Variability in Titan's Mesospheric HCN and Temperature Structure as Observed by ALMA

Titan, Saturn's largest moon, possesses a substantial atmosphere that is characterized by a thermal structure similar to that of the Earth, though colder and more vertically extended (up to ∼1500 km), resulting from its comparatively reduced insolation and low gravity (see Flasar et al. 2014; Yelle et al. 2014; Hörst 2017, and references therein). Through observations by the Voyager 1 and 2 and Cassini spacecraft and direct measurements by the Huygens probe during its descent in 2005, Titan's atmospheric behavior and dynamics have been well studied through roughly half of its ∼29.5 yr seasonal cycle (see, for example, Smith et al. 1982; Lindal et al. 1983; Vervack et al. 2004; Flasar et al. 2005; Fulchignoni et al. 2005; Shemansky et al. 2005; Yelle et al. 2006; Teanby et al. 2008, 2012; Snowden et al. 2013; Vinatier et al. 2015; Teanby et al. 2017, 2019; Sylvestre et al. 2020; Creecy et al. 2021; Seignovert et al. 2021; Sharkey et al. 2021). Additionally, measurements from ground-based observatories have helped to bridge gaps between missions to the Saturnian system (e.g., Kim et al. 2000; Geballe et al. 2003; Gurwell 2004; Moreno et al. 2005). However, the nature of short-term variability in the dynamical state of Titan's upper atmosphere, the seasonal evolution of its stratosphere (∼80–300 km), and the contribution of many different chemical and dynamical components to Titan's atmospheric radiative budget still remain enigmatic after the end of the Cassini/Huygens mission.

Models of Titan's upper atmosphere—consisting of the mesosphere (typically defined as ∼300–600 km), thermosphere (∼600–1000 km), and ionosphere (>1000 km)—attempt to simulate Titan's atmospheric thermal structure through the balance of heating by solar EUV/UV photons, electron and ion precipitation, interactions with Saturn's magnetosphere and plasma sheet, thermal conduction, and cooling by rotational and vibrational emission from hydrogen cyanide (HCN) and various hydrocarbons produced via photochemistry (Yelle 1991; Müller-Wodarg et al. 2000, 2008; Yelle et al. 2008; Bell et al. 2010, 2011; Westlake et al. 2011; Bell et al. 2014; Snowden & Yelle 2014; Snowden & Higgins 2021). Additionally, atmospheric wave activity induced by gravitational tides may produce significant temperature perturbations in the upper atmosphere (Strobel 2006). These predictions, along with inferences of temperature from in situ density profiles obtained with the Cassini Ion and Neutral Mass Spectrometer (INMS) and Huygens Atmospheric Structure Instrument (HASI) and measurements derived through stellar occultations from the ground and the Cassini Ultraviolet Imaging Spectrograph (UVIS), revealed large variability in Titan's middle and upper atmospheric thermal structure on both diurnal and seasonal timescales. Further, observations from the Cassini Composite Infrared Spectrometer (CIRS), Imaging Science Subsystem (ISS), and Visible and Infrared Mapping Spectrometer (VIMS) instruments covering roughly half a Titan year indicated that Titan's radiative energy budget may not be fully balanced on both temporal (seasonal) and spatial (hemispheric) scales (Li 2015; Creecy et al. 2019, 2021), though further observations are required to determine the nature and effects of this imbalance on timescales of a Titan year or more.

Vertical perturbations of ∼10–30 K on ∼50–100 km scales (on order 1–2 scale heights) were present in the Huygens/HASI thermospheric measurements, indicating strong wave activity (Fulchignoni et al. 2005; Aboudan et al. 2008). Ionospheric measurements with the INMS revealed 30–60 K variations in temperature over a number of Cassini flybys between 2004 and 2010 and further evidence of wave activity on large (150–450 km) vertical scales (Müller-Wodarg et al. 2006; Snowden et al. 2013). These perturbations in temperature were found to be much larger in the upper atmosphere than in the lower mesosphere and stratosphere (Lorenz et al. 2014). Additionally, INMS measurements revealed warmer upper atmospheric temperatures on Titan's nightside and while it was within Saturn's plasma sheet; cooler temperatures were found on both the dayside and when Titan was in Saturn's magnetospheric lobe, though no strong correlation was found between temperature and latitude, longitude, solar zenith angle, or local solar time (Cui et al. 2009; Westlake et al. 2011; Snowden et al. 2013). UV occultations sounding Titan's middle atmosphere measured a mesopause at ∼650 km (Shemansky et al. 2005; Liang et al. 2007) and temperature inversions between 350 and 500 km attributed to Titan's seasonally variable detached haze layer (Sicardy et al. 2006; Lavvas et al. 2009), which was also detected with ISS and HASI (Porco et al. 2005; Fulchignoni et al. 2005; West et al. 2011).

As a strong contributor to both cooling Titan's upper atmosphere and producing further complex organic species, HCN provides substantial insight into Titan's innately linked chemical and dynamical systems. HCN was first detected on Titan by Voyager 1 (Hanel et al. 1981) and has been routinely observed in the millimeter regime (see, for example, Paubert et al. 1987; Hidayat et al. 1997; Marten et al. 2002; Courtin et al. 2011; Rengel et al. 2022). Following observations of CH₄ fluorescence in Titan's mesosphere—resulting in additional measurement of Titan's mesopause near 600 km (Kim et al. 2000)—HCN fluorescent emission was observed with the NIRSPEC instrument on Keck II by Geballe et al. (2003) and subsequently reanalyzed by Yelle & Griffith (2003) and Kim et al. (2005) to produce vertical profiles in Titan's mesosphere and thermosphere. These measurements and those from the Cassini VIMS, UVIS, and INMS instruments revealed Titan's HCN mole fraction to range between ∼3 × 10⁻⁴ and 3 × 10⁻³ at 1000 km, along with evidence for diurnal variability (Shemansky et al. 2005; Magee et al. 2009; Koskinen et al. 2011; Adriani et al. 2011; Vinatier et al. 2015; Cui et al. 2016). In the stratosphere and mesosphere, the variability of Titan's HCN abundance has been studied using Cassini/CIRS, revealing dramatic enhancements at high latitudes larger than an order of magnitude throughout Titan's seasonal cycle (Teanby et al. 2007; Vinatier et al. 2015; Teanby et al. 2019). Further discrepancies exist between these observations and predictions from photochemical models (e.g., Loison et al. 2015; Willacy et al. 2016; Vuitton et al. 2019), raising additional questions pertaining to the role of HCN in Titan's chemical and radiative balance.

After the end of the Cassini/Huygens mission, ground-based observations continue to provide insight into Titan's complex atmospheric composition and dynamics on short-term and seasonal timescales. In particular, the Atacama Large Millimeter/submillimeter Array (ALMA) has proven to be a powerful facility to study planetary atmospheres and allows for the study of Titan's stratosphere through thermosphere by way of strong rotational emission lines from a variety of molecular species. Previous ALMA observations from 2012 to 2015 allowed for the study of temperature and HCN abundance in Titan's stratosphere during its northern spring (Molter et al. 2016; Serigano et al. 2016; Thelen et al. 2018, 2019a, 2019b), while spectral Doppler shift measurements revealed short-term variability in Titan's wind field in the middle and upper atmosphere (Lellouch et al. 2019, hereafter L19; Cordiner et al.2020). Additionally, through the combination of carbon monoxide (CO) and HCN emission lines, L19 retrieved Titan's vertical temperature and HCN abundance profiles near the equator during 2016, showing a distinct mesopause at ∼800 km.

Here we analyze ALMA observations of Titan from 2012, 2014, 2015, and 2017 (L_S ∼ 34°–89°) to determine the thermal structure and HCN abundance throughout Titan's upper atmosphere. These results are compared to previous measurements from the Cassini and Voyager missions, ground-based observations, and model predictions of Titan's atmosphere. We describe the observations and data reduction in Section 2 and the radiative transfer modeling process in Section 3. The results, comparisons to previous measurements and models, and discussion are presented in Section 4, followed by concluding remarks in Section 5.

Data from the ALMA main array, comprised of up to 50 12 m antenna dishes, were obtained from the ALMA Science Archive ⁸ covering the period 2012–2017. Observations of Titan were chosen such that the ALMA spatial resolution, often denoted by the FWHM of the point-spread function (PSF)—i.e., the synthesized beam—was <05 (roughly half the angular size that Titan's solid body plus extended atmosphere subtends on the sky). The spectral resolution was high enough to resolve the ∼10 MHz wide HCN line core, enabling measurement of Titan's upper atmospheric temperature profile (L19). The same rotational transition of HCN was measured across multiple observations so as to directly compare the derived vertical HCN and temperature profiles; in this case, the HCN J = 4−3 transition at 354.505 GHz was used. The resulting parameters for selected archival observations with the qualities specified above are presented in Table 1, including a mix of dedicated Titan observations from 2012, 2016, and 2017 and short observations from 2014 and 2015, where Titan was used as a flux calibration source. As the 2016 data were previously analyzed by L19, the observational parameters and spectra are only shown here for context and comparison.

The ALMA observations were reduced and calibrated in the Common Astronomy Software Application (CASA) package (Jaeger 2008) using the Joint ALMA Observatory (JAO) pipeline scripts provided with each observation. Typical pipeline calibration procedures included the correction of complex visibility gain and phase measurements and amplitude calibration using strong (sub)millimeter continuum sources, such as quasars or solar system moons (including Titan itself). For observations from 2014 and 2015, where Titan was used as a flux calibration target, the JAO scripts were modified to remove commands flagging spectral windows containing HCN emission lines, which are often removed during calibration procedures due to their strong, broad line wings. Calibrated visibilities were corrected to Titan's rest velocity frame using ephemeris data from the JPL Horizons System ⁹ and deconvolved to remove interferometric artifacts using the CASA tclean task. During imaging, pixel sizes were set to one-fourth to one-fifth of the ALMA synthesized beam size. The Högbom clean algorithm (Högbom 1974) was used, and additional weighting to long antenna baselines was applied using the Briggs weighting scheme with a robust parameter of 0.5 (Briggs 1995). The data were cleaned to a flux threshold of twice the rms noise.

Individual spectra were extracted from pixels at Titan's nadir and western limb and converted to radiance units for use during radiative transfer modeling using the procedures described in Thelen et al. (2018, 2019b). The nadir spectra of Titan's strong, rotational HCN emission lines are particularly sensitive to the mesospheric temperature structure (L19); comparisons of HCN nadir spectra are shown in Figure 1, with the inset showing differences in the line core depth between each observation. Simultaneously modeling both limb and nadir spectra allows for the retrieval of Titan's vertical thermal profile while mitigating degeneracy imposed by variability in the vertical HCN volume mixing ratio (VMR) profile (L19). To ensure that the HCN line core was fixed at the rest frequency, small spectral line shifts were added to the extracted limb spectra (on the order of 1–2× the ALMA channel spacing; Table 1) to account for Doppler shifts induced by upper atmospheric winds found in high-resolution observations of various molecular species (L19; Cordiner et al. 2020).

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As rotational HCN emission lines are pressure-broadened by Titan's atmosphere and thus vary in line shape due to changes in the atmospheric state with altitude (and as a result of the ALMA beam size and latitude), radiative transfer models were calculated independently for spectra from each observation to determine both the continuous vertical HCN VMR and temperature profiles from the top of the stratopause to the lower thermosphere. Modeling was performed using the Nonlinear optimal Estimator for MultivariatE spectral analySIS (NEMESIS) radiative transfer code developed by Irwin et al. (2008). Synthetic HCN spectra were calculated implicitly using NEMESIS in line-by-line mode using spectral line and partition function data from the Cologne Database for Molecular Spectroscopy ¹⁰ (Müller et al. 2001) and HITRAN catalog ¹¹ (Gordon et al. 2022). Line broadening and temperature-dependence coefficients, collisionally induced absorption pairs, and initial gas profiles for N₂, CH₄, and H₂ were taken from Serigano et al. (2016), Molter et al. (2016), Thelen et al. (2018), and references therein. A priori vertical profiles for CO, HCN, and HC₃N were used from ALMA observations of Titan in 2012, 2014, and 2015 presented in previous analyses (Molter et al. 2016; Serigano et al. 2016; Thelen et al. 2018, 2019a,2019b). The temperature and HCN a priori vertical profiles used here are shown in Figure 2.

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Models of Titan's (sub)millimeter thermal emission were initialized using temperature profiles from Cassini Radio Science observations sensitive to Titan's troposphere and stratosphere at altitudes <100 km (Schinder et al. 2012, 2020). Spectral windows adjacent to the HCN J = 4−3 emission line were then used to determine offsets between the data and model continuum to derive a multiplicative corrective scaling factor (if needed), as described in Thelen et al. (2018); these corrective factors were on order 0.90–1.10, a typical uncertainty for flux density calibrations of ALMA with quasars and solar system objects (Fomalont et al. 2014).

As previous studies retrieved Titan's vertical temperature (using CO) and HCN profiles throughout Titan's stratosphere (Serigano et al. 2016; Molter et al. 2016; Thelen et al. 2018, 2019b), we parameterized NEMESIS models of HCN spectra such that the vertical temperature and HCN abundance were allowed to vary continuously above the stratopause (∼250–350 km) in a grid of 108 atmospheric layers. Layers were separated vertically by 3–40 km, increasing with altitude. Simple a priori vertical profiles above the stratosphere were used to avoid imparting artificial vertical structure on the retrieved profiles. These include an isothermal profile at 160 K at altitudes >600 km and a simple vertical HCN gradient in log(VMR) from its condensation altitude at ∼80 km (Marten et al. 2002) up to a VMR = 2 × 10⁻⁴ at 1100 km (Vuitton et al. 2007). Subsequent model runs incorporated perturbations to the HCN profile by an order of magnitude in either direction or used the photochemical model profile of Loison et al. (2015) to test the sensitivity of the retrievals to vertical structure (Molter et al. 2016; L19). Unique vertical features, such as a small temperature inversion around 400 km (Figure 3), were considered valid if they manifested in the majority of retrievals. The correlation length for temperature and HCN profiles was set to 1.5 and 3.0 scale heights, respectively, so as to prevent unphysical vertical oscillations in the retrieved HCN profiles (Molter et al. 2016; Thelen et al. 2018). The 1σ errors on the vertical thermal profile were initialized at 1 K above the stratopause and raised to 5–10 K in the upper atmosphere, while the HCN a priori errors were set at 100% of the HCN VMR.

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As in L19, the HCN abundance and temperature were retrieved simultaneously by fitting both nadir and limb spectra at once. While previous observations have shown Titan's thermospheric temperature and HCN abundance to vary with longitude (Snowden et al. 2013; Cui et al. 2016), we found the retrieved profiles from the east and west limbs to fall within the corresponding retrieval errors. To accurately model emission from the ALMA beam shape, 20 emission angles were used for each viewing geometry, as described in Thelen et al. (2018). ¹² A representative set of emission angle weights are shown in Figure 4 for the observation from 2017 May 8 (ALMA Project Code 2016.A.00014.S). A smaller range of data points was modeled near the HCN line center at the native ALMA spectral resolution, allowing for higher weighting of the line core during the χ² minimization of iterative spectral inversions, thereby optimizing the sensitivity of the retrievals to the relevant atmospheric pressures. Derivatives calculated for the HCN J = 4−3 limb and nadir spectral radiances as a function of both HCN VMR and temperature are shown in Figure 5 (see Section 2.1 of Irwin et al. 2008 for a complete description of functional derivatives in the NEMESIS code). These functional derivatives illustrate the sensitivity of the HCN line core to Titan's temperature profile up to ∼1000 km and the complementary nature of limb and nadir spectral sensitivity to HCN abundance that helps to break the degeneracy in HCN and temperature retrievals (L19). The resulting best-fit limb and nadir spectra are shown in Figure 6; see the Supplementary Materials of L19 for the analysis of data from 2016.

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The retrieved vertical profiles corresponding to the best-fit spectral models presented in Figure 6 are shown in Figures 7 and 8, accompanied by the retrieved temperature and HCN profiles from 2016 adapted from L19. As with previous observations of Titan's mesosphere and thermosphere, substantial variability is present between the four profiles derived here and that of L19 above ∼400 km (0.01 mbar), reaching maximum differences of 10–30 K. The differences across our retrieved temperatures are much larger than those found for the stratosphere during the same period with similar ALMA observations (generally ∼5 K from Earth year to year; Figure 2, left; Thelen et al. 2018). A distinct mesopause was found for all four observations analyzed here and in the profile derived from 2016 data by L19, as indicated by the presence of absorption in the HCN line core (Figure 1, inset) that is distinctly sensitive to the pressure, temperature, and HCN abundance of the atmosphere >600 km (Figure 5). The location and temperature of these relative minima are largely different from one observation to the next. This is also clearly visible in the raw spectra as differences in the relative depth and shape of the HCN line cores. The warmest and lowest temperature minimum, 153.7 ± 4.0 K at 563 ± 10 km (∼3.8 × 10⁻⁴ mbar), was found in 2014 (L_S ∼ 56°). The coldest and highest mesopause, 134.9 ± 3.4 K at 812 ± 42 km (∼3.3 × 10⁻⁶ mbar), was found in 2015 (L_S ∼ 68°), just under 26 Titan days (413 Earth days) later. The 2012 and 2017 measurements and those found by L19 are bracketed by these extremes, resulting in a distinct and rapid separation between the 2012 and 2014 (L_S ∼ 34°–56°) observations and those after 2015 (L_S ≥ 68°). The upper atmospheric temperatures vary between 153 K (2016; L19) and 165 ± 4.4 K (2014) at 1200 km (2.0 × 10⁻⁸ mbar), while the largest temperature difference in the lower mesosphere is found at ∼450 km (4.6 × 10⁻⁴ mbar) between 2015 and 2016, where the temperature varies between 155 ± 3.8 and 169 ± 3.0 K. Authors L19 noted a stable, quasi-isothermal region in their retrieved profiles from ∼450–600 km, which was also evident in the a priori profiles they employed based on data from Cassini/CIRS. Similarly, a small temperature inversion was found to be stable in our 2012, 2015, and 2017 results, as shown in the inset in Figure 7. These inversions may be attributed to Titan's detached or transient haze layers, as discussed further in Section 4.2.2.

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The retrieved vertical HCN profiles, as shown in Figure 8, show similarly large variability between the four observations analyzed here. In particular, the 2012 spectrum required substantially lower HCN abundance throughout the atmosphere above ∼600 km (2.5 × 10⁻⁴ mbar), resulting in an order of magnitude less HCN in the lower atmosphere than the other retrievals and ∼30× less than the 2014 and 2016 (L19) profiles near 1100 km (7.6 × 10⁻⁸ mbar). Attempts to fit the spectrum from 2012 using the retrieved HCN profiles from 2014 a priori resulted in large reduced χ² values for spectral fits and required significant or physically unlikely deviations from the input profiles. This suggests that the atmosphere in Titan's low northern latitudes during 2012 was depleted of HCN above the mesopause (occurring near this location in 2012) by a factor of 2–30. Similarly, the 2015 and 2017 retrievals require roughly an order of magnitude less HCN above ∼800 km (7.4 × 10⁻⁶ mbar) than those of 2014 and 2016 (L19). The retrieved HCN VMR values at 1000 km, along with the mesopause altitudes and temperatures, are plotted as a function of time in Figure 9. While the distinct transition between the 2014 and 2015 mesopause locations and magnitudes is evident here, a correlation with the HCN abundance is not apparent.

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4.1. Comparisons to Previous Studies

Our results are compared to previous measurements and predictions of Titan's thermal profile in Figure 10. Titan's mesospheric temperature profiles appear to transition between two states, with a mesopause at ∼600 or 800 km, similar to previous measurements by Kim et al. (2000) and Liang et al. (2007; revised from Shemansky et al. 2005) in 1999 and 2004, respectively. Our profiles also agree with the altitudes of the mesopause predicted by the atmospheric models of Lellouch et al. (1990) and Yelle et al. (1997), though the magnitude of the mesopause temperature is only matched by the Lellouch et al. (1990) model in 2015; our 2014 profile is significantly warmer (∼20 K) than the results of Yelle et al. (1997), despite the similar location of the inversion. These comparisons may reflect the differences in the incorporated physics and inclusion of HCN cooling between the two models (Yelle 1991), while the discrepancy with the model of Yelle et al. (1997) may be due to assumptions of or seasonal variations in aerosol heating and CH₄ or C₂H₆ abundances, which affect the temperature profile more strongly at ∼600 km (Yelle 1991). Our retrieved mesopause temperatures are generally warmer than previous observations from earlier in Titan's seasonal cycle, though they occur at similar altitudes, with the exception of the profile derived from HASI measurements during the Huygens descent, where a temperature minimum of 152 K was found at ∼490 km. We find warmer upper atmospheric temperatures than the average profile measured by Cassini/INMS, though within one standard deviation (125–165 K at 1200 km) observed over a large number of flybys (Snowden et al. 2013). The retrieved profiles are generally cooler than the models of Bell et al. (2010, 2014) and do not exhibit a mesopause at ∼900 km, as seen there; however, the temperatures above those altitudes are generally in good agreement. Our retrieved temperature profiles tend to relax toward the a priori value of 160 K above ∼1100 km, where the spectra are less sensitive to deviations in the temperature profile (Figure 5), consistent with the thermospheric model of Müller-Wodarg et al. (2008) at 1000 km for low northern latitudes (164 ± 6 K at 20° N). The profiles derived from ALMA observations between 2012 and 2017 are consistent with the range of values found through contemporaneous observations with the Cassini/CIRS instrument in the lower mesosphere (see Mathé et al. 2020, as depicted by the range of temperatures in orange in Figure 10, and Vinatier et al. 2020). Compared with the stratosphere, which varies significantly at high latitudes and on seasonal timescales (Sylvestre et al. 2020; Sharkey et al. 2021), the derived mesospheric temperature profiles from ALMA observations present evidence for larger-magnitude thermal variability within low-latitude regions on shorter timescales.

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Previous measurements of Titan's upper atmospheric HCN by ground-based observations and Cassini are compared to our retrieved 2014 and 2015 (L_S ∼ 56° and 68°) profiles in Figure 11. Results from Cassini UVIS and VIMS observations (Shemansky et al. 2005; Koskinen et al. 2011; Adriani et al. 2011) and the NIRSPEC instrument on Keck II (Yelle & Griffith 2003; Kim et al. 2005) show generally higher HCN abundances in the upper atmosphere than found with the Cassini INMS instrument (Magee et al. 2009; Cui et al. 2016) and predicted by photochemical models (Willacy et al. 2016; Vuitton et al. 2019; Dobrijevic et al. 2021). While our 2014 (L_S ∼ 56°) retrieval and that of L19 in 2016 (L_S ∼ 82°) fall somewhere in the middle of these prior observations, the 2012, 2015, and 2017 results deviate further still above ∼900 km, extending the range of measured HCN in Titan's upper atmosphere from ∼3 × 10⁻⁵ to 3 × 10⁻³, with relative maxima occurring at altitudes of ∼950–1100 km. While atmospheric nitriles (e.g., HC₃N) have been found to vary with latitude up to 2 orders of magnitude between Titan's equator and winter pole in the stratosphere (Teanby et al. 2007; Vinatier et al. 2015), we find that the temporal HCN variability is far more significant in the upper atmosphere throughout its year compared to contemporaneous retrievals in the stratosphere at low latitudes by the Cassini CIRS instrument (Teanby et al. 2019; Coustenis et al. 2020; Mathé et al. 2020; Vinatier et al.2020).

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4.2. Interpretations

4.3. Considerations and Future Work

Through the analysis of yearly ALMA observations of Titan from 2012 to 2017 (L_S ∼ 34°–89°), we found large variability in the temperature and HCN abundance of Titan's mesosphere and thermosphere. The largest variation in mesospheric temperatures occurs between 2014 and 2015 (L_S ∼ 56° and 68°), where the mesopause shifts dramatically upward by 250 km from 563 ± 10 to 812 ± 42 km and cools from 153.7 ± 4.0 to 134.9 ± 3.4 K (Figure 9). The observed mesopause at both ∼600 and ∼800 km during this epoch agrees with prior observations by Cassini and Keck II and corroborates the results of models of Titan's atmosphere; however, the large variability observed here provides further challenges for modeling Titan's thermal structure in the mesosphere and stratosphere. Smaller temperature changes in the thermosphere and lower mesosphere may be explained by previously observed perturbations in exogenic energy sources, such as charged particle precipitation and interactions with Saturn's magnetosphere, which occur on timescales shorter than the observations analyzed here. Though latitudinal gradients in temperature were not derived from these observations, dynamical instabilities related to Titan's thermospheric jet (L19; Cordiner et al. 2020) may account for differences in the 2016 and 2017 (L_S ∼ 82° and 89°) profiles. In the lower mesosphere, we retrieve small inversions in the 2012, 2015, and 2017 temperature profiles that may be indicative of the location of transient or detached haze layers located between ∼350 and 420 km following Titan's spring equinox, similar to prior observations during this time period by Cassini and 1 Titan yr before by Voyager 2 and ground-based observations.

Similarly large variability is found in Titan's HCN abundance, which extends the observed thermospheric HCN by an order of magnitude, with measurements from ALMA and Cassini now comprising VMRs from ∼3 × 10⁻⁵ to 3 × 10⁻³ above 900 km. While the range in HCN abundance from year to year may account for deviations in the temperature profile from 2012—which is relatively warm, while the HCN abundance is fairly low—and minor temperature differences in the lower mesosphere and thermosphere, the temperatures retrieved for 2014, 2015, and 2017 require energy sources in addition to HCN cooling, solar UV/EUV heating, and wind shear. These sources may include variability in the abundances of Titan's other atmospheric species produced through photochemistry, such as C₂H₂ or C₂H₆, wave activity, or energy transfer from Titan's lower atmosphere during the seasonal transition to northern summer.

Future observations with ALMA may help to further elucidate the connection between Titan's upper atmospheric temperature and HCN abundance through dedicated, high-resolution observations over short timescales (i.e., <10 Earth days), where diurnal differences and those due to Titan's orbital position relative to the Saturnian plasma environment are relevant. While the variability of Titan's thermosphere was explored during the Cassini era, mesospheric temperature differences with latitude, longitude, and local time may be assessed through the unique capabilities of ground-based (sub)millimeter facilities such as ALMA. In particular, these observations are pertinent in the period around Titan's upcoming autumnal equinox in 2025, where similarly large variations in mesospheric temperature may manifest.

The authors wish to acknowledge the insightful discussion and comments from E. Lellouch regarding ALMA data analysis and radiative transfer modeling and the helpful suggestions from two anonymous reviewers that improved the quality of the final manuscript.

A.E.T. was funded by an appointment to the NASA Astrobiology Postdoctoral Program at Goddard Space Flight Center, administered by the USRA and ORAU through a contract with NASA. C.A.N. and M.A.C. were supported for their contributions to this work by NASA's Solar System Observations (SSO) Program. N.A.T. was funded by the UK Science and Technology Facilities Council.

This paper makes use of the following ALMA data: ADS/JAO.ALMA#2012.0.00727.S, 2 012.1.00635.S, 2 013.1.00446.S, 2 015.1.01023.S, and 2016.A.00014.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

Some of the figures within this paper were produced using IDL color blind–friendly color tables (see Wright 2017).