 753–767. 10.1016/j.icarus.2012.09.006

Nixon. C.A. 2024. ACS Earth Space Chem 29, 8(3), 406-456. Doi: 10.1021/acsearthspacechem.2c00041.

Samuelson. R. E. et al. 1983. JGR: Space Physics, 88(A11), 8709-8715. Doi: 10.1029/JA088iA11p08709.

Teanby. N.A. et al. 2012. Nature, 491, 732. Doi: 10.1038/nature11611

Teanby. N.A. et la. 2017. Nature Communications, 8, 1586.Doi:10.1038/s41467-017-01839-z

Teanby. N.A. et al. 2019. Geophysical Research Letters, 46, 3079-3089. Doi: 10.1029/2018GL081401.

Thomas. P.C. et al. 2016. Icarus, 264, 37-47. DOI: 10.1016/j.icarus.2015.08.037.

Wilson, R. et al. 2015. JGR Space Physics 120 (8). DOI:10.1002/2014JA020557.

Vuitton. V. et al. 2007. Icarus 191. doi: 10.1016/j.icarus.2007.01.028.

Vuitton.V. et al. 2019. Icarus, 324, 120-197. Doi: 10.1016/j.icarus.2018.06.013

Vuitton. V. et al. (2024), ‘Chapter 6 : Titan’s Atmospheric Structure, Composition, Haze, and Dynamics’ In Titan after Cassini–Huygens, COSPAR Scientific Symposium Series.