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I recently was looking through one of my trade journals and they were talking about methane hydrates. The structure of them (circular?) and the placement of them (between grains of sand) made me think of these photos from Mars. I did a google on methane hydrates nodules and came up with the following information. The stuff in CAPS are my highlights. Food for thought. Need to get back to work, but will look into what levels of methane are in Mar's atmoshpere.

Methane hydrate can form in rocks or sediments of any type given suitable pressures, temperatures, and supplies of water and methane (please see our discussion on Necessary Conditions for Methane Hydrate Formation). Although natural methane hydrate has been most commonly observed occurring as DISSEMINATED GRAINS, other forms are known, including massive layers of pure hydrate up to 4 meters thick, nodules that grow and displace surrounding sediments, veins filling small fractures, thin layers along bedding planes, and as a cement binding sedimentary grains together.

Similarly, hydrate is typically found with uniform distribution, showing clear, but subtle vertical trends of increasing or decreasing abundance. However, examples of heterogeneous distribution with zones of sparse or no hydrate interspersed with zones of high concentration are also common.

Although the factors that control the ultimate type, distribution, and amount of hydrate are still poorly understood, perhaps the primary controls are 1) the porosity and permeability and 2) the degree of lithification of the enclosing medium. The geologic environment in which the sediments/rock exist largely determines these aspects.

In the late 1960s, the global view of clathrate science began to change dramatically when "solid natural gas" or methane hydrate was observed as a naturally-occurring constituent of subsurface sediments in the giant gas fields of the Western Siberia basin.

Shortly thereafter, hydrate was also found in shallow, sub-permafrost sediments on the North Slope of Alaska. Soon, scientists, particularly those in the former Soviet Union, began to speculate that the low temperature/high pressure conditions necessary for hydrate formation should exist extensively around the globe, not only in permafrost regions, but also under deep oceans. The global hunt for methane hydrate was on.

FROM ANOTHER SITE:

Most natural gas hydrate is formed from biogenic methane, excreted by BACTERIA that eat organic matter that has been washed into (or died in) the ocean. This type of hydrate is concentrated where there is a rapid accumulation of organic detritus and also where there is a rapid accumulation of sediments (which protect detritus from oxidation).

Hydrates also form when faults permit natural gas (or other gases) to migrate from deeper inside the Earth's crust to the surface of the seabed at places with appropriate temperature and pressure levels.

These processes can also cause hydrates to form below permafrost, which acts as a cap to prevent further upward migration of gas into the atmoshere.