http://SaturnianCosmology.Org/ mirrored file For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== */PLEASE NOTE:/* * */Information circulated on the cambridge-conference network is for/* */scholarly use only. The attached text may not be reproduced/* */or transmitted without prior permission of the copyright holder./* ------------------------------------------------------------------------ * CCNet-ESSAY, 7 July 1999 ------------------------ NEED FOR A NEW RISK ASSESSMENT FOR AIRBURST IMPACTS? From Kjeld Engvild, Risoe National Laboratory > Most evaluations of the risks of asteroid or comet impacts state that there is little danger associated with those many cases where the object explodes in the air and never reaches the ground (Chapman and Morrison 1994). There is only significant global risk in those rare cases where a large object hits the surface - less than once per 100,000 years. However, the experience of the last 100,000 years has been a large number of climate excursions overlapped by the beginning and end of an ice age. The ice age climate excursions occurred about every 3,000 years with differences in mean temperatures of up to 7 degrees centigrade (Dansgaard et al 1993). Climate excursions on a smaller scale have also characterized the latest 6,000 years. This has been shown by decade long, worldwide narrowing of tree rings (Baillie 1995, 1999). Baillie has described at least 6 well dated episodes: 540 AD, 44 BC, 207 BC, 1159 BC, 1628 BC and 2345 BC. Only the episode 1628 BC can be explained by volcanism: the explosion of Santorini north of Crete destroying the minoan civilization. The AD 540 event lasting from 536-545 is best described in the historical records; it involved reduced sunlight, mists or "dry" fogs, crop failures, famines in China and the Mediterranean, and plagues. The impact of comet Shoemaker-Levy on Jupiter was a series of high airbursts resulting in huge plumes covering tens of thousands of km squared. The plumes were extremely black ("gunk"), they were visible for months and the particle haze in the Jupiter atmosphere has remained for years (Friedson et al 1999). Could a similar airburst over the earth cause significant climatic excursions? Let us examine a degassed comet 0.2 km diameter, density 1, mass 4.2 million tons, impacting at 15 km/sec; such an object should hit about once every 1400 years (Solar system collisions) and release an energy of 88 megatons TNT of energy. Assuming 75% of the object to be dust (the other being ice), the explosion would blow 3 million tons carbon, silicon, magnesium and sulfur and other stuff more than 1,000 km above the earth (Boslough and Crawford 1997) Entrained with the explosion will be 1 million tons of ice from the comet and millions of tons of water vapor from the atmosphere. Most of the dust will be ground to about 0.1 micrometers; some dust will be oxidized to silicates and MgO by atmospheric oxygen. 5-10 million tons of material will be injected high into the stratosphere and cover most of the globe within a week. This amount is of course several orders of magnitude lower than the dust and sulfate injections which can cause volcanic winters. The difference is in the location. Most stuff from the volcanoes will not reach the stratosphere, while most stuff from the asteroid and the entrained material will reach 50 or 100 km altitude. The amount corresponds to 10-20 mg of smoke per square meter. It will take several years before most of the particles have reached ground level.The dust particles will act as nucleation centers for ice formation causing much additional haze in the stratosphere. The result is a much increased earth albedo causing earth surface cooling over several years. Perhaps enough to cause significant climate effects with reduced yields in agriculture? Boslough and Crawford (1997) have modeled a "Tunguska" size impact and conclude that it will be dangerous for satellites in orbit. This is of course regrettable, but one would like to know if more serious consequences might be foreseen, such as "cosmic winters", drastically lowered food (rice and maize) production, and large scale population migrations as happened in the years after the 540 AD event in Europe. References Baillie M 1999. Exodus to Arthur. Batsford London Baillie MGL 1995. A slice through time: Dendrochronology and precision dating. Batsford, London Boslough MBE Crawford DA 1997. Ann New York Acad Sci 822: 236-282. Chapman CR Morrison D 1994. Nature 367: 33-40. Dansgaard W et al 1993. Nature 364: 218 Friedson AJ West RA et al 1999. Icarus 138: 141-156. Web Impact hazards: http://impact.arc.nasa.gov Planetary defense workshop: http://www.llnl.gov/planetary/planetary1.html Shoemaker-Levy: http://www.jpl.nasa.gov/sl9 Solar system collisions: http://janus.astro.umd.edu/astro/impact.html Kjeld Engvild Plant Biology and Biogeochemistry Risoe National Laboratory ------------------------------------------------------------------------ CCCMENU CCC for 1999 /The content and opinions expressed on this Web page do not necessarily reflect the views of nor are they endorsed by the University of Georgia or the University System of Georgia./