MODELING AN IMPACT
Since 1980, bolide impacts (bolide is a term of ignorance, meaning any object from space [comet, asteroid, Klingon Warship]) have been considered prime suspects for the forcing mechanism of some or all major extinction events in the earth's history.
What effects would a major bolide impact have on the earth? Modeling an impact is not an easy thing to do because there are so many variables to be considered.
EARLY MODEL
In the mid-1980’s the model was based on an asteroid with a roughly 10km. diameter, a stony composition and a density of roughly 3.09 gm/cc. Its speed was estimated at 25 km/sec.
Based on these assumptions, the impact force would equal about 100 million megatons. Approximately 1 x 10 11 greater than the Hiroshima bomb. This would produce a initial crater 50 to 70 km in diameter, increasing to about 125 km after wall collapse.
If the bolide hit in mid-ocean it was thought that it would produce a tidal wave (tsunami) 4 to 5 km high in the open ocean, but of much lesser height on the continental shelf.
Stratospheric pollution would be caused by the Atmospheric Canon Effect. As the bolide passes through the atmosphere it causes a bow shock wave - punching a cylindrical hole in the atmosphere. This cylindrical hole is basically a vacuum (extreme low pressure region). A split second after impact, the surrounding air (high pressure) with dust, vapor, etc. rushes in to fill the low pressure region and "fires" debris into upper atmosphere. In any case, dust and vapor kicked up into the stratosphere would equal to about 10% the mass of the projectile (approx. 1017 grams or 1012 tons). This does not include fast falling ejecta. It is important for the debris to reach the stratosphere, because once there it will remain suspended in the atmosphere for a long period of time. If the impact ejecta does not reach the stratosphere, the atmosphere will clear quickly.
The pollution of the stratosphere would reduce incident solar radiation (sunlight) by up to 99% for 3 to 6 months. (NOTE, more recent studies indicate a much shorter dark time – if 3 to 6 months total extinction would result, i.e., we would not be here!).
The expected results of this massive pollution of the stratosphere would include:
Acid Rain (see also notes on Acid Rain) – possibly of the concentration of battery acid. Affects terrestrial foliage and also calcareous microflora and microfauna in oceans (the increased acidity of ocean water dissolves plant and animal shells leading to death – which explains the mass extinction of micro- fauna and flora which occurred at time of dino extinction).
The Black out due to the dust causes:
Collapse of photosynthesis. The base of the food chain is lost causing mass starvation amongst herbivores, leading to starvation among carnivores.
Drastic temperature drop – subfreezing on continents, followed within weeks by major increase in temperature due to the greenhouse effect caused by large amounts of CO2 and water now in atmosphere. Widespread snowfall ( up to 6 m ???) on continents.
NOTE: This is basically a nuclear winter scenario without the radiation - Carl Sagan came up with the idea of nuclear winter following the modeling of the K/T impact.
RECENT MODEL
More recently, models have begun to take into account the geology at the impact site, now confirmed as the Chicxulub Impact site –Yucatan Peninsula. Remains of crater, while buried by roughly 63 million years of sediment, appear on geophysical surveys. Recent radiometric dates place this crater right at the K/T boundary.
There are various size estimates for the crater, with the very large 175 mi. (280 km) diameter gradually becoming more widely accepted. At this size, the crater would have been 35 mi. (56 km) deep following impact. [NOTE: a more conservative estimate of a 110 mi. (176 km) diameter with an initial depth of roughly 18 mi. (29 km) following the impact also has many supporters].
Using the large diameter estimate, modelers now take more account of the geology of the Yucatan in predicting results. This impact site is made up of predominantly carbonate and sulfate sedimentary rocks above the igneous silicate basement rock. Roughly speaking, the composition of these rocks works out to 35-40% dolomite ((Ca,Mg)2CO3), 25-35% limestone (CaCO3), and 25-30% anhydrite (CaSO4), with all other sedimentary rock types being negligible. However you look at it, this rock contains lots of CO2 and SO2.
IMPACT!
The impact is now modeled as having two separate parts - the "Warm Fireball" and the "Hot Fireball".
'Warm Fireball" vapors are dominated by H2O, with the total mount of superheated water generated dependent on the speed and size of the bolide. If the bolide was 10 km in diameter and traveling at 20 km/sec, then 340 Gigatons of water vapor would be generated. If the bolide was 14 km in diameter and travelling at 38 km/sec, then it would have generated up to 670 Gigatons of water vapor.
This roughly 500 Gigaton fireball expands to a volume of about 400,000 km3. The temperature would not exceed about 1750oC. The "Warm Fireball" would expand about 100km. before the initial expansion of the "Hot Fireball".
The "Hot Fireball" would be similar to the impact event of the comet Shoemaker-Levy 9 with Jupiter as seen from earth. The "Hot Fireball" would rapidly blow up to the top of the atmosphere and spread its debris around the planet (similar to that envisioned in the Atmospheric Canon Model). Unlike the "Warm Fireball", the "Hot Fireball" would contain large amounts of CO2, SO2 derived from the vaporization of the sedimentary rocks at the impact site, and silicate vapors from the igneous basement rock and the projectile itself. Total vapor produced by the "Hot Fireball" is estimated at 32,000 gigatons with about 580 gigatons being CO2, SO2 and H2O.
Temperatures at the Chicxulub impact site would have risen to greater than 3000oK for several minutes, which would have been sufficient to vaporize rock. This heat would be transferred to the upper atmosphere by the "Hot Fireball" and would have warmed the stratosphere to about 800oK on a global scale.
The long term effect would have been caused by the sulfate aerosols in the atmosphere. These would have caused an intense drop in global temperature. Here the estimate depends on the amount of time it takes the oceans to equilibrate with the atmospheric temperature change. If equilibrium could be achieved within 15 years (rapid equilibrium) then the model indicates a drop to near freezing temperatures globally within 3 years, a condition which would continue for about 2 years. If, on the other hand, equilibrium would not be reached for 150 years, then the oceans would moderate the climatic effects to a drop in global average temperature of only about 5oC, but that average temperature would be maintained for about 150 years.
Obviously, a major impact would devastate the areas for 100's ( if not 1000's) of kilometers around the impact site, and cause tremendous climatic disruptions.