OPS5
Session assets
Introduction:
Titan's atmosphere features intricate chemical and meteorological dynamics, particularly the seasonal development of polar stratospheric clouds, which were extensively studied by the Cassini mission. These clouds were initially observed during the northern winter, completely covering the North Polar Region between 2004 and 2012 [1]. As Titan transitioned into northern spring, the northern cloud dissipated, and a similar cloud emerged over the South Pole from 2012 until the mission concluded in 2017 [2, 3, 4].
Spectroscopic analyses have revealed that these clouds are composed of various ices, including hydrocarbons such as C6H6, as well as nitriles like HCN [5, 2, 6].
Nevertheless, the full seasonal behavior of these polar clouds and the relationship between their northern and southern occurrences remain poorly understood. Key questions persist: What processes lead to their formation? How do their structure and composition evolve? Are the northern and southern clouds manifestations of a single overarching system? Is there an asymmetry between them? And what becomes of the condensed chemical species?
Results & Discussion:
To address these questions, we employ a microphysical cloud model that simulates key processes—nucleation, condensation, and sublimation—for six primary species involved in Titan’s cloud dynamics: CH4, C2H2, C2H6, C6H6, HCN, and HC3N. This model is integrated with the Titan Planetary Climate Model (Titan PCM) [7, 8], which provides atmospheric constraints and simulates three-dimensional transport and mixing.
Our simulations indicate that Titan's polar clouds begin forming in early autumn at altitudes around 330 km, confined within the polar vortex, triggered by pronounced stratospheric cooling and a buildup of trace gases driven by the planet's general circulation (Figure 1). During autumn and winter, the clouds undergo significant evolution in altitude, extent, and composition. Enhanced atmospheric subsidence during late autumn causes the cloud layer to descend to below 160 km and spread horizontally to about 59° latitude. By late winter, the cloud reaches a final development phase before fully dissipating roughly four years after the spring equinox, driven by warming in the stratosphere and depletion of condensable species in the polar region.
Figure 1. Vertical evolution of the mass mixing ratio of ices at Titan’s South Pole (> 60°S) in the Titan PCM. Vertical lines correspond to key phases in the polar cloud’s evolution.
While these clouds facilitate the formation of ices in Titan’s lower atmosphere, their surface precipitation rates remain minimal compared to that of methane. Due to their high-altitude origin, most of the cloud particles re-evaporate before reaching the surface.
We will present Titan’s polar cloud cycle, the observed variations in altitude and composition between early autumn and late winter, as well as the connection between the clouds observed in the northern and southern hemispheres.
References: [1] Le Mouélic et al (2012) Planet. Space Sci., 60, 1. [2] de Kok et al (2014) Nature, 514, 7520. [3] West et al (2016) Icarus, 270. [4] Le Mouélic et al (2018) Icarus, 311. [5] Griffith et al (2006) Science, 313, 5793. [6] Vinatier et al (2018) Icarus, 310. [7] de Batz de Trenquelléon et al (2025a) Planet. Sci. J., 6, 4. [8] de Batz de Trenquelléon et al (2025b) Planet. Sci. J., 6, 4.
How to cite: de Batz de Trenquelléon, B., Rannou, P., Vinatier, S., and Lebonnois, S.: Complex Seasonal Cycle of Titan's Polar Clouds, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1312, https://doi.org/10.5194/epsc-dps2025-1312, 2025.
Titan is the largest moon of Saturn, with a radius around 2575 km, and it is surrounded by a thick atmosphere composed of nitrogen, methane, and many other organic compounds. The temperature and pressure profiles on Titan enable the condensation of several gases in the atmosphere, including methane, forming clouds. Here we focus on the methane clouds.
Titan has been observed up close during the Cassini mission, between 2004 and 2017. Some of the methane clouds observed then seem convective (see Figure 1 for an example). We want to reproduce these clouds in a numerical model, to try to answer some of these questions:
- what are the conditions that enable convective methane clouds on Titan? (temperature, methane concentration, global scale influence, topography, ...)
- do we form shallow convection? deep convection?
- how does convection organize itself on Titan?
- what is the size of the cloud particles?
- how do the scales of the convective systems on Titan and on Earth relate?
Figure 1 : Tropospheric methane clouds over the south pole of Titan, July 2004 (Cassini/ISS, infrared filters). The cloud diameter is around 500 km.
To tackle these questions, we need a mesoscale model of Titan's atmosphere, i.e. a local model (approximately above a 100 km zone). The current Titan mesoscale model used at LMD couples the physics of the Titan-PCM model (Trenquelléon et al. 2025a) with the dynamics of the WRF model (a weather forecasting model also used for Earth previsions), see for instance Lefevre, Bonnefoy, and Spiga 2024. In our study, we include the new microphysics modules (i.e. the aerosols and clouds modules, described in Trenquelléon et al. 2025b) and use the last version of WRF (WRF-V4). The microphysics schemes we use enable to study the formation of cloud particles, and their size distribution. We use a horizontal resolution of a few kilometers: this enables us to resolve the clouds, i.e. to consider each grid cell to be either completely a cloud or completely clear, without subcell cloud parameterization. Such a model is called a cloud resolving model. For Titan, several cloud resolving models have been developped (Hueso and Sánchez-Lavega 2006, Barth and Rafkin 2010), based on other dynamical cores and physical schemes.
Our first test cases are a "warm bubble" and a "cold bubble", over a very small domain (40x40 km, the top of the model is 30 km high). Such simulations are useful to check the behavior of the model. We obtain respectivelly a rising and a falling air parcel, with for each case methane condensation (when the air reaches colder atmospheric layers for the warm bubble case, and due to low temperatures in the bubble for the cold bubble case).
Our next steps will be to perform more testing (e.g. abundant methane zone). Then, we will study the different formation mechanisms for clouds (in particular convective clouds): solar heating, topographic clouds, methane accumulation due to the global circulation, lake evaporation, ... We will try to explore the parallels and discrepancies between Earth's water convective clouds and Titan's methane convective clouds.
References
Barth, Erika L. and Scot C. R. Rafkin (Apr. 1, 2010). “Convective cloud heights as a diagnostic for methane environment on Titan”. In: Icarus. Cassini at Saturn 206.2, pp. 467–484. issn: 0019-1035. doi: 10.1016/j.icarus.2009.01.032. url: https://www.sciencedirect.com/science/article/pii/S0019103509000591
Hueso, R. and A. Sánchez-Lavega (July 2006). “Methane storms on Saturn’s moon Titan”. In: Nature 442.7101. Number: 7101 Publisher: Nature Publishing Group, pp. 428–431. issn: 1476-4687. doi: 10.1038/nature04933. url: https://www.nature.com/articles/nature04933
Lefevre, Maxence, Léa Bonnefoy, and Aymeric Spiga (July 3, 2024). Mesoscale Modelling of Titan’s Shangri-La region. doi: 10.5194/epsc2024-408. url: https://meetingorganizer.copernicus.org/EPSC2024/EPSC2024-408.html
Trenquelléon, Bruno de Batz de et al. (Mar. 31, 2025a). “The New Titan Planetary Climate Model. I. Seasonal Variations of the Thermal Structure and Circulation in the Stratosphere”. In: The Planetary Science Journal 6.4. Publisher: IOP Publishing, p. 78. issn: 2632-3338. doi: 10.3847/PSJ/adbbe7. url: https://iopscience.iop.org/article/10.3847/PSJ/adbbe7/meta
Trenquelléon, Bruno de Batz de et al. (Mar. 31, 2025b). “The New Titan Planetary Climate Model. II. Titan’s Haze and Cloud Cycles”. In: The Planetary Science Journal 6.4. Publisher: IOP Publishing, p. 79. issn: 2632-3338. doi: 10.3847/PSJ/adbb6c. url: https://iopscience.iop.org/article/10.3847/PSJ/adbb6c/meta
How to cite: Moisan, E., Spiga, A., and Chatain, A.: Developping a new cloud resolving model for Titan’s methane clouds, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1616, https://doi.org/10.5194/epsc-dps2025-1616, 2025.
Introduction:
The Cassini mission has observed surface changes that may represent the formation and dissipation of ephemeral lakes, i.e., transient lakes that dry-up on a geologically short timescale. As with ephemeral lakes on Earth, Titan’s ephemeral lakes result from the interaction of a generally arid climate with the variability of annual weather. Understanding the physics of ephemeral lakes will provide insight into the local, arid hydrological (methanological) cycle on Titan.
We have limited observations of the formation and dissipation of ephemeral lakes on Titan. Our best evidence is for two events: astorm and related ponding of liquid methane on the equator and the formation of a lake in the Arrakis Planitia region. Here we focus on the ephemeral lake in Arrakis Planitia, with particular interest in the mechanisms for the removal of the lake.
Arrakis Planitia Observations:
The formation of an ephemeral lake at Arrakis Planitia began in 2004. From 2004 to 2005, the Imaging Science Subsystem on the Cassini Mission along with a handful of ground-based telescopes observed both extensive cloud activity and darkening of the surface in the Arrakis Planitia region of Titan. Turtle et al. (2009) [1] identified darkening in Arrakis Planitia likely due to liquid methane rain produced by the storm seen in late 2004. The surface was observed to still be dark ∼9 months after the cloud activity [1,2], thus the presumed surface liquid remained at this time. By January 2007 (27 months after the cloud activity), the previously darkened region had brightened [2] however, there were at least some pockets of liquid standing on the surface in May 2007 (31 months after the cloud activity) as evidenced by specular reflections of sunlight observed by VIMS [2]. The darkening observed in Arrakis Planitia is in one region and corresponds with topography observed by the Cassini Synthetic Aperture Radar (SAR). The observations suggest that at least a few decimeters of liquid had pooled in this area.
Investigating the Evaporative Loss of the Ephemeral Lake:
We created a map of Arrakis Planitia as the surface boundary condition for our simulations, which were conducted using the Mesoscale Titan WRF (mtWRF) model. We then integrated new estimates of the volume and longevity of the Arrakis Planitia ephemeral pond into mtWRF, and we simulated several ephemeral lake scenarios for Arrakis Planitia. The simulations were run for 10 tsols (i.e., Titan days), to allow the environment to reach a quasi-steady state.
With these simulations we estimated lake-scale evaporation rates and compared these rates with the observed surface changes. In Figure 1, we show the evaporation rate as a function of initial lake depth for three different Titan solar longitudes, that correspond to the beginning, middle, and end of the ephemeral lake. In each plot, the curves represent the average lake evaporation rate over multiple tsols, starting at the end of tsol 1. In the top row of Figure 1, the evaporation rate changes over a diurnal cycle. The simulation of Ls =142 (middle plot in Figure 1) is now further into southern winter, when the amount of insolation has begun to drop [3], and thus the evaporation rate has dropped by roughly half compared to the Ls = 305 evaporation rates. For the Ls = 35 simulations (bottom row of Figure 1), the diurnal cycle is completely gone. Instead, the evaporation of methane from the lake surface reaches a steady, day-long rate that is similar in magnitude to the nighttime evaporation rates seen in Ls = 305 simulations. This is what we expect for surface liquid methane in the polar south during the winter.
Rafkin et al., (2020) [4] have shown that convectively driven storms can form in the southern polar regions and can deposit up to 0.3 meters equivalent of methane rain over regions spanning 200 km. With pooling, it is possible to get ephemeral lake depths of a meter or greater. Thus, the delivery of methane rain and the range of lake depths that we have studied is consistent with modeling of storms on Titan.
For shallow lake depths, it may be possible for the ephemeral lake to disappear solely due to evaporation. Around 0.14 meters of liquid methane is evaporated per Earth year at the start of the ephemeral lake existence (i.e., 2005 or Ls = 305). Since most of the lake was gone by 2007 (Soderblom, 2024), two years later, an initial lake depth of roughly 0.3 meters could be removed in that time solely through evaporation. These estimates, however, are rather optimistic, since as time moves forward from 2005 to 2007, the evaporation rates over the lakes decreases. Deeper initial lakes, however, would also require infiltration of liquid methane to lower lake levels fast enough to match Cassini observations.
Conclusion:
Mesoscale modeling of the ephemeral lake at Arrakis Planitia demonstrates that evaporation may be able to completely remove the shallowest possible depths for this lake. A deeper lake, however, requires infiltration of methane liquid into the subsurface to explain the observed changes in the lake extent. Regardless, evaporation remains an important mechanism for the ephemeral lake evolution, particularly at the early stages of the lake’s existence, when the evaporation rates were double the rates experienced during polar night.
References:
[1] Turtle, E. P. et al. Cassini imaging of Titan’s high-latitude lakes, clouds, and south-polar surface changes. Geophys. Res. Lett. 36, L02204 (2009).
[2] A Soderblom, J.M., (2024) personal communication.
[3] Lora, J. M.. et al. Insolation in Titan’s troposphere. Icarus 216, 116–119 (2011).
[4] Rafkin, S. C. R. & Soto, A. Air-sea interactions on titan: Lake evaporation, atmospheric circulation, and cloud formation. Icarus 351, 113903 (2020).
How to cite: Soto, A., Soderblom, J., and Steckloff, J.: Potential Evaporation of Ephemeral Lakes on Titan, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1096, https://doi.org/10.5194/epsc-dps2025-1096, 2025.
1. Introduction
Cassini’s RADAR altimetry data enabled the investigation of suspended particles and nitrogen gas bubbles in Titan’s hydrocarbon lakes and seas. These bodies, composed primarily of methane and ethane with dissolved nitrogen, may contain inclusions such as water ice, organic particles (e.g., tholins, nitriles), and nitrogen gas. Solid particles can originate from atmospheric haze, fluvial erosion, or precipitation, and may accumulate through sedimentation or surface runoff. Nitrogen bubbles are thought to form via supersaturation processes, triggered by temperature changes or increased ethane concentration during rainfall, leading to nitrogen exsolution and bubble formation near the seabed [1-3].
We present a methodology to constrain the size and density of both solid and gaseous inclusions using Cassini RADAR altimetric returns. A physical model based on Mie scattering and radiative transfer theory, compares modeled and observed surface-to-volume power ratios (SVR) to assess the inclusion detectability in Titan's liquid environments [4].
Figure 1. Selected bursts from T91 altimetric observation acquired by the Cassini RADAR.
2. Dataset, modelling and assumptions
We analyze altimetric data from the Cassini RADAR acquired at Ku-band (13.78 GHz, λ = 2.17 cm) acquired during fly-by T91 (Fig. 1). The data were processed using the Cassini Processing of Altimetric Data (CPAD), including incoherent averaging and range compression [5]. To enhance the nominal 35-m resolution, we applied super-resolution (SR) method based via Burg algorithm [6,7], improving surface/subsurface peak separation.
Nadir-looking radar observations in altimetry mode allow for the analysis of suspended inclusions in a homogeneous liquid medium. The received waveform enables surface, subsurface, and volume-scattered power measurement from which constrain inclusion size and density. With the objective of obtaining an expression for the Surface-to-Volume scattering power Ratio (𝑆𝑉𝑅), we model a liquid column containing uniformly distributed spherical particles and evaluate the volume backscattered power based on radiative equilibrium. Reflected and transmitted powers are defined using the surface Fresnel coefficient, dependent on the host dielectric constant, and a two-way transmissivity term. The total volume backscattering cross section includes both this transmissivity and the individual particle backscatter cross sections. Signal attenuation from particle scattering and absorption—mainly influenced by size and loss tangent—is also considered. The model assumes identical dielectric properties, no polarization or multiple scattering effects, and neglects shadowing, valid for particles smaller than the radar wavelength. The final expression, obtained for the case of a beam limited configuration, is
where 𝑃𝑆 and 𝑃𝑉 are the power reflected by the surface interface and through the volume respectively, 𝜎𝑜𝑉 and 𝑘𝑒 are the volume backscattering and extinction coefficients , 𝜃3,𝑑𝐵2 is the antenna beamwith at -3 dB, 𝑧1 and 𝑧2 are the integration depths [4].
Figure 2. Two of the selected altimetric observations acquired during fly-by T91 over Ligeia Mare, before and after super-resolution processing, in black and blue respectively. The area highlighted in green refers to the integrated waveform, while the gray one to the integrated noise.
3. Methodology and SVR Measurement
SVR was measured using T91 data over Ligeia Mare, where a consistent seabed echo was detected at approximately 160 m depth [5]. To improve seabed detectability, windowing has applied to the waveform, leading to degradation of the range resolution and limiting the available integration window. This required the application of super-resolution techniques to widen the final integration window between the surface and the subsurface, as shown in Fig.2.
The presence of volume scattering has then been evaluated by comparing the measured SVR with the integrated Signal-to-Noise Ratio (SNR𝑖𝑛𝑡). The measured volume power resulted falling between the integrated noise level across all observations, with lesser variation than 2-𝜎, indicating no clear evidence of volume scattering. Therefore, measured SVR served as an upper limit, constraining the possible size and density of inclusions. This upper bound defines a solution space where, for a given density, a maximum particle size is established—and vice versa—based on the modeled relationship between volume scattering and inclusion properties.
4. Results
Assumed a host medium composed of a liquid mixture of nitrogen-methane-ethane, with dielectric properties defined by a relative permittivity ε′ₕ = 1.72 and loss tangent tgδ = 3.5×10⁻⁵. Our analysis includes both suspended particles and nitrogen gas bubbles as potential inclusions. In Fig. 3, the measured SVR value of 45.92 dB delineates the boundary between two regimes: Non-Detection (ND) zone, where no volume scattering is observed, the Detection Zone (DZ), where scattering would be detectable by the radar, and an Extinction Zone (EZ), where excessive scattering or absorption would fully attenuate the signal, is also shown. By incorporating particle radius estimates from previous literature, the EZ allows us to define the maximum volume fraction at which inclusions become undetectable due to attenuation. Conversely, the DZ indicates the specific volume density required to match the measured SVR.
Figure 3. The figure shows the SVR derived from T91 Cassini RADAR data, modeled using a methane/ethane/nitrogen liquid host with nitrogen gas bubbles. The blue area represents combinations of particle size and density consistent with the absence of volume scattering. Horizontal lines indicate particle sizes from previous studies, while the black solid lines mark the measured SVR, setting upper limits on bubble density for a given size—or maximum size at varying densities.
For nitrogen-gas bubbles with radius of 0.65 mm from laboratory data, the maximum undetectable volume fraction was estimated at 0.19% [2]. For solid inclusions, the study found that particles smaller than approximately 0.34 mm (water ice), 0.5 mm (nitriles), and 0.84 mm (tholins) remain undetectable without causing extinction of the radar signal. Considering a realistic particle size range of 0.025–1.25 mm, the maximum detectable volume fractions were calculated to be 0.02%–3.49% for water ice, 0.03%–1.43% for nitriles, and 0.16%–1.52% for tholins. These results offer constraints on inclusions properties and inform detectability limits of radar systems, guiding future missions planning to Titan or similar icy moons [4].
References
[1] Barnes et al. 2011
[2] Farnsworth et al., 2019
[3] Cordier and Liger-Belair, 2018
[4] Gambacorta et al., 2024
[5] Mastrogiuseppe et al., 2014
[6] Raguso et al., 2024
[7] Gambacorta et al., 2022
How to cite: Gambacorta, L., Mastrogiuseppe, M., Carmela Raguso, M., Poggiali, V., Cordier, D., and Farnsworth, K. K.: Titan lakes and seas: estimation of nitrogen and solid particleS content via Cassini RADAR data, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1891, https://doi.org/10.5194/epsc-dps2025-1891, 2025.
Titan is the only extraterrestrial environment known to support bodies of standing liquid on its surface. The Cassini mission provided important composition measurements of a few lakes and seas, which suggest they may contain anywhere from 5 - 80 % methane and 8 - 79 % ethane mixtures, in addition to dissolved nitrogen from the atmosphere. Cassini also measured trace amounts of higher-order hydrocarbons, such as propane, ethylene, and acetylene, in Titan’s atmosphere. When these trace species rain down onto the surface and mix with the lakes and seas, they create unique chemical conditions and alter the phase behavior, solubility, and stability of Titan’s surface liquids. In an effort to study these environments, we present our experimental work done in the Astrophysical Materials Lab at Northern Arizona University (NAU). We studied two different trace species, ethylene and acetylene, in methane-ethane mixtures under a nitrogen atmosphere of 1.5 bar to replicate Titan lakes. Specifically, we incorporated 1-10% additions of either ethylene or acetylene to the methane-ethane-nitrogen system and performed cooling and warming cycles of our sample between 70 - 100 K. Through a combination of visual inspection and Raman spectroscopy, we found that these mixtures undergo phase changes at different temperatures than their individual end members. The observed phase changes under these conditions have exciting implications for Titan’s lakes and seas, including compositional stratification of surface liquids, bubbles, and ice formation. We also measured the solubilities of both ethylene and acetylene in pure methane and pure ethane at 95 K. We will present the results of these experiments, in addition to thermodynamic models and how they relate to Titan environments.
This work was supported by NASA SSW grant 80NSSC21K0168, the John and Maureen Hendricks Foundation, and the Lowell Observatory Slipher Society.
How to cite: Thieberger, C., Hanley, J., Engle, A., Tan, S., Grundy, W., Lindberg, G., Steckloff, J., and Tegler, S.: Laboratory Experiments of Ethylene and Acetylene in Titan Lakes, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1178, https://doi.org/10.5194/epsc-dps2025-1178, 2025.
Introduction
Saturn’s largest moon, Titan, has a dense atmosphere comprised mostly of nitrogen and methane. The photolysis and ionization of these major components
leads to complex chemical reactions, which create substantial diversity among Titan’s minor atmospheric constituents. Remote sensing and molecular pectroscopy historically have been a critical tool for detecting trace gases in Titan’s atmosphere and help corroborate predictions of Titan’s atmospheric composition from photochemical models. Following the Voyager and Cassini missions, which provided a wealth of spectroscopic studies of Titan’s atmosphere, ground-based measurements have been useful for detecting elusive trace gases. The Echelon-Cross-Echelle Spectrometer (EXES) is a high-resolution (R ∼ 90, 000) mid-infrared spectrometer that was previously operated aboard NASA’s Stratospheric Observatory For Infrared Astronomy (SOFIA)(1 ). EXES benefited from the high altitude flights during the SOFIA mission to make observations above the bulk of the atmosphere to avoid strong telluric absorption lines that inhibit ground based mid-IR spectrometers such as its sister instrument TEXES.
Here we present EXES observations of Titan which were made in an attempt to detect two trace gases, triacetylene (C6H2) and dicyanoacetylene (C4N2). C6H2 is an important polyyne and is predicted to form readily from the addition of the ethynyl (C2H) radical with diacetylene (C4H2). It remains yet to
be detected, though, and the previous upper limit study was limited by the lower spectral resolution of Voyager’s IRIS (R ∼ 145)(2 ). Delpech et al. 1994 derived an upper-limit of 6 × 10−11 which would be detectable by EXES.
Gas-phase C4N2 formation is primarily completed through C3N addition to HCN or, alternatively, CN addition to HC3N(3 ). The ice-phase C4N2, which is formed through solid-state photochemical reactions on the surface of HC3N ice grains, has been detected in spectra measured by Voyager’s IRIS and CIRS
during the Cassini mission (4, 5 ), yet C4N2 in the gas-phase remains elusive to spectroscopic detections. Again, previous studies of the gas-phase upper limits (3σ = 1.53 × 10−9) were performed using spectra collected by CIRS (R ∼ 1240) which has a resolving power significantly lower than EXES(6 ). The high-resolution of EXES will help improve on the upper limits of both of these species and allow for an updated comparison to photochemical model predictions of their vertical profiles in Titan’s atmosphere.
Observations and Modeling
Mid-infrared observations of Titan were made in June of 2021, using EXES. These observations aim to detect the ν11 out-of-plane bending mode of C6H2 at 621 cm−1 and the perpendicular ν9 stretch of the gasphase C4N2 at 472 cm−1. Figure 1 shows a small portion of the EXES spectrum measured at the 621 cm−1 spectral setting. In this region there are strong emission features from diacetylene (C4H2) and propyne (C3H4) which must be fit before analyzing the C6H2 upper limits. Highlighted in the blue box is the region where the ν11 vibrational mode for C6H2 should be present.
To model the collected spectra, we use the arch-NEMESIS radiative transfer package which is a new Python implementation of the NEMESIS radiative transfer code (7, 8 ). The radiative transfer modeling of the measured spectra occurs in two steps. The initial step is to retrieve the atmospheric profiles of the aerosols and known gases using the archNEMESIS optimal estimation algorithm. For the 621 cm−1 spectral setting, the vertical profiles of C4H2, C3H4, and aerosol continuum are retrieved, however, at the 472 cm−1 region, there are no emission features to fit and just the continuum level is retrieved by adjusting the aerosol profile. For both spectral regions, we use a temperature profile and initial gas profiles defined in Vuitton et al. 2019 photochemical model (3 ). The quality of each retrieval is determined by a goodnessof-fit metric (χ2) which compares the residual of the modeled spectrum to the noise of the measurement. Following the retrieval, we derive the upper limits by building forward models of the spectral regions where the abundance of each target species is iteratively increased and a subsequent χ2 is determined. We then take the difference, Δχ2, between the retrieved and updated forward model χ2 to find where the abundance causes significant deviation from the retrieved spectrum. Step-profiles, which have a cutoff altitude and constant abundance above this cutoff, were used to determine the upper-limits for each species. This method has been applied for many different upper limits studies of gases predicted in Titan’s atmosphere (9, 10 ).
Results
Based on these observations, C6H2 and gas-phase C4N2 remain undetected and therefore, we derive the upper limits to their atmospheric abundance. We improve upon the upper limits of C6H2 and C4N2 by an order of magnitude for both species. Figure 2 shows Δχ2 increase sharply with increased abundance for both C6H2 and C4N2. For C6H2 the 3σ upper limit (Δχ2 = 9) is on the order of 10−11 and for C4N2, 10−10. These new upper limits improve on the previously derived upper limits by an order of magnitude for each target species. More work is still being done to precisely determine the upper limits and compare these values to the current photochemical model predictions of their abundance. The values of the 1σ, 2σ, and 3σ upper limits for each species will be reported in the presentation. The upper limits derived improved upon the previous upper limits by an order of magnitude and we are currently working on comparing these upper limits to photochemical models of Titan’s atmospheric composition to build a better understanding of the chemical pathways in Titan’s atmosphere which will also be discussed in the presentation.
Acknowledgments
The material is based upon work supported by NASA under award number 80GSFC24M0006.
References
1. Richter et al., 2018
2. Delpech et al., 1994
3. Vuitton et al., 2019
4. Samuelson et al, 1997
5. Anderson et al, 2016
6. Jolly et al., 2015
7. Alday et al, 2025
8. Irwin et al., 2008
9. Nixon et al., 2010
10. Teanby et al., 2013
How to cite: McQueen, Z., DeWitt, C., Jolly, A., Alday, J., Teanby, N., Vuitton, V., Lavvas, P., Penn, J., Irwin, P., and Nixon, C.: Using SOFIA’s EXES to improve the upper limits for C6H2 and C4N2 in Titan’s atmosphere, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1950, https://doi.org/10.5194/epsc-dps2025-1950, 2025.
Titan’s substantial atmosphere is primarily composed of molecular nitrogen (N2) and methane (CH4), which are dissociated by solar UV photons and subsequently generate a vast chemical network of trace gases. The composition of Titan’s atmosphere is markedly different than that of Saturn, including both the complex molecular inventory and the hitherto measured isotopic ratios – including that of nitrogen (14N/15N). Atmospheric and interior evolution models (e.g., Mandt et al., 2014) indicate that the atmospheres of Saturn and Titan did not form in the same manner or from the same constituents, and that Titan’s atmospheric N2 may have originated from its interior as NH3. The evolution of 14N/15N in Titan’s atmosphere over time does not result in a value comparable to that measured on Saturn and instead is closer to cometary values; this indicates that the origin of Titan’s atmosphere appears to be from protosolar planetesimals enriched in ammonia and not from the sub-Saturnian nebula. However, selective isotopic fractionation of molecular species in Titan’s atmosphere complicates this picture, as the isotopic ratios may vary as a function of altitude (Figure 1). To further constrain the evolution of Titan’s atmosphere – and indeed, its origin – isotopic ratios must be measured throughout its atmosphere, instead of being interpreted from bulk values likely only representative of the stratosphere.
While the measurement of Titan’s 14N/15N in N2 (167.7; Niemann et al. 2010) places it firmly below the lower limit derived for Saturn (~350; Fletcher et al., 2014), Titan’s atmospheric nitriles (e.g., HCN, HC3N, CH3CN) are further enriched in 15N, resulting in ratios closer to 70 (Molter et al., 2016; Cordiner et al., 2018; Nosowitz et al., 2025). The variation in nitrogen isotopic ratios between the nitriles and N2 is thought to be the result of higher photolytic efficiency of 15N14N compared to N2 in the upper atmosphere (~900 km), resulting in increased 15N incorporated into nitrogen-bearing species (Liang et al., 2007; Dobrijevic & Loison, 2018; Vuitton et al., 2019). As these species are advected to lower altitudes, the nitrogen isotope ratio may vary vertically (Figure 1, red and black profiles), but previous measurements have only presented bulk atmospheric isotope ratios primarily representing Titan’s stratosphere (Figure 1, blue lines).
Recent observations with the Atacama Large Millimeter/submillimeter Array (ALMA) have allowed for the derivation of vertical abundance profiles of Titan’s trace atmospheric species and measurements of N, D, and O-bearing isotopologues (Molter et al., 2016; Serigano et al., 2016; Cordiner et al., 2018; Thelen et al., 2019; Nosowitz et al., 2025). However, vertical isotopic ratio profiles have yet to be derived. Here, we utilize observations acquired with ALMA in July 2022 containing high sensitivity measurements of the HC15N J=4–3 transition at 344.2 GHz (~ 0.87 mm) to investigate vertical variations in the 14N/15N of Titan’s HCN. We compare the results of the vertical 14N/15N profile to those predicted by photochemical models to determine the impact of the isotopic-selective photodissociation of nitrogen-bearing molecular species in Titan’s atmosphere, and the impact of the Saturnian and space environments that vary between model implementations.
Figure 1. 14N/15N profile for HCN predicted by photochemical models from Vuitton et al. (2019; black line) and Dobrijevic & Loison (2018; red line). Blue colored bars in the lower atmosphere represent previous HCN nitrogen isotope ratios from Cassini, Herschel, and ground-based (sub)millimeter observations (see Molter et al., 2016, and references therein). Measurements are offset vertically for clarity, and all refer to HC14N/HC15N measurements for the bulk stratosphere.
References:
Cordiner et al., 2018, The Astrophysical Journal Letters, 859, L15.
Dobrijevic & Loison, 2018, Icarus, 307, 371.
Fletcher et al., 2014, Icarus, 238, 170.
Liang et al., 2007, The Astrophysical Journal Letters, 644, L115.
Mandt et al. 2014, The Astrophysical Journal Letters, 788, L24.
Molter et al., 2016, The Astronomical Journal, 152, 42.
Niemann et al., 2010, Journal of Geophysical Research, 115, E12006.
Nosowitz et al., 2025, The Planetary Science Journal, 6, 107.
Serigano et al., 2016, The Astrophysical Journal Letters, 821, L8.
Thelen et al., 2019, The Astronomical Journal, 157, 219.
Vuitton et al., 2019, Icarus, 324, 120.
How to cite: Thelen, A., de Kleer, K., Teanby, N., Hofmann, A., Cordiner, M., Nixon, C., Nosowitz, J., and Irwin, P.: Investigating the Vertical Variability of Titan’s 14N/15N in HCN, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1211, https://doi.org/10.5194/epsc-dps2025-1211, 2025.
Titan’s entire stratosphere is in superrotation (Flasar et al. 2005) and appears to rotate about an axis offset from its solid body rotation axis by around 4o (Achterberg et al. 2008). The stratospheric tilt axis has been estimated previously through temperature measurements (Achterberg et al. 2011; 2008), composition retrievals (Sharkey et al. 2020; Teanby 2010), and by analysis of stratospheric haze (Kutsop et al. 2022; Roman et al. 2009; Snell and Banfield 2024; Vashist et al. 2023) and a polar cloud (West et al. 2016). Despite this, the mechanism causing the tilt is not well understood. This challenge is further heightened as Titan General Circulation Models (GCMs) are yet to resolve a tilt consistent with observations (e.g., Lombardo and Lora (2023a; 2023b)).Understanding the cause of Titan’s stratospheric tilt may provide insight into the underlying dynamics that drive superrotation in Titan’s atmosphere and the behaviour of superrotating atmospheres in general. Furthermore, due to the strength of Titan’s zonal winds, the offset of the stratospheric rotation axis may have a significant effect on the atmospheric descent of the upcoming Dragonfly mission to Titan. Thus, improved constraints on the tilt axis may better inform the landing site calculations for Dragonfly.
We determine the evolution of Titan’s stratospheric tilt axis over 13 years (Ls = 293—93o), which spans almost half a Titan year. The tilt was determined by inspecting zonal symmetry in the (i) thermal and (ii) composition structure of Titan’s stratosphere. These two independent methods probe different latitude regions. We use infrared observations acquired by the Composite Infrared Spectrometer (CIRS) (Flasar et al. 2004; Jennings et al. 2017; Nixon et al. 2019) instrument onboard the Cassini spacecraft, which toured the Saturn system from 2004 to 2017. We use nadir CIRS observations acquired at a low apodised spectral resolution (FWHM∼13.5–15.5 cm−1). This data set provides excellent spatial coverage of Titan’s middle atmosphere throughout the Cassini mission and achieves the best horizontal spatial resolution of any of the CIRS observations. Despite the subtle and often blended spectral features in these data, Wright et al. (2024) show that they can be reliably forward modelled. Vertical profiles of temperature and gas volume mixing ratios (VMRs) are estimated from CIRS FP3/4 spectra using the Non-linear Optimal Estimator for MultivariatE Spectral AnalySIS (NEMESIS) radiative transfer and retrieval code (Irwin et al. 2008). The observations probe pressure levels of ~10—10-3 mbar in Titan’s atmosphere, with peak contributions at around 1 mbar. These data enable us to reveal Titan’s stratospheric thermal and composition structure in the highest meridional resolution to date and facilitate an independent study of the tilt offset of Titan’s stratosphere.
We find that the tilt axis in the mid-latitudes (from (i)) and the equatorial region (from (ii)) are in good agreement, which supports the theory that Titan’s entire stratosphere is tilted relative to its solid body (Achterberg et al. 2008). In addition to this, we present the best evidence yet that the pointing direction of Titan’s stratospheric tilt axis is constant in the inertial reference frame (Wright et al. in press), consistent with previous studies (Achterberg et al. 2011; Kutsop et al. 2022; Sharkey et al. 2020; Snell and Banfield 2024). The tilt azimuth is determined to be 121± 7o West of the sub-solar point at Titan’s northern spring equinox (Ls = 0o). Put another way, the pointing direction of the tilt axis would appear constant to an observer looking down on the Solar System.
In addition, we present new evidence that the magnitude of Titan’s stratospheric tilt axis may have a seasonal dependence, oscillating between values of approximately 2o to 10o with a period similar in length to half a Titan year. If this pattern is real, it suggests that the tilt of Titan’s stratosphere is impacted by seasonal forcing, even though the direction of the tilt remains constant.
Fig 1: Schematic showing the direction of Titan’s stratospheric tilt axis from Wright et al. (in press). Titan and Saturn are shown at some example times in their orbit. The tilt direction is determined to be approximately constant in the inertial reference frame, that is, fixed with respect to the Titan-Sun vector at northern spring equinox (Ls = 0◦). The approximate size of the tilt magnitude, β, is indicated by font size.
References:
Achterberg, R. K., et al. 2008. Icarus 197 (2): 549–55. https://doi.org/10.1016/j.icarus.2008.05.014.
Achterberg, R. K., et al. 2011. Icarus 211 (1): 686–98. https://doi.org/10.1016/j.icarus.2010.08.009.
Flasar, F. M., et al. 2005. Science 308 (5724): 975–78. https://doi.org/10.1126/science.1111150.
Flasar, F. M., et al. 2004. Space Science Reviews 115 (1–4): 169–297. https://doi.org/10.1007/s11214-004-1454-9.
Irwin, P.G.J., et al. 2008. Journal of Quantitative Spectroscopy and Radiative Transfer 109 (6): 1136–50. https://doi.org/10.1016/j.jqsrt.2007.11.006.
Jennings, D. E., et al. 2017. Applied Optics 56 (18): 5274. https://doi.org/10.1364/AO.56.005274.
Kutsop, N. W., et al. 2022. The Planetary Science Journal 3 (5): 114. https://doi.org/10.3847/PSJ/ac582d.
Lombardo, N. A., and J. M. Lora. 2023a. Journal of Geophysical Research: Planets 128 (12): e2023JE008061. https://doi.org/10.1029/2023JE008061.
Lombardo, N. A., and Juan M. Lora. 2023b. Icarus 390 (January):115291. https://doi.org/10.1016/j.icarus.2022.115291.
Nixon, C. A., et al. 2019. The Astrophysical Journal Supplement Series 244 (1): 14. https://doi.org/10.3847/1538-4365/ab3799.
Roman, M. T., et al. 2009. Icarus 203 (1): 242–49. https://doi.org/10.1016/j.icarus.2009.04.021.
Sharkey, J., et al. 2020. Icarus 337 (February):113441. https://doi.org/10.1016/j.icarus.2019.113441.
Snell, C., and D. Banfield. 2024. The Planetary Science Journal 5 (1): 12. https://doi.org/10.3847/PSJ/ad0bec.
Teanby, N. A. 2010. Faraday Discussions 147:51. https://doi.org/10.1039/c001690j.
Vashist, Aadvik S, et al. 2023. The Planetary Science Journal 4 (6): 118. https://doi.org/10.3847/PSJ/acdd05.
West, R. A., et al. 2016. Icarus 270 (May):399–408. https://doi.org/10.1016/j.icarus.2014.11.038.
Wright, L., et al. 2024. Experimental Astronomy 57 (2): 15. https://doi.org/10.1007/s10686-024-09934-y.
Wright, L., et al. in press. The Planetary Science Journal. https://doi.org/10.3847/PSJ/adcab3.
How to cite: Wright, L., Teanby, N., Irwin, P., Nixon, C., Lombardo, N., Lora, J., and Mitchell, D.: The Rise and Fall of a Mid-West Tilt: Seasonal Evolution of Titan’s Stratospheric Tilt Axis, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-175, https://doi.org/10.5194/epsc-dps2025-175, 2025.
Titan, Saturn’s largest satellite, is host to an intricate chemical and dynamical environment. Its atmosphere is composed primarily of molecular nitrogen (N2) and methane (CH4) that form the basis for a complex organic chemistry. But there are known to be temporal and spatial variations in the abundances of the chemical species that compose Titan’s atmosphere. Zonal winds traverse an object along lines of latitude, parallel to the equator. Titan’s zonal winds can reach speeds up to ~ 350 m/s in the upper stratosphere (z ~ 250-350 km) and are responsible for horizontal mixing. They are primarily a result of the seasonally varying solar forcing, and they play an important role in Titan’s global climate and atmospheric circulation (Hörst 2017). The superrotation of Titan’s atmosphere can dominate the transport and mixing of high-altitude photochemical species as well as produce significant east-west asymmetries in the thermosphere.
Considering its global importance, there remain major gaps in our understanding of the detailed behavior of Titan’s wind field. From Hubbard et al. (1993) who first inferred the presence of zonal winds on Titan using stellar occultation observations to de Batz et al. (2025) who improved upon current GCM models, there have been many studies of Titan’s zonal winds. One of the more recent observational studies using ALMA, Lellouch et al. (2019), found a thermospheric, equatorial jet, which is difficult to reproduce in current atmospheric models and raises many questions about the origin of such a fast jet.
In this work, we investigate the temporal evolution of Titan’s high-altitude winds, focusing mainly on the less well studied equatorial region, to help inform our understanding of their physical basis, origin, and climatological implications. Using archival ALMA band 7 (275-373 GHz; ~0.8-1.1 mm) data from 2012-2016, we determined Titan’s zonal wind speeds as a function of latitude and time. We measured the Doppler shifts of molecular emission lines, an example of the Doppler shift is shown for an archival ALMA dataset in Figure 1, by fitting a Moffat function to the emission line profile in each spatial pixel. The data cubes have sufficient spectral resolution (< 300 kHz) to accurately measure Doppler shifts and spatial resolution (< 0.7”) to resolve the East and West limbs of Titan. Using the methods of Cordiner et al. (2020), the data were deconvolved and analyzed to obtain the intrinsic zonal wind speed profile as a function of latitude for each dataset. These profiles will be collated into a time series and compared with current Titan climate and circulation models such as those described by Newman et al. (2011) and Lora et al. (2015). Due to their different contribution functions, different molecules sound wind speeds at different altitudes (Lellouch et al. 2019): HC3N probes the altitude range z ~ 500-900 km, where an equatorial wind speed variation of 116 ± 3 m/s was previously found between 2016-2017 (Cordiner et al. 2020). We extend the altitudinal range of our analysis and include other molecules such as CH3CN (probing altitudes z ~ 200-450 km). Through this work, we further improve the constraints on the temporal variability of Titan’s wind field and better determine the conditions that lead to the formation of the thermospheric jet.
Figure 1: Left: ALMA archival HC3N (J=39-38) continuum subtracted, integrated emission map of Titan observed in 2015, from ALMA project 2013.1.00446.S. The 'x' notes the regions of the limbs where the East and West limb spectra were extracted. Right: Extracted East and West limb spectra, showing a strong Doppler shift due to high-altitude, prograde zonal winds.
This work was supported by NSF grant AST-2407709
References:
Hörst 2017, J. Geophys. Res. Planets, 122, 432-482
Hubbard et al., 1993, A&A, 269, 541-563
Bruno de Batz de Trenquelléon et al., 2025, Planet. Sci. J. 6, 78
Lellouch et al., 2019, Nature Astronomy, 5, 614-619
Cordiner et al., 2020, ApJL, 904, L12
Newman et al., 2015, Icarus, 213, 636-654
Lora et al., 2015, Icarus, 250, 516-528
How to cite: Nosowitz, J., Cordiner, M., Nixon, C., Thelen, A., Cosentino, R., and Charnley, S.: Temporal Variability of Titan’s Equatorial Zonal Winds Between 2012-2016, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1905, https://doi.org/10.5194/epsc-dps2025-1905, 2025.
NASA’s fourth New Frontiers mission, Dragonfly (Turtle & Lorenz 2024), will land a robotic octocopter on Saturn’s moon Titan in 2034 to study its prebiotic chemistry, constrain its habitability, and search for potential chemical biosignatures. The target landing site will be a portion of the Shangri-La sand sea just south of Selk Crater, an 80-km impact structure (Lorenz et al. 2021). Taking advantage of Titan’s low gravity and thick air, Dragonfly will aerially traverse to over twenty distinct landing sites on dune, interdune, and icy crater terrains (Lorenz et al. 2018).
Figure 1. Dragonfly is a large 4.5-meter long, almost 1000 kg aerial rover with a sophisticated and high-mass scientific payload. It will spend its 3.3-year nominal surface mission exploring and sampling Titan’s sand dunes, interdunes, and the icy 80-km Selk Crater impact structure.
On the ground at those sites we will employ our four scientific instruments: a mass spectrometer (DraMS Trainer et al. 2022), a gamma-ray/neutron spectrometer (DraGNS), seven cameras (DragonCam), and a geophysical and meteorological suite (DraGMet). The DrACO sampling system will drill into the surface and ingest surface samples by means of a hydraulic vacuum cleaner. The vehicle spends over 99% of its time on the ground, and therefore while it can and will fly, the term “relocatable lander” brings to mind a better sense for the operations that we expect to execute on Titan.
We seek to determine how far organic chemistry has progressed, to ground-truth the global methane meteorological cycle, to measure the modes and rates of surface geologic processes, to constrain when and where water and organics might have mixed, and to look for evidence that either water- or hydrocarbon-based life may have existed on Titan (Barnes et al. 2021). Our science objectives with respect to organic chemistry are to measure compositions of materials in different geologic settings and to determine presence and abundance of key molecules for Earth-like life. We have goals to characterize Titan’s methane cycle by constraining the atmospheric methane moisture budget at our tropical landing site and assaying the abundance of stored liquid methane in the near-subsurface.
To place our samples into context, we will need to assess their provenance which we will do by determining the conditions for aeolian transport, by determining the transport mode and history of clastic saterials, and by establishing the geologic context of sampled materials. Our fourth goal is to determine where and how liquid water may have mixed with organic material, which we will do with a seismometer to measure current lithospheric activity and, if Titan is sufficiently active seismically, to constrain the depth to Titan’s liquid-water ocean. Other instrumentation will determine the availability of near-surface water ice and constrain past geologic processes.
Dragonfly finally has a goal to search for any potential chemical biosignatures on Titan as may have been produced by either extinct or extant life. Our objectives on this front are to determine enantiomeric abundance of chiral molecules, to determine if patterns exist in molecular masses and distribution, and to follow up on Huygens hydrogen indications with more and more accurate hydrogen profiles of the lower atmosphere.
Dragonfly recently passed its Critical Design Review and we are fabricating hardware ahead of the launch period that opens on 2028 July 5.
Figure 2. Dragonfly is a single-element mission, and will communicate to Earth directly by means of an 85-cm radial line slot array high-gain antenna, as can be seen here deployed on the ventral (top) side of the vehicle.
REFERENCES
Barnes, J. W., Turtle, E. P., Trainer, M. G., et al. 2021,
Science Goals and Objectives for the Dragonfly Titan
Rotorcraft Relocatable Lander, The Planetary Science
Journal, 2, 130, doi: 10.3847/PSJ/abfdcf
Lorenz, R. D., Turtle, E. P., Barnes, J. W., et al. 2018,
Dragonfly: a Rotorcraft Lander Concept for scientific
exploration at Titan, Johns Hopkins APL Technical
Digest, 374
Lorenz, R. D., MacKenzie, S. M., Neish, C. D., et al. 2021,
Selection and Characteristics of the Dragonfly Landing
Site near Selk Crater, Titan, The Planetary Science
Journal, 2, 24, doi: 10.3847/PSJ/abd08f
Trainer, M. G., Brinckerhoff, W. B., Grubisic, A., et al.
2022, Dragonfly Mass Spectrometer Investigation at
Titan, in The Astrobiology Science Conference
(AbSciCon) 2022, 239–03
Turtle, E. P., & Lorenz, R. D. 2024, Dragonfly: In Situ
Aerial Exploration to Understand Titan’s Prebiotic
Chemistry and Habitability, in 2024 IEEE Aerospace
Conference, IEEE, 1–5
How to cite: Barnes, J. W., Turtle, E. P., Trainer, M. G., Lorenz, R. D., Murchie, S. L., MacKenzie, S. M., Pontefract, A., and Dragonfly Science Team, T.: NASA's Dragonfly Rotorcraft Lander Mission to Titan, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-209, https://doi.org/10.5194/epsc-dps2025-209, 2025.
1. Introduction
Titan’s weather is particularly active at all spatial scales: global (e.g., super-rotating winds), intermediate / mesoscale (e.g., convective thunderstorms, air-sea circulations), local (e.g., turbulence in the planetary boundary layer, the atmospheric layer in direct contact with the surface). The aeolian environment of Titan is active, with a fifth of Titan’s surface being covered by dunes fields and the organic dust particles at the surface being potentially transported by wind circulations at all scales. In 2034 NASA’s Dragonfly quadcopter will land and explore Selk impact crater structure in the dark Shangri-La region [1], characterized by a surface rich in organic dust particles organized as dune fields whose peculiar morphology probably involves two distinct wind regimes that remain to be understood [2].
2. Model
To study the near-surface dynamics, a new model was developed based on the Weather-Research Forecast (WRF) non-hydrostatic dynamical core [3] and coupled with the LMD Titan PCM physics package [4, 5]. The domain of interest is centered on Selk crater and Shangri-La region. The horizontal resolution is set to 5 km. Synthetic high-resolution topography and surface characteristic maps such as albedo, thermal inertia and surface roughness, set constant in the PCM respectively to 0.2, 335 J K−1 m−3 and 5 cm, were created based on Cassini SAR images. The meteorological fields initial states are taken from LMD Titan PCM outputs. The mesoscale boundary conditions are forced with the LMD Titan GCM.
3. Results
Fig 1 shows Snapshots map of horizontal winds 10 m above local surface (m s−1) at noon. The topography, plains and mountains, affects the direction of the surface winds. The vast majority of the wind is between ± 0.5 m s−1, consistent with observations. The high mountains are able to engender mountain waves with horizontal wind reaching 1.0 m s−1. The diurnal cycle has an impact on the amplitude and direction of wind, with preferably anabatic winds during the day and katabatic winds at night. Different synthetic topography maps were tested and will be presented. The impact of albedo, thermal inertia and surface roughness spatial variability due to terrain type of these parameters will be discussed. The near-surface dynamics seasonal variability is sensitive to the seasonal variability of the Hadley cell between roughly 100 and 400 km. At both Equinoxes, the Hadley cell will be composed of a cell in each hemisphere, whereas is northern winter, for example, there will only have a large cell with a jet in the Northern Hemisphere. This seasonal variability will affect the tropospheric circulation and therefore the near-surface winds at the synoptic scale. To capture this variability at the mesoscale level, five solar longitudes were selected, thought to be representative of each four region and will be discussed.
Figure 1: Snapshots map of horizontal winds 10 m above local surface (m s−1 ) at noon in the center of the domain. The red circle represents the landing site of Dragonfly [1].
References
[1] Lorenz, R. D. et al. Selection and Characteristics of the Dragonfly Landing Site near Selk Crater, Titan. Planet. Sci. J. 2, 24 (2021).
[2] Malaska, M. J. et al. Geomorphological map of the Afekan Crater region, Titan: Terrain relationships in the equatorial and mid-latitude regions. Icarus 270, 130–161 (2016).
[3] Skamarock, W. C. & Klemp, J. B. A time-split nonhydrostatic atmospheric model for weather research and forecasting applications. Journal of Computational Physics 227, 3465–3485 (2008).
[4] de Batz de Trenquelléon, B., L. Rosset, J. V. d’Ollone, S. Lebonnois, P. Rannou, J. Burgalat, and S. Vinatier The New Titan Planetary Climate Model. I. Seasonal Variations of the Thermal Structure and Circulation in the Stratosphere. The Planetary Science Journal, 6, 78. (2025)
[5] de Batz de Trenquelléon, B., P. Rannou, J. Burgalat, S. Lebonnois, and J. V. d’Ollone The New Titan Planetary Climate Model. II. Titan’s Haze and Cloud Cycles. The Planetary Science Journal, 6, 79. (2025)
How to cite: Lefevre, M., Bonnefoy, L., Spiga, A., Lebonnois, S., and de Batz de Trenquelléon, B.: Mesoscale Modelling of Titan’s Shangri-La reigon:the impact of surface properties on surface winds, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-320, https://doi.org/10.5194/epsc-dps2025-320, 2025.
The NASA Dragonfly mission, selected in 2019 as the 4th mission of the New Frontiers mission program, is set to explore Saturn's largest moon, Titan, in the mid-2030s [1]. Titan remains a prime target for astrobiological research due to its dense atmosphere, active organic chemistry, and suspected subsurface liquid ocean beneath an icy crust [2]. One of the Dragonfly’s key science goals is to detect potential biosignatures and to study the coupling between Titan’s internal, surface, and atmospheric layers through the characterization of the transport of materials across the entire body with the tectonic processes that might drive or influence this global circulation. To date, the composition and thickness of internal layers – the outer ice shell, global ocean, possible high-pressure ice phases, and silicate mantle – remain poorly constrained, limiting our understanding of Titan’s habitability potential [3,4].
To address this, the mission will carry the DraGMet SEIS instrument (uniaxial velocity-sensing short-period seismometer and 3-axis geophones mounted on each skid), designed to perform seismic measurements to probe Titan’s subsurface and deeper internal structure. Titan displays a wide variety of surface features, such as large equatorial dune fields [5], mountain chains [6], and impact craters [7], along with multiple lines of evidence of resurfacing processes [8,9], as documented by the Cassini-Huygens mission (2004–2017). These observations suggest ongoing internal and surface geological activity that could produce signals recordable by a single-station seismometer. Among the anticipated seismic sources, icequakes generated by tidal-induced stress are also expected to occur in icy satellite environments [10,11,12]. Our recent investigations focus on the detectability of seismic body wave reverberations of such events within the icy crust under realistic noise conditions, which includes updated experimental data on DraGMet short-period seismometers’ instrumental self-noise under cryogenic temperatures and models of environmental noise based on Titan’s atmospheric dynamics.
As a first step, we select a seismic source event whose source mechanism and location are consistent with diurnal tidal stress models and surface observations [13]. The event seismic magnitude is set to Mw 4.0, representing an end-member scenario consistent with predictions on Titan seismicity modeled with tidal dissipation energy budgets [14]. Given the uncertainty in ice thickness estimated to range between 50 and 200km [15], we also construct a suite of 1D self-consistent internal structures and anelastic attenuation scenarios, assuming a homogeneous outer water-ice shell (Figure 1).
We simulate high-frequency waveforms with the wavenumber integration method of CPS (Computer Programs in Seismology; [18]), where results are displayed in Figure 2, and then compare the amplitude of the P-wavetrain window content with noise models in the spectral domain.
Atmospheric turbulence is considered as one of Titan's dominant environmental noise sources [22,23]. To enable direct comparisons with synthetic waveform amplitudes, the expected noise level range induced by atmospheric turbulence was modeled for two end-member surface conditions: a highly porous regolith-like material and a rigid icy substrate [24]. In the Fourier domain, environmental noise is predominant at high frequencies (i.e.,≥1Hz), whereas instrumental noise tends to dominate at lower frequencies.
Our results suggest that a Mw≥4.0 event, relatively strong and marginal on Titan, could be detectable within the frequency band where body waves dominate (i.e., 0.1–1Hz) under a low-attenuation scenario. Regardless of the attenuation model, detection becomes increasingly difficult at higher frequencies when the surface is poorly consolidated (i.e., analogous to Martian regolith), limiting the usable frequency range to around 2Hz, or even below 1Hz if the ice is highly attenuating. In the worst-case scenario, involving a thick and strongly attenuating ice shell, the amplitude of these phases is significantly reduced, severely hindering their detection even within the body-waves frequency band (Figure 2).
When multiples are detectable – for instance, during nearby events, strong enough magnitude, or in low-attenuation ice – reverberations can be used to estimate ice thickness. Assuming a uniform crustal thickness, the time separation between multiples can be directly linked to the ice thickness (see Figure 3). The picking of these multiples using this method is illustrated in Figure 2 with dashed-red lines.
References
[1] Barnes et al. (2021). Planet. Sci. J., 2, 130. [2] Iess et al. (2012). Science, 337, 457-459. [3] Vance et al. (2018). Astrobiology, 18, 37-53. [4] Marusiak et al. (2021). Planet. Sci. J., 2, 150. [5] Lorenz et al. (2006). LPSC XXXVII, abstract #1249. [6] Radebaugh et al. (2007). Icarus, 192, 77-91. [7] Wood et al. (2005). LPSC XXXVI, abstract #1117. [8] Tomasko et al. (2005). Nature, 438, 765-778. [9] Moore et al. (2011). Icarus, 212, 790-806. [10] Greenberg et al. (1998). Icarus, 135, 64-78. [11] Smith-Konter and Pappalardo (2008). Icarus, 198, 435-451. [12] Pappalardo et al. (1997). Icarus, 135, 276-302. [13] Burkhard et al. (2022). Icarus, 371, 114700. [14] Panning et al. (2021). SEG Techn. Progr., 3539-3542. [15] Vance et al. (2018). JGR: Planets, 123, 180-205. [16] Crotwell et al. (1998). Seism. Res. Lett., 70, 154-160. [17] Stähler et al. (2018). JGR: Planets, 123, 206-232. [18] Herrmann (2013). Seism. Res. Lett., 84, 1081-1088. [19] Cammarano et al. (2006). JGR: Planets, 111, E12009. [20] Kuroiwa (1964). Contrib. Inst. Low Temp. Sci., A18, 49-62. [21] Dapré and Irving (2024). Icarus, 408, 115806. [22] Jackson et al. (2020). JGR: Planets, 125, e2019JE006238. [23] Lorenz et al. (2021). Planet. Space Sci., 206, 105320. [24] Onodera et al. (2025). 56th LPSC, abstract #1145.
How to cite: Delaroque, L., Kawamura, T., Lucas, A., Rodriguez, S., Onodera, K., Shiraishi, H., Yamada, R., Tanaka, S., Panning, M., and Lorenz, R.: Preparing Dragonfly to Titan: Detection of Body Waves to Constrain Ice Shell Thickness, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1311, https://doi.org/10.5194/epsc-dps2025-1311, 2025.
1. Introduction
The thick atmosphere of Titan is home to complex, tightly coupled dynamics, photochemistry and microphysics. Methane is abundent, and the conditions of pressure and temperature allow for the development of a methane cycle similar to that of water on Earth, with convective methane clouds forming in the troposphere [1]. In addition to the methane cycle, stratiform hydrocarbon clouds at the poles, or HCN clouds at high altitude have also been observed [2]. Knowledge of Titan’s clouds mostly comes from the data collected during the Cassini-Huygens mission (2014-2017). In this context, climate models are invaluable tools for understanding the mechanisms at work and deepening our interpretation of these observations. The characteristics and conditions of formation of Titan’s clouds depends on many parameters and mechanisms, among which the atmospheric conditions, but also the properties of the photochemical aerosols that serve as privileged cloud condensation nuclei. The aim of this presentation is to assess the impact of these different factors on cloud formation and composition, and their repercussions on Titan's climate.
2. Methods
The Titan Planetary Climate Model (Titan PCM [3]), first developed at the Pierre-Simon Laplace Institute (IPSL), is a 3D model that can simulate Titan’s Climate at a global scale. By integrating couplings between dynamics, radiative transfer, chemistry and microphysics, it enables to study the interaction between these various processes and their influence on each other. In particular, it now includes a microphysical model in moments for haze and clouds [4]. Nevertheless, discrepancies with observations persist, and in its current state, the model only takes into account the condensation of a limited number of species (CH4, C2H2, C2H6, HCN) in the form of ice only, and independently of each other. Certain parameters, such as wettability or density of the aerosols, also remain poorly constrained. We want to further develop the microphysical model in order to improve the description of clouds on Titan and better understand the mechanisms of cloud formation and the methane cycle.
3. Results and objectives
Previous studies have hypothesized the existence of different cloud layers in the troposphere, with solid mehane cloud forming around 25 km of altitude, and liquid methane-nitrogen clouds closer to the surface, around 10 km [5,6]. While the model reproduces cloud formation at these two altitudes at a latitude and period coherent with observations, it does not consistently discriminate two distinct layers between 10 and 30 km (see Fig. 1). Therefore, forthcoming studies will focus on the implementation of the various phases of the droplets (solid and liquid), which would enable the impact of mixtures and interactions between species to be taken into account at a later stage. First, we will investigate the formation of liquid-phase clouds in the troposphere. In particular, the role of ethane and nitrogen in the condensation and stabilization of liquid methane will be studied. Model results will be compared with observations. These developments will first be tested in a 1D study before being incorporated into the 3D model.
Fig. 1 : Comparison between the modeled cloud extinction profile at 0.7 μm (in red) and the modeled temperature profile (in black dashed line) at low latitudes (30°N) during the middle of northern summer (Ls=135°). The extinction peak between 30-80 km corresponds to a mist layer of minor species condensates, while the extinction below is associated with methane clouds. The altitude scale on the right axis is approximate.
References
[1] Turtle, E. P., J. E. Perry, J. M. Barbara, A. D. Del Genio, S. Rodriguez, S. Le Mouélic, C. Sotin, et al. « Titan’s Meteorology Over the Cassini Mission: Evidence for Extensive Subsurface Methane Reservoirs ». Geophysical Research Letters 45, no 11 (2018): 5320‑28. https://doi.org/10.1029/2018GL078170.
[2] West, R. A., Del Genio, A. D., Barbara, J. M., Toledo, D., Lavvas, P., Rannou, P., . . . Perry, J. (2016). « Cassini Imaging Science Subsystem observations of Titan’s south polar cloud ». Icarus, 270 , 399-408. doi:10.1016/j.icarus.2014.11.038
[3] De Batz De Trenquelléon, Bruno, Lucie Rosset, Jan Vatant d’Ollone, Sébastien Lebonnois, Pascal Rannou, Jérémie Burgalat, et Sandrine Vinatier. « The New Titan Planetary Climate Model. I. Seasonal Variations of the Thermal Structure and Circulation in the Stratosphere ». The Planetary Science Journal 6, no 4 (1 avril 2025): 78. https://doi.org/10.3847/PSJ/adbbe7.
[4] De Batz De Trenquelléon, Bruno, Pascal Rannou, Jérémie Burgalat, Sébastien Lebonnois, et Jan Vatant d’Ollone. « The New Titan Planetary Climate Model. II. Titan’s Haze and Cloud Cycles ». The Planetary Science Journal 6, no 4 (1 avril 2025): 79. https://doi.org/10.3847/PSJ/adbb6c.
[5] Wang, Chia C., Sushil K. Atreya, et Ruth Signorell. « Evidence for Layered Methane Clouds in Titan’s Troposphere ». Icarus 206, no 2 (avril 2010): 787‑90. https://doi.org/10.1016/j.icarus.2009.11.022.
[6] Curtis, Daniel B., Courtney D. Hatch, Christa A. Hasenkopf, Owen B. Toon, Margaret A. Tolbert, Christopher P. McKay, et Bishun N. Khare. « Laboratory Studies of Methane and Ethane Adsorption and Nucleation onto Organic Particles: Application to Titan’s Clouds ». Icarus 195, no 2 (juin 2008): 792‑801. https://doi.org/10.1016/j.icarus.2008.02.003.
How to cite: Rosset, L., Chatain, A., Jaziri, Y., Carrasco, N., de Batz de Trenquelléon, B., Petetin, C., and Moisan, E.: Cloud formation and composition on Titan with a Planetary Climate Model, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-108, https://doi.org/10.5194/epsc-dps2025-108, 2025.
Titan hosts quite Earth-like weather patterns. This is mainly due to the fact that its surface pressure is very close to the Earth’ (1.5 bar) and that methane is abundant and with atmospheric conditions near its triple point. It can therefore be a gas, a liquid or a solid with small variations in the temperature or pressure. We then observe similar landscapes and weather processes as on Earth: clouds, rain, rivers, lakes, seas (Hayes, 2016, Ann Rev EPS; Turtle et al., 2018, GRL)…
The Cassini-Huygens mission gave us insights on the surface conditions and topography on Titan. The future Dragonfly mission will allow an in-depth analysis of an equatorial location just after the Northern winter solstice with many meteorological sensors (Barnes et al., 2021, PSJ).
In the meantime, and to help prepare future Dragonfly operations, we can run climate models in Titan’s conditions to investigate typical weather patterns in the atmosphere. Here we focus on surface-atmosphere local interactions with an idealized mesoscale model (with a horizontal resolution of 1-4 km) based on the WRF (Weather Research and Forecasting) model developed by NCAR for Earth weather predictions. We call it mtWRF (“mesoscale Titan WRF”). This model can in particular simulate a methane lake (Rafkin & Soto, 2020, Icarus), in 3D (Chatain et al., 2024, Icarus), with topography (Moisan et al., PSJ, in review) and includes the diurnal influence of radiations (Chatain et al., 2022, PSJ). We use input atmospheric profiles from the global scale Titan Atmospheric Model (Lora et al., 2022, Icarus) at various locations and seasons. We investigate the influence of topography with idealized maps as well as synthetic topography maps based on radar data at specific locations on Titan (Bonnefoy et al., EPSC 2024).
Results show that lakes cool down by evaporating methane into the atmosphere. This induces a local cooling of the atmosphere and strong winds at the shores. We observe strong diurnal, latitudinal and seasonal variations, showing that solar heating plays a major role.
These results will help us understand the general absence of waves in lake observations, the possibility of fog in depressions, the evaporation rate of ponds after heavy rains and the intensity of winds created by local slopes.
References:
Barnes, J. W., Turtle, E. P., Trainer, M. G., Lorenz, R. D., MacKenzie, S. M., Brinckerhoff, W. B., Cable, M. L., Ernst, C. M., Freissinet, C., Hand, K. P., Hayes, A. G., Hörst, S. M., Johnson, J. R., Karkoschka, E., Lawrence, D. J., Gall, A. Le, Lora, J. M., McKay, C. P., Miller, R. S., … Stähler, S. C. (2021). Science goals and objectives for the Dragonfly Titan rotorcraft relocatable lander. Planetary Science Journal, 2(4), 130. https://doi.org/10.3847/PSJ/abfdcf
Bonnefoy, L. E., Lefèvre, M., Chatain, A., Spiga, A., Hayes, A. G., Rodriguez, S., & Lucas, A. (2024). Synthetic topography, roughness, albedo, and thermal inertia maps for mesoscale modeling on Titan. Europlanet Science Congress 2024, 907. https://doi.org/10.5194/epsc2024-907
Chatain, A., Rafkin, S. C. R., Soto, A., Hueso, R., & Spiga, A. (2022). Air – Sea Interactions on Titan: Effect of Radiative Transfer on the Lake Evaporation and Atmospheric Circulation. The Planetary Science Journal, 3(10), 232. https://doi.org/10.3847/PSJ/ac8d0b
Chatain, A., Rafkin, S. C. R., Soto, A., Moisan, E., Lora, J. M., Le Gall, A., Hueso, R., & Spiga, A. (2024). The impact of lake shape and size on lake breezes and air-lake exchanges on Titan. Icarus, 411(December 2023), 115925. https://doi.org/10.1016/j.icarus.2023.115925
Hayes, A. G. (2016). The Lakes and Seas of Titan. Annual Review of Earth and Planetary Sciences, 44(1), 57–83. https://doi.org/10.1146/annurev-earth-060115-012247
Lora, J. M., Battalio, J. M., Yap, M., & Baciocco, C. (2022). Topographic and orbital forcing of Titan’s hydroclimate. Icarus, 384(May), 115095. https://doi.org/10.1016/j.icarus.2022.115095
Moisan, E., Chatain, A., Rafkin, S. C. R., Soto, A., Mackenzie, S. M., & Spiga, A. (n.d.). Ramparts around lakes on Titan impact winds and methane evaporation. In Revision for the Planetary Science Journal.
Rafkin, S. C. R., & Soto, A. (2020). Air-sea interactions on Titan: Lake evaporation, atmospheric circulation, and cloud formation. Icarus, 351, 113903. https://doi.org/10.1016/j.icarus.2020.113903
Turtle, E. P., Perry, J. E., Barbara, J. M., Del Genio, A. D., Rodriguez, S., Le Mouélic, S., Sotin, C., Lora, J. M., Faulk, S., Corlies, P., Kelland, J., MacKenzie, S. M., West, R. A., McEwen, A. S., Lunine, J. I., Pitesky, J., Ray, T. L., & Roy, M. (2018). Titan’s Meteorology Over the Cassini Mission: Evidence for Extensive Subsurface Methane Reservoirs. Geophysical Research Letters, 45(11), 5320–5328. https://doi.org/10.1029/2018GL078170
How to cite: Chatain, A., Bonnefoy, L. E., Moisan, E., Rafkin, S., Soto, A., Lora, J., Spiga, A., Lefèvre, M., and Hueso, R.: The local effect of surface liquid methane and topography on Titan’s weather, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-2044, https://doi.org/10.5194/epsc-dps2025-2044, 2025.
Meteoroid entry into planetary atmospheres generates bow shocks, resulting in high-temperature gas conditions. In shocked gas, high temperatures accelerate chemical reactions, leading to significant compositional changes. However, as the gas expands and cools, the reaction rate decreases (cf. Arrhenius's law) and eventually becomes slower than the cooling timescale, causing chemical reactions to freeze out. Thus, the final chemical composition is governed by two key fliud dynamical processes: shock heating and subsequent cooling.
However, many previous studies have estimated the final chemical products under the assumption of equilibrium neglecting fluid dynamics. In this paper, we perform three-dimensional hydrodynamic simulations of meteoroid entry using the Athena++ code, coupled with chemical kinetics calculations via Cantera to model the non-equilibrium chemistry triggered by atmospheric entry. Our aerodynamical simulations reveal the formation of complex shock structures, including secondary shock waves, which influence the thermodynamic evolution of the gas medium. By tracking thermodynamic parameters along streamlines, we analyze the effects of shock heating and subsequent expansion cooling on chemical reaction pathways.
Our results demonstrate that chemical quenching occurs when the cooling timescale surpasses reaction rates, leading to the formation of distinct chemical products that deviate from equilibrium predictions. We show that the efficiency of molecular synthesis depends on the object’s size and velocity, influencing the composition of the post-entry gas mixture. Applying our model to Titan, we demonstrate that organic matter can be synthesized in the present environment of Titan. Also, we find that nitrogen, the dominant atmospheric component, remains stable, while water vapor is efficiently removed, a result inconsistent with equilibrium chemistry assumptions. Moreover, we compare our simulation results with laser experiments and find good agreement in chemical yields. Subsequent impact on the ground surface generates vaporized gas, which can also contribute to atmospheric alteration. Finally, we assess the relative contributions of atmospheric entry heating and impact-induced vaporization in driving atmospheric evolution.
How to cite: Miyayama, R., Schaefer, L., Kobayashi, H., and Zorzi, A.: Modeling Atmospheric Alteration on Titan: Hydrodynamics and Shock-Induced Chemistry of Meteoroid Entry, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-85, https://doi.org/10.5194/epsc-dps2025-85, 2025.
The far ultraviolet (FUV) dayglow emissions have been observed from Titan and Earth by the Cassini Ultraviolet Imaging Spectrograph (UVIS) and Global-scale Observations of the Limb and Disk (GOLD), respectively, operating in the same wavelength range between 135–165 nm. Both Solar System Objects (SSO) are fascinating environments where N2 is the most abundant thermosphere-ionosphere (T-I) species. They both exhibit similar UV spectra. The UV spectra consist of strong emissions from the Lyman-Birge-Hopfield (LBH) band system a 1Πg → X 1Σg+ of N2 excited from photoelectrons. The dayglow’s of both SSOs are most intense at its own ionosphere altitude [900 km at Titan vs 120-150 km at Earth] with similar thermodynamic conditions for atmospheric N2 density [1010-1011 cm-3]. Titan and Earth have vastly different T-I temperatures (150 K vs ~700K) and the presence of minor constituents [ CH4 on Titan vs O, O2 on Earth]. The FUV dayglow spectra of both SSOs are shown over plotted in Fig. 1. Titan’s nitrogen dominated atmosphere is the densest of any moon in the solar system and Earth’s nitrogen dominated atmosphere has the densest N2 content of any planet in the solar system. We propose to present a new model analysis of the full FUV (115–190 nm) disk airglow observations of Titan’s and Earth’s atmosphere based on UV laboratory optically thin electron impact fluorescence studies of the LBH band system. The experiments will be performed at SSO atmospheric T-I density and temperature that must occur to accurately determine the relevant emission cross sections. The experimental apparatus at the University of Colorado has installed a new high energy resolution electron gun to study the threshold energy (10-30 eV) behavior of the LBH band system, where photoelectron distribution functions peak in energy.
This work is under contract to the University of Colorado by NSF Aeronomy and NASA Cassini Data Analysis Program Offices.
Fig. 1 A comparison of GOLD (2019) and Cassini UVIS (2009)
FUV dayglow spectra between 136-155 nm, both containing the LBH bands.
The rotational temperatures of both SSO are very different and represent
the upper atmosphere T-I kinetics temperature.
How to cite: ajello, J., royer, E., aryal, S., evans, S., eastes, R., holsclaw, G., veibell, V., and esposito, L.: Comparison of Planetary UV Dayglow: Analysis of Titan-Cassini and Earth-GOLD UV Airglow Observations By Laboratory Spectroscopy of N2, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1081, https://doi.org/10.5194/epsc-dps2025-1081, 2025.
Introduction: The Huygens probe gave unprecedented information on the properties of Titan’s aerosols (vertical distribution, opacity as a function of wavelength, phase function, single scattering albedo) by in-situ measurements [1]. Being the only existing in-situ atmospheric probing for Titan, this aerosol model currently is the reference for many Titan studies (e.g. by being applied as physical input in radiative transfer models of the atmosphere). A reanalysis of the DISR dataset, corroborated by data from the Downward- Looking Visible Spectrometer (DLVS), was carried out by the same group [2], leading to significant changes to the indications given by [1]. Recent work from [3] also refine the optical properties of Titan’s haze particle, leading in particular to a change in their required fractal dimension.
Here we present the analysis of the Emission Phase Function observation (EPF) performed by VIMS during the Cassini Titan flyby T88 (November 2012) in terms of aerosol optical properties.
Observations: An EPF observes the same spot on the surface (and thus the same atmosphere) with the same incidence angle but with different emergence and phase angles. In this way, our VIMS EPF allows, for the first time, to have direct information on the phase function of Titan’s aerosols, as well as on other important physical parameters as the behavior of their extinction as a function of wavelength and the single scattering albedo (also as a function of wavelength) for the whole VIMS range (0.8-5.2 μm). The T88 EPF is composed of 26 VIMS datacubes spanning a phase angle range approximately from 0°to 70°.
Model: We used the radiative transfer model described in [4] as baseline, updated with improved methane (+ related isotopes) spectroscopy and aerosol description [5]. By changing the aerosol description in the model, we found the combination of aerosol optical parameters that fits best a constant aerosol column density over the whole set of the VIMS datacubes of the EPF sequence of observations.
Result: We confirmed that the results from [2] and [3] do improve the fit for what concerns the vertical profile and the extinction as a function of wavelength. However, a different phase function with respect to what they propose must be employed, especially towards the backscattering region. This has important implications in terms of aerosol physical (size and structure) and chemical (composition and mixing with liquids) properties.
References: [1] Tomasko et al. (2008) Planetary and Space Science, 56, 669. [2] Doose et al. (2016) Icarus, 270, 355. [3] Coutelier et al. (2021) Icarus, 364, 114464. [4] Hirtzig et al. (2013) Icarus, 226, 470. [5] Es-sayeh et al. (2023), PSJ, in reviews.
Figure 1: Location and geometries (incidence, emergence, and phase angles, and spatial sampling) of the T88 EPF VIMS sequence of observations.
How to cite: Rodriguez, S., Es-sayeh, M., Rannou, P., Coutelier, M., Maltagliati, L., Cornet, T., Le Mouélic, S., and Sotin, C.: A new description of Titan’s aerosol optical properties from the analysis of VIMS Emission Phase Function observations, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-257, https://doi.org/10.5194/epsc-dps2025-257, 2025.
I. Introduction
Many evidence suggest the presence of an internal global ocean on Titan [1,2]. Notably, the Permittivity, Wave and Altimetry (PWA) analyzer on board the Huygens’ probe in 2005 might have detected a Schumann-like resonance (SR) [3]. On earth, SR are extremely low frequency waves propagating between the Earth’s surface and the ionosphere [4]. Theoretically, SR could be observed on other planets and therefore, can be used as a tool to obtain information on the various planetary cavities [5]. Although the detection of SR by Huygens is still in doubt [6], the Dragonfly mission (NASA) to Titan, will embark an electrical-field sensor designed to detect the first three harmonics of SR, if any [7]. On Titan, SR are believed to propagate between the ionized atmospheric layer (60–70 km altitude) and a subsurface layer (40–80 km depth), presumably a subsurface salty water ocean. Thus, the detection of SR by Dragonfly would provide better constraints on the depth of this buried ocean and would therefore be key to better assess Titan’s habitability.
Fig.1: Structure and parameters of Titan’s cavity [8]
A numerical model has been develop to accurately approximate the behavior of Titan's planetary cavity (see figure 1) [8]. This model showed that the assumptions on SR made on Huygens observations, does not provide any specific constraint on the depth of Titan’s ocean in the range 5-200 km contrary to what is advanced in [3]. This work present the extension on what can be done with this model in order to investigate notably the uncertainty at estimating Titan's crust thickness. Current simulations are including the actual EFIELD electrodes along with the drone body of Dragonfly to study their effect on measurements of the location and polarization of the possible sources of SR.
II. Numerical simulations of the EFIELD experiment
With the current design of the EFIELD experiment, frequencies could be measured in the range 1-100 Hz which allows the measurement of the first three SRs with their corresponding quality factor. Considering the three input parameters of our model: the thickness of the ice crust z_c, its conductivity σ_c and its permittivity ε_c, their existing domains can be integrated into the model to compute the measurement uncertainty of the EFIELD experiment. 
Fig.2: Standard deviation of the EFIELD inversion against the thickness of the ice crust z_c for two different uncertainties on σ_c in percentage of z_c
For example on figure 2, the standard deviation at estimating z_c with the current uncertainties on the electromagnetic properties of the ice crust from Hamelin et al. [9] can be computed. This allows us to investigate the potential results from another experiment of the Dragonfly Geophysics and Meteorology Package (DraGMet): the DIEL experiment which aims at measuring the electrical properties of Titan's surface. On figure 2, a better constraint on σ_c (in red) which could be measured with DIEL compared to the existing constraint from [9] (in blue) would greatly decrease the measurement uncertainty on z_c.
The current simulations are taking into account the actual EFIELD electrodes accommodated on the Dragonfly (conductive) body. Similarly as with the electromagnetic properties of the surface, the drone could influence the inversion of the EFIELD experiment and thus the estimation of the thickness of the ice crust z_c. On figure 3, the deformation of the equipotentials and the differential potential between the two probes for a SR at f=36 Hz with a vertical polarization are shown around a simplification of the drone body.
(a)
(b)
Fig.3: Influence of the drone body with a Schumann resonance at f=36 Hz} on the equipotential (a) and the differential potential between the two probes (b)
The EFIELD experiment will measure the electrical potential at the two probes. Given the vertical polarization of a single mode as shown on figure 3, the Schumann resonance can be accurately measured in both frequency and power and the effect of the metallic body properly quantified. But realistically, the wave polarization is unknown and the SRs are a spectrum with various modes instead of individual peaks. The next step is to include a full wave spectrum with various polarizations. Given the different orientation and altitude of the drone and the two EFIELD electrodes, at least two components of the electrical field could be measured with the third component captured by rotating the drone. Further studies could also include the sources of the SRs. Such further developments will be presented at EPSC2025.
References
[1] R.-M. Baland, T. Van Hoolst, M. Yseboodt, ¨O. Karatekin, Astronomy & Astrophysics 530, A141 (2011).
[2] L. Iess et al., Science 337, 457–459 (2012).
[3] C. Béghin et al., Icarus 218, 1028–1042 (2012).
[4] W. O. Schumann, Zeitschrift für Naturforschung A 7, 149–154 (1952).
[5] F. Simões et al., The Astrophysical Journal 750, 85 (2012).
[6] R. D. Lorenz, A. Le Gall, Icarus 351, 113942 (2020).
[7] J. W. Barnes et al., The Planetary Science Journal 2, 130 (2021).
[8] P. Lagouanelle, A. Le Gall, Icarus 428, 116372 (2025).
[9] M. Hamelin et al., Icarus 270, 272–290 (2016).
How to cite: Lagouanelle, P., Reuchin, E., Le Gall, A., Deprez, G., Berthelier, J.-J., and Lorenz, R. D.: Measuring Schumann Resonances on Titan: expected performances of future EFIELD/Dragonfly observations, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-152, https://doi.org/10.5194/epsc-dps2025-152, 2025.