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CCNet-ESSAY, 7 July 1999
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NEED FOR A NEW RISK ASSESSMENT FOR AIRBURST IMPACTS?

From Kjeld Engvild, Risoe National Laboratory <kjeld.engvild@risoe.dk>

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

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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.