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The following appears in the Jan/Feb 96 Planetary Report (Vol XVI, #1), from
The Planetary Society. The Answer is from Richard P. Binzel of MIT
Question: "Could the Cretaceous/Tertiary Impact have affected the tilt,
rotation and/or orbit of Earth and, consequently, it's climate?"
Answer: "Impacts are responsible for giving all planets their rotation and
axial tilt. Planetary rotation is a natural outcome of the process of
accretion, the growing of planets through relatively low velocity collisions
among planetsimals. It has been suggested that very large collisions near the
end of a planet's formation might dictate its final rotation state. k Thus a
late, but great, collision might account for the slow "backward" roation of
Venus compared with the somewhat similar rotation rates and axial tilts of
Earth and Mars.
"Although devastating to the biosphere, the K/T impactor was puny as far
planet earth was concerned. The rotating Earth has a great deal of momentum
implying that it takes a large force to spin it up, slow it down, or change
its titl. Even assuming a large size (10 kilometer diameter), dense iron-rich
impactor striking at a high velocity and optimum angle, the calculated effects
on our planet are quite small. The K/T impactor could have change Earth's
rotation and tilt by no more than one ten-millionth of its present value.
Measured over a year, this change would amount to a difference of about three
seconds in the length of time for Earth to complete 365 rotations.
"The effect on Earth's orbit is even less. The total duration of a century
(the time for Earth to m ake 100 complete orbits) would be altered by only
about one second. Thus any long-term effect on Earth as a planet is
astonishingly small compared to the the temporary havoc wreaked on the
biosphere by the immediate heat and dust of the impact."
Gary Bull (firstname.lastname@example.org)