Origin, evolution, and fate of Titan’s polar clouds

Domain: physics
Published: 2025-12-04
Editorial outcome: minor_revision

Abstract
Saturn’s moon Titan exhibits a rich interplay of atmospheric chemistry and meteorology, including the seasonal formation of polar stratospheric clouds, first observed by Voyager and later by the Cassini mission. Using a global climate model with haze and cloud microphysics, we investigate their origin, evolution, and fate. We find that cloud formation begins in early autumn, triggered by rapid radiative cooling and chemical enrichment within the stratospheric polar vortex. Initially extending to 336 km altitude and composed of benzene and hydrogen cyanide ices, these clouds descend to lower altitudes during autumn and winter, evolving in composition as additional species condense, before dissipating in spring. This process reflects a unified, hemisphere-spanning seasonal mechanism shaping Titan’s climate. Our model explains observed seasonal patterns and predicts the onset of the next northern polar cloud in 2027. These results provide a predictive framework for future Titan observations, missions, and long-term climate evolution studies.

Introduction
Polar clouds were long detected in the northern polar region shortly after the spring equinox, using the InfraRed Interferometer Spectrometer onboard Voyager (November 1980; L_S = 9.1°). Clouds were first inferred from the opacity they generate in the low stratosphere1,2 and, in a more specific way, with spectroscopic signatures of condensate species such as C4N2, HC3N, and C2H2 3,4. Later, tropospheric polar ice clouds were directly imaged in the southern polar region during spring and summer, using large ground-based telescopes5–7 (from October 1998 to January 2005; L_S = 216.8°–300.6°). These scattered events constituted the first hints, still indecipherable, of a polar cloud cycle.

At the very beginning of the Cassini mission in the Saturnian system, both the Imaging Science Subsystem (ISS) and the Visual and Infrared Mapping Spectrometer (VIMS) monitored convective tropospheric clouds in the southern polar regions8 (from April to December 2004; L_S = 290.3°–299.3°). The same year, VIMS carried out its first observations of Titan’s northern hemisphere. In December 2004, it revealed the edge of a first polar cloud structure9. This cloud, initially observed between latitude 51°N and the terminator at 68°N, was estimated to lie between 30 and 65 km in altitude, in the region where CH4 and C2H6 can condense10. In December 2006 (L_S = 326.7°), while the north polar region gradually emerged from the polar night, it brought to light a much more complex and widespread cloud structure, covering the entire polar region11,12. Spectral analyses from the Composite Infrared Spectrometer (CIRS) and VIMS revealed the presence of HCN ice and HC3N ice within this cloud, with cloud top reaching 165 km at 70°N and 150 km at 62°N13,14. This structure began to dissipate gradually in March 2009 (L_S = 355.7°) as Titan approached equinox, and was observed until June 2010 (L_S = 10.5°), when only residual traces remained above the north pole12,14,15. A season later, shortly after the southern autumnal equinox, a large stratospheric cloud structure was identified above the south pole by ISS16,17, beginning in April 2012 (L_S = 32.1°). This detection was later confirmed by VIMS18 and CIRS19. Images acquired in May 2012 (L_S = 33.7°) show a cloud centered above the south pole, at latitudes higher than 80°S, and measured at an altitude of 300 ± 10 km17, with a top at 322 ± 25 km16. It remains unclear whether the cloud formed in April 201216 or, as suggested by limb images taken in September 201117 (L_S = 25.8°), whether its formation began earlier, following a sharp decrease in stratospheric temperature, along with a significant increase in molecular gas concentrations detected by CIRS18,20,21 over Titan’s south polar region between May 2011 (L_S = 21.7°) and February 2012 (L_S = 30.8°). Until the end of the Cassini mission in 2017, the cloud evolved in both horizontal extent and vertical thickness14,16. In April 2013 (L_S = 44.2°), it reached 75°S latitude and expanded to 58°S by April 2017 (L_S = 89.7°), while its top decreased in altitude compared to 2012. Spectral analysis revealed the presence of condensed HCN from 201218, followed by the first detection of C6H6 ice in 201322.

Despite an abundance of high-quality observations, the formation of the northern polar cloud and the final stages of the southern polar cloud during winter could not be observed due to the timing of the Cassini mission. As a result, the formation mechanisms of these clouds and their complete seasonal cycle are not yet fully understood. For instance, it is not clearly established if the south and the north clouds are two ends of the polar system, and if the cloud cycle is symmetric between the two polar regions. Furthermore, the composition and size of the cloud particles, the fate of the chemical species once condensed and their role in surface-atmosphere interactions are not yet determined.

We explore the mechanisms governing the formation and evolution of Titan’s polar clouds using the Titan Planetary Climate Model23,24 (Titan PCM), coupled with a microphysical haze and cloud model (“Methods”). The Titan PCM predicts three main condensation layers24. The first, located between 5 and 25 km altitude, consists of «massive tropospheric clouds» primarily composed of methane. These clouds develop within the ascending branches of the atmospheric circulation, at mid-latitudes in the spring and summer hemisphere. The second layer, situated near the tropopause between 30 and 75 km altitude, corresponds to a cold region oversaturated with trace hydrocarbons (e.g., C2H6) and nitriles (e.g., HCN) that persist throughout the year. This region of condensed hydrocarbons and nitriles is present across the entire planet, with greater density in the polar regions during spring and summer (Fig. 1 and Supplementary Fig. 1). In the following sections, we will refer to this layer as the «condensed mist». Finally, the third condensation layer, which is the main focus of this study, lies at high-altitude, between 75 and 350 km. We will refer to these high-altitude structures, typical of Titan’s polar regions, as «polar clouds».

The fundamental differences between these three condensation regions lie in their respective lifetimes and cloud particle sizes: a few Titan days for the massive tropospheric clouds, roughly half a Titan year for the polar clouds, and a continuous presence throughout the year for the condensed mist. The corresponding droplet sizes are larger than 100, 10, and 1 µm, respectively.

The Titan PCM predicts a similar evolution of the northern and southern polar clouds, with quantitative differences which can be related to Titan’s orbital eccentricity (Fig. 1 and Supplementary Fig. 1). Since more observations are available for the southern polar cloud, particularly regarding its genesis and early stages of evolution, we focus primarily on the evolution of this cloud. We find that the timeline of the southern polar cloud in the Titan PCM, and compared at each step ((a) through (j)) with observations, can be described as follows.

Results
Formation processes of polar clouds on Titan
Before the southern autumnal equinox in August 2009 (L_S = 0.0°), no polar cloud is present above the south pole (Fig. 1a). This is confirmed by observations from the T51 flyby (March 26, 2009; L_S = 355.7°), which show a clear sky over the south pole (Fig. 2a).

At the beginning of autumn, the model predicts that the descent of polar air from the atmospheric circulation cell brings down to the core of the newly formed polar vortex the chemical species produced several hundred kilometers higher (Supplementary Fig. 2), along with Titan’s photochemical aerosols. This process is illustrated in the model by a marked increase in poleward horizontal fluxes of chemical species starting in August 2011 (Supplementary Fig. 3; L_S = 25.3°), and has been characterised by CIRS18,20,21 (Supplementary Fig. 4). The thermal infrared emission to space of these atmospheric compounds, combined with the beginning of the polar night at the south pole, leads to significant cooling in the stratosphere. Between March 2009 (L_S = 355.7°) and October 2011 (L_S = 27.3°), the stratosphere cooled by 30.3 K above the south pole at an altitude of 200 km (Supplementary Fig. 5). The combination of this chemical enrichment (Supplementary Figs. 2–4) and cooling (Supplementary Fig. 5) triggers the nucleation of C6H6 on aerosols in October 2011, followed by its condensation (Fig. 1b). Consequently, the enhancement in scattered sunlight above the south pole observed on September 201117 is more likely attributable to a local enrichment of haze caused by the circulation, rather than to condensation marking the initial stages of the southern polar cloud formation, which actually begins later, at the end of 2011.

At the beginning of 2012, the southern stratospheric temperature continued to decrease (Supplementary Fig. 5), promoting further cloud development and condensation at higher altitudes. Once the nucleation threshold of C6H6 is reached in October 2011, the condensation of HCN on cloud condensation nuclei becomes easier (“Methods”), beginning a few months later, in March 2012 (L_S = 31.5°). The model therefore predicts that HCN can condense in Titan’s stratosphere with temperatures near 130 K, consistent with previous estimates18, and that cloud properties can be reproduced under these conditions, approximately 30 K warmer than predicted by earlier models25. The onset of HCN condensation, which produces an ice mixing ratio ten times higher than that of C6H6 (Supplementary Fig. 6), coincides with the first observations of the southern polar cloud in April 201216 (L_S = 32.1°).

The modeled condensed species are confined within the polar vortex (Fig. 2 and Supplementary Fig. 2), trapped by a strong jet stream that creates a barrier, preventing mixing between polar and mid-latitude air masses. This mechanism thus limits the latitudinal extent of the southern polar cloud to poleward regions (Fig. 2b). Observational evidences of zonal winds and molecular abundances26 confirmed that the stratospheric cloud cannot extend into or beyond the polar vortex, where molecular abundances are significantly lower and temperatures higher, inhibiting mid-latitudes cloud formation.

Few months later, in mid-2012, the cloud reaches titanic proportions in the model. It now extends from 130 to 336 km in altitude (Fig. 1), with a thickness exceeding 200 km above the pole and a peak condensation near 300 km, similar to the cloud altitudes observed between May 2012 (L_S = 33.7°) and April 2013 (L_S = 44.0°) by ISS16,17 and VIMS18. The cloud remains confined within the polar vortex, covering the south pole up to 79°S latitude (Fig. 2c and Supplementary Fig. 2), as observed during the T84 flyby (June 06, 2012; L_S = 34.5°). The modeled cloud is structured in distinct layers: a layer of pure C6H6 ice between 315 and 336 km, a mixture of C6H6 and HCN ices between 150 and 315 km, and finally a layer of dominantly HCN ice between 130 and 150 km (Supplementary Fig. 6).

Seasonal variations of Titan’s polar clouds
As autumn progresses, the temperature in the polar upper stratosphere (>200 km) increases over time (Supplementary Fig. 5) due to enhanced subsidence, which strengthens the adiabatic warming in this region23. While the top of the cloud continues to be supplied with chemical species, they now condense at lower altitudes, following locations where temperatures remain cold enough, thereby reducing the vertical extent of the cloud (Fig. 1). Between 2013 and 2017, the top of the cloud descends from 275 km in July 2013 (L_S = 47.7°) to 225 km in February 2015 (L_S = 65.3°), and down to 160 km in April 2017 (L_S = 89.7°), with a vertical extent of 80 km (Fig. 1d–f), in good agreement with the recent observations16.

Below 200 km altitude, the weaker zonal winds reduce the intensity of the polar vortex, resulting in a less pronounced confinement of chemical species enrichment toward the pole23,27,28 (Supplementary Fig. 2). This allows the cloud to extend in latitude, reaching 75°S in July 2013, 68°S in February 2015, and 59°S in April 2017 (Fig. 2d–f), in agreement with the observations14. This mechanism may explain why the polar cloud is observed at lower altitudes farther from the pole14,16,22.

The Titan PCM predicts that C6H6 primarily condenses between 160 and 275 km in July 2013, between 120 and 225 km in February 2015, and between 100 and 160 km in April 2017 (Supplementary Fig. 6). This is consistent with the detections of C6H6 ice at 280 km in July 2013 and between 170 and 250 km in February 201522. HCN condenses at lower altitudes: mainly between 150 and 260 km in July 2013, 110 and 205 km in February 2015, and 75 to 145 km in April 2017 (Supplementary Fig. 6). The evolution of the cloud toward lower and colder altitudes enables other species to condense in the model (Fig. 1). From December 2016 onwards (L_S = 85.5°), HC3N begins to condense between 90 and 130 km (Supplementary Fig. 6), indicating that by late autumn, the cloud is composed of a variety of ices formed from chemical species present in Titan’s atmosphere.

During most of the winter period (southern hemisphere: 2017–2022, L_S = 90.0–146.5°; northern hemisphere: 2002–2006, L_S = 270.0–326.7°), the cloud is in a steady state and undergoes very little change. The cloud top remains at a maximum altitude of about 145 km (Fig. 1g), continuously supplied with chemical species by the descending air from the global circulation, which persists throughout autumn and winter. However, it can be observed that in winter, the polar vortex weakens in the model (Fig. 2g and Supplementary Fig. 2), leading to a faster horizontal expansion of the cloud, which reaches latitudes down to about 55°S in May 2022 (L_S = 146.5°).

At lower altitudes (<75 km), the condensed mist condensation intensifies (Fig. 1g), which is consistent with the observation of northern winter C2H6 clouds between 35 and 60 km in December 20049 (L_S = 299.3°).

Final phase and dissipation of Titan’s polar clouds
At the end of winter (southern hemisphere: 2022–2025, L_S = 146.5–180.0°; northern hemisphere: 2006–2009, L_S = 326.7–0.0°), subsidence weakens in the model (Supplementary Figs. 2 and 3). Adiabatic heating decreases, giving way to dominant radiative cooling. As a result, the temperature in the cloud region decreases slightly (Supplementary Fig. 7). Simultaneously, a small secondary meridional wind cell emerges in the winter hemisphere (>70°), opposite to the main meridional circulation cell, at altitudes between 100 and 400 km (Supplementary Fig. 2). This cell, associated with the overall destabilization of the main cell, is unstable, forming and dissipating rapidly within the cloud region, and preferentially enriches the high latitudes (~75°) in molecules rather than the pole. The cooling, combined with this circulation cell, enables the cloud to redevelop at latitudes near 75°S, extending vertically between 75 and 160 km (Fig. 1h). The situation is symmetrical between the southern and northern winter hemispheres. Therefore, the late winter ring-shaped structure observed in the north by Cassini during the T51 flyby (March 26, 2009; L_S = 355.7°), linked to the cloud dissipation of its inner part and to the depletion of aerosols at the pole through wet deposition14,26, would also be reinforced by the recent development of the cloud at high latitudes (Fig. 2h).

The model now provides an explanation for the observation of a stable winter cloud layer in the northern polar region, located around 100 ± 20 km altitude, above the condensed mist, as detected by VIMS between December 2006 and May 200926 (L_S = 326.7–357.1°). Furthermore, according to the model, the C6H6 and HCN condense mainly poleward (>70°), while HC3N extends to lower latitudes (beyond 55°). As a result, the cloud is more opaque above the pole than at mid-latitudes. This was observed at the north pole at the end of winter in May 2009 (L_S = 357.1°), where the clouds extended from 64°N up to the north pole and were surrounded by a diffuse hood reaching down to 55°N12.

After the southern spring equinox in April 2025 (L_S = 180.0°), the south pole emerges from the polar night, leading to a warming of the region. Moreover, due to the reversal of atmospheric circulation, the pole is no longer supplied with chemical species in the Titan PCM23 (Supplementary Figs. 2 and 3), as observed by CIRS27. The cloud therefore begins to dissipate (Fig. 1i). This event in the south polar region mirrors the gradual dissipation observed for the northern polar cloud after the northern spring equinox in August 200914 (L_S = 0.0°). Polar clouds therefore, remain for some time after the spring equinox. This persistence was confirmed by Voyager’s observations of the north pole a few months after the northern spring equinox, in November 1980 (L_S = 9.1°), which still revealed condensate signatures in this region3,4. Eventually, the polar cloud completely disappears in the model four years after the equinox (Fig. 2j). This conclusion is supported in particular by the T104 flyby in August 2014 (L_S = 59.9°), which revealed the appearance of small convective clouds and surface features such as lakes and seas in the northern hemisphere14, as well as by ground-based southern hemisphere observations from mid-spring onward in 1988 that detected tropospheric convective clouds5–7.

Methods
Model
The Titan Planetary Climate Model (Titan PCM) is based on previous modeling studies of Titan’s atmosphere23,24, while incorporating higher resolution and an expanded set of chemical species in the cloud microphysics module. The atmospheric domain is discretized on a 32-longitude × 64-latitude grid (11.25° × 2.81°) with 55 vertical levels extending from the surface to approximately 500 km altitude. At each grid point, the dynamical core computes the transport of atmospheric constituents (gases, aerosols, and ices) 16,000 times per Titan day, corresponding to a dynamical timestep of 1 min 30 s. The physical parameterizations compute haze and cloud microphysics, boundary-layer turbulent mixing, dry convective adjustment, and subsurface thermal conduction 400 times per Titan day. Radiative transfer is evaluated 40 times per Titan day, while the photochemical solver is executed four times per Titan day.

Haze processes
The haze microphysics module integrated into the Titan PCM employs a moment-based representation of a bimodal aerosol population, allowing simultaneous treatment of small spherical particles and large fractal aggregates while avoiding the computational cost of bin-resolved schemes24,31. The model calculates production, coagulation, and sedimentation for both aerosol modes. Injection of spherical aerosols produced by high-altitude photochemistry is imposed at the model top according to a Gaussian production profile. Intramodal and intermodal Brownian coagulation for both spherical and fractal aerosols is explicitly solved. Sedimentation is treated using a flux-form scheme with a first-order slip-flow correction. Aerosol properties are prescribed as follows:
• Aerosol density: ρ = 800 kg m−3;
• Production radius of spherical aerosols: r_p = 10 nm;
• Monomer radius: r_m = 50 nm;
• Fractal dimension of aerosols: D_f = 2.0;
• Electric charge of aerosols: n_e = 15 μm−1.

Cloud processes
The cloud microphysics module is designed to simulate the physical processes of nucleation, condensation/sublimation, and sedimentation24,32 for six key species involved in Titan’s cloud formation: CH4, C2H2, C2H6, C4H2, HCN, and HC3N. When the nucleation of one species is triggered, all species can condense simultaneously onto aerosols acting as cloud condensation nuclei (CCN). However, the model does not consider the co-condensation of species onto aerosols33,34, and they therefore condense independently of each other.

Photochemistry and radiative transfer
The photochemical solver is based on updated spectroscopic data and includes only neutral chemistry23. The reaction scheme is solved independently in each 1D column, extending from the surface to 1300 km altitude, and comprises 43 chemical species participating in 323 reactions.

The radiative transfer scheme employs a correlated-k method with 23 narrow bands in the visible range and 23 in the infrared, spanning wavelengths from 0.3 to 10,000 μm23. Absorption by CH4, C2H2, C2H6, and HCN is included. Spectroscopic tables are generated on a 2D grid of 10 pressure bins (10−1–106 Pa) and eight temperature bins (60–200 K). Collision-induced absorption is treated as continuum opacity, and Rayleigh scattering is accounted for. Radiative transfer is fully coupled with the chemical and microphysical modules. The radiative transfer equations are solved under the plane-parallel approximation using a two-stream source-function method.

Initial conditions and simulation setup
The reference simulation presented here includes both haze and cloud microphysics, fully coupled to the radiative transfer module. The simulation was initialized without any pre-existing clouds and the modeled clouds were allowed to evolve freely over a period of 50 Titan years. The interannual variations in the polar cloud cycle become negligible after 30 years, and we present here the average of the last four years of simulation.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of Nature Communications Article NCOMMS-25-66908-T\nSummary\nThis paper builds on previous work by the authors that developed a sophisticated model of planetary climate for Titan (Titan\nPCM), which for the first time includes a fully integrated treatment of chemistry, dynamics and cloud microphysics for multiple\nspecies. In earlier publications the model was described and benchmarked against measurements of global temperature\nand gas abundances by the Cassini spacecraft: this work goes a step further and compares the vertical and latitudinal extent\nof polar stratospheric ice clouds against the Cassini observations.\nOverall, the work is of a very high quality, and both the text and figures are remarkably clear and well described. The results\nand conclusions of the paper are highly significant, showing for the first time that the size and composition of the polar\nstratospheric ice clouds can be well accounted for. The authors make testable hypotheses about how the clouds should\ntherefore develop in future years and suggest that observations with JWST and ground-based telescopes can verify the\npredictions. This model represents a significant advance in the state of the art and will prove an important tool for future\nunderstanding of Titan’s climate.\nOverall, I recommend the paper for publication in Nature Communications with only some minor changes that can be\nsupervised by the editor.\nComments\nL142: change “enriches in molecules the high latitudes around 75oS…” to “enriches the high latitudes around 75oS in\nmolecules…”\nFig 2: the provenance of the images is unclear here. At a minimum, the caption should be expanded to say which images\nare from Cassini ISS, VIMS or other instruments, and the references to papers 13, 15, 18 should be tagged to individual\nimage, so as to make this work more self-contained. Better yet, a Table (perhaps as a Supplement) giving fuller information\nfor each of the images could be provided.\nExtended Data Figure 2: the x-axis here is in L_s, not years as in the other figures, making intercomparison difficult. I\nsuggest adding the corresponding Cassini years, either at the top or bottom x-axis.\nEND REVIEW\n(Remarks on code availability)\nI briefly looked at the code, which appears complete. I have not attempted to download, compile and run this very large\nFORTRAN suite. However, it appears that some improvements could be made to make it more easily useable by outsiders:\n- The README file appears to be a log of code changes, not an actual how-to guide to install and run the code. It should be\nrenamed CHANGES.TXT or similar.\n- A separate user guide (or installation and user guides) and should be provided, both as text, ad preferably as Pdf as well,\nwith figures. This should include some examples of inputs and outputs and command-line examples of execution.\n- A list of related publications should be provided in a PUBLICATIONS.TXT or similar.", "reviewer_2": "This paper describes the annual evolution of Titan’s polar stratospheric clouds as simulated using the new Titan Planetary\nClimate Model. After a brief but thorough introduction that reviews the present understanding of Titan’s stratospheric cloud\nformation and evolution, the authors describe the evolution of middle-atmosphere polar cloud in their model. This provides\ninsight into the cloud evolution during parts of the year lacking observations, suggests potential mechanisms driving the\nobserved cloud patterns, and estimates of surface precipitation rates of each of the species. I'm especially excited to see\nexplanations of the late-winter annular opacity feature and new estimates of precipitation distribution. While this paper\nconstitutes an important contribution to the field, several revisions are necessary to make it accessible for readers who\nare not already intimately familiar with Cassini’s observations of Titan’s polar clouds.\nGeneral comments\nThe authors may want to consider including solar longitudes along with each date referenced in the paper. This would make\nit significantly easier for readers to understand the seasonal evolution being described, and to compare this work to other\nmodeling work and future observations. For example, Figure 2 would be significantly clearer if Ls was included along with\nthe flyby and date labels. Several extended data figures would also greatly benefit from including both date and Ls.\nSimilarly, it would be very helpful if the times a-j were marked when possible in extended data Figs 2-7.\nThis paper differentiates low altitude (below 75 km) and higher altitude (above 75 km) middle-atmosphere cloud, but it is not\nalways clear which of these is being discussed. It would be helpful to add a sentence or two to the introductory section to\nexplain this. Without clarifying this point, several figures and statements could appear contradictory. For example, e. line 81:\n“in July 2009, no cloud is present” appears to contract Figure 1, which shows condensation at low altitudes. Also, line 156.\nSince condensation is highly dependent on the vapor abundance, it would be helpful to include vapor abundance profiles for\neach of the condensing species. It would be especially helpful to have plots of modeled and measured vapor abundance\nprofiles for southern polar fall. Some of these were included in a previous paper, but as far as I know benzene and\ncyanoacetylene were not.\nSince this paper discusses both northern and southern hemispheres, I suggest using the terms “poleward” and\n“equatorward” rather than words like southward and northward, or above or below, to avoid confusion (for example lines\n130-131).\nSpecific comments major and minor comments\nLines 19–21: By stating that the cloud forms at 200 km, this abstract might be read as contradicting previous analyses of\nobservations that first identified the cloud around 300 km (de Kok+2014, West+2016, Hanson+2025). Since this paper\nactually takes a more nuanced perspective, it might be worthwhile to reword the abstract to avoid confusion by readers who\nare less acquainted with the field.\nLine 30: Khanna (2005) does not appear to have done a radiative transfer analysis, but rather “identified” propionitrile based\non the existence of an absorption peak at 221 cm-1 that resembled to the large partial feature at the long-wavelength end of\nthe IRIS spectra. Although they did not reanalyze the Voyager IRIS data, Samuelson+2007 later “unambiguously ruled out”\ncrystalline propionitrile as the source of the 221 cm-1 feature in the data from Cassini/CIRS, which has a larger spectral\nrange that includes the entire 221cm-1 absorption feature. Since later analyses have also cast significant doubt on the\nidentification of propionitrile, and since it has not been identified in an subsequent observations, it is misleading to include it\nin this list.\nLine 56–57: Neither of the cited references (de Kok+2014, Vinatier+2018) appear to support the detection of HC3N ice.\nPerhaps there’s an additional reference missing here?\nLines 68–70: Does the model simulate these species condensing together (whether co-condensation as suggested by\nAnderson+2014,2018 or “layered” condensation) or separately? How does condensation of one species affect condensation\nof the others?\nLines 95–97: The paper referenced (West et al. 2016) seems to assert that the high-altitude scattering observed in\nSeptember 2011 is due to haze, not cloud (condensates). Hanson et al. 2025 also suggest that the first image with clear\nevidence of cloud formation is from April 2012. Both of these papers also locate the cloud at an altitude of 300 km. Do the\nauthors consider their model results to be consistent with these observations, or are they suggesting that the Cassini\nobservations have been misinterpreted? It would help to have more discussion of this discrepancy.\nLines 99–100: It would be more useful to list the absolute temperature here, and state how that compares to previous\nestimates by de Kok+2014 and Hanson+2024. Second, the paper referenced here (Hanson+2024) arrives at their\ntemperature estimate by identifying the conditions that reproduce retrieved cloud properties. Are the authors asserting that\nthey also reproduce the measured cloud properties at a higher temperature, or simply that HCN condenses at a higher\ntemperature?\nLines 115–121: How does this compare with the altitude estimates from ISS observations (Hanson+2025)?\nLine 127: Is the 145 km referring to the maximum cloud top altitude?\nLine 138: It might be helpful to also list here the corresponding time period (2006–2009?) for the northern hemisphere,\nsince this is the period covered by Cassini observations.\nLines 140–142: Does the secondary cell not always appear? I thought all the climate models (TAM, TitanWRF, Titan PCM,\netc) more or less agreed that the end of winter always features a small secondary cell that gradually grows to overtake the\nprimary cell by early spring, but the phrase “occasionally appears” makes it sound like it occurs during some years but not\nothers.\nLines 143–145: This says the cloud redevelops, but the figure 1 caption says (h) is when cloud dissipation begins. Should\nthis say something like “shortly before line h in Figure 1”?\nLines 138–151: Paragraph (h) seems to be saying that the small counterrotating meridional overturning cell that forms in late\nwinter produces both the annular cloud (~70°N) and depleted haze (poleward of ~80°N) seen by (e.g.) Le Mouelic+2012,\nRannou+2012, and Le Mouelic+2018. I assume the changes in the cloud distribution are caused by upwelling at the poles,\nand upper-level convergence and downwelling near 70° latitude. It’s unclear to me whether the authors are attributing the\npolar haze depletion to the same mechanism, or whether the winter cloud removes haze particles through wet deposition\nand leaves a region of depleted haze. I’m concerned that this paragraph might be unclear to readers who are unfamiliar with\nthe 2009 VIMS observations being reference, or readers who are less familiar with Titan’s polar circulation. It might help to\nexplicitly mention the “meridional overturning” or “Hadley circulation” for clarity, and reference one of the papers that\ndiscusses the development of the secondary cell (e.g. Rannou+2006, Lora+2015, Shultis+2022, etc).\nLines 160–161: It would strengthen the argument in the next paragraph to also mention the similar observations of\ntropospheric convective clouds in south polar summer.\nLines 164–167: Will JWST or ELT be able to identify the composition or species in this cloud?\nLine 168: What causes the particles to re-evaporate? Is slightly warmer temperatures at Ls = 30 and 40° or due to limited\nvapor lower in the stratosphere? I think extended data Fig. 4 illustrates this evaporation more clearly than Fig 6.\nLine 171: I’m guessing “subsists” is a typo, should probably say “precipitates”?\nLines 171–173: Extended data Fig. 7c might be easier to interpret if the species were scaled as in Extended data Fig 2. As it\nis currently, the latitudinal dependence is unclear.\nFigure 2: Is this figure just showing opacity above 75 km? The caption also references white dashed circles, but I cannot see\nthem in the figure.\nLine 315 (Extended data Fig 6 caption): I assume this should say cloud “particle” since the particles in question are ice.\nFig. 7c in the extended data shows concerning precipitation asymmetries that are never mentioned or explained in the\ncaption or text. It looks like the precipitation from the CH4 and C2 species might only represent half a year. This figure either\nneeds to be explained further or fixed.\n(Remarks on code availability)\nI'd like to encourage the authors to publish relevant portions of the model output in a public archive as previous Titan GCM\npapers have done.", "reviewer_3": "This manuscript presents the simulated evolution of polar stratospheric clouds in Titan’s atmosphere, using a global three-\ndimensional model that includes coupling between dynamics and microphysics. This is, I believe, the first global model that\ncan simulate these clouds, and the model appears to reproduce limited observations very well. Thus, this is an important\nresult that warrants publication. However, I think a few issues need to be addressed before publication (but these should be\nrelatively easy to address).\nMajor Comment\n1. There is a recent paper, Hanson et al. (The evolution of Titan's cold southpolar cloud. Geophysical Research Letters,\nhttps://doi.org/10.1029/2024GL113415), that needs to be cited. Further, the observation estimates in fig 3 of this paper could\nbe included in Fig 1 of this paper, and maybe fig 2 can be modified to show model-data comparison of the extent of the\ncloud.\n2. The manuscript is full of statements relating the cloud to the polar vortex and meridional circulation, but there are no plots\nof the latter and no evidence to support causal statements E.g. lines 84, 94, 103, and 109 have statements on vortex, some of\nwhich related extent of cloud to vortex edge and mixing, while 141-142 talks about meridional circulation. I don’t doubt the\nmodel shows these features, but these need to be shown.\n3. The one aspect I am not convinced by is mixing by vortex edge limits the cloud extent. I don’t know how the edge has\nbeen defined or where it is in the model (as not shown), but my understanding is that the peak winds are much further\nequatorward of the extent of the cloud. Adding a few wind contours to fig 2 could address this and previous points.\n4. Why are labels (a), (b) etc introduced in the text and fig 1? I think it would be simpler if the different paragraphs just\nreferred to time in Ls, and make lines for Ls in fig 1 more frequent and at regular intervals. This would make the description\neasier to follow as one would know how far along the annual cycle the discussion is without needing to know what letter\nrefers to a particular time.\n5. What happened between (b) and (c) to cause the cold top to go from 200 to over 300 km?\nMinor Comments\nLine 89: “30 K is lost” I don’t think lost is the correct word. The area has “warmed by 30 K” would be better.\nLine 100: Why the difference from ref 25?\n(Remarks on code availability)"}, "author_response": "Response: we would like to express our sincere gratitude to the reviewers for their constructive comments and valuable suggestions. Their expertise and dedication have greatly contributed to improving the quality of our manuscript, and we are deeply appreciative of the time they devoted to this review process.\n\n[...]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Review of Nature Communications Article NCOMMS-25-66908-T\nSummary\nThis paper builds on previous work by the authors that developed a sophisticated model of\nplanetary climate for Titan (Titan PCM), which for the first time includes a fully integrated\ntreatment of chemistry, dynamics and cloud microphysics for multiple species. In earlier\npublications the model was described and benchmarked against measurements of global\ntemperature and gas abundances by the Cassini spacecraft: this work goes a step further and\ncompares the vertical and latitudinal extent of polar stratospheric ice clouds against the Cassini\nobservations. Overall, the work is of a very high quality, and both the text and figures are\nremarkably clear and well described. The results and conclusions of the paper are highly\nsignificant, showing for the first time that the size and composition of the polar stratospheric ice\nclouds can be well accounted for. The authors make testable hypotheses about how the clouds\nshould therefore develop in future years and suggest that observations with JWST and ground-\nbased telescopes can verify the predictions. This model represents a significant advance in the\nstate of the art and will prove an important tool for future understanding of Titan’s\nclimate.Overall, I recommend the paper for publication in Nature Communications with only\nsome minor changes that can be supervised by the editor.", "reviewer_2": "The authors have made major changes that significantly improve the clarity of the manuscript, correct some minor factual errors, and add some additional details, and they have provided insightful responses to my previous comments. This revised manuscript presents a much fuller picture of the south polar cloud’s lifecycle that will be vital for future work on Titan’s polar stratosphere. I have a few minor comments.\nGeneral comments:\n1. It’s very helpful that the authors included more dates and solar longitude, and I think this will make the paper more useful to a wider variety of readers. The second decimal of precision (0.01) of solar longitude is not that meaningful in most cases since it corresponds to a resolution of seven hours (a month roughly corresponds to 1 degree of solar longitude). I recommend matching the precision of Ls to the precision of the actual time/date being referenced.\n2. Like other models, TitanPCM does not appear to capture the cold “polar shoulder” from radio occultation temperature profiles (based on the previous paper, ref 23). Can you estimate how much this could the lower temperatures affect the sublimation/precipitation rates? If so, this would be an interesting detail to include.\nSpecific minor comments:\nLine 41: should be “in the region”\nLine 60: It is unclear whether “thickness” here refers to optical thickness or cloud top altitude (especially since there are no measurements of the lower limit of the cloud).\nLines 82–97: Very nice explanation of the three condensation layers. This significantly clarifies the rest of the paper and will be very helpful for readers less familiar with Titan’s atmosphere.\nLine 103: It might be helpful to indicate here that the letters refer to the stages of cloud formation, for example “We find that the timeline of the southern polar cloud in the Titan PCM, and compared at each step (a through j) with observations, can be described as follows.”\nLine 106: Wasn’t the equinox in August 2009?\nLine 132: What does it mean that HCN is “ten times more intense than” benzene? Is this referring to the ice mixing ratio or the condensation rate, or some other metric?\nLine 191: Clarify that this is referring to a meridional circulation cell to distinguish from the zonal polar vortex discussed elsewhere.\nLines 237–238: This says the sublimating particles release vapor into the troposphere, but both the dramatic decrease in particle sizes shown in supplementary figure 8 and abundances in figure 6 appear to show the sublimation occurring largely in the stratosphere (above 50 km). From Supplementary Fig 9, it does appears that only ethane is re-released in the troposphere, below 50 km.\nLines 253–255: It’s not clear how this anticipates future observations by Dragonfly, since that probe will be confined to low latitudes.", "reviewer_3": "I am satisfied by the revisions the authors have made, and I think the manuscript is suitable for publication."}, "author_response": "Response: we would like to once again express our sincere gratitude for the time you dedicated to us and for your constructive feedback, which has greatly contributed to improving our paper."}]

Decision letter