Temporal Variability of Titan’s Equatorial Zonal Winds Between 2012-2016

Titan, Saturn’s largest satellite, is host to an intricate chemical and dynamical environment. Its atmosphere is composed primarily of molecular nitrogen (N₂) and methane (CH₄) that form the basis for a complex organic chemistry. But there are known to be temporal and spatial variations in the abundances of the chemical species that compose Titan’s atmosphere. Zonal winds traverse an object along lines of latitude, parallel to the equator. Titan’s zonal winds can reach speeds up to ~ 350 m/s in the upper stratosphere (z ~ 250-350 km) and are responsible for horizontal mixing. They are primarily a result of the seasonally varying solar forcing, and they play an important role in Titan’s global climate and atmospheric circulation (Hörst 2017). The superrotation of Titan’s atmosphere can dominate the transport and mixing of high-altitude photochemical species as well as produce significant east-west asymmetries in the thermosphere.

Considering its global importance, there remain major gaps in our understanding of the detailed behavior of Titan’s wind field. From Hubbard et al. (1993) who first inferred the presence of zonal winds on Titan using stellar occultation observations to de Batz et al. (2025) who improved upon current GCM models, there have been many studies of Titan’s zonal winds. One of the more recent observational studies using ALMA, Lellouch et al. (2019), found a thermospheric, equatorial jet, which is difficult to reproduce in current atmospheric models and raises many questions about the origin of such a fast jet.

In this work, we investigate the temporal evolution of Titan’s high-altitude winds, focusing mainly on the less well studied equatorial region, to help inform our understanding of their physical basis, origin, and climatological implications. Using archival ALMA band 7 (275-373 GHz; ~0.8-1.1 mm) data from 2012-2016, we determined Titan’s zonal wind speeds as a function of latitude and time. We measured the Doppler shifts of molecular emission lines, an example of the Doppler shift is shown for an archival ALMA dataset in Figure 1, by fitting a Moffat function to the emission line profile in each spatial pixel. The data cubes have sufficient spectral resolution (< 300 kHz) to accurately measure Doppler shifts and spatial resolution (< 0.7”) to resolve the East and West limbs of Titan. Using the methods of Cordiner et al. (2020), the data were deconvolved and analyzed to obtain the intrinsic zonal wind speed profile as a function of latitude for each dataset. These profiles will be collated into a time series and compared with current Titan climate and circulation models such as those described by Newman et al. (2011) and Lora et al. (2015). Due to their different contribution functions, different molecules sound wind speeds at different altitudes (Lellouch et al. 2019): HC₃N probes the altitude range z ~ 500-900 km, where an equatorial wind speed variation of 116 ± 3 m/s was previously found between 2016-2017 (Cordiner et al. 2020). We extend the altitudinal range of our analysis and include other molecules such as CH₃CN (probing altitudes z ~ 200-450 km). Through this work, we further improve the constraints on the temporal variability of Titan’s wind field and better determine the conditions that lead to the formation of the thermospheric jet.

Figure 1: Left: ALMA archival HC₃N (J=39-38) continuum subtracted, integrated emission map of Titan observed in 2015, from ALMA project 2013.1.00446.S. The 'x' notes the regions of the limbs where the East and West limb spectra were extracted. Right: Extracted East and West limb spectra, showing a strong Doppler shift due to high-altitude, prograde zonal winds.

 

This work was supported by NSF grant AST-2407709

References:

Hörst 2017, J. Geophys. Res. Planets, 122, 432-482

Hubbard et al., 1993, A&A, 269, 541-563

Bruno de Batz de Trenquelléon et al., 2025, Planet. Sci. J. 6, 78

Lellouch et al., 2019, Nature Astronomy, 5, 614-619

Cordiner et al., 2020, ApJL, 904, L12 

Newman et al., 2015, Icarus, 213, 636-654

Lora et al., 2015, Icarus, 250, 516-528