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Re: DINOSAUR digest 1757

Quoting ptnorton:

">such as an impact by a meteor, or a small comet or whtaever it could have

According to Frank Kyte (Nature v. 396, 1998, pp. 237-39), who analyzed what
he believed was a piece of the K/T impactor, it was a carbonaceous chondrite
(aka, a stony meteorite).

And from TH:

>(not piddly little bumps like Chixculub...)<

Obviously some tongue in cheek on that, but it's worth noting that,
according to the American Geophysical Union, Chixculub is perhaps one of the
largest craters produced in the inner solar system in the last 4 billion


Well, maybe last 3 billion... What was special about the Chixculub impact
is that it was a really late hit of its size - nothing on the Moon
which is this big and this recent, for example. The really big ones
that have been implicated in seismically triggered, antipodal
vulcanism (the Caloris basin on Mercury, maybe Mare Orientale on the
Moon and Argyre or Hellas Planitiae on Mars) were rather earlier,
when there were a lot more potential impactors. (A law of orbital 
dynamics in the solar system: an object in orbitt remains in orbit
until it hits something...). All of the mare-sized impacts on the
Moon do seem to go back to the first 0.5 Gyr (that may be an astronomism,
Gyr=1000Myr=10^9 years as a time span not a backwards date).

Just our luck. Or just their luck. Whichever.

Bill Keel
Astronomy, University of Alabama