Chris Peterson wrote:aristarchusinexile wrote:Water vapour emitted with methane, and methane emitted by itself .. wow, Mars is an active planet beneath all that dust and rock.
Water vapor doesn't really suggest activity, since it's very likely that a large amount of frozen water is stored close to the surface. The absence of water vapor would be the tricky question to answer. Methane is strongly
suggestive of activity, since methane is less stable than water, so it is likely being manufactured by some active process. However, there are methane storage mechanisms, so the presence of this gas doesn't absolutely require activity. Stored methane (in clathrates, for example) would certainly raise all sorts of interesting questions about Mars's geologic history.
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Methane clathrate , also called methane hydrate or methane ice, is a solid form of water that contains a large amount of methane within its crystal structure (a clathrate hydrate). Originally thought to occur only in the outer regions of the Solar System where temperatures are low and water ice is common, significant deposits of methane clathrate have been found under sediments on the ocean floors of Earth.
Methane clathrates are common constituents of the shallow marine geosphere, and they occur both in deep sedimentary structures, and as outcrops on the ocean floor. Methane hydrates are believed to form by migration of gas from depth along geological faults, followed by precipitation, or crystallization, on contact of the rising gas stream with cold sea water. Methane clathrates are also present in deep Antarctic ice cores, and store a record of atmospheric methane concentrations, dating to 800,000 years ago. The ice-core methane clathrate record is a primary source of data for global warming research, along with oxygen and carbon dioxide.
While it is stable at a temperature of up to around 0°C, at higher pressures methane clathrates remain stable up to 18 °C. The average methane clathrate hydrate composition is 1 mole of methane for every 5.75 moles of water, though this is dependent on how many methane molecules "fit" into the various cage structures of the water lattice. The observed density is around 0.9 g/cm³. One liter of methane clathrate solid would therefore contain, on average, 168 liters of methane gas (at STP).
Methane forms a structure I hydrate with two dodecahedral (20 vertices thus 20 water molecules) and six tetradecahedral (24 water molecules) water cages per unit cell. The hydration value of 20 can be determined experimentally by MAS NMR.
Methane clathrates are restricted to the shallow lithosphere (i.e. < 2000 m depth). Furthermore, necessary conditions are found only either in polar continental sedimentary rocks where surface temperatures are less than 0 °C; or in oceanic sediment at water depths greater than 300 m where the bottom water temperature is around 2 °C. In addition, deep lakes may host gas hydrates as well, e.g. the freshwater Lake Baikal, Siberia. Continental deposits have been located in Siberia and Alaska in sandstone and siltstone beds at less than 800 m depth. Oceanic deposits seem to be widespread in the continental shelf and can occur within the sediments at depth or close to the sediment-water interface. They may cap even larger deposits of gaseous methane.
The presence of clathrates at a given site can often be determined by observation of a "Bottom Simulating Reflector" (BSR), which is a seismic reflection at the sediment to clathrate stability zone interface caused by the unequal densities of normal sediments and those laced with clathrates.
The size of the oceanic methane clathrate reservoir is poorly known, and estimates of its size decreased by roughly an order of magnitude per decade since it was first recognized that clathrates could exist in the oceans during the 1960s and 70s. The highest estimates (e.g. 3×1018 m³) were based on the assumption that fully dense clathrates could litter the entire floor of the deep ocean. However, improvements in our understanding of clathrate chemistry and sedimentology have revealed that hydrates only form in a narrow range of depths (continental shelves), only at some locations in the range of depths where they could occur (10-30% of the GHSZ), and typically are found at low concentrations (0.9-1.5% by volume) at sites where they do occur. Recent estimates constrained by direct sampling suggest the global inventory lies between 1×1015 and 5×1015 m³ (1 quadrillion to 5 quadrillion). This estimate, corresponding to 500-2500 gigatonnes carbon (Gt C), is smaller than the 5000 Gt C estimated for all other fossil fuel reserves but substantially larger than the ~230 Gt C estimated for other natural gas sources. The permafrost reservoir has been estimated at about 400 Gt C in the Arctic, but no estimates have been made of possible Antarctic reservoirs. These are large amounts. For comparison the total carbon in the atmosphere is around 700 gigatons.
These modern estimates are notably smaller than the 10,000 to 11,000 Gt C (2×1016 m³) proposed by previous workers as a motivation considering clathrates as a fossil fuel resource. Lower abundances of clathrates do not rule out their economic potential, but a lower total volume and apparently low concentration at most sites does suggests that only a limited percentage of clathrates deposits may provide an economically viable resource.
Methane clathrates in continental rocks are trapped in beds of sandstone or siltstone at depths of less than 800 m. Sampling indicates they are formed from a mix of thermally and microbially derived gas from which the heavier hydrocarbons were later selectively removed. These occur in Alaska, Siberia as well as Northern Canada. In 2008, Canadian and Japanese researchers extracted a constant stream of natural gas from a test project at the Mallik gas hydrate field in the Mackenzie River delta. This was the second such drilling at Mallik: the first took place in 2002 and used heat to release methane. In the 2008 experiment, researchers were able to extract gas by lowering the pressure, without heating, requiring significantly less energy.
The sedimentary methane hydrate reservoir probably contains 2–10× the currently known reserves of conventional natural gas. This represents a potentially important future source of hydrocarbon fuel. However, in the majority of sites deposits are likely to be too dispersed for economic extraction. Other problems facing commercial exploitation are detection of viable reserves; and development of the technology for extracting methane gas from the hydrate deposits. To date, there has only been one field commercially produced where some of the gas is thought to have been from Methane clathrates, Messoyakha Gas Field.
Methane clathrates (hydrates) are also commonly formed during natural gas production operations, when liquid water is condensed in the presence of methane at high pressure. It is known that larger hydrocarbon molecules such as ethane and propane can also form hydrates, although as the molecule length increases (butanes, pentanes), they cannot fit into the water cage structure and tend to destabilise the formation of hydrates.
Once formed, hydrates can block pipeline and processing equipment.
Methane is a powerful greenhouse gas. Despite its short atmospheric half life of 7 years, methane has a global warming potential of 62 over 20 years and 21 over 100 years. The sudden release of large amounts of natural gas from methane clathrate deposits has been hypothesized as a cause of past and possibly future climate changes. Events possibly linked in this way are the Permian-Triassic extinction event, the Paleocene-Eocene Thermal Maximum. Climate scientists such as James Hansen expect that methane clathrates in the permafrost regions will be released as a result of global warming, unleashing powerful feedback forces which may cause runaway climate change that cannot be controlled. Recent research carried out in 2008 in the Siberian Arctic has shown millions of tons of methane being released with concentrations in some regions reaching up to 100 times above normal.
Since methane clathrates are stable at a higher temperature (−20 vs −162 °C) than LNG, there is some interest in converting natural gas into clathrates rather than liquifying it when transporting it by seagoing vessels. Accordingly, the production of NGH from NG at the terminal would require a smaller plant than LNG would.
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