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Hundreds of lakes and a few seas of liquid hydrocarbons have been observed by the Cassini spacecraft to cover the polar regions of Titan. A significant fraction of these lakes or seas could possibly be interconnected with subsurfa...
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Hundreds of lakes and a few seas of liquid hydrocarbons have been observed by the Cassini spacecraft to cover the polar regions of Titan. A significant fraction of these lakes or seas could possibly be interconnected with subsurface liquid reservoirs of alkanes. In this paper, we investigate the interplay that would happen between a reservoir of liquid hydrocarbons located in Titan's subsurface and a hypothetical clathrate reservoir that progressively forms if the liquid mixture diffuses throughout a preexisting porous icy layer. To do so, we use a statistical-thermodynamic model in order to compute the composition of the clathrate reservoir that forms as a result of the progressive entrapping of the liquid mixture. This study shows that clathrate formation strongly fractionates the molecules between the liquid and the solid phases. Depending on whether the structures I or II clathrate forms, the present model predicts that the liquid reservoirs would be mainly composed of either propane or ethane, respectively. The other molecules present in the liquid are trapped in clathrates. Any river or lake emanating from subsurface liquid reservoirs that significantly interacted with clathrate reservoirs should present such composition. On the other hand, lakes and rivers sourced by precipitation should contain higher fractions of methane and nitrogen, as well as minor traces of argon and carbon monoxide.
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Ethane is expected to be the dominant photochemical product on Titan's surface and, in the absence of a process that sequesters it from exposed surface reservoirs, a major constituent of its lakes and seas. Absorption of Cassini's...
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Ethane is expected to be the dominant photochemical product on Titan's surface and, in the absence of a process that sequesters it from exposed surface reservoirs, a major constituent of its lakes and seas. Absorption of Cassini's 2.2 cm radar by Ligeia Mare however suggests that this north polar sea is dominated by methane. In order to explain this apparent ethane deficiency, we explore the possibility that Ligeia Mare is the visible part of an alkanofer that interacted with an underlying clathrate layer and investigate the influence of this interaction on an assumed initial ethane-methane mixture in the liquid phase. We find that progressive liquid entrapment in clathrate allows the surface liquid reservoir to become methane-dominated for any initial ethane mole fraction below 0.75. If interactions between alkanofers and clathrates are common on Titan, this should lead to the emergence of many methane-dominated seas or lakes. (C) 2015 Elsevier Inc. All rights reserved.
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Observations of lakes, fluvial dissection of the surface, rapid variations in cloud cover, and lake shoreline changes indicate that Saturn's moon Titan is hydrologically active, with a hydrocarbon-based hydrological cycle dominate...
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Observations of lakes, fluvial dissection of the surface, rapid variations in cloud cover, and lake shoreline changes indicate that Saturn's moon Titan is hydrologically active, with a hydrocarbon-based hydrological cycle dominated by liquid methane. Here we use a numerical model to investigate the Titan hydrological cycle - including surface, subsurface, and atmospheric components - in order to investigate the underlying causes of the observed distribution and sizes of lakes in the north polar region. The hydrocarbon-based hydrological cycle is modeled using a numerical subsurface flow model and analytical runoff scheme, driven by a general circulation model with an active methane-cycle. This model is run on synthetically generated topography that matches the fractal character of the observed topography, without explicit representation of the effects of erosion and deposition. At the scale of individual basins, intermediate to high permeability (10(-8)-10(-6) cm(2)) aquifers are required to reproduce the observed large stable lakes. However, at the scale of the entire north polar lake district, a high permeability aquifer results in the rapid flushing of methane through the aquifer from high polar latitudes to dry lower polar latitudes, where methane is removed by evaporation, preventing large lakes from forming. In contrast, an intermediate permeability aquifer slows the subsurface flow from high polar latitudes, allowing greater lake areas. The observed distribution of lakes is best matched by either a uniform intermediate permeability aquifer, or a combination of a high permeability cap at high latitudes surrounded by an intermediate permeability aquifer at lower latitudes, as could arise due to karstic processes at the north pole. The stability of Kraken Mare further requires reduction of the evaporation rate over the sea to 1% of the value predicted by the general circulation model, likely as a result of dissolved ethane, nitrogen, or organic solutes, and/or a climatic lake effect. These results reveal that subsurface flow through aquifers plays an important role in Titan's hydrological cycle, and exerts a strong influence over the distribution, size, and volatile budgets of Titan's lakes. (C) 2016 Elsevier Inc. All rights reserved.
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Microphysical models describe the way aerosols and clouds behave in the atmosphere. Two approaches are generally used to model these processes. While the first approach discretizes processes and aerosols size distributions on a ra...
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Microphysical models describe the way aerosols and clouds behave in the atmosphere. Two approaches are generally used to model these processes. While the first approach discretizes processes and aerosols size distributions on a radius grid (bin scheme), the second uses bulk parameters of the size distribution law (its mathematical moments) to represent the evolution of the particle population (moment scheme). However, with the latter approach, one needs to have an a priori knowledge of the size distributions. Moments scheme for Cloud microphysics modeling have been used and enhanced since decades for climate studies of the Earth. Most of the tools are based on Log-Normal law which are suitable for Earth, Mars or Venus. On Titan, due to the fractal structure of the aerosols, the size distributions do not follow a log-normal law. Then using a moment scheme in that case implies to define the description of the size distribution and to review the equations that are widely published in the literature. Our objective is to enable the use of a fully described microphysical model using a moment scheme within a Titan's Global Climate Model. As a first step in this direction, we present here a moment scheme dedicated to clouds microphysics adapted for Titan's atmosphere conditions. We perform comparisons between the two kinds of schemes (bin and moments) using an annual and a diurnal cycle, to check the validity of our moment description. The various forcing produce a time-variable cloud layer in relation with the temperature cycle. We compare the column opacities and the temperature for the two schemes, for each cycles. We also compare more detailed quantities as the opacity distribution of the cloud events at different periods of these cycles. Results show that differences between the two approaches have a small impact on the temperature (less than 1 K) and range between 1% and 10% for haze and clouds opacities. Both models behave in similar way when forced by an annual and by a diurnal cycles. We note that in our model, the diurnal cycle produces a remarkable asymmetry between the morning and the evening, that can be associated to morning/evening limb asymmetry observed with ground-based telescopes.
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In May of 2012 images of Titan obtained by the Cassini Imaging Science Subsystem (ISS) showed a newly formed cloud patch near the southern pole. The cloud has unusual morphology and texture suggesting that it is formed by condensa...
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In May of 2012 images of Titan obtained by the Cassini Imaging Science Subsystem (ISS) showed a newly formed cloud patch near the southern pole. The cloud has unusual morphology and texture suggesting that it is formed by condensation at an altitude much higher than expected for any of the known organics in Titan's atmosphere. We measured the altitude to be 300 10 km from images when the feature was on the limb. Limb images suggest that the initial stages of the formation began in late 2011. It was just visible in images obtained in 2014 but is not expected to be visible in the future due to enveloping darkness as the season progresses. The feature has a slightly different color than the surrounding haze. Its optical thickness is near 2 at 889 nm wavelength and the particle imaginary refractive index must be less than 5 x 10(-4) at that wavelength. Wind vectors derived from a time series show that it is rotating about a center offset by 4.5 degrees from Titan's solid-body spin axis, consistent with that found from the temperature field by Achterberg et al. (Achterberg, R.K., Conrath, B.J., Gierasch, P.J., Flasar, F.M., Nixon, C.A. [20084 Icarus 197, 549-555) and subsequent measurements. The feature rotates at an angular velocity near the rate expected for transport of angular momentum from the low latitudes to the pole. The clumpy texture of the feature resembles that of terrestrial cloud fields undergoing open cell convection, an unusual configuration initiated by downwelling. (C) 2014 Elsevier Inc. All rights reserved.
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Based on comprehensive mapping of the region, the recent theories of Xanadu's origin are examined and a chronology of geologic processes is proposed. The geologic history of Titan's Xanadu region is different from that of the othe...
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Based on comprehensive mapping of the region, the recent theories of Xanadu's origin are examined and a chronology of geologic processes is proposed. The geologic history of Titan's Xanadu region is different from that of the other surface units on Saturn's moon. A previously proposed origin of western Xanadu from a giant impact in the early history of the moon is difficult to confirm given the scarcity of morphologic indications of an impact basin. The basic topographic structure of the landscape is controlled by tectonic processes that date back to the early history of Titan. More recently, the surface is intensely reworked and resurfaced by fluvial processes, which seem to have leveled out and compensated height differences. Although the surface age seems young at first view, the underlying processes that created this surface and the topographic structure appear to be ancient.
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All landforms on Titan that are unambiguously identifiable can be explained by exogenic processes (aeolian, fluvial, impact cratering, and mass wasting). Previous suggestions of endogenically produced cryovolcanic constructs and f...
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All landforms on Titan that are unambiguously identifiable can be explained by exogenic processes (aeolian, fluvial, impact cratering, and mass wasting). Previous suggestions of endogenically produced cryovolcanic constructs and flows have, without exception, lacked conclusive diagnostic evidence. The modification of sparse recognizable impact craters (themselves exogenic) can be explained by aeolian and fluvial erosion. Tectonic activity could be driven by global thermal evolution or external forcing, rather than by active interior processes. A lack of cryovolcanism would be consistent with geophysical inferences of a relatively quiescent interior: incomplete differentiation, only minor tidal heating, and possibly a lack of internal convection today. Titan might be most akin to Callisto with weather: an endogenically relatively inactive world with a cool interior. We do not aim to disprove the existence of any and all endogenic activity at Titan, nor to provide definitive alternative hypotheses for all landforms, but instead to inject a necessary level of caution into the discussion. The hypothesis of Titan as a predominantly exogenic world can be tested through additional Cassini observations and analyses of putative cryovolcanic features, geophysical and thermal modeling of Titan's interior evolution, modeling of icy satellite landscape evolution that is shaped by exogenic processes alone, and consideration of possible means for supplying Titan's atmospheric constituents that do not rely on cryovolcanism.
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Titan, with its thick, nitrogen-dominated atmosphere, has been seen from satellite and terrestrial observations to harbour methane clouds. To investigate whether atmospheric features such as clouds could also be visible from the s...
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Titan, with its thick, nitrogen-dominated atmosphere, has been seen from satellite and terrestrial observations to harbour methane clouds. To investigate whether atmospheric features such as clouds could also be visible from the surface of Titan, data taken with the Side Looking Imager (SLI) on-board the Huygens probe after landing have been analysed to identify any potential atmospheric features. In total, 82 SLI images were calibrated, processed and examined for features. The calibrated images show a smooth vertical radiance gradient across the images, with no other discernible features. After mean-frame subtraction, six images contained an extended, horizontal feature that had a radiance value that lay outside the 95% confidence limit of the predicted radiance when compared to regions higher and lower in the images. The change in optical depth of these features were found to be between 0.005 and 0.014. It is considered that these features most likely originate from the presence of a fog bank close to the horizon that rises and falls during the period of observation. (C) 2016 Elsevier Inc. All rights reserved.
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Density-driven circulation in Titan's seas forced by solar heating and methane evaporation/precipitation is simulated by an ocean circulation model. If the sea is transparent to sunlight, solar heating can induce anti-clockwise gy...
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Density-driven circulation in Titan's seas forced by solar heating and methane evaporation/precipitation is simulated by an ocean circulation model. If the sea is transparent to sunlight, solar heating can induce anti-clockwise gyres near the sea surface and clockwise gyres near the sea bottom. The gyres are in geostrophic balance between the radially symmetric pressure gradient force and Coriolis force. If instead the sea is turbid and most sunlight is absorbed near the sea surface, the sea gets stratified in warm seasons and the circulation remains weak. Precipitation causes compositional stratification of the sea to an extent that the sea surface temperature can be lower than the sea interior temperature without causing a convective overturning. Non-uniform precipitation can also generate a latitudinal gradient in the methane mole fraction and density, which drives a meridional overturning with equatorward currents near the sea surface and poleward currents near the sea bottom. However, gyres are more ubiquitous than meridional overturning. (C) 2015 Elsevier Inc. All rights reserved.
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To date, there has been no conclusive observation of ongoing endogenous volcanic activity on Saturn's moon Titan. However, with time, Titan's atmospheric methane is lost and must be replenished. We have modeled one possible mechan...
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To date, there has been no conclusive observation of ongoing endogenous volcanic activity on Saturn's moon Titan. However, with time, Titan's atmospheric methane is lost and must be replenished. We have modeled one possible mechanism for the replenishment of Titan's methane loss. Cryolavas can supply enough heat to release large amounts of methane from methane clathrate hydrates (MCH). The volume of methane released is controlled by the flow thickness and its areal extent. The depth of the destabilisation layer is typically approximate to 30% of the thickness of the lava flow (approximate to 3 m for a 10-m thick flow). For this flow example, a maximum of 372 kg of methane is released per m(2) of flow area. Such an event would release methane for nearly a year. One or two events per year covering similar to 20 km(2) would be sufficient to resupply atmospheric methane. A much larger effusive event covering an area of approximate to 9000 km(2) with flows 200 m thick would release enough methane to sustain current methane concentrations for 10,000 years. The minimum size of "cryo-flows" sufficient to maintain the current atmospheric methane is small enough that their detection with current instruments (e.g., Cassini) could be challenging. We do not suggest that Titan's original atmosphere was generated by this mechanism. It is unlikely that small-scale surface MCH destabilisation is solely responsible for long-term (> a few Myr) sustenance of Titan's atmospheric methane, but rather we present it as a possible contributor to Titan's past and current atmospheric methane. (C) 2016 Published by Elsevier Inc.
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