29:50 Modern Astronomy
Fall 1999
Lecture 41 ...December 8, 1999
Lectio Ultima
Omnis pulchrima in exitu eius...Cicero
Tedious details:
(1) Exam this Friday, 10 problems like homework problems. Attach brain before leaving
your dorm room; bring a calculator.
(2) Final exam in this room on Friday December 17; 50 questions. First 25 over last
third of the course. Second 50 comprehensive. All questions content and concepts. No
math.
Watch the skies! Watch the skies!
Another meteor shower; The Geminids. Usually 2nd best of year.
Peaks December 13-14. Roof will be open if clear, starting at 8PM. Description of
shower from textbook: ``rates increase for several days before maximum, then drop off
quickly.'' The Geminids are debris from the asteroid Phaethon.
Remember lunar eclipse January 20, 9PM - midnight. Middle of totality
at 10:44 PM CST.
Asteroids and BIG Impacts II
Image of Mathilde
1. Last time discussed existence of Apollo asteroids or Earth-orbit-crossing asteroids.
From study of their orbits, we WILL get nailed eventually by one of these. The rates of
collisions are estimated to be in the following ballpark.
0.5 - 1 km diameter: once every 100,000 years.
2 - 5 kilometers: once every 5 million years
5 - 10 kilometers: every 50 million years
Sky and Telescope artists conception of impact of 5 - 10 kilometer object.
BIG IMPACT
Such a collision would produce a global climate catastrophe; huge amounts of dust & debris thrown into high atmosphere, would block sunlight for years, cause nuclear winter. Also Texas-size tidal waves, probable disruption of ocean circulation patterns. The technical astrophysical term for this is a doggone mess!.
2. But is it true? The Case of the Cretaceous-Tertiary Extinctions. Occurred 65 million years ago. Extinction of dinosaurs, rise of mammals. Also 60 % of plant and animal species, evidently quite rapidly.
Indication that extinctions had an astronomical origin came about 20 years
ago. At geological level corresponding to the ``K-T extinctions'', there is a layer of clay
enhanced in the element Iridium. Abundance in meteors far higher than in Earth. Led to
suggestions that a Big Impact was responsible for these extinctions.
The Smoking Gun...Chicxulub Crater.
In the last few years geologists have discovered what they think is the impact crater,
in the gulf of Campeche off Yucutan. Chicxulub crater is 180 kilometers in diameter
(probable 
kilometer impactor) which occurred 65 MYr ago.
Gravity image of Chucxulub crater.
Chicxulub Crater
All over the Caribbean basin, little glass spheres found at this level which are molten
debris from the impact.
3. Other Extinctions.
Louis Leakey, in his book ``The Sixth Extinction'', describes that there have been five
major extinction events since the Cambrian Period, with the K-T being the last but not the
biggest.
Transparency with extinction events.
The so-called ``End-Permian'' evidently resulted in extinction of 90 % of species then
present. These great extinction events occur about every 50 - 100 million years...at the
rate that we know BIG GUYS hit the Earth.
I think there would be powerful motivation for considering the possibility that these other extinction events also had an astronomical origin.
4. Final Ruminations
(a) Life in the Universe
(b) The things I didn't talk about
5. That's it. Walk with Ursus...
