When an asteroid enters earth's atmosphere there are two
occurences of extreme shock. The first occurs as the asteroid enters the
atmosphere. The second, more important one, occurs when the
ejecta plume (the ejected debris caused by an impact) enters
the atmosphere (Zahnle, 1990).
This shock causes the breakdown of the oxygen (O2) and the
nitrogen (N2) molecules found in our atmosphere.
Through a series of chemical reactions the
dissociation of the nitrogen and oxygen forms NO which is then converted into NO2. These two
molecules can produce acid rain (HNO3) (Prinn and Fegley, 1987).
In 1987 Prinn and Fegley determined the atmospheric consequences of a "large" comet
impact versus a "small" comet impact. They assumed the "large" impact comet had a mass of
1.25 x 1016 Kg travelling at a velocity of 65 Km/s. As for the "small" impact asteroid, they
assumed it had a mass of 5 x 1014 Kg travelling at a velocity of 20 Km/s. It is important to note
that these two objects are possible bolides that hit the earth 65 million years ago.
Conclusions of the Large Impact Scenario
If the comet scenario occurred, 7 x 1040 molecules of NO would have been produced and
subsequently converted into acid rain. (See chemical reactions) This would have
caused a global dispersal of acid rain with a pH of 0-1.5.
On the continents the acid rain would have weathered the soil removing many of the insoluble
elements ( for e.g. Be+2, Al+3, Hg+2, Cu+, Fe+2, Fe+3, Ti+3, Pb+2, Cd+2, Mn+2, Sr+2). These elements
would end up in soil water, streams, rivers, lakes, etc., causing a problem as some of these
elements are known for their toxicity towards plants and animals (e.g. Al, Be, Ti, Hg)
As for the oceans, the global acid rain would lower the pH of the mixed layer (the top 75 M of
the ocean) to a pH of 7.8, breaking down the calcareous shells of organisms that thrive
in the mixing zone.
Conclusions of the Small Impact Scenario
If the "small" asteroid scenario were to occur, the amount of acid rain produced would be similar
to the "large" comet scenario but only near the impact site. The global pH change would be rather
insignificant.
