![]() In shallow water, pressure oscillations from waves of sufficient height and wave length may penetrate into the sediment to a depth equal to the wave amplitude 21. Identified triggers for the eruption of pockmarks include earthquakes 18, 19, tides 20 and storm waves 12. The post-eruption phase can either be a period of continuous seepage through the created vents or a dormant period until the critical pressure is exceeded again. When the interstitial gas pressure exceeds the load of the overlying sediment and water column, the gas erupts locally, suspending sediment into the water column and a crater remains at the eruption site. ![]() The formation mechanism of gas induced pockmarks can be divided into three phases 8: During the first phase, pressure is built up as gas rises from deeper reservoirs and accumulates below the seabed. In contrast to continuous diffusive seepage, the presence of pockmarks indicates a more vigorous escape of fluids from the seabed. Many occurrences of pockmarks can be linked to the seepage of fluids including carbon dioxide and groundwater but the majority of reported pockmarks have been related to the expulsion of biogenic methane from the microbial degradation of organic matter 9, 17. The emergence of pockmarks and associated fluid seepage influences the entire local environment 12, especially seabed flow structures 13, morphodynamics 14, biogeochemistry 15 and ecology 16. Their morphological expression varies from isolated elliptical features to coherent clusters or strings. The bedforms are described as cone-shaped craters in the seabed 8, 9 with diameters in the order of centimeters up to 100 s of meters and depths from a few decimeters to 10 s of meters 7, 9, 10, 11. Since first detections in the 1970’s and the coining of the term 1, pockmarks have been documented in lakes 2, estuaries 3, on continental shelves 4 as well as in coastal 5 and deep sea environments 6, 7. Pockmarks are morphological expressions of vigorous fluid escape from subaqueous sediments. These observations most probably describe a reoccurring phenomenon in shallow shelf seas, which may have been overlooked before because of the transient nature of shallow water bedforms and technology limitations of high resolution bathymetric mapping. Conservative estimates amount to 5 kt of methane, equivalent to 67% of the annual release from the entire North Sea. Due to the shallow water depths and energetic conditions at the presumed time of eruption, a large fraction of the released gas must have been emitted to the atmosphere. Storm waves are suggested as the final trigger for the eruption of the gas. Subseafloor structures and high methane concentrations of up to 30 μmol/l in sediment pore water samples suggest a source of shallow biogenic methane from the decomposition of postglacial deposits in a paleo river valley. The gas source and the trigger for the simultaneous outbreak remain speculative. The time of emergence can be confined to 3 months in autumn 2015, suggesting a very dynamic genesis. Covering an area of around 915 km 2, up to 1,200 pockmarks per square kilometer have been identified. In 2019, data from the LADEE mission revealed that OH and/or H 2O existed on the Moon, beyond the permanently shadowed regions, and is expelled through micrometeorite impacts.A series of multibeam bathymetry surveys revealed the emergence of a large pockmark field in the southeastern North Sea. These areas are so cold that water within them gets trapped and can’t evaporate. The majority of signals come from permanently shadowed regions – craters at the Moon’s poles that never see sunlight. Since then, numerous missions have made similar detections, including our Lunar Reconnaissance Orbiter and LCROSS missions. In the late 2000s, a number of missions including the Indian Space Research Organization’s Chandrayaan-1, and NASA’s Cassini and Deep Impact detected hydration on the lunar surface – but these missions could not determine if the signals were hydroxyl (OH) or water (H 2O).ĭuring this same time period, using ultra high precision laboratory equipment, scientists also found water molecules locked in glasses and minerals in the samples returned by the Apollo missions. This indicates that water is widely distributed across the lunar surface.ĭid we already know water existed on the Moon? NASA recently announced that - for the first time - we’ve confirmed the water molecule, H 2O, in sunlit areas of the Moon.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |