# Titan Surface Temperatures during the Cassini Mission

D. Jennings, T. Tokano, V. Cottini, C. Nixon, R. Achterberg, F. Flasar, V.

Kunde, P. Romani, R. Samuelson, M. Segura, et al.

► To cite this version:

D. Jennings, T. Tokano, V. Cottini, C. Nixon, R. Achterberg, et al. Titan Surface Temperatures during the Cassini Mission. The Astrophysical Journal Letters, 2019, 877 (1), pp.L8. 10.3847/2041-8213/ab1f91. [hal-02354916](https://hal.science/hal-02354916v1)

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# Titan Surface Temperatures during the Cassini Mission

D. E. Jennings¹, T. Tokano², V. Cottini¹‚³ ID, C. A. Nixon¹ ID, R. K. Achterberg¹‚³, F. M. Flasar¹, V. G. Kunde¹, P. N. Romani¹, R. E. Samuelson¹‚³, M. E. Segura¹‚³, N. J. P. Gorius⁴, E. Guandique¹‚⁵, M. S. Kaelberer¹‚⁵, and A. Coustenis⁶ ID
¹Goddard Space Flight Center, Greenbelt, MD 20771, USA; donald.e.jennings@nasa.gov

²Institut für Geophysik und Meteorologie, Universität zu Köln, Albertus-Magnus-Platz, D-50923 Köln, Germany

³Department of Astronomy, University of Maryland, College Park, MD 20742, USA

⁴The Catholic University of America, Washington, DC 20064, USA

⁵ADNET Systems, Inc., Bethesda, MD 20817, USA

⁶Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique (LESIA), Observatoire de Paris, CNRS, UPMC Univ. Paris 06, Univ. Paris-Diderot, 5, place Jules Janssen, F-92195 Meudon Cedex, France

Received 2019 April 22; revised 2019 May 7; accepted 2019 May 7; published 2019 May 20

## Abstract

By the close of the *Cassini* mission in 2017 the Composite Infrared Spectrometer had recorded surface brightness temperatures on Titan for 13 yr (almost half a Titan year). We mapped temperatures in latitude from pole to pole in seven time segments from northern mid-winter to northern summer solstice. At the beginning of the mission the warmest temperatures were centered at 13 S where they peaked at 93.9 K. Temperatures fell off by about 4 K toward the north pole and 2 K toward the south pole. As the seasons progressed the warmest temperatures shifted northward, tracking the subsolar point, and at northern summer solstice were centered at 24 N. While moving north the peak temperature decreased by about 1 K, reaching 92.8 K at solstice. At solstice the fall-off toward the north and south poles were 1 K and 3 K, respectively. Thus the temperature range was the same 2 K at the two poles. Our observed surface temperatures agree with recent general circulation model results that take account of methane hydrology and imply that hemispherical differences in Titan's topography may play a role in the north-south asymmetry on Titan.

**Key words:** planets and satellites: individual (Titan) – radiation mechanisms: thermal – radiative transfer

## 1. Introduction

During its 13 yr duration, the *Cassini* mission collected data over nearly half of a Titan year, from northern mid-winter in 2004 to beyond summer solstice in 2017. This period covered the large temperature swings associated with the crucial seasonal reversal between northern and southern hemispheres. During that time, the Composite Infrared Spectrometer (CIRS) on board *Cassini* made measurements over the globe of Titan's surface brightness temperatures. With the *Cassini* mission completed the full CIRS data set can be examined to derive a comprehensive picture of Titan's seasonal evolution. A look at the final CIRS surface temperature data set is especially important in view of recent studies using a general circulation model (GCM) that includes the effects of orbital eccentricity and geography on the methane hydrological cycle (Tokano 2019). Seasonal anomalies reported previously (Jennings et al. 2016) in Titan's surface temperatures are explained by the updated GCM model. Here we report new CIRS results from 2015 until the end of the mission in 2017 and combine them with our previously reported measurements to complete our comparison with GCM simulations.

Knowledge of surface temperatures is essential for understanding processes near Titan's surface. Visible and near-infrared sunlight heats the surface directly, penetrating the atmospheric haze, thereby driving the temperature at the bottom of the atmosphere. Solar insolation varies with latitude, season, and time of day (Tokano 2005; Cottini et al. 2012). The lowest layer of the troposphere is the realm of weather, where winds and precipitation are driven by heat from the surface (Rannou et al. 2006; Rodriguez et al. 2011; Lora et al. 2015; Tokano 2019). Methane evaporation and condensation patterns at the surface, which determine surface liquid and lake

distribution, depend strongly on surface temperature (Mitchell et al. 2009; Hayes et al. 2011; Tan et al. 2015). On a global scale, seasonal progression of surface temperatures plays a part in the redistribution of volatiles through atmospheric circulation.

A window of transmission at 520–540 cm⁻¹ (19 µm) in Titan's atmospheric spectrum permits thermal radiation from the surface to reach space. At this wavelength the collisionally induced H₂–N₂ opacity is near minimum and the N₂–CH₄ opacity is low (Courtin et al. 1995). Samuelson et al. (1981) originally identified this spectral window using data from the Infrared Interferometer Spectrometer (IRIS) on *Voyager 1* recorded during the flyby of Saturn and Titan in 1980 November. Flasar et al. (1981) mapped the latitude distribution of IRIS surface brightness temperatures and showed that, at the near-equinox epoch of *Voyager*, it was approximately symmetric north and south. Temperatures dropped from 93 K near the equator to 91 K at both 60 N and 70 S. Radio Science occultation measurements by *Voyager* also gave a near-surface temperature of 93 K (Tyler et al. 1981). As laboratory results and modeling of opacity sources improved (Dore et al. 1986; Courtin 1988; Borysow & Tang 1993) the IRIS surface temperatures were refined, yielding 93.5 ± 0.5 K at the equator with a symmetrical decrease of approximately 2 K toward high latitudes (Courtin & Kim 2002). In the post-*Voyager* era, it was recognized that the radiative time constant of the troposphere, much greater than a Titan year, would not support a substantial seasonal variation (Flasar et al. 1981, 2014; Hunten et al. 1984). The observed north-south symmetry of the surface temperatures was thought to represent the near-surface steady state. In this scenario, the 2–3 K droop toward the poles resulted from long-term circulation in the troposphere (Flasar et al. 1981; Hunten et al. 1984). However, it became clear when
thermal properties of the surface were taken into account in the modeling (Tokano 2005) that the surface reacts to insolation on a much shorter timescale than the lower troposphere. Cassini observations confirmed the prediction of marked seasonal changes of temperatures at the surface (Jennings et al. 2011, 2016).

## 2. Observations and Analysis

Our CIRS data ranged in time from 2004 December to 2017 September. The measurements from 2015 to 2017 have not previously been reported. CIRS was a Fourier transform spectrometer that covered $10^{-1500} \text{ cm}^{-1}$ at resolutions $0.5-15 \text{ cm}^{-1}$ (Kunde et al. 1996; Flasar et al. 2004; Jennings et al. 2017). The CIRS field of view below $600 \text{ cm}^{-1}$ was approximately Gaussian with a FWHM of 2.5 mrad. To maximize sensitivity we used the lowest resolution $15 \text{ cm}^{-1}$ spectra. In some cases higher resolution spectra ($0.5$ or $3 \text{ cm}^{-1}$) were smoothed to a resolution of $15 \text{ cm}^{-1}$. Selection and reduction of spectra followed the procedure that we have described elsewhere (Jennings et al. 2016). We performed latitudinal mapping by averaging radiance spectra in $18^\circ$ latitude bins from $90$ N to $90$ S. All longitudes were averaged in each bin (diurnal effects were averaged and dayside solar reflection is negligible at this wavelength). The effective latitude in each bin was determined as the weighted average of the latitudes of the spectra in that bin. A bin width of $10^\circ$ was chosen as a trade-off between signal-to-noise and latitude discrimination. To keep the CIRS field of view close to the bin size we limited the range between the spacecraft and Titan to a maximum of $140,000$ km. An exception was made in 2017, when the range was extended to $200,000$ km in order to include adequate data, particularly at high latitudes, during the final orbits of the mission. Emission angles were usually limited to less than $50^\circ$ to restrict the field-of-view footprint and to restrain the atmospheric opacity. Exceptions were made at high north and south latitudes in the 2004–06 data and in the far south in the 2017 data, where there were no spectra below $50^\circ$ emission angle. In these cases emission angles of $50^\circ-70^\circ$ were used. We averaged the spectra in seven time periods: (1) 2004 October to 2006 July; (2) 2006 September to 2008 May; (3) 2008 November to 2010 June; (4) 2010 December to 2012 November; (5) 2013 April to 2014 September; (6) 2015 January to 2016 November; and (7) 2017 February to 2017 September. These seven periods were centered on solar longitudes $L_s = 313^\circ, 335^\circ, 0^\circ, 26^\circ, 49^\circ, 73^\circ$, and $90^\circ$. (Note that $L_s = 26^\circ$ and $49^\circ$ are modified from the $28^\circ$ and $53^\circ$ listed in our previous publications.) Periods up to two years in duration were long enough to collect sufficient data and short enough to resolve the seasonal evolution. Over all seven periods the number of spectra in the latitude bins ranged from 19 to 8858, with bins nearer the equator tending to have more spectra. Uncertainties were calculated from the number of spectra in each bin. In the resultant spectrum from each bin we selected a single $15 \text{ cm}^{-1}$ wide spectral channel centered at $530 \text{ cm}^{-1}$ and converted its radiance to brightness temperature. We assumed that the surface emissivity at $19 \mu\text{m}$ was unity (Jennings et al. 2011). Reflected solar radiation is negligible at $19 \mu\text{m}$.

The surface brightness temperatures were corrected for atmospheric attenuation and emission using the approach described previously (Jennings et al. 2011, 2016). Our atmospheric model was based on the temperature profile

recorded by the *Huygens* Atmospheric Structure Instrument (HASI) during the *Huygens* descent (Fulchignoni et al. 2005). The HASI profile was modified to account for latitude dependence and seasonal variation during the mission derived from measurements by CIRS and Radio Science (Coustenis et al. 2010, 2016, 2018; Achterberg et al. 2011; Schinder et al. 2012; Achterberg 2018). We included the winter stratospheric temperature minima seen at high latitudes in both the north and south, and also the very cold temperatures found after 2012 at the south pole (Achterberg et al. 2014; Jennings et al. 2015; Coustenis et al. 2016). Temperatures near the base of the troposphere were transitioned to the temperature at the surface. Corrections were made for haze opacity as a function of altitude and latitude (de Kok et al. 2007; Cottini et al. 2012), and also for variation of $CH_4-N_2$ and $H_2-N_2$ opacities with altitude (see Jennings et al. 2016 and references therein). Temperature corrections were usually less than $0.2$ K, except for those at $70-80$ S and $80-90$ S in the last three periods (2013–17). During those latter periods the cold stratosphere in the vicinity of the south pole required corrections of $0.3-0.8$ K, depending on emission angle.

## 3. Results and Discussion

Our measured surface temperatures are presented in Figure 1. From the seven latitude maps some seasonal characteristics can be recognized immediately. First, the peak temperatures in the vicinity of the equator were always $93-94$ K and decreased toward the poles by $1-4$ K, depending on the season. Second, over the course of the mission the warmest latitudes moved from the southern to the northern hemisphere. Third, the polar north warmed by about $2$ K and the polar south cooled by about $2$ K. These features continue trends that we have seen in our earlier, partial data sets. Our measurement at $10$ S in 2004–06 ($L_s = 313^\circ$) compares well with the HASI measurement at the surface, $93.65 \pm 0.25$ K (Fulchignoni et al. 2005). Our measurements also agree with near-surface temperatures reported by Schinder et al. (2012) from Cassini radio occultations.

Our now-finalized data set allows us to perform a more comprehensive comparison with models. Figure 1 shows predictions from a recent GCM study that includes the methane hydrological cycle and examines the effects of geography, both uniform and non-uniform (Tokano 2019). Non-uniform geography incorporates observed global variations in topography, albedo, emissivity, and thermal inertia. Also shown in the figure is the result from an earlier GCM (Tokano 2005) that did not account for condensed methane on the surface (labeled “dry”). In past reports (Jennings et al. 2011, 2016) we compared CIRS surface temperatures with the earlier dry case study. Using data up to 2012 ($L_s = 313^\circ-26^\circ$) we found that the observations agreed approximately with the dry model if the surface thermal inertia was assumed to be low in the south and high in the north (Figure 1 shows the dry, low thermal inertia scenario). However, beginning in 2013, in the $L_s = 49^\circ$ period, temperatures in the north deviated significantly from the dry model, falling below the predicted spring increase by about $1$ K. Although Saturn’s orbital eccentricity can be expected to give rise to cooler northern summers in the present epoch (Tokano 2019), that cannot be the cause of the observed lag because orbital eccentricity was included in the dry model (Tokano 2005). In our previous paper (Jennings et al. 2016) we suggested that the cooler than expected temperatures were due
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Measured surface brightness temperatures compared with GCM predictions. Observations are shown in blue, with dashed lines indicating observations at emission angles above 50°. Data were averaged in 10° latitude bins in each of seven time periods covering the *Cassini* mission. The error bar on each average is one standard deviation. Latitudes are plotted south (negative) to north (positive). The solar longitudes $L_s$ refer to the center date in each period. The temperature axis is the same for the first six periods, but is extended for the seventh period to include the coldest southern value. Data in the first five periods (2004–2014) are the same as those presented previously (Jennings et al. 2016), but with some high latitude points added to $L_s$ = 313 and 26. Two cases are shown from the recent GCM study by Tokano (2019) for uniform (red) and non-uniform (green) topography. A dry surface, low thermal inertia case from Tokano (2005, Scenario 1) is shown as a brown-dashed line. The three models were averaged over the observation periods.</figcaption></figure>
to evaporative cooling of wet surfaces, both seas and land, at northern latitudes during spring warming (Lora et al. 2015; Jennings et al. 2016; Tokano & Lorenz 2016). Le Gall et al. (2016) came to a similar conclusion from 2.2 cm Cassini radiometer studies. The figure demonstrates that when methane surface hydrology is included in the GCM (Tokano 2019), the observed overall behavior of the surface temperatures is accounted for and in particular the depressed northern spring temperatures are explained. Tokano (2019) reported this agreement with his model results using CIRS data up to 2014. As can be seen in Figure 1, during the final two periods, $L_s = 73^\circ$ and $90^\circ$, the model predictions continued to match the cooler measurements in the north.

On the whole, the CIRS measurements are better described by both the uniform and non-uniform cases than by the dry case. This implies that methane surface hydrology plays a crucial role in driving seasonal surface temperatures. The differences between the two geography cases, primarily driven by topography, are most pronounced in the wetter north (Tokano 2019). By early northern spring ($L_s = 26^\circ$) the non-uniform predictions in the north are well below the uniform predictions. In mid-northern spring ($L_s = 49^\circ$) the measurements above 40 N start to deviate toward the non-uniform case. Then in late northern spring ($L_s = 73^\circ$) up to the solstice ($L_s = 90^\circ$) the difference between the uniform and non-uniform cases becomes less. In those two final periods the measurements between the equator and 40 N conformed more closely to the non-uniform case, while at the higher, polar latitudes the data are nearer the uniform case. The drop in observed temperatures at 0–40 N after 2015 may have been caused by rainfall events like those seen by Turtle et al. (2011). Such events might be indicated by the sharp downward spikes at 30 N in the non-uniform model cases for $L_s = 26^\circ, 49^\circ$ and $73^\circ$. Evaporative cooling following rainfall events might have depressed mid-latitude surface temperatures late in the spring. The Tokano (2019) GCM predicts that in the present epoch the maximum temperatures in the north are never as warm as they are in the south. The south, in contrast to the north, tends to be dry most of the year (Lora & Mitchell 2015; Lora & Ádámkovics 2017; Birch et al. 2018; Tokano 2019). Therefore, there is not much difference among the models in the south and the measurements do not distinguish among them. An exception occurs in the two earliest periods ($L_s = 313^\circ$ and $335^\circ$), where at the south pole the dry prediction is significantly higher than the other two cases and also above the data. A dip in temperatures at the pole might be evidence of permanent surface wetness there. The south pole is expected to retain moisture more persistently than the rest of the southern hemisphere (Tokano 2019).

An anomalous warming occurred at southern latitudes in 2015–16 ($L_s = 73^\circ$). The temperatures at 50–90S were significantly above all three of the model predictions. Apparently this warming did not happen during the previous 2013–14 ($L_s = 49^\circ$) or following 2017 ($L_s = 90^\circ$) periods (Coustenis et al. 2019). We are not certain what would cause such a transient warming in the polar south in late fall, but one possible mechanism is the release of latent heat as a result of ethane condensation near the surface. This process is not included in the models. It is natural to assume that the effect was related to the dynamic atmospheric structure that developed at the south pole at mid-to-late fall (Achterberg et al. 2014; Jennings et al. 2015; West et al. 2016; Coustenis

et al. 2019). At that time the descending branch of the global meridional circulation was forming at the south pole. Strong, cold downwelling forced trace gases to accumulate and condense in the cold stratosphere (Bampasidis et al. 2012; Teanby et al. 2012, 2017; Coustenis et al. 2013, 2016, 2018; Vinatier et al. 2015, 2018; Sylvestre et al. 2018). In particular, solid particles of ethane descended in the warmer troposphere and re-evaporated, building up abundance at the pole. As deep cooling began in late fall, ethane recondensed near the surface (Rannou et al. 2006), perhaps as mist or fog. The release of latent heat temporarily warmed the surface. Eventually this effect was overcome by winter cooling. We cannot be sure that near-surface ethane condensation would be sufficient to cause the observed heating. Compared to methane, which is included in the model of Tokano (2019), the rate of ethane condensation is 1000 times smaller (Rannou et al. 2006; Anderson et al. 2014). On the other hand, methane condensation may have also played a role in releasing latent heat. Methane condensation near the south pole may not have been fully modeled for the time of the strong polar vortex in late fall. An alternative explanation for the apparent warming in the far south is that the opacity of the warmer stratosphere may have increased during 2015–16, producing an error in the atmospheric correction. Between 2013 and 2017 the temperature at 0.5–5 mbar at high southern latitudes dipped to a low and then recovered (Coustenis et al. 2019). Any enhanced condensation during that time would have briefly raised the opacity and cause the warm stratosphere to contribute extra radiance, beyond that in the atmospheric correction model.

As in our previous reports, we attempted to characterize the distribution and seasonal evolution of Titan's surface temperatures by creating an analytical description of the measurements. We present this as a formula that summarizes the latitudinal dependence of surface temperatures as a function of time:

$$T(L, Y) = (93.53 - 0.095Y) \cos[(L + (0.85 - 3.2Y)) \times (0.0029 - 0.00006Y)]. \quad (1)$$

Here $T(L, Y)$ is the surface brightness temperature, $L$ is the latitude, and $Y$ is the time in years measured from Titan equinox (2009 August 11 → 2009.61). This analytical model is only valid over the range of the data, i.e., for $L = -90$ to +90 and $Y = -4.9$ to 8.1 (2004 October to 2017 September). The formula was derived by first fitting the observations in each of the seven periods in Figure 1 to a separate cosine. Fitting was performed by adjusting the amplitude, phase, and width of each cosine to minimize the standard deviation. These parameters correspond, respectively, to the peak temperature, the shift of the peak from the equator, and the drop-off toward the poles. From examining the seven curves we determined that all three parameters had varied systematically during the seasons. We performed a linear fit ($a+bY$) to the seven values of each parameter and used the resultant six coefficients to create formula (1). We found that this analytical expression is a good representation of the surface temperatures measured by CIRS over the Cassini mission. Its standard deviation from the entire data set is 0.4 K. The worst fit was for 2010–12 ($L_s = 26^\circ$), which had a standard deviation of 0.7 K. We note that this was the time, about two years after equinox, when dramatic changes were occurring globally in the atmosphere (see, for example, Teanby et al. 2012).

Formula (1) summarizes the seasonal trends in the surface temperatures. The northward shift of the peak temperature that
we have previously reported (Jennings et al. 2011, 2016) continued through late northern spring to the solstice. The latitude of maximum temperature (center of north-south symmetry) closely tracked the subsolar point from 13 S in 2005 to 24 N in 2017. We find a non-zero phase offset that corresponds to a small seasonal lag at equinox, about 0.1 Titan months. Figure 1 shows that a small lag is consistent with the non-uniform GCM case, although it is not as clear that there is a lag in the uniform case (see also Tokano 2019, Figure 3). Our seasonal lag agrees with that reported by Janssen et al. (2016) and is also in line with the observations of *Voyager 1* IRIS near the previous equinox in 1980 November (Flasar et al. 1981; Courtin & Kim 2002). A small lag is consistent with the low thermal inertia required of the surface for diurnal variations (Cottini et al. 2012). Our equatorial temperature of 93.5 ± 0.4 K at equinox ($L_s = 0°$) agrees with the *Voyager* IRIS measurement, 93 ± 1 K (Courtin & Kim 2002). From early to late in the mission the maximum amplitude of the cosine dropped by about 1 K, from 93.9 to 92.8 K. The time-dependent width in formula (1) signifies a “flattening” of the temperature distribution as the peak moved north, i.e., a more gradual fall-off toward the poles. An examination of the measurements and of the fitted cosines suggests that, although the peak temperature decreased, the average of north and south pole temperatures in each period remained approximately the same throughout all seven periods.

Our formula can be used to inspect the change in surface temperature at any particular latitude over the two-season mission. For example, at the *Huygens* landing site, 10 S, the temperature ranged from 93.9 to 92.5 K between 2005 and 2017. Because the peak temperature decreased as it shifted northward, the semi-annual temperature variation was strongly dependent on latitude. The temperature variation was greatest at 40 S (93.6–91.7 K), where the surface was relatively dry, and was smallest at 40 N (92.6–92.7 K), where surface temperatures were moderated by the wetter north (Lora et al. 2015; Tokano 2019). Changes in surface condensation must be much less at mid-northern than at mid-southern latitudes. At 40 S between winter and summer, vapor pressures would have increased by a factor of 1.26 for methane and 1.61 for ethane while at 40 N they would have varied by only factors of 1.01 and 1.03. Any northward migration of volatiles while the south is warmer will not be fully undone later in the yearly cycle because the northern temperatures never get as warm as in the south. The temperature ranges at the poles were virtually identical: 91.9–89.8 K at 80 S and 89.9–91.9 K at 80 N. Both poles, at their coldest, reached the triple points for methane (90.7 K) and ethane (90.3 K). The poles may at times harbor both liquid and solid phases. Overall our results indicate that the northern hemisphere presents a cooler, more benign surface environment, consistent with the buildup of condensed volatiles. We note that Coustenis et al. (2019) have reported that in 2017, in the south near the pole, temperatures remained low in the stratosphere at pressures greater than 0.5 mbar. At the same time, southern stratospheric gas abundances had markedly decreased from what they had been in 2014–2015. Late fall may have been a period of enhanced condensation in the atmosphere and deposition on the surface.

Our global surface temperatures support the picture of a north-south hemispherical asymmetry on Titan, with a generally cooler north in the present epoch. Our agreement with model predictions that include the influence of methane

hydrology (Tokano 2019) lend strong support to the conclusion that liquid methane, on land as well as in seas, is more prevalent on Titan's northern surfaces than in the south and moderates the surface temperatures in the north. Unlike the north, most of the southern hemisphere is not wet all year round. This permits the south to be warmer than the north and to exhibit wider temperature swings. The poles on the other hand do not share the hemispherical asymmetry. The minimum and maximum temperatures at the north and south poles are quite similar. This implies that the two poles are both moist. As demonstrated by Tokano (2019), orbital eccentricity alone is not enough to cause strong accumulation of methane at the north pole at the expense of the south pole and topography may play an important role in the north-south asymmetry on Titan.

The authors acknowledge support from NASA's *Cassini* mission and *Cassini* Data Analysis Program. T.T. was supported by DFG grant TO269/4-2. The authors thank C. M. Anderson for illuminating discussions during the course of this work.

## ORCID iDs

V. Cottini https://orcid.org/0000-0003-0839-5855

C. A. Nixon https://orcid.org/0000-0001-9540-9121

A. Coustenis https://orcid.org/0000-0003-3414-3491

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