•Experientially measure e-IDR rate constants relevant to Titan.•Establish trends in benzene analogs to aid prediction.•Correlation between e-IDR rate constant and negative Hammett σpara ...values.•Correlation between temperature dependence and negative Hammett σpara values.
An in-depth study of the effects of functional group substitution on benzene’s electron–ion dissociative recombination (e-IDR) rate constant has been conducted. The e-IDR rate constants for benzene, biphenyl, toluene, ethylbenzene, anisole, phenol, and aniline have been measured using a Flowing Afterglow equipped with an electrostatic Langmuir probe (FALP). These measurements have been made over a series of temperatures from 300 to 550K. A relationship between the Hammett σpara values for each compound and rate constant has indicated a trend in the e-IDR rate constants and possibly in their temperature dependence data. The Hammett σpara value is a method to describe the effect a functional group substituted to a benzene ring has upon the reaction rate constant.
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.
•Cloud resolving simulations are initialized with soundings from a global atmospheric model of Titan.•The impact of deep convection on the large-scale environment is diagnosed.•Titan's large-scale environmental response to deep convection is analogous to Earth.•Tunable parameters for a deep convective parameterization in global models are derived.
Titan's abundant lakes and seas exchange methane vapor and energy with the atmosphere via a process generally known as air-sea interaction. This turbulent exchange process is investigated with an ...atmospheric mesoscale model coupled to a slab model representation of an underlying lake. The impact of lake size, effective lake mixed layer depth, background wind speed, air-lake temperature differential, and atmospheric humidity on air-sea interaction processes is studied through dozens of two-dimensional simulations. The general, quasi-steady solution is a non-linear superposition of a very weak background plume circulation driven by the buoyancy of evaporated methane with a stronger opposing thermally direct (sea breeze) circulation driven by the thermal contrast between the cold marine layer over the lake and the warmer inland air. The specific solution depends on the value of selected atmosphere and lake property parameters, but the general solution of the superposition of these two circulations is persistent. Consistent with previous analytical work of others, the sensible heat flux and the latent heat flux trend toward opposite and equal values such that their ratio, the Bowen ratio, approaches −1.0 in most, but not all, of the quasi-steady state solutions. Importantly, in nearly all scenarios, the absolute magnitude of the fluxes trends toward very small values such that the equilibrium solution is also nearly a trivial solution where air-sea energy exchange is ~3 W m−2 or less. In all cases, a cool, moist, and statically stable shallow marine layer with nearly calm winds and small turbulent flux exchanges with a colder underlying lake is produced by the model. The temperature of the lake, the marine properties of the air, and the strength of the sea breeze depends on the initial conditions and to a lesser degree, the boundary conditions.
•Evaporation from Titan's lakes involves a complex process of thermodynamics and dynamics.•A cold, stable marine layer commonly forms in the vicinity of lakes.•Condition for cloud formation is difficult to achieve.•The equilibrium solution tends toward very small turbulent fluxes and near calm winds.
•We present simulations of Titan’s middle and lower atmosphere with the Titan Atmospheric Model (TAM) GCM.•Vertical and latitudinal temperature profiles from the surface through the stratopause are ...reproduced.•Superrotation develops naturally in the model.•Comparison to observations indicates the prevalence of dry conditions at low latitudes.•Polar and equatorial precipitation are consistent with observed clouds, but mid-latitude cloudiness remains a puzzle.
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.
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.
•Topographic and orbital forcing tested individually and together in a Titan GCM with coupled surface hydrology.•Topography causes warmer surface at poles and regional precipitation heterogeneity.•Topography does not force notable asymmetry in meridional circulation or in atmospheric methane transport.•Atmospheric methane transport responds to orbital forcing but distribution of surface liquids is dominated by surface hydrology.
Titan's labyrinth terrains are an organic-rich, topographically elevated, highly dissected and puzzling geomorphic unit. How these features came to be composed of organics and remain elevated may ...hold clues about Titan's complicated history, and in particular the dynamics and composition of Titan's crust. One subtype of labyrinth terrains, the radially networked labyrinth terrains, is found in Titan's mid-latitudes. They are dome-shaped with radial drainage patterns and appear to be a clustering of uplifted, organic-rich dissected plateaux. We use scaling relationships to determine whether they formed as elevated surfaces that were uplifted by solid-state diapirs or cryomagmatic laccoliths at depth. Based on the large, variable spacing between features, we find it unlikely that they formed via density-driven diapirism. Instead, their dimensions suggest that they are cryomagmatic intrusions that formed near the most prominent rheological contrast in the ice shell, the brittle–ductile transition (BDT). At that location, we argue that intrusions spread horizontally and inflate, forming large cryomagmatic laccoliths (upturned, large saucer-shaped sills). The intrusions would flex the overlying lithosphere and surficial sedimentary layers (likely undifferentiated plains), resulting in prominent domed features that are susceptible to erosion and incision by methane rain and wind. This process then leads to the highly dissected, dome-shaped labyrinth terrains. To determine whether the laccoliths formed near Titan's BDT, we calculate lithospheric strength envelopes for a pure water ice shell with and without an insulating methane clathrate crust using two conductive heat flows: 4 and 7 mW/m2. The plausible range for the BDT for an ice shell with a 1 km thick methane clathrate crust is 12–34 km for the 4 mW/m2 heat flow. This agrees well with the expected intrusion depths of the laccoliths (21–28 km) associated with a cluster of radial labyrinths in Titan's northern mid-latitudes, as derived from a scaling relationship relating intrusion depth to dome width. We find that methane clathrate thicknesses of 2 km or greater result in a BDT that is generally too shallow (3–24 km) to match our observations. If we consider the higher heat flow, these BDTs are even shallower. Although methane clathrate is known to be stronger than ice, its low thermal conductivity significantly raises the underlying ice shell's temperature, which results in a significantly thinner lithosphere that is not able to support these large plateaux. We conclude that in the location of this radial labyrinth terrain cluster there may be intrusive cryomagmatic activity approximately 21 km deep within the water ice shell, with a putative surface methane clathrate crust, if it exists atop a conductive ice shell, that is constrained to be <2 km thick.
•Titan's northern radial labyrinths appear to be uplifted undifferentiated plains.•Their dimensions support a cryomagmatic intrusion origin, rather than diapirism.•The (BDT) brittle–ductile transition is shallower under a methane clathrate crust.•The widths of these features scale with the depth of intrusion and the BDT.•Titan's methane clathrate crust is likely <2 km thick in the region of study.
Methane rain on Saturn's moon Titan makes it the only place, other than Earth, where rain interacts with the surface. When and where that rain wets the surface changes seasonally in ways that remain ...poorly understood. Here we report the discovery of a bright ephemeral feature covering an area of 120,000 km2 near Titan's north pole in observations from Cassini's near‐infrared instrument, Visual and Infrared Mapping Spectrometer on 7 June 2016. Based on the overall brightness, spectral characteristics, and geologic context, we attribute this new feature to specular reflections from a rain‐wetted solid surface like those off of a sunlit wet sidewalk. The reported observation is the first documented rainfall event at Titan's north pole and heralds the arrival of the northern summer (through climatic evidence), which has been delayed relative to model predictions. This detection helps constrain Titan's seasonal change and shows that the “wet‐sidewalk effect can be used to identify other rain events.”
Plain Language Summary
Cassini arrived in the Saturnian system in the southern summers of 2004. As expected, the Cassini team observed cloud cover, storms, and precipitation on the south pole. Like Earth, Titan has an axial tilt (27°) and its seasons vary over its year (30 Earth years). Ever since this shift in season began, the Cassini team eagerly waited for observations indicating cloud cover and precipitation that went missing from the northern latitudes. Our rainfall observation at the north pole is a major finding for two important reasons. First, this discovery observation heralds the much awaited arrival of the north polar summer rainstorms on Titan. This atmospheric phenomenon has been delayed compared to the theoretical predictions and was perplexing Titan researchers and climate modelers especially because the north pole hosts most of Titan's lakes and seas. Second, it is extremely difficult to detect rainfall events on Titan due to its thick atmospheric haze and very limited opportunities to view the surface (and its changes). We have used a novel phenomenon—the smoothening of a previously dry, rough surface by a thin layer of fluid after rainfall, similar to a wet sidewalk—as evidence for rainfall events on the surface of Titan.
Key Points
We report the discovery of a bright ephemeral feature near Titan's north pole in observations from the Cassini's near‐infrared instrument, VIMS
Detections of broad specular reflections is a new technique for monitoring the occurrence of precipitation across Titan's surface
Our detection of a wet surface near Titan's north pole is the first evidence of rain during north polar summer
•We have generated the first, complete geomorphologic map of Titan's polar regions.•Most of Titan's lake depressions can be found with a single plains unit, suggesting that the unit is conducive to ...the formation of such features.•Uplands materials are not uniform in composition, where mountains and dissected uplands have different radar backscattering behaviors.•The observed landscape appears as an underlying bedrock material that was mantled by vast sedimentary deposits. Superposed on these sedimentary deposits are steep-sided depressions, canyons and seas.•We offer a conceptual model that is able to explain the distribution and superposition relationships among our map units, where in Titan's distant past the poles were submerged under larger liquid bodies.
We present a geomorphologic map of Titan's polar terrains. The map was generated from a combination of Cassini Synthetic Aperture Radar (SAR) and Imaging Science Subsystem imaging products, as well as altimetry, SARTopo and radargrammetry topographic datasets. In combining imagery with topographic data, our geomorphologic map reveals a stratigraphic sequence from which we infer process interactions between units. In mapping both polar regions with the same geomorphologic units, we conclude that processes that formed the terrains of the north polar region also acted to form the landscape we observe at the south. Uniform, SAR-dark plains are interpreted as sedimentary deposits, and are bounded by moderately dissected uplands. These plains contain the highest density of filled and empty lake depressions, and canyons. These units unconformably overlay a basement rock that outcrops as mountains and SAR-bright dissected terrains at various elevations across both poles. All these units are then superposed by surficial units that slope towards the seas, suggestive of subsequent overland transport of sediment. From estimates of the depths of the embedded empty depressions and canyons that drain into the seas, the SAR-dark plains must be >600m thick in places, though the thickness may vary across the poles. At the lowest elevations of each polar region, there are large seas, which are currently liquid methane/ethane filled at the north and empty at the south. The large plains deposits and the surrounding hillslopes may represent remnant landforms that are a result of previously vast polar oceans, where larger liquid bodies may have allowed for a sustained accumulation of soluble and insoluble sediments, potentially forming layered sedimentary deposits. Coupled with vertical crustal movements, the resulting layers would be of varying solubilities and erosional resistances, allowing formation of the complex landscape that we observe today.
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