Scientists Think They May Have Solved The Siberian Crater Mystery
A trio of mysterious gaping holes in northern Siberia has spawned many theories about the craters’ origin, but scientists have suggested some concrete explanations.
Air samples taken at the bottom of one of the craters that have recently appeared in Siberia seem to support fears that the hole was formed by methane released from melting permafrost. If so this is very bad news for the planet’s future, indicating frighteningly high emissions of the powerful greenhouse gas.
What others thought it happened?
Methane, not an alien spacecraft crash, is probably responsible for the 30-meter-wide crater that suddenly appeared in Siberia in mid-July.
While air normally contains just 0.000179 percent methane, air near the bottom of the crater was composed of 9.6 percent methane, Nature reports. Researchers suspect that rising summer temperatures in the region thawed permafrost in the ground. As the soil defrosted, methane gas trapped in the permafrost pooled underground before bursting to the surface and ripping a hole in the ground.
Scientists believe that as regional temperatures rise, these kinds of craters could become more common.
ScienceNews | T. Sumner
I found siberiantimes.com and dailymail.co.uk for information about the hole. The latter was updated July 21 and contains excellent pictures and much speculation as of that date. The Science News article above contributes very useful new information by identifying methane at a relatively high concentration in the bottom of the pit. The molecular weight of methane (CH4) is 16, while the molecular weight of nitrogen (N2) is 28, so methane is lighter than air and will tend to dissipate upwards rather than stay at depth. Finding it at the bottom of the pit means it has an active source to maintain these concentrations.
This area does produce natural gas, so this could be methane leaking upward through the rocks overlying the gas field and trapped beneath a permafrost layer until it built up enough pressure to breach the permafrost cap. Methane also bonds with ice to form methane hydrates (Methane clathrate is the listing in Wikipedia) in cold temperatures, but I don’t know whether if would form in shallow permafrost. I have heard claims that global warming may release this methane from ocean-floor sediments and other claims relative to thawing permafrost, but don’t know enough to verify it. In any case, methane availability is not surprising here. If leakage from the deep natural gas field is the source of this, the warming of the permafrost would not be necessary. However, it may be the warming has had an effect. There would be a considerable amount of thermal mass in the ice of the permafrost to overcome to be releasing the deeper gas, though warming the upper part of the permafrost may weaken it both by thinning the permafrost as some melts and also because warmer ice is physically weaker.
The pictures clearly show soil (permafrost) was ejected to create the hole. The daily mail article linked above repeats a claim that a methane explosion could be created by a mixture of methane, water, and salt. I don’t follow the chemistry of that as an explosion would require oxidation of the methane to produce heat as a way to cause expansion. The reaction seems to end with the same amount of gas molecules (one molecule methane combines with two molecules oxygen to produce one molecule carbon dioxide and two molecules water), so the other component of an explosion in which more gas is produced than originally present does not apply here. The heat would vaporize the water so that would add to the expanding gases of the explosion. The problem with having an explosion at depth to blow off the permafrost cap is getting enough oxygen to the methane to make an explosive mixture. Methane has an upper explosive limit above which there is too much methane concentration to ignite, even if oxygen is present. It is like in older cars when the engine is “flooded”, meaning there is too much gasoline for the available air for it to ignite. It may be the salt will function in some manner as a catalyst if the claim that a mixture of salt, methane, and water is explosive is true. I just have never heard of this before.
My speculation based on the limited information I have is the hole formed as a “burp” due to build up of gas pressure below the permafrost cap as methane progressively escaped the deep gas field. Weakening or thinning the permafrost may have had a role, or may not. Once a local spot begins to lift because the gas pressure equals the weight of the overlying permafrost plus whatever strength it has, the flexing of this permafrost would create a crack. As the gas enters the crack, its pressure remains unchanged at it rises to shallower depths because its density is very small compared to the density of the permafrost. The pressure restraining the gas mostly comes from the overlying weight and this is directly due to the thickness of permafrost above the tip of the growing crack. Thus, the difference between the driving gas pressure and the resisting soil weight becomes greater the farther the crack extends, until it finally breaks through to the ground surface and drains the methane “bubble” below. Only once it breaks through to the ground surface can it mix with enough oxygen to burn and it may have ignited at that point, possibly burning back into the ground to create the larger hole like the central hole in a large candle following its wick. I don’t know how large a hole may have been pushed out by the out-rushing gas as the gas “bubble” emptied. Viscous friction with the rushing gas would entrain some of the permafrost from the walls of the hole. This entrained material would help erode the walls more to enlarge the hole as long as enough gas remains in the “bubble” to support a large outflow.
I don’t see the methane burning in the hole itself when the off-gassing is most intense. Once the rate of gas release slows, it may be possible in such a large diameter hole for a convective “chimney” effect to develop where the burning gas in the center rises due to its heat while drawing air downward along the sides of the hole to support the central combustion. I don’t know whether this is possible, I am making an analogy to “fire storms” drawing in air from the sides and don’t know whether this could allow air to actually be drawn downwards along the sides as I am speculating. This scenario, if it did occur, would certainly help enlarge the original hole. If this scenario is not possible (combustion experts with knowledge of fluid mechanics would have to answer that), then the size of the hole would have to be ascribed almost entirely to the outflow from the initial “burp”. A weak outward flow in the wasting stage would allow enough air into the hole to produce some ongoing burning without my “chimney” effect if the methane was already burning. This may have caused some enlargement of the hole and smoothing of the sides.
The Daily Times quoted a Dr. Plekhanov as saying the removed materials may have been as much as 80% ice (by volume). That sounds high from looking at the photos of the shallow soil, but Dr. Plekhanov likely has more information and information from deeper in the hole than me. He also is suggesting a “‘build-up of excessive pressure’ underground” as a cause of the hole. Certainly pingos have a core of almost solid ice. When they melt, they leave a characteristic lake in the central depression, as noted by Dr. Chris Fogwill. It is certainly possible this actually was a pingo before the methane burp and the methane escaped at this location because the lower density of the ice core exerted less restraining weight to hold down the underlying gas “bubble”.
Tony Cooley | Geologist, geotechnical and environmental engineer, science lover. Work for KY Dept of Environmental Protection in Solid Waste Branch as environmental engineer and hydrogeologist.