http://SaturnianCosmology.Org/ mirrored file For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== *Asteroids & Tsunamis * *By Michael Paine * Special to space.com posted: 06:07 pm ET 05 November 1999 Big asteroids can be extra deadly when they strike the ocean, carving aquaticcraters and sending huge waves in all directions. These tsunami can wreakdestruction on shores thousands of miles away. Bad news for people livingin coastal areas, but it could be a lucky break for the rest of mankind:The same impact on land would throw dust high into the atmosphere and couldblock sunlight for many months, possibly causing global starvation andmass extinctions. Dangerous waves The surface of water is verygood at transferring energy, in the form of waves, across great distances.In 1960, for example, an earthquake near Chile created a series of wavesthat crossed the Pacific Ocean and killed several hundred people 10,000miles away in Japan. These waves, which are generatedfrom a major disturbance to the water surface, are known as tsunami. (Mostscientists don't like the popular name "tidal wave" because tsunami havenothing to do with the tides. However, tsunami sometimes surge ashore likea huge, fast-moving tide rather than breaking like a classic surfing wave.) Tsunamican travel at around 400 mph in deep water. When they reach shallow waterthey slow down, and that's when the real danger begins. The front of thewave slows first and the effect is like a pile-up on a freeway, with therear of the wave catching up to the front. The wave increases in heightfrom this bunching effect. The final height of the wave depends on severalfactors, but the shape of the sea floor has the greatest impact. Estuaries,harbours, cliffs, reefs, and the topography of the continental shelf allplay a role. For a typical shoreline,the final tsunami height is usually about three times its height in deepwater, but in some locations the ratio (known as "run-up factor") reaches40. In other words, a 1-foot wave in deepwater can amplify to a 40-footwave at a shoreline that is exceptionally vulnerable to tsunami, as aresome parts of Hawaii. Splashdown If an asteroid collides with the Earth there is a good chance it will hit an ocean, simply because two-thirds of the Earth's surface is covered bywater. A gigantic explosion occurs and the asteroid is pulverised and vaporised, along with a huge volume of water. This creates a crater in the water surfacethat quickly fills. The filling process generates a series of tsunami thatradiate across the ocean. The effect is similar to a pebble thrown intoa pond, though with a 50,000-mph impact, we're not talking ripples here. Based on NASA estimates,about once every 2,000 years an asteroid with a diameter of about 100 yardscan be expected to hit one of Earth's oceans. Larger asteroids collidewith the Earth much less frequently -- a 500-yard rock from space mighthit an ocean once every 80,000 years and a 1,000-yard (1 k) asteroid perhapsonce every 200,000 years. *Atomic bombs and oceanimpacts* The largest aboveground H-bombtest by the United States was like a firecracker compared to an asteroidimpact. That "Bravo" explosion at Bikini Atoll in 1954 was equivalent tofifteen megatons (million tons) of TNT but was only about one-thousandthof the energy of a 500-yard asteroid moving at 50,000 mph. The Bikini Atoll H-bomb testsenabled scientists to develop computer models of the destructive effects(on shipping) of explosions at the water surface. In the early 1990s thesemodels were applied to asteroid impacts. Initial results suggested thateven relatively small impacts could pose a grave tsunami threat over largeareas of ocean. More recent modelling indicatesthat the tsunami generated by an asteroid impact tend to dissipate, ordie out, rapidly (the computer program, developed by Sandia National Laboratories,accurately predicted the consequences of the plummet of Comet Shoemaker-Levy9 into Jupiter in 1994). According to this work, a500-yard-diameter asteroid is predicted to generate a water crater nearly3 miles in diameter. At a distance of 10 miles from "ground zero" the resultingdeepwater tsunami will be about 200 yards high, but by the time the wavehas travelled 100 miles it will be reduced to a height of about 14 yards.After 1,000 miles it will have dropped to less than 1 yard in height. Dueto the amplification in shallow water, however, this size tsunami couldstill become a 120-foot wave at a vulnerable shore. Extra hazard to coastal areas Due to the extra hazard of tsunami,locations such as Hawaii are at much greater risk from asteroid impactsthan inland areas. Rough calculations suggest that a coastal location witha typical tsunami run-up factor of three has about three times the riskof devastation from an asteroid-generated tsunami than the risk of a directblast to an inland location. Locations with an extreme tsunami run-up factorof 40 have about 70 times the risk compared with an inland location. People in these vulnerablelocations need not lose sleep, however, because the odds of a major asteroid-generatedtsunami in any one year are about one in 200,000. On the other hand, asastronomer Duncan Steel has pointed out, asteroid impacts don't run toa timetable like busses. The estimate of impact tsunamirisk is based on the limited search for Near Earth Asteroids carried outso far and assumes that impacts are randomly distributed in time. Thereis some evidence that impacts may come in clusters (some busses seem todo the same). If this is the case, then it is well worth finding out ifwe are approaching the next barrage so that coastal areas can be betterprepared. Climate disruption The comparison between coastaland inland locations is not entirely fair because the biggest danger froman asteroid impact is not from the direct blast but from the after-effects.In particular, the temporary cooling of the Earth due to huge quantitiesof dust released into the atmosphere from a land impact can disrupt cropproduction and lead to global starvation. The giant plumes from theJupiter impact of Comet Shoemaker-Levy 9 clearly showed how a comet orasteroid tunnels through the atmosphere and creates a temporary chimney.This draws the impact debris into the upper atmosphere. Scientists areonly beginning to understand this effect in the case of an impact intoEarth's oceans. An ocean impact by a 500-yard-diameterasteroid will vaporise about 20 cubic miles of water. At first sight thisappears to be insignificant since it is less than one tenth of the totalamount of water that evaporates from the world's oceans every day (assuming1 inch of rain over 10 percent of the Earth's surface each day). Scientists caution, however,that an ocean impact would send the water vapour high into the atmosphere,compared with the lower atmosphere, or troposphere, in the case of evaporation.The upper stratosphere is normally extremely dry and the effects of a suddeninjection of a large quantity of water vapour are simply unknown. Othereffects of concern are greenhouse warming (water vapour is a strong greenhousegas) and ozone depletion. Unlike evaporation, an ocean impact would sendsalt (sodium chloride) into the air. The chlorine in the salt may affectupper atmosphere ozone levels in the same way as chlorofluorocarbons. The same impact on land wouldpulverise an equivalent amount of rock (20 cubic miles -- about 1,000 timesthe volume of the asteroid) and send much of it into the upper atmosphere,where it would circulate around the globe and disrupt agriculture for manymonths. A lesson from violent volcanoes In 1815 a volcano on the Indonesianisland of Tambora exploded and produced a crater similar in size to thatfrom a 500-yard asteroid. About 20 cubic miles of ejecta was released (forcomparison, the Mount St. Helens explosion in 1980 released about a quarterof a cubic mile of ejecta). In the case of Tambora, ithas been estimated that 10,000 people died directly from the explosionand 80,000 more died in the region from indirect effects, such as starvation.In addition, the ash is thought to have caused the "year without a summer"in 1816, when there were widespread crop failures across /North America/.The final death toll was probably in the hundreds of thousands. A similarevent today might kill millions. Because of the chimney effect,an asteroid impact is much more efficient at sending dust into the upperatmosphere than a volcanic explosion, and the climatic disruption is probablymuch greater with an asteroid impact. Even so, the events of 1815 serveas a clear warning of the global danger from land impacts by asteroids. With much less dust releasedinto the atmosphere, an ocean impact will have very different, and perhapsless damaging, effects than a land impact. If an asteroid struck thickice formations, such as Antarctica or the extensive ice sheets of the lastIce Age, the result would likely be similar to a water impact. It's possible that our specieshas been saved from extinction several times because a large asteroid hitthe ocean or ice rather than the land. Every million years or so it canbe expected that a mile-wide asteroid will hit the Earth. A land impactwould probably cause severe climatic disruption and regional extinctions./If/the global effects of an ocean/ice impact are less severe than one on land,then the impact by a mile-wide asteroid into the ocean might not be ashazardous to life. Evidence of ocean impacts Past impacts with water or iceare very difficult to detect, because they leave very little evidence.One such impact is known to have occurred in the South Pacific Ocean, nearChile, about 2 million years ago. This event -- known as "Eltanin" afterthe ship that discovered the deposits -- involved an asteroid between 1and 3 miles in diameter that would have created a water crater at least40 miles across. Tsunami would have swamped coasts around the Pacific andwould even have reached some Atlantic coastlines. Assuming a typical run-upfactor of three, the coast of Chile would have been inundated by 250-yard-hightsunami. Likely results for other locations: Hawaii 90-yard tsunami (probablyhigher due to the greater run-up factor); California, 60 yards; Japan andAustralia, 25 yards; New Zealand; 120 yards. Despite this presumed destructionto coastal areas, there is no evidence of global climate change or regionalextinctions around this time, when our early ancestors, Australopithecus,were roaming Africa. We don't know whether they would have been wiped outif the Eltanin asteroid had struck land in South America or Africa, insteadof splashing into the ocean. To solve that puzzle, to understand whichtype of impact most threatens our existence, we need a much better understandingof the consequences of asteroid impacts. *Acknowledgements* I am grateful to the followingscientists for providing comment on this article: Erik Asphaug, Universityof Southern California, Elisabeth Pierazzo, University of Arizona, DavidCrawford, Sandia National Laboratories. This article does not necessarilyrepresent their views. -- Michael Paine DMCA/Copyright © 2007 Imaginova Corp. All rights reserved..