Publications

Authors: Olim, E., Lora, J.M., and Battalio, J.M.

 Journal: Icarus 425, 116290.

Model Configuration: Coupled Hydrology

Abstract: Methane precipitation is a key component of the climate on Titan, and has been shown to impact surface features. Recent general circulation models (GCMs) have reproduced Titan’s hydroclimate, including precipitation, with increasing accuracy, yet characterization of their simulated precipitation events is lacking. We investigate the characteristics and evolution of methane storms simulated over 40 Titan years using the Titan Atmospheric Model, a validated GCM. Storms are identified and tracked using the density-based spatial clustering of applications with noise (DBSCAN) algorithm, allowing them to be followed through time and space. We find that storms follow seasonality expected from observations and prior modeling, occur preferentially in the summer hemisphere, and tend to start over high topography. The population of storms is bimodal in traits corresponding to intensity, area, and duration, with a large population of small, short-lived, and weakly precipitating storms and a smaller population of exceptionally large, long-lasting, and intense storms. These largest storms tend to evolve similarly over their lifetimes, peaking early in intensity and in the middle of their lives in area. We also find temporal clustering of storms, in alignment with observations and the proposed relaxation-oscillation model of Titan’s methane precipitation. These storm clusters emerge quasi-periodically following long dry spells during which evaporation of surface methane recharges atmospheric moisture. Approximately five clusters occur per Titan year, and their locations are strongly seasonal. Overall, our quantitative descriptions of storms and storm clusters over a long timescale provide additional insight into Titan’s methane cycle and surface features, and may assist in the planning of future missions such as Dragonfly.

Authors: Lora, J.M.

 Journal: Icarus 422, 116241.

Model Configurations: “Aquaplanet”, “Wetlands”, and Coupled Hydrology

Abstract: Titan’s surface and lower atmosphere support a hydrologic cycle that influences various aspects of the icy moon’s appearance and evolution. Here, we review the state of knowledge around this methane cycle, focusing on its relationship to the circulation of the troposphere and to the distribution of surface liquids. Titan’s meridional circulation consists mainly of Hadley cells, with an intertropical convergence zone—in which clouds and precipitation are promoted—that oscillates with latitude seasonally. There are separate regions at the poles wherein precipitation occurs in summertime. Overall, the character of precipitation depends on the amount of liquid available to evaporate on the surface, and a realistic liquid distribution (that is, with liquids limited to polar regions) leads to highly sporadic seasonal precipitation. This also produces a latitudinal profile of near-surface humidity wherein the poles are more humid than the lower latitudes. The lower latitude humidity reflects the horizontal transport by the Hadley circulation, but is cut off from the high near-surface humidity at the poles. Polar moist convection humidifies the mid-levels and from there the low latitudes, and equatorward, downgradient transport of moisture is accomplished by traveling storm systems in the high mid-latitudes. These waves in some cases interact with the convection to communicate the effects of latent heating nearly globally. Separately, surface and subsurface hydrology are important processes that lead to the observed distribution of liquids in polar basins, and furthermore indicate the influence of a subsurface methane table interacting with the climate system. Precipitation at lower latitudes largely runs off or infiltrates into the surface; runoff at higher latitudes feeds some of the low-lying polar basins; and subsurface methane flow regulates the distribution of near-surface methane such that the seas are surface exposures of, and other polar areas sustain evaporation from, a shallow methane table. Finally, we discuss the possible long-term evolution of surface liquids, including the influence of Croll-Milankovitch cycles and their effect on atmospheric moisture transport by eddies; whether or not Titan’s surface features indicate past cycling of polar liquids, slower secular trends, or something else entirely remains unresolved.

Authors: Chatain, A., Rafkin, S., Soto, A., Moisan, E., Lora, J.M., Le Gall, A., Hueso, R., and Spiga, A.

 Journal: Icarus  11, 115925

Model Configuration: Coupled Hydrology

Abstract: Titan, the largest moon of Saturn, has many lakes on its surface, formed mainly of liquid methane. Like water lakes on Earth, these methane lakes on Titan likely profoundly affect the local climate. Previous studies (Rafkin and Soto, 2020; Chatain et al., 2022) showed that Titan’s lakes create lake breeze circulations with characteristic dimensions similar to the ones observed on Earth. However, such studies used a model in two dimensions; this work investigates the consequences of the addition of a third dimension to the model. Our results show that 2D simulations tend to overestimate the extension of the lake breeze over the land, and underestimate the strength of the subsidence over the lake, due to divergence/convergence geometrical effects in the mass conservation equations. In addition, 3D simulations including a large scale background wind show the formation of a pocket of accelerated wind behind the lake, which did not form in 2D simulations. An investigation of the effect of shoreline concavity on the resulting air circulation shows the formation of wind currents over peninsulas. Simulations with several lakes can either result in the formation of several individual lake breeze cells (during the day), or the emergence of a large merged cell with internal wind currents between lakes (during the night). Simulations of several real-shaped lakes located at a latitude of 74°N on Titan at the autumn equinox show that larger lakes trigger stronger winds, and that some sections of lakes might accumulate enough methane vapor to form a thin fog. Additionally, we adapted the Turbulent Kinetic Energy closure scheme of the model to better represent the extremely low turbulence at the surface of Titan, of 2×10⁻⁴ m².s⁻² above the land, and inferior to 3×10⁻⁵ m².s⁻² above the lake. The addition of a third dimension, along with adjustments in the parametrizations of turbulence and subsurface land temperature, results in a reduction in the magnitude of the average lake evaporate rate, namely to ∼6 cm/Earth year.

Authors: Lombardo, N.A. and Lora, J.M.

 Journal: JGR Planets 128, e2023JE008061.

Model Configuration: Middel Atmosphere

Abstract: The thermal and dynamical structure of Titan’s middle atmosphere (the stratosphere and mesosphere) has been observed to evolve over seasonal timescales. Measurements from the Composite Infrared Spectrometer on the Cassini spacecraft indicated the presence of a westerly jet with the strongest winds exceeding 200 m s⁻¹ near the autumn and spring poles. The strength of the winds varied substantially with latitude and altitude, and weakened throughout most of winter. The strong winds also served to trap short-lived trace molecules near the winter pole, leading to a chemically enriched environment. Here, to better understand the evolution of the middle atmosphere jet, we quantify the heat and zonal momentum budgets in Titan’s stratosphere and mesosphere over the course of a Titan year using a three-dimensional general circulation model. We confirm that the dominant heating balance is between the net radiative and adiabatic heating rates, and also show that the convergence of sensible heat by the atmospheric flow is important at low stratospheric altitudes above the winter pole. We show that the polar jet is maintained by the convergence of zonal momentum by the mean meridional flow, while the low-latitude winds are maintained by an up-gradient transport of momentum by eddies that occur on time scales of less than one Titan day. The heat and momentum budgets we determine here will be useful in constraining the factors controlling the evolution of Titan’s middle atmosphere over the coming decades.

Authors: Lewis, N.T., Lombardo, N.A., Read, P.L., and Lora, J.M.

 Journal: Planet. Sci. J. 4, 149.

Model Configuration: Middle Atmosphere

Abstract: We investigate the characteristics of equatorial waves associated with the maintenance of superrotation in the stratosphere of a Titan general circulation model. A variety of equatorial waves are present in the model atmosphere, including equatorial Kelvin waves, equatorial Rossby waves, and mixed Rossby–gravity waves. In the upper stratosphere, acceleration of superrotation is strongest around solstice and is due to interaction between equatorial Kelvin waves and Rossby-type waves in winter hemisphere midlatitudes. The existence of this “Rossby–Kelvin”-type wave appears to depend on strong meridional shear of the background zonal wind that occurs in the upper stratosphere at times away from the equinoxes. In the lower stratosphere, acceleration of superrotation occurs throughout the year and is partially induced by equatorial Rossby waves, which we speculate are generated by quasigeostrophic barotropic instability. Acceleration of superrotation is generally due to waves with phase speeds close to the zonal velocity of the mean flow. Consequently, they have short vertical wavelengths that are close to the model’s vertical grid scale and therefore likely to be not properly represented. We suggest that this may be a common issue among Titan general circulation models that should be addressed by future model development.

Authors: Birch, S., Parker, G., Corlies, P., Soderblom, J.M., Miller, J.W., Palermo, R.V., Lora, J.M., Ashton, A.D., Hayes, A.G. and Perron, J.T.

 Journal: PNAS 120 e2206837120.

Model Configuration: Coupled Hydrology

Abstract: Alluvial rivers are conveyor belts of fluid and sediment that provide a record of upstream climate and erosion on Earth, Titan, and Mars. However, many of Earth’s rivers remain unsurveyed, Titan’s rivers are not well resolved by current spacecraft data, and Mars’ rivers are no longer active, hindering reconstructions of planetary surface conditions. To overcome these problems, we use dimensionless hydraulic geometry relations—scaling laws that relate river channel dimensions to flow and sediment transport rates—to calculate in-channel conditions using only remote sensing measurements of channel width and slope. On Earth, this offers a way to predict flow and sediment flux in rivers that lack field measurements and shows that the distinct dynamics of bedload-dominated, suspended load-dominated, and bedrock rivers give rise to distinct channel characteristics. On Mars, this approach not only predicts grain sizes at Gale Crater and Jezero Crater that overlap with those measured by the Curiosity and Perseverance rovers, it enables reconstructions of past flow conditions that are consistent with proposed long-lived hydrologic activity at both craters. On Titan, our predicted sediment fluxes to the coast of Ontario Lacus could build the lake’s river delta in as little as ~1,000 y, and our scaling relationships suggest that Titan’s rivers may be wider, slope more gently, and transport sediment at lower flows than rivers on Earth or Mars. Our approach provides a template for predicting channel properties remotely for alluvial rivers across Earth, along with interpreting spacecraft observations of rivers on Titan and Mars.

Authors: Lombardo, N.A. and Lora, J.M.

 Journal: Icarus 390, 115291.

Model Configuration: Middle Atmosphere

Abstract: Titan’s atmosphere exhibits variations in composition as it progresses through its seasons. Past observations have shown a substantial enrichment in short-lived molecules in the winter stratosphere above 100 km. Seasonal variations in Titan’s stratospheric dynamics also lead to a transient detached haze layer (DHL) above 400 km. The seasonal variations in aerosol opacity and molecular abundance lead to varying radiative heating rates in both the shortwave and longwave spectral regions. In this paper, we report on the effects of a new dataset for aerosol opacity and trace gas abundance derived from Cassini observations on simulations of Titan’s stratospheric dynamics with the Titan Atmospheric Model (TAM). We find that including seasonally varying radiative species (SVRS) decreases the autumn and winter polar stratopause temperature by up to 10 K poleward of 60°. We also find a similar increased seasonality in the zonal winds with the early autumn polar jet strengthening by 60 m s⁻¹, while the mid winter jet is unaffected. While including the observationally derived SVRS dataset increases the strength of the seasonal variations of the polar jet, it does not substantially affect the timing of the onset or dissipation of the jet or other seasonal phenomena.

Authors: Lewis-Merrill, R.A., Moon, S., Mitchell, J.L. and Lora, J.M.

 Journal: Planet. Sci. J. 3, 223

Model Configuration: Coupled Hydrology

Abstract: Present-day environmental conditions on Titan, the largest moon of Saturn, may do active geomorphic work on its surface. On Earth, the hydrologic water cycle erodes and weathers its continents. Deluges over elevated terrain create debris flows and sheetfloods that spread into alluvial fans as the topographic slope decreases. Mars also shows evidence of past fluvial erosion, but fluvial activity cannot be ongoing in the present. On Titan, however, fluvial erosion is likely ongoing. In this study, we focus on understanding the environmental controls on the spatial distributions of alluvial fans, a type of fluvial depositional feature observed globally on the surface of Titan. To do this, we utilize probabilistic models to determine the strength of spatial correlations between spatial distributions of alluvial fans and present-day environmental factors. We find that the spatial distribution of alluvial fans on Titan correlates well with several present-day environmental conditions, including average precipitation, precipitation variability, and elevation. Based on our model, we also provide predictions of the likelihood of alluvial fan occurrences for areas of Titan not mapped with Cassini, which may be of interest for future missions to Titan.

Authors: Lora, J.M., Battalio, J.M., Yap, M. and Baciocco, C.

 Journal: Icarus 384, 115095.

Model Configuration: Coupled Hydrology

Abstract: The cause of the hemispheric asymmetry of Titan’s methane lakes and seas is the subject of ongoing debate. A leading hypothesis posits that seasonal insolation asymmetries caused by Saturn’s eccentric orbit lead to differences in net precipitation over the two poles, perhaps mediated by asymmetric atmospheric transport of moisture. But topographic variations have also been proposed to contribute, albeit without considering the importance of surface hydrology. Here we present general circulation model simulations including a synchronously coupled surface and ground hydrology scheme, testing the separate and combined influences of topography and orbital forcing on Titan’s hydroclimate. We find that, while topography leads to warmer polar regions relative to a flat surface which in turn enhance methane loss to the atmosphere, the overall effect on the global distribution of surface methane liquid is minor. In particular, topography does not force any notable asymmetry in the meridional circulation, nor does it affect the seasonality of the methane cycle, though it does increase the regional heterogeneity of average precipitation at mid-latitudes. We also find that Titan’s atmospheric methane transport robustly responds to orbital forcing, in agreement with previous results, but this is insufficient to overcome the distribution of surface liquids dictated by surface hydrology. We conclude that Croll-Milankovitch cycles are plausible on Titan, but potentially not the dominant driver of the current distribution of liquids; relatedly, our results suggest that the volume of the large seas and lakes has not varied substantially on millennial timescales.

Authors: Comola, F., Kok, J.F., Lora, J.M., Cohanim, K., Yu, X., He, C., McGuiggan, P., Hörst, S.M. and Turney, F.

 Journal: Geophys. Res. Lett. 49, e2022GL097913.

Model Configuration: Coupled Hydrology

Abstract: Titan, the largest moon of Saturn, is characterized by gigantic linear dunes and an active dust cycle. Much like on Earth, these aeolian processes are caused by the wind-driven saltation of surface grains. It is still unclear, however, how saltation on Titan can occur despite the typically weak surface winds and the potentially cohesive surface grains. Here, we explore the hypothesis that saltation on Titan may be sustained at lower wind speeds than previously thought, primarily through granular splash rather than aerodynamic lifting of surface grains. We propose a saltation mass flux parameterization for Titan and use it to quantify sediment transport with a general circulation model. The results suggest that Titan’s prevailing circulation can generate highly intermittent yet significant saltation, with mass fluxes of the order of 10⁴ kg m⁻¹ year⁻¹, and that Titan dunes may be formed primarily by fine grains, approximately 0.1 mm in size.

Authors: Rafkin, S., Lora, J.M., Soto, A. and Battalio, J.M.

 Journal: Icarus 373, 114755.

Model Configuration: Coupled Hydrology

Abstract: The deep convective cloud–environment feedback loop is likely important to Titan’s global methane, energy, and momentum cycles, just as it is for Earth’s global water, energy, and momentum budgets. General circulation models of Titan’s atmosphere are unable to explicitly simulate deep convection and must instead parameterize the impact of this important subgrid-scale phenomenon on the model-resolved atmospheric state. The goal of this study is to better quantify through cloud resolving modeling the effects of deep convective methane storms on their environment and to feed that information forward to improve parameterizations in global models. Dozens of atmospheric profiles unstable with respect to deep moist convection are extracted from the global Titan Atmospheric Model (TAM) and used to initialize the cloud-resolving Titan Regional Atmospheric Modeling System (TRAMS). Mean profiles of heating/cooling and moistening/drying of the large-scale environment in TRAMS indicate that Titan’s deep convection forces the environment in a manner analogous to Earth: Large-scale subsidence of the environmental air warms and dries the environment, but clouds can also moisten the environment through the detrainment and evaporation of condensate near cloud top. Relative humidity profiles and characteristic convective time scales are derived to guide the tuning of the deep convective parameterization implemented in TAM, as described in a companion paper. The triggering of convection, the dry convective mixing of the planetary boundary layer, and the entrainment of environmental air into rising air parcels are found to be critical to determining whether a deep convective cloud will form. Only profiles with relatively large convective available potential energy (CAPE) and well mixed planetary boundary layers with high relative humidity were found to produce storms. Environments with low-level thermal inversions and planetary boundary layers with low relative humidity or rapidly decreasing moisture with height failed to generate deep convection in TRAMS despite positive CAPE.

Authors: Battalio, J.M., Lora, J.M., Rafkin, S. and Soto, A. 

 Journal: Icarus 373, 114623.

Model Configuration: Coupled Hydrology

Abstract: The impact of methane convection on the circulation of Titan is investigated in the Titan Atmospheric Model (TAM), using a simplified Betts–Miller (SBM) moist convection parameterization scheme. We vary the reference relative humidity (RHsbm) and relaxation timescale of convection (tau) parameters of the SBM scheme. Titan’s atmosphere is mostly insensitive to changes in tau, but convective instability and precipitation are highly impacted by changes in RHsbm. Convection behavior changes from infrequent (<1 per Titan year), intense events at summer solstice that quickly encompass the entire globe at low RHsbm to near-continuous precipitation at the poles during summer at high RHsbm (85%). The intermediate regime (RHsbm=70%–80%) consists of frequent events (10 per Titan year) of moderate intensity that are limited in meridional extent to their respective hemisphere. Using results from the Titan Regional Atmospheric Modeling System (TRAMS) and observations, we tune the parameters of the SBM parameterization with optimum values of RH=80% and tau=28800 s. We present a simulated decadal climatology that qualitatively matches observations of Titan’s humidity and cloud activity and generally resembles previous results with TAM. Comparing this simulation to one without moist convection demonstrates that convection strengthens the meridional circulation, warms the mid-levels and cools the surface at the poles, and magnifies zonal-mean global moisture anomalies.

Authors: Battalio, J.M. and Lora, J.M.

 Journal: Nature Astronomy 5, 1139–1147

Model Configuration: Coupled Hydrology

Abstract: Annular modes explain much of the internal variability of Earth’s atmosphere but have never been identified as influential on other planets. Using data assimilation datasets for Mars and a general circulation model for Titan, we demonstrate that annular modes are prominent in the atmospheres of both worlds, capturing a larger fraction of their respective variabilities than Earth’s. One mode describes latitudinal shifts of the jet on Mars, as on Earth, and vertical shifts of the jet on Titan. Another describes pulses of mid-latitude eddy kinetic energy on all three worlds, albeit with somewhat different characteristics. We demonstrate that this latter mode has predictive power for regional dust activity on Mars, revealing its usefulness for understanding Martian weather. The similarity of annular variability in dynamically diverse worlds suggests its ubiquity across the Solar System, potentially extending to exoplanets.

Authors: Battalio, J.M. and Lora, J.M.

 Journal: Geophys. Res. Lett 48, e2021GL094244.

Model Configuration: Coupled Hydrology

Abstract: One of the first large cloud systems ever observed on Titan was a stationary event at the southern pole that lasted almost two full Titan days. Its stationary nature and large extent are puzzling given that low-level winds should transport clouds eastward, pointing to a mechanism such as atmospheric waves propagating against the mean flow. We use a composite of 47 large convective events across 15 Titan years of simulations from the Titan Atmospheric Model to show that Rossby waves trigger polar convection—which halts the waves and produces stationary precipitation—and then communicate its impact globally. In the aftermath of the convection, forced waves undergo a complicated evolution, including cross-equatorial propagation and tropical-extratropical interaction. The resulting global impact from convection implies its detectability anywhere on Titan, both via surface measurements of pressure and temperature and through remote observation of the outgoing longwave radiation, which increases by ~0.5% globally.

Authors: Faulk, S.P., Lora, J.M., Mitchell, J.L., Milly, P.C.D.

 Journal: Nature Astronomy 4, 390–398.

Model Configuration: Coupled Hydrology

Abstract: Planetary surfaces beyond Earth’s are impacted by surface hydrology, and exhibit fluvial and lacustrine features. Titan in particular harbours a rich hydroclimate replete with valley networks, lakes, seas and putative wetlands, all of which are pronounced in the lower-elevation polar regions. However, understanding of Titan’s global climate has heretofore neglected the hydraulic influence of Titan’s large-scale topography. Here we add a surface hydrology model to an existing Titan atmospheric model, and find that infiltration, groundmethane evaporation, and surface and subsurface flow are fundamental to simultaneously reproducing Titan’s observed surface liquid distribution and other aspects of its climate system. We propose that Titan’s climate features infiltration into unsaturated low- and mid-latitude highlands and surface or subsurface flow into high-latitude basins, producing the observed polar moist climes and equatorial deserts. This result implies that a potentially massive unobserved methane reservoir participates in Titan’s methane cycle. It also illustrates the importance of surface hydrology in Titan climate models, and by extension suggests the influence of surface hydrology in idealized models of other planetary climates, including the climates and palaeoclimates of Earth, Mars and exoplanets.

Authors: Lora, J.M., Tokano, T., Vatant d’Ollone, J., Lebonnois, S. and Lorenz, R.D.

 Journal: Icarus 333, 113-126.

Model Configuration: “Wetlands”

Abstract: Cassini-Huygens provided a wealth of data with which to constrain numerical models of Titan. Such models have been employed over the last decade to investigate various aspects of Titan’s atmosphere and climate, and several three-dimensional general circulation models(GCMs) now exist that simulate Titan with a high degree of fidelity. However, substantial uncertainties persist, and at the same time no dedicated intercomparisons have assessed the degree to which these models agree with each other or the observations. To address this gap, and motivated by the proposed Dragonfly Titan lander mission, we directly compare three Titan GCMs to each other and to in situobservations, and also provide multi-model expectations for the low-latitude environment during the early northern winter season. Globally, the models qualitatively agree in their representation of the atmospheric structure and circulation, though one model severely underestimates meridional temperature gradients and zonal winds. We find that, at low latitudes, simulated and observed atmospheric temperatures closely agree in all cases, while the measured winds above the boundary layer are only quantitatively matched by one model. Nevertheless, the models simulate similar near-surface winds, and all indicate these are weak. Likewise, temperatures and methane content at low latitudes are similar between models, with some differences that are largely attributable to modeling assumptions. All models predict environments that closely resemble that encountered by the Huygens probe, including little or no precipitation at low latitudes during northern winter. The most significant differences concern the methane cycle, though the models are least comparable in this area and substantial uncertainties remain. We suggest that, while the overall low-latitude environment on Titan at this season is now fairly well constrained, future in situ measurements and monitoring will transform our understanding of regional and temporal variability, atmosphere-surface coupling, Titan’s methane cycle, and modeling thereof.

Authors: Mackenzie, S.M., Lora, J.M. and Lorenz, R.D.

 Journal: JGR Planets 124, 1728-1742.

Model Configuration: Dry version

Abstract: Like Earth, the atmosphere of Titan, Saturn’s largest moon, is affected by the ability of the surface to store and release heat. Modeling efforts have shown that global temperature distributions and wind profiles differ between simulations describing Titan’s surface with a uniform thermal inertia. Cassini-Huygens demonstrated, however, that a variety of morphologies and compositions make up the Titanian landscape. Using data from Cassini RADAR and the Visible and Infrared Mapping Spectrometer, we classified the surface into five terrain types: dune, lake, hummocky, plains, and labyrinth. We estimated the thermal and physical properties (conductivity, specific heat, and density) for each type, creating a 1° × 1° global map of thermal inertia values. For the lakes, as the depth of convection is not yet known, we considered both still and convective bodies. Four simulations of the Titan Atmospheric Model were run with different surface thermal properties: a low homogeneous thermal inertia, a moderate homogeneous, the heterogeneous map with still lakes, and the heterogeneous map with convective lakes. In dry regimes (i.e., without the hydrological cycle), the differences between the four cases were generally minimal, suggesting that general circulation models can use a single (moderate) value for surface thermal inertia value for large-scale investigations that do not consider diurnal variations. Given the importance of hydrological processes and regional spatial diversity on Titan, future work should consider the effects of nonuniform thermal inertia on Titan’s climate on local and regional scales.

Authors: Turtle, E.P., Perry, J.E., Barbara, J.M., Del Genio, A.D., Rodriguez, 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.

 Journal: Geophys. Res. Lett. 45, 5320-5328.

Model Configuration: “Wetlands”

Abstract: Cassini observations of Titan’s weather patterns over >13 years, almost half a Saturnian year, provide insight into seasonal circulation patterns and the methane cycle. The Imaging Science Subsystem and the Visual and Infrared Mapping Spectrometer documented cloud locations, characteristics, morphologies, and behavior. Clouds were generally more prevalent in the summer hemisphere, but there were surprises in locations and timing of activity: Southern clouds were common at midlatitudes, northern clouds initially appeared much sooner than model predictions, and north polar summer convective systems did not appear before the mission ended. Differences from expectations constrain atmospheric circulation models, revealing factors that best match observations, including the roles of surface and subsurface reservoirs. The preference for clouds at mid-northern latitudes rather than near the pole is consistent with models that include widespread polar near-surface methane reservoirs in addition to the lakes and seas, suggesting a broader subsurface methane table is accessible to the atmosphere.

Authors: Lora, J.M., Kataria, T. and Gao, P.

Journal: ApJ 853, 58

Model Configuration: Dry version

Abstract: With the discovery of ever smaller and colder exoplanets, terrestrial worlds with hazy atmospheres must be increasingly considered. Our solar system’s Titan is a prototypical hazy planet, whose atmosphere may be representative of a large number of planets in our Galaxy. As a step toward characterizing such worlds, we present simulations of exoplanets that resemble Titan but orbit three different stellar hosts: G, K, and M dwarf stars. We use general circulation and photochemistry models to explore the circulation and chemistry of these Titan-like planets under varying stellar spectra, in all cases assuming a Titan-like insolation. Due to the strong absorption of visible light by atmospheric haze, the redder radiation accompanying later stellar types produces more isothermal stratospheres, stronger meridional temperature gradients at mbar pressures, and deeper and stronger zonal winds. In all cases, the planets’ atmospheres are strongly superrotating, but meridional circulation cells are weaker aloft under redder starlight. The photochemistry of hydrocarbon and nitrile species varies with stellar spectra, with variations in the FUV/NUV flux ratio playing an important role. Our results tentatively suggest that column haze production rates could be similar under all three hosts, implying that planets around many different stars could have similar characteristics to Titan’s atmosphere. Lastly, we present theoretical emission spectra. Overall, our study indicates that, despite important and subtle differences, the circulation and chemistry of Titan-like exoplanets are relatively insensitive to differences in the host star. These findings may be further probed with future space-based facilities, like WFIRST, LUVOIR, HabEx, and OST.

Authors: Faulk, S.P., Mitchell, J.L., Moon, S. and Lora, J.M.

 Journal: Nature Geoscience 10, 827–831.

Model Configuration: “Wetlands”

Abstract: Geomorphic features typically associated with extreme rainfall events in terrestrial settings, including extensive fluvial features and alluvial fans, have been detected on Titan’s surface. Methane flow from precipitation on Titan can transport sediments and potentially erode the icy bedrock, but averaged precipitation rates from prior global-scale modelling are too low by at least an order of magnitude to initiate sediment transport of observed grain sizes at low latitudes. Here, we quantify the regional magnitude, frequency and variability of extreme rainfall events from simulations of present-day Titan, with a general circulation model coupled to a land model partially covered by wetlands reservoirs that can capture Titan’s regionally varying hydroclimate. We find that the most extreme storms tend to occur in the mid-latitudes, where observed alluvial fans are most concentrated. Storms capable of sediment transport and erosion occur at all latitudes in our simulations, consistent with the observed global coverage of fluvial features. Our results demonstrate the influential role of extreme precipitation in shaping Titan’s surface. We therefore suggest that, similarly to Earth but differently from Mars, active geomorphic work may be ongoing in the present climate on Titan.

Authors: Lora, J.M. and Ádámkovics, M. 

 Journal: Icarus 286, 270-279.

Model Configuration: “Aquaplanet”, “Wetlands”, and Simple Hydrology

Abstract: We retrieve vertical and meridional variations of methane mole fraction in Titan’s lower troposphere by re-analyzing near-infrared ground-based observations from 17 July 2014 UT (Ádámkovics et al., 2016). We generate synthetic spectra using atmospheric methane profiles that do not contain supersaturation or discontinuities to fit the observations, and thereby retrieve minimum saturation altitudes and corresponding specific humidities in the boundary layer. We relate these in turn to surface-level relative humidities using independent surface temperature measurements. We also compare our results with general circulation model simulations to interpret and constrain the relationship between humidities and surface liquids. The results show that Titan’s lower troposphere is undersaturated at latitudes south of 60°N, consistent with a dry surface there, but increases in humidity toward the north pole indicate appreciable surface liquid coverage. While our observations are consistent with considerably more liquid methane existing at the north pole than is present in observed lakes, a degeneracy between low-level methane and haze leads to substantial uncertainty in determining the extent of the source region.

Authors: McDonald, G.D., Hayes, A.G., Ewing, R.C., Lora, J.M., Newman, C.E., Tokano, T., Lucas, A., Soto, A. and Chen, G.

Journal: Icarus 270, 197-210.

Configuration: Simple Hydrology

Abstract: Wind-blown dunes are a record of the climatic history in Titan’s equatorial region. Through modeling of the climatic conditions associated with Titan’s historical orbital configurations (arising from apsidal precessions of Saturn’s orbit), we present evidence that the orientations of the dunes are influenced by orbital forcing. Analysis of 3 Titan general circulation models (GCMs) in conjunction with a sediment transport model provides the first direct intercomparison of results from different Titan GCMs. We report variability in the dune orientations predicted for different orbital epochs of up to 70°. Although the response of the GCMs to orbital forcing varies, the orbital influence on the dune orientations is found to be significant across all models. Furthermore, there is near agreement among the two models run with surface topography, with 3 out of the 5 dune fields matching observation for the most recent orbital cycle. Through comparison with observations by Cassini, we find situations in which the observed dune orientations are in best agreement with those modeled for previous orbital configurations or combinations thereof, representing a larger portion of the cycle. We conclude that orbital forcing could be an important factor in governing the present-day dune orientations observed on Titan and should be considered when modeling dune evolution.

Authors: Mitchell, J.L. and Lora, J.M.

Journal: Ann. Rev. Earth Planet. Sci. 44, 353-380.

Configuration: “Wetlands”

Abstract: Over the past decade, the Cassini-Huygens mission to the Saturn system has revolutionized our understanding of Titan and its climate. Veiled in a thick organic haze, Titan’s visible appearance belies an active, seasonal weather cycle operating in the lower atmosphere. Here we review the climate of Titan, as gleaned from observations and models. Titan’s cold surface temperatures (∼90 K) allow methane to form clouds and precipitation analogously to Earth’s hydrologic cycle. Because of Titan’s slow rotation and small size, its atmospheric circulation falls into a regime resembling Earth’s tropics, with weak horizontal temperature gradients. A general overview of how Titan’s atmosphere responds to seasonal forcing is provided by estimating a number of climate-related timescales. Titan lacks a global ocean, but methane is cold-trapped at the poles in large seas, and models indicate that weak baroclinic storms form at the boundary of Titan’s wet and dry regions. Titan’s saturated troposphere is a substantial reservoir of methane, supplied by deep convection from the summer poles. A significant seasonal cycle, first revealed by observations of clouds, causes Titan’s convergence zone to migrate deep into the summer hemispheres, but its connection to polar convection remains undetermined. Models suggest that downwelling of air at the winter pole communicates upper-level radiative cooling, reducing the stability of the middle troposphere and priming the atmosphere for spring and summer storms when sunlight returns to Titan’s lakes. Despite great gains in our understanding of Titan, many challenges remain. The greatest mystery is how Titan is able to retain an abundance of atmospheric methane with only limited surface liquids, while methane is being irreversibly destroyed by photochemistry. A related mystery is how Titan is able to hide all the ethane that is produced in this process. Future studies will need to consider the interactions between Titan’s atmosphere, surface, and subsurface in order to make further progress in understanding Titan’s complex climate system.

Authors: Lora, J.M. and Mitchell, J.L. 

 Journal: Geophys. Res. Lett. 42, 6213-6220.

Model Configuration: “Wetlands”

Abstract: The hemispheric asymmetry of Titan’s surface methane has been proposed to be a consequence of orbital forcing affecting Titan’s hydrologic cycle, but the mechanism behind asymmetrical transport of moisture remains to be examined. Using general circulation model simulations of Titan’s atmosphere, we show that atmospheric moisture transport by three-dimensional tropospheric eddies is critical in generating Titan’s surface liquid asymmetry. Comparison of axisymmetric and three-dimensional simulations demonstrates that a significant asymmetry only develops in the latter case. Analysis of the components of the three-dimensional moisture transport reveals that nonaxisymmetric eddies transport methane away from the poles and into the midlatitudes, where they transfer moisture into the cross-equatorial transport by the mean meridional circulation, producing an atmospheric “bucket brigade.” Because these high-latitude, baroclinic eddies are more intense in the south than in the north, the net transport is preferentially northward, with the northern hemisphere gaining surface liquid at the expense of the southern hemisphere.

Authors: Lora, J.M., Lunine, J.I. and Russell, J.L.

 Journal: Icarus 250, 516-528.

Model Configuration: Simple Hydrology and “Aquaplanet”

Abstract: Simulation results are presented from a new general circulation model (GCM) of Titan, the Titan Atmospheric Model (TAM), which couples the Flexible Modeling System (FMS) spectral dynamical core to a suite of external/sub-grid-scale physics. These include a new non-gray radiative transfer module that takes advantage of recent data from Cassini–Huygens, large-scale condensation and quasi-equilibrium moist convection schemes, a surface model with “bucket” hydrology, and boundary layer turbulent diffusion. The model produces a realistic temperature structure from the surface to the lower mesosphere, including a stratopause, as well as satisfactory superrotation. The latter is shown to depend on the dynamical core’s ability to build up angular momentum from surface torques. Simulated latitudinal temperature contrasts are adequate, compared to observations, and polar temperature anomalies agree with observations. In the lower atmosphere, the insolation distribution is shown to strongly impact turbulent fluxes, and surface heating is maximum at mid-latitudes. Surface liquids are unstable at mid- and low-latitudes, and quickly migrate poleward. The simulated humidity profile and distribution of surface temperatures, compared to observations, corroborate the prevalence of dry conditions at low latitudes. Polar cloud activity is well represented, though the observed mid-latitude clouds remain somewhat puzzling, and some formation alternatives are suggested.

Authors: Lora, J.M., Lunine, J.I., Russell, J.L, and Hayes, A.G.

 Journal: Icarus 250, 264-273.

Model Configuration: Simple Hydrology

Abstract: We investigate the effects of varying Saturn’s orbit on the atmospheric circulation and surface methane distribution of Titan. Using a new general circulation model of Titan’s atmosphere, we simulate its climate under four characteristic configurations of orbital parameters that correspond to snapshots over the past 42 kyr, capturing the amplitude range of long-period cyclic variations in eccentricity and longitude of perihelion. The model, which covers pressures from the surface to 0.5 mbar, reproduces the present-day temperature profile and tropospheric superrotation. In all four simulations, the atmosphere efficiently transports methane poleward, drying out the low- and mid-latitudes, indicating that these regions have been desert-like for at least tens of thousands of years. Though circulation patterns are not significantly different, the amount of surface methane that builds up over either pole strongly depends on the insolation distribution; in the present-day, methane builds up preferentially in the north, in agreement with observations, where summer is milder but longer. The same is true, to a lesser extent, for the configuration 14 kyr ago, while the south pole gains more methane in the case for 28 kyr ago, and the system is almost symmetric 42 kyr ago. This confirms the hypothesis that orbital forcing influences the distribution of surface liquids, and that the current observed asymmetry could have been partially or fully reversed in the past. The evolution of the orbital forcing implies that the surface reservoir is transported on timescales of ∼30 kyr, in which case the asymmetry reverses with a period of ∼125 kyr. Otherwise, the orbital forcing does not produce a net asymmetry over longer timescales, and is not a likely mechanism for generating the observed dichotomy.