Titan's shape is characterized by a difference between the long equatorial radius and the polar radius that is several hundred meters larger than that predicted by the flattening due to its spin ...rate. The North polar region is covered by large mare filled with hydrocarbons, including ethane. Moreover global circulation models predict ethane precipitation on the polar areas. This study shows that the shape of Titan can be explained by the subsidence associated with the substitution of methane with ethane‐rich liquids percolating into the crust which, as suggested by evolution models, may be composed of methane clathrate hydrates. Such substitutions have been observed in laboratory experiments. This process would provide an additional methane source as required for sustaining the presence of this constituent in Titan's atmosphere through its history. A 270 m subsidence of the polar caps is explained by the circulation of 1.5 to 6 × 1018kg of ethane in the top three kilometers of an initially methane‐clathrate crust. This process would have operated during the last 300–1200 Myr at the present ethane production rate.
Key Points
Ethane can substitute methane in clathrate hydrates on Titan
The densification of clathrates by this exchange induces subsidence
The subsidence is consistent with Titan's shape as measured by the Cassini probe
By comparison with the Earth-like planets and the large icy satellites of the Solar System, one can model the internal structure of extrasolar planets. The input parameters are the composition of the ...star (Fe/Si and Mg/Si), the Mg content of the mantle (Mg#
=
Mg/Mg
+
Fe), the amount of H
2O and the total mass of the planet. Equation of State (EoS) of the different materials that are likely to be present within such planets have been obtained thanks to recent progress in high-pressure experiments. They are used to compute the planetary radius as a function of the total mass. Based on accretion models and data on planetary differentiation, the internal structure is likely to consist of an iron-rich core, a silicate mantle and an outer silicate crust resulting from magma formation in the mantle. The amount of H
2O and the surface temperature control the possibility for these planets to harbor an ocean. In preparation to the interpretation of the forthcoming data from the CNES led CoRoT (Convection Rotation and Transit) mission and from ground-based observations, this paper investigates the relationship between radius and mass. If H
2O is not an important component (less than 0.1%) of the total mass of the planet, then a relation
(
R
/
R
Earth
)
=
a
(
M
/
M
Earth
)
b
is calculated with
(
a
,
b
)
=
(
1
,
0.306
)
and
(
a
,
b
)
=
(
1
,
0.274
)
for
10
−2
M
Earth
<
M
<
M
Earth
and
M
Earth
<
M
<
10
M
Earth
, respectively. Calculations for a planet that contains 50% H
2O suggest that the radius would be more than 25% larger than that based on the Earth-like model, with
(
a
,
b
)
=
(
1.258
,
0.302
)
for
10
−2
M
Earth
<
M
<
M
Earth
and
(
a
,
b
)
=
(
1.262
,
0.275
)
for
M
Earth
<
M
<
10
M
Earth
, respectively. For a surface temperature of 300 K, the thickness of the ocean varies from 150 to 50 km for planets 1 to 10 times the Earth's mass, respectively. Application of this algorithm to bodies of the Solar System provides not only a good fit to most terrestrial planets and large icy satellites, but also insights for discussing future observations of exoplanets.
A mass-radius (M-R) relationship is proposed for solid planets and solid cores ranging from 1 to 100 Earth-mass planets. It relies on the assumption that solid spheres are composed of iron and ...silicates, around which a variable amount of water is added. The M-R law has been set up assuming that the planetary composition is similar to the averaged composition for silicates and iron obtained from the major elements ratio of 94 stars hosting exoplanets. Except on Earth for which a tremendous amount of data is available, the composition of silicate mantles and metallic cores cannot be constrained. Similarly, thermal profiles are poorly known. In this work, the effect of compositional parameters and thermal profiles on radii estimates is quantified. It will be demonstrated that uncertainties related to composition and temperature are secondary compared to the effect of the amount of water. The super-Earth family includes four classes of planets: iron-rich, silicate-rich, water-rich, or with a thick atmosphere. For a given mass, the planetary radius increases significantly from the iron-rich to the atmospheric-rich planet. Even if some overlaps are likely, M-R measurements could be accurate enough to ascertain the discovery of an Earth-like planet. The present work describes how the amount of water can be assessed from M-R measurements. Such an estimate depends on several assumptions including (1) the accuracy of the internal structure model and (2) the accuracy of mass and radius measurements. It is shown that if the mass and the radius are perfectly known, the standard deviation of the amount of water is about 4.5%. This value increases rapidly with the radius uncertainty but does not strongly depend on the mass uncertainty.
Saturn's largest satellite, Titan, has a massive nitrogen atmosphere containing up to 5 per cent methane near its surface. Photochemistry in the stratosphere would remove the present-day atmospheric ...methane in a few tens of millions of years. Before the Cassini-Huygens mission arrived at Saturn, widespread liquid methane or mixed hydrocarbon seas hundreds of metres in thickness were proposed as reservoirs from which methane could be resupplied to the atmosphere over geologic time. Titan fly-by observations and ground-based observations rule out the presence of extensive bodies of liquid hydrocarbons at present, which means that methane must be derived from another source over Titan's history. Here we show that episodic outgassing of methane stored as clathrate hydrates within an icy shell above an ammonia-enriched water ocean is the most likely explanation for Titan's atmospheric methane. The other possible explanations all fail because they cannot explain the absence of surface liquid reservoirs and/or the low dissipative state of the interior. On the basis of our models, we predict that future fly-bys should reveal the existence of both a subsurface water ocean and a rocky core, and should detect more cryovolcanic edifices.
Ceres: Evolution and current state McCord, Thomas B.; Sotin, Christophe
Journal of Geophysical Research,
20/May , Letnik:
110, Številka:
E5
Journal Article
Recenzirano
Odprti dostop
We modeled several thermal evolution scenarios for Ceres to explore the nature of large, wet protoplanets and to predict current‐day evidence that might be found by close inspection, such as by the ...Dawn mission. The density for Ceres is near 2.1, suggesting a water content between 17% and 27% by mass. Short‐ and long‐lived radioactive nuclide heating is considered. Even if only long‐lived radionuclide heating is assumed, the water ice in Ceres melts quickly and a water mantle forms, but an approximately 10‐km crust does not melt. The circulating warm water would alter the silicates. As heat is lost by conduction through the frozen crust, water begins to freeze out at the base of the crust. Solid‐state convection begins and transports more heat as well as perhaps material dissolved or entrained in the water to or near the surface. Ceres' water layer eventually (but perhaps not entirely) freezes, forming a layered density structure with perhaps some liquid water remaining today. Our differentiated models are in agreement with the recently measured difference between the equatorial and polar radii. We find that Ceres' existence and evolution depend critically on it containing water at formation, and this depends strongly on the combination of when it accreted and the amount of 26Al present in the pre‐Ceres ∼1‐km‐sized objects; slightly more 26Al or earlier accretion produces a dry Vesta‐like object. Melting and freezing plus mineralization would lead to several dimensional changes over time, creating topographic features, zones of weakness, and perhaps disruptions in the crust.
•3D convection simulations tailored to specific structure of high-pressure ice mantle of large icy moons.•Melting is introduced, melt extraction is assumed instantaneous.•HP mantle found globally ...cold but small areas always at/close to the melting point in hot upwellings.•Heat flux from rock core accomodated by melting at rock core interface and melt extraction throughout the HP ice mantle.
While the existence of a buried ocean sandwiched between surface ice and high-pressure (HP) polymorphs of ice emerges as the most plausible structure for the hundreds-of-kilometers thick hydrospheres within large icy moons of the Solar System (Ganymede, Callisto, Titan), little is known about the thermal structure of the deep HP ice mantle and its dynamics, possibly involving melt production and extraction. This has major implications for the thermal history of these objects as well as on the habitability of their ocean as the HP ice mantle is presumed to limit chemical transport from the rock component to the ocean. Here, we describe 3D spherical simulations of subsolidus thermal convection tailored to the specific structure of the HP ice mantle of large icy moons. Melt production is monitored and melt transport is simplified by assuming instantaneous extraction to the ocean above. The two controlling parameters for these models are the rheology of ice VI and the heat flux from the rock core. Reasonable end-members are considered for both parameters as disagreement remains on the former (especially the pressure effect on viscosity) and as the latter is expected to vary significantly during the moon’s history. We show that the heat power produced by radioactive decay within the rock core is mainly transported through the HP ice mantle by melt extraction to the ocean, with most of the melt produced directly above the rock/water interface. While the average temperature in the bulk of the HP ice mantle is always relatively cool when compared to the value at the interface with the rock core (∼ 5 K above the value at the surface of the HP ice mantle), maximum temperatures at all depths are close to the melting point, often leading to the interconnection of a melt path via hot convective plume conduits throughout the HP ice mantle. Overall, we predict long periods of time during these moons’ history where water generated in contact with the rock core is transported to the above ocean.
This paper describes a new approach based on variational principles to calculate the radial distribution of tidal energy dissipation in any satellite. The advantage of the model with respect to ...classical solutions, is that it relates in a straightforward way the radial distribution of the time-averaged dissipation rate to its sensitivity to the corresponding distribution of viscoelastic parameters. This method is applied to Io-, Europa-, and Titan-like interiors, and it is tested against the results obtained by two classical methods by determining global dissipation as well as radial and lateral distributions within satellite interiors. By exploring systematically the different parameters defining the interior models, we demonstrate that the presence of a deep ocean below an outer ice layer strongly influences the tidal dissipation distribution in both the outer ice layer and in the innermost part of the satellite. On the one hand, the ocean by imposing a large radial displacement at the base of the outer ice I layer, controls the distribution of tidal strain rate within the outer layer, making the tidal strain rate field very weakly sensitive to the viscosity variations. Conversely, in the high-pressure ice layer below the ocean, both tidal strain rate and dissipation are very sensitive to any variation of the ice viscosity. On the other hand, for identical structures of the mantle and of the core, the presence of a subsurface ocean reduces the strength of dissipation in the silicate mantle. The existence of a liquid layer within Europa makes models of the silicate mantle less dissipative than the predictions for Io.
Liquid ethane is present in the lakes and seas observed on Titan's surface by the Cassini‐Huygens mission. While interaction between liquid hydrocarbons and water ice is expected to result in the ...formation of clathrate hydrates, such reaction (and its kinetics) has not yet been demonstrated for cryogenic liquids under relevant planetary conditions. In this paper, we report the first experimental evidence for rapid formation of clathrates upon direct contact of liquid ethane with water ice at 1 bar using micro‐Raman spectroscopy. Kinetics experiments conducted in the temperature range 150–173 K yield an activation energy of 14.8 ± 2.2 kJ/mol, which is suggestive of a diffusion‐controlled mechanism for clathrate formation. This implies that a clathrate reservoir can form within seasonal time scales on Titan if liquid ethane comes into contact with a pre‐existing icy bedrock, which may hold important implications for the structure and dynamics of Titan's crust and its global evolution.
Plain Language Summary
Saturn's moon Titan has a dense atmosphere and liquid lakes (of methane and ethane) on its surface. These organic compounds are the main constituents of natural gas on Earth. Water is completely frozen and comprises most of Titan's bedrock materials. Under the right conditions, it is common for water ice to form new solids that trap natural gas in their structures, called clathrate hydrates. On Earth, clathrate hydrates are often found in the permafrost and deep ocean. However, Titan is so much colder that these natural gases exist as liquids. We do not yet know if clathrates can form under such conditions, and if so, how quickly. In this study, we prepared small grains of water ice, bathed them in liquid ethane, and monitored the rate at which clathrate hydrates appear. Our data confirm that clathrates would indeed form on Titan from an ethane lake or rain event onto water ice, and they would do so very quickly (in just a few Earth years). This suggests that clathrate hydrates can be an important material on Titan that may be encountered by future missions there.
Key Points
Rapid formation of clathrate hydrates from interaction between liquid ethane and water ice at 1 bar is demonstrated for the first time
Kinetics data at 150–173 K yield an activation energy of 14.8 kJ/mol, implying a diffusion‐controlled mechanism for clathrate formation
Extrapolation to Titan's conditions suggests that ethane‐clathrate caps can form on seasonal time scales, which may explain polar depressions
Earth, Jupiter's moon Io and Saturn's tiny moon Enceladus are the only solid objects in the Solar System to be sufficiently geologically active for their internal heat to be detected by remote ...sensing. Interestingly, the endogenic activity on Enceladus is only located on a specific region at the south pole, from which jets of water vapor and ice particles have been observed Spencer, J.R., and 9 colleagues, 2006. Science 311, 1401–1405; Porco, C.C., and 24 colleagues, 2006. Science 311, 1393–1401. The current polar location of the thermal anomaly can possibly be explained by diapir-induced reorientation of the satellite Nimmo, F., Pappalardo, R.T., 2006. Nature 441, 614–616, but the thermal anomaly triggering and the heat power required to sustain it over geological timescales remain problematic. Using a three-dimensional viscoelastic numerical model simulating the response of Enceladus to tidal forcing, we explore the effect of a low-viscosity anomaly in the ice shell, localized to the south polar region, on the tidal dissipation patterns. We demonstrate that only interior models with a liquid water layer at depth can explain the observed magnitude of dissipation rate and its particular location at the south pole. Moreover, we show that tidally-induced heat must be generated over a relatively broad region in the southern hemisphere, and it is then transferred toward the south pole where it is episodically released during relatively short resurfacing events. As large tidal dissipation and internal melting cannot be induced in the south polar region in the absence of a pre-existing liquid decoupling layer, we propose that liquid water must have been present in the interior for a very long period of time, and possibly since the satellite formation. Owing to the orbital equilibrium requirement Meyer, J., Wisdom, J., 2007. Icarus 188, 535–539, sustaining some liquid water at depth is impossible if heat is continuously emitted at a rate of 4–8 GW at the south pole. Based on that requirement, we propose that the current thermal emission is not in equilibrium with the heat production, and that the thermal emission rate is abnormally high at present time. Alternatively, continuous dissipation at a rate of 1–2 GW in the ice shell at the south pole should be sufficient to induce internal melting and it could sustain a layer of liquid water at depth over geologic timescales.
Saturn's moon Enceladus emits a plume of water vapour and micrometre-sized ice particles from a series of warm fissures located near its south pole. This geological activity could be powered or ...controlled by variations in the tidal stresses experienced by Enceladus as it moves around its slightly eccentric orbit. The specific mechanisms by which these varying stresses are converted into heat, however, are still being debated. Furthermore, it has proved difficult to find a clear correlation between the predicted tidal forces and measured temporal variations in the plume's gas content or the particle flux from individual sources. Here we report that the plume's horizontally integrated brightness is several times greater when Enceladus is near the point in its eccentric orbit where it is furthest from Saturn (apocentre) than it is when near the point of closest approach to the planet (pericentre). More material therefore seems to be escaping from beneath Enceladus' surface at times when geophysical models predict its fissures should be under tension and therefore may be wider open.
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