Stability of Martian Clathrates
Ö. KARATEKIN, E. GLOESENER
Royal Observatory of Belgium
Introduction
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The recent detection of methane at an
average in the martian atmosphere from
orbit and Earth-based observations
suggests that methane currently is being
produced.
Methane is released locally on Mars.
Seasonal variation and process in subpermafrost regions, and/or wide-spread
surface activity, is implied
High CH4!
High H2O!
High CH4!
Low H2O!
High CH4!
Low H2O!
 Some release zones are correlated with geologically interesting features:
Hydrated old terrain rich in phyllosillicates and carbonates
 Methane may have different origins: volcanism, hydrogeochemical/thermal
activity, biological activity, external supply by meteorites and comets.
 A possibility is that methane is released to the atmosphere from subsurface
clathrate layers.
Clathrates
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Gas hydrates are solids that form from a combination
of water and one or more hydrocarbon or
nonhydrocarbon gases.
In physical appearance, gas hydrates resemble
packed snow or ice. (Because all three common
hydrate structures consist of about 85% water on a
molecular basis)
In a gas hydrate, the gas molecules are “caged”
within a crystal structure composed of water
molecules Therefore gas hydrates are often called
“gas clathrates.”
A gas hydrate, such as methane hydrate, is a
crystalline solid known as a clathrate. The word
“clathrate” has its origins in the Latin word clatratus,
meaning “to enclose with bars.”
It follows then that clathrates are a class of chemical
substances made of two unique materials, one of
which encloses the other in an open, latticelike cage.
There is no chemical bonding to hold the two
materials together, only the physical structure. The
most abundant naturally forming clathrate is methane
hydrate.
Gas
molecules
Water molecule cages
Hydrate crystal unit structures
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All common natural gas hydrates belong to the three crystal structures, cubic structure I (sI) (a) ,
cubic structure II (sII) (b) , or hexagonal structure H (sH) (c).
There are four requirements for generation of natural gas hydrates: (1) low temperature, (2) high
pressure, (3) the availability of methane or other small nonpolar molecules, and (4) the availability
of water. Without any one of these four criteria, hydrates will not be stable.
Gas hydrate formation is usually described as a crystallization process with nucleation, growth,
agglomeration, and breakage. Gas is dissolved in water, and nucleation starts primarily at the
gas–water interface, where the gas concentration is highest.
When the clathrates are warmed or depressurized, they decompose and dissociate into water
and methane gas. Since each volume of hydrate can contain as much as 184 volumes of gas
(STP), hydrates are currently considered a potential unconventional energy resource.
Pressure–Temperature Diagrams of
the CH4 + H2O system
The diagrams uses symbols of I, LW, H, V to
represent ice, liquid water, hydrate and vapor. A
two-component system such as methane +
water is represented on a pressure–temperature
diagram as an area (for two phases), a line
(three phases), or a point (four phases).
The pressures and temperatures of the LW–H–V
and the I–H–V lines mark the limits to hydrate
formation. At higher temperatures or lower
pressures of both lines, hydrate cannot form and
the system will contain only aqueous and
hydrocarbon fluid phases, while hydrate
formation can occur to the left of LW–H–V and I–
H–V.
The presence of thermodynamic inhibitors (e.g.,
salts, alcohols, or glycols) causes a change in
the pressure–temperature diagram,
Martian Clathrates
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WE use a numerical code (CSMHYD Colorado school of Mines, Sloan et Koh, 2008) to calculate
phase curves of simple (CH4)and mixed hydrates (CH4-CO2) as a function of methane fraction in
the gaseous phase.
The presence of CO2 in the methane clathrates decreases the pressure at which they are formed.
More in the hydrates means that the mixed clathrates can be formed closer to the surface.
I+V
I+H
Stability Zone
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The region of the crust which satisfies the thermodynamic stability criteria yields the
hydrate thermodynamic stability zone (HSZ).
The boundaries of hydrate stability zone is determined by the intersection of the phase
boundary and the geothermal gradient (Shown in red)
In practice, the prediction of HSZ is not straightforward task even on the Earths, since it
depends on several factors, such as the availability and percentages of organic matter,
presence of inhibitors such as salts, porosity, thermal state and pressure (Saturation of the
soil), and so on. All estimates of natural gas hydrates are not well defined, and therefore
somewhat speculative on the Earth.
Stability zones of clathrates as a function of surface temperature
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Calculations at lattitudes of 0º, 30º, 60º et 90º with Ts= 218, 211, 179 et 154 K, q = 0,03 W/m². Ps = 610 Pa
Stability zones of clathrates as a function of surface temperature
Effect of Seasons and Obliquity Changes
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Propagation of Temp oscillations based on thermal properties of
the soil:
k = therm. cond. P = échelle de tps des variations,
ρ = masse vol., c = chaleur spécifique
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Seasonal Variations ~ 4.4 m (Affects lattitudes above ~50o ).
Obliquity Variations ~ 1500 m (Affects mostly Equatorial regions)
Effect of Composition
• Inhibitor in liquid phase: NaCl (25 wt %)
Summary and Perspectives
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The depth of the stability zone of simple and mixed clathrates in
Martian crust are calculated. The stability zone of CH4 and
CH4+CO2 clathrates vary between few meters and several km
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The effects of seasonal and long term surface temperature
variations (due to climates changes) can be observable most
easily at the polar and equatorial regions respectively.
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The presence of salts (NaCl) in water decreases the stability
zone,
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We are currently working on taking
into account the regional and
temporal variations in heat flux, soil
properties and surface temperature.
The diffusion within the crust and the
affect of inhibitors will be studied.
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