Day 10

Louis Lackey

Day 10 notes

The introductory lecture was about Chicxulub. A 110 mile wide crater was formed by a 6 mile meteorite, 65 million years ago. This caused the extinction of the dinosaurs and allowed mammals to rise to the top of the food chain. The Mexican oil company PEMEX kept this a secret until 1981. Scientific development depends on political and social conditions.

Chapter 9

Asteroids, comets, and dwarf planets

Section 9.1

asteroids and meteorites

Asteroids are rocky leftovers from planet formation. The largest is Ceres, ~1000km. There are 150,000 listed in catalogs, and probably over a million with a diameter over 1km. Small are more common than large. All the asteroids together wouldn't add up to a small terrestrial planet. Asteroids are cratered and not round. Some asteroids have their own moons.

Most asteroids orbit in a belt between mars and Jupiter Trojan asteroids follow Jupiter’s orbit. Near earth asteroids cross earth's orbit. Jupiter's gravity through resonances prevented the belt's planetesimals from accreting into a planet.

Most meteorites are pieces of asteroids. A meteorite is a rock from space that falls through earth's atmosphere. A meteor is a bright trail left by a meteorite. Primitive meteorites have been unchanged in composition since they first formed 4.6 billion years ago. Processed meteorites have experienced volcanism and differentiation. Some meteorites are from the moon and mars.

Section 9.2

Comets

Comets formed beyond the frost line. The nucleus is a dirty snowball. Most do not have tails. Most are perpetually frozen in the outer solar system. Only comets that enter the inner solar system have tails. Comets have a dust tail and a plasma tail. The dust tail is left behind the movement of the rock, pushed by photons. The plasma tail is sun-ionized molecules, facing away from the sun, pushed by the sun. The comet has an atmosphere from the heated nucleus called a coma. Small particles from comets are left behind and cause meteor showers. Comets come from the Oort cloud and Kuiper belt. The cloud is huge, distant, and random, is a sphere, and the comets orbit in any direction. The belt is 30-100AU and orbits in a disk, flat plane, orbiting the same direction as planets.

Section 9.3

Pluto

Pluto's orbit is tilted and elliptical. It is much smaller, and not a gas giant. It has more in common with comets than the other planets. In 2005 astronomers discovered Eris, an iceball that rivals Pluto. In 2006 it was ruled that Pluto, Eris, and similar objects are dwarf planets. Its largest moon is nearly as large as itself. It is very cold, 40K, Pluto has a thin nitrogen atmosphere that freezes to the surface when it is far enough from the Sun. Very little is known about these objects.

Section 9.4

Collisions

Comet SL9 caused a string of violent impacts on Jupiter in 1994, reminding us of the danger. Fossil records show mass extinctions. The most recent was 65 million years ago, ending the dinosaurs.

Iridium is very rare on earth but very common in meteorites. There is a worldwide iridium layer 65 million years deep in the crust, all dinosaur fossils are below this layer. A meteorite 10Km in size would cause a mass extinction from climate change. Major impacts are rare, but guaranteed to happen sooner or later. Jovian planets, especially Jupiter, deflect comets away from us.