1. Comets and Asteroids

2. Angular Momentum

linear momentum = mass x velocity

conserved!

Remember Newton's first law:

anything at rest (or in uniform motion) stays at rest (or in uniform motion), unless acted on by a force.

Analogy for spinning objects:

angular momentum = mass x velocity x size

conserved!

Consequences:

For a given mass which is spinning ,

if it gets smaller (more compact) it spins faster

if it gets bigger (less compact) it spins slower

3. Top 10 list for the Solar System:

What are the salient features of the solar system that a formation model must explain?

(1) Planet orbits are nearly circular

(2) Planet orbital planes are nearly the same

(3) Planets' orbital direction is the same as Sun's spin

(4) Planets' spin direction mostly the same as the Sun's

(5) Most moons revolve in same direction as planets' spin

(6) Each planet is isolated in space

(7) Planetary mass depends on solar distance

(8) Planetary composition depends on solar distance

terrestrial vs. jovian planets
(9) Asteroids are ancient, but unlike planets, are unevolved

(10) Comets also ancient, but do not orbit in the ecliptic plane

4. Solar System Formation

Nebular contraction

- gravitational collapse of proto-solar nebula

- flattening of the proto-solar nebula due to:

conservation of angular momentum

Hence the over-all spin and orbits of the planets and sun.

An aside:

Compare solar system to another on a very different scale: the Milky Way (our galaxy) is also highly flattened!

Condensation theory

- the role of dust

planetesimal formation via particle accretion

``snow balls''

- planet growth via accretion ...

... of planetesimals

... of gas (if massive enough and cool enough)

effects of temperature gradients in the collapsing nebula

- the inner nebula was hotter than the outer nebular

(a natural consequence of collapse)

only the heavier elements could condense into planetesimals in the inner, hotter regions

- but the lighter elements are more abundant!

differentiation in the solar system:

outer planets: more massive. They could accumulate the lighter, more abundant elements (H, He)

inner planets: less massive and made mostly of heavy elements

"Post-game" clean up:

- the solar wind

blow out any remaining gas

- gravitational forces

suck up, kick out, or 'shepherd' remaining debris

comets

asteroids (the asteroid belt)

5. Destruction

Collisions with some remnant planetesimal, e.g. an asteroid:

1 km - sized asteroid = 10⁶ megaton H bomb

IF it hits Earth.

One such object recently seen 0.5 light-seconds from Earth.

Probability:

collision every 10⁵-10⁶ years with a 1 km - sized asteroid.

BUT! 1 megaton equivalents once per century.

(many more smaller asteroids than bigger ones)

Example: Tunguska, 1908 (Siberia)

Dinosaurs demise?

What about us?

(a) The proto-solar nebula was composed mostly of Hydrogen and Helium.

(b) Dust played a major role in the formation of planetesimals.

(c) Overall, the proto-solar nebula was spinning at some rate.

(d) Jovian planets could not have existed prior to the onset of the solar wind.

(e) The proto-solar nebula was cold and lifeless.

(a) The dinosaur extinctions were not definitively caused by an asteroid impact millions of years ago.

(b) There is no evidence for large meteor impacts on Earth.

(c) There is no evidence for large meteor impacts on any planet in the solar system.

(d) What harm could a little 1km asteroid impact do anyway?

(e) The 1908 Tunguska event was due to an early Soviet weapons program.