Periodic Reporting for period 2 - TLALOC (Titan’s Lakes and Lower Clouds: investigation of the enigmatic methane cycle with a new advanced model.)
Reporting period: 2023-09-01 to 2024-08-31
Titan, Saturn’s largest moon, is the only place beyond Earth that hosts a hydrological cycle. At Titan’s surface at -180ºC, the flowing element is methane. The Cassini-Huygens mission brought unprecedented insights on this methane cycle, with radar cartography of many methane lakes at the poles, images of clouds and riverbeds, and proofs of rain. However, this cycle is not completely understood. Open questions remain on the presence of methane reservoirs and on why global climate models do not fairly reproduce clouds and rain. The goal of TLALOC was to develop a competitive Titan system model in collaboration with the U.S. (SwRI) and two E.U. laboratories (UPV/EHU and LMD/Sorbonne University) in order to close key scientific knowledge gaps about Titan’s methane cycle.
To better understand the methane cycle, we investigated key physical process (evaporation, cloud condensation, precipitation) with a mesoscale climate model. We started from an Earth atmospheric model (the Weather Research Forecast model, WRF) and modify it to feature Titan’s extreme conditions. This work also improves our knowledge on the climate models used for the Earth their limits. The NASA Dragonfly mission will explore Titan’s surface by the mid 2030s with a large instrumented drone. Having an advanced mesoscale model of Titan will be fundamental to drive the mission operations and understand many of the atmospheric measurements.
The TLALOC project aimed to: (1) quantify the effect of lakes and wetlands on the local atmospheric moisture; (2) characterize the effect of solar insolation on the methane cycle at the local scale; (3) understand the formation mechanism and dynamical evolution of storm systems; and (4) confront local and global atmospheric conditions predicted by the models to Cassini and Earth-based observations. The main conclusion from this project is that lakes strongly affect their environment. Methane evaporation cools down the lake surface by 2°C compared to the shore, creating continual winds from the lake to the land with diurnal and seasonal variations. The cold humid marine above the lake enables the formation of fog as observed from Cassini data. The marine layer slows down the evaporation of the lake and decreases the formation of clouds and precipitations in Global Climate Models to values closer to observations.
To better understand the methane cycle, we investigated key physical process (evaporation, cloud condensation, precipitation) with a mesoscale climate model. We started from an Earth atmospheric model (the Weather Research Forecast model, WRF) and modify it to feature Titan’s extreme conditions. This work also improves our knowledge on the climate models used for the Earth their limits. The NASA Dragonfly mission will explore Titan’s surface by the mid 2030s with a large instrumented drone. Having an advanced mesoscale model of Titan will be fundamental to drive the mission operations and understand many of the atmospheric measurements.
The TLALOC project aimed to: (1) quantify the effect of lakes and wetlands on the local atmospheric moisture; (2) characterize the effect of solar insolation on the methane cycle at the local scale; (3) understand the formation mechanism and dynamical evolution of storm systems; and (4) confront local and global atmospheric conditions predicted by the models to Cassini and Earth-based observations. The main conclusion from this project is that lakes strongly affect their environment. Methane evaporation cools down the lake surface by 2°C compared to the shore, creating continual winds from the lake to the land with diurnal and seasonal variations. The cold humid marine above the lake enables the formation of fog as observed from Cassini data. The marine layer slows down the evaporation of the lake and decreases the formation of clouds and precipitations in Global Climate Models to values closer to observations.
In this project we developed a mesoscale climate model of Titan and focused on the evaporation of lakes, which is the first major step in the atmospheric methane cycle. We started from a version of the WRF model adapted to Titan by the U.S. host (SwRI, Rafkin & Soto 2020) and progressively developed its capacities.
1) In a first study, we added a radiative transfer module to WRF modelling solar and infrared fluxes. Although Titan is 9.5 times farther away from the Sun than the Earth and has a thick hazy atmosphere, radiation has a major impact on the atmospheric circulation. We found that the breeze flowing from the cold air above the lake to the warmer air above the land shows strong diurnal variations driven by radiative processes (Chatain et al, PSJ, 2022; presented at LPSC 2022, EGU 2022, the French PNP 2022, EGU 2023).
2) In a second study, we improved the model to run in 3 dimensions instead of 2. The results (Chatain et al, Icarus 2024; presented at DPS 2022, OPAG 2022, Titan Through Time 2023, the Spanish CPESS 2023, AOGS 2023) show that the divergence/convergence of winds and vapour mixing in 3D strongly impact the atmospheric circulation above the lake, the methane evaporation budget and the lake equilibrium temperature. We also observed that the turbulent kinetic energy (TKE) computed by the 2D model and based on studies of Earth atmosphere was strongly overestimated. We therefore corrected this effect to adapt the TKE to Titan’s sluggish atmosphere.
3) In a third study, we investigated the abundant small lakes on Titan that are surrounded by ramparts. Our results show that surface roughness on the ramparts has a very significant effect in the dynamics, as it slows down the surface wind, affecting evaporation rates, temperatures and the overall climate above lakes. Simulations with topography show that high ramparts deform the local lake breeze created by the lake, but do not stop it (Moisan, Chatain et al, submitted to PSJ in Feb. 2024; presented at Titan Through Time 2023, the Spanish CPESS 2023, DPS 2023, the French conference Elbereth 2024).
4) The fourth study investigated how the implementation of realistic topography in 3D simulations of lakes affects the methane budget and could lead to the formation of methane fog (Chatain et al, to be submitted to JGR: Planets in 2024; presented at DPS 2023 and FAIRPLAY workshop 2023).
5) With a fifth study we investigated cloud dynamics on Titan from image analysis. The analysis of a cloud system observed over few hours by the Cassini spacecraft allowed us to quantify the very slow evolution of a polar cloud, giving insights on its physics (presented at the Spanish CPESS 2023). In parallel, we submitted proposals to observe cloud systems in the coming years with the JWST to investigate on the frequency of such events. This investigation is continued after the end of the TLALOC project in the framework of a 2023-2026 PhD thesis at LMD co-supervised by the TLALOC researcher.
6) Finally, we started to develop a model that includes microphysics to study the formation and evolution of clouds in the lakes region. This investigation will also be continued with the PhD thesis at LMD.
1) In a first study, we added a radiative transfer module to WRF modelling solar and infrared fluxes. Although Titan is 9.5 times farther away from the Sun than the Earth and has a thick hazy atmosphere, radiation has a major impact on the atmospheric circulation. We found that the breeze flowing from the cold air above the lake to the warmer air above the land shows strong diurnal variations driven by radiative processes (Chatain et al, PSJ, 2022; presented at LPSC 2022, EGU 2022, the French PNP 2022, EGU 2023).
2) In a second study, we improved the model to run in 3 dimensions instead of 2. The results (Chatain et al, Icarus 2024; presented at DPS 2022, OPAG 2022, Titan Through Time 2023, the Spanish CPESS 2023, AOGS 2023) show that the divergence/convergence of winds and vapour mixing in 3D strongly impact the atmospheric circulation above the lake, the methane evaporation budget and the lake equilibrium temperature. We also observed that the turbulent kinetic energy (TKE) computed by the 2D model and based on studies of Earth atmosphere was strongly overestimated. We therefore corrected this effect to adapt the TKE to Titan’s sluggish atmosphere.
3) In a third study, we investigated the abundant small lakes on Titan that are surrounded by ramparts. Our results show that surface roughness on the ramparts has a very significant effect in the dynamics, as it slows down the surface wind, affecting evaporation rates, temperatures and the overall climate above lakes. Simulations with topography show that high ramparts deform the local lake breeze created by the lake, but do not stop it (Moisan, Chatain et al, submitted to PSJ in Feb. 2024; presented at Titan Through Time 2023, the Spanish CPESS 2023, DPS 2023, the French conference Elbereth 2024).
4) The fourth study investigated how the implementation of realistic topography in 3D simulations of lakes affects the methane budget and could lead to the formation of methane fog (Chatain et al, to be submitted to JGR: Planets in 2024; presented at DPS 2023 and FAIRPLAY workshop 2023).
5) With a fifth study we investigated cloud dynamics on Titan from image analysis. The analysis of a cloud system observed over few hours by the Cassini spacecraft allowed us to quantify the very slow evolution of a polar cloud, giving insights on its physics (presented at the Spanish CPESS 2023). In parallel, we submitted proposals to observe cloud systems in the coming years with the JWST to investigate on the frequency of such events. This investigation is continued after the end of the TLALOC project in the framework of a 2023-2026 PhD thesis at LMD co-supervised by the TLALOC researcher.
6) Finally, we started to develop a model that includes microphysics to study the formation and evolution of clouds in the lakes region. This investigation will also be continued with the PhD thesis at LMD.
Our work has led to an important update of the estimated evaporation rate of Titan’s lakes, which is considerably smaller than what suggested previous models. This value is in better agreement with observations of lakes from Cassini, and, combined with Global Climate Models, suggests that methane evaporation over moist terrains is also needed to accumulate additional moisture in the atmosphere at the global scale and explain the observations of atmospheric methane clouds.
Our work shows that differential evaporation between lakes and dry lands leads to a differential temperature and forms winds. We also expect that this effect could also be important between wet terrains and dry lands. Lakes are concentrated at the poles, but wet lands can be found at lower latitudes, and in particular at the landing site of NASA’s Dragonfly mission. We also show that topography create local winds along slopes. More investigations are needed to conclude on the wind conditions at the Dragonfly landing site, and it will be of major importance to have a robust local atmospheric model during the mission operations in the mid 2030s. The development of our Titan WRF model in this project is a key step in that direction.
Our latest results suggest the possibility to form fog above lakes, in agreement with observations by the Cassini mission. We expect to refine these conclusions in the future with a study on the very small turbulence of the marine layer above the lakes. Except for fogs, our simulations do not predict the formation of clouds below 15 km, the top of the model. As previous works suggested that clouds would condense around 30 km, we started to develop a model with a higher upper limit, microphysics and improved stormy initial and boundary conditions to try to reproduce large storm clouds observed on Titan, and rain.
Our work shows that differential evaporation between lakes and dry lands leads to a differential temperature and forms winds. We also expect that this effect could also be important between wet terrains and dry lands. Lakes are concentrated at the poles, but wet lands can be found at lower latitudes, and in particular at the landing site of NASA’s Dragonfly mission. We also show that topography create local winds along slopes. More investigations are needed to conclude on the wind conditions at the Dragonfly landing site, and it will be of major importance to have a robust local atmospheric model during the mission operations in the mid 2030s. The development of our Titan WRF model in this project is a key step in that direction.
Our latest results suggest the possibility to form fog above lakes, in agreement with observations by the Cassini mission. We expect to refine these conclusions in the future with a study on the very small turbulence of the marine layer above the lakes. Except for fogs, our simulations do not predict the formation of clouds below 15 km, the top of the model. As previous works suggested that clouds would condense around 30 km, we started to develop a model with a higher upper limit, microphysics and improved stormy initial and boundary conditions to try to reproduce large storm clouds observed on Titan, and rain.