Lecture #21: Comet Tales
I. Asteroid hits
-- die-offs from Cretaceous to Tertiary era, 65 million years ago
-- iridium in soil
-- crater in Yucatan-- 180 km (so at least a 10 km asteroid)
-- 1 km asteroid hit every 100,000 years
-- 100 m asteroid hit every 100 years
II. Cratering rates
-- comets and meteors both cause craters on Moon (no atmosphere)
-- no atmospheric shielding means all material reaches surface,
explains why lunar soil has so much carbonaceous chondrites
-- lack of geologic activity means large craters remain indefinitely,
unlike Earth where they only last a few million years
-- craters are always round (explosive)
-- cratering rates useful for dating ancient
lava flows (lunar maria, for example)
III. Comets
-- sounds a little like "comb", why? (hair, Greek kome)
-- structure: nucleus, coma, tail (type I is ionized gas, also called
plasma tail, tends to be a little blue, type II is dust, also called
dust tail, is whiter and slower, so curves noticeably as the dust is
still in orbit and leads to meteor showers). A good example is
comet West.
-- the tail points away from the Sun, not away from the direction of motion
(notice where the Sun is in this picture of comet Ikeye-Seki.
IV. Comet orbits
-- elliptical around Sun (as always)
-- many are nearly parabolic (eccentricity near 1)
-- what would hyperbolic (eccentricity > 1) orbit imply?
-- eccentricities much larger than 1 are never seen, so no comets come
from outside the solar system
-- the most famous is comet Halley, which
returns about once a human lifetime (next time: 2061) and was used to confirm that
Kepler's laws applied to more than just planets