Unit 3 Intro/ Chp 6: When two Planets Collide

Question 1

What characteristic is used to differentiate gas planets from terrestrial planets?

Answer 1

• a terrestrial planet is a celestial body that is composed primarily of silicate rocks or metals and has a solid surface.
• This distinguishes them from gas giants, which are primarily composed of gases.

Question 2

What elements are common in the rocky planets?

Answer 2

the composition of that ‘rocky’ material is pretty much common to them all: a lot of silicon (Si), oxygen (O), aluminum (Al), magnesium (Mg), sulfur (S), and iron (Fe).

Question 3

What is the most abundant rock type?

Answer 3

basalt: an igneous rock, the primary product of volcanic lava, fine grained, dark grey to black

Question 4

Do planets close to the Sun experience more or fewer bombardments? Why?

Answer 4

The closer a planet is to the Sun – which has a massive gravitational attraction – the more often will it be bombarded by incoming asteroids and comets.

Question 5

Name the four hypothesis, which one is favoured?

Answer 5

1. the fission hypothesis.
2. the condensation hypothesis.
3. the capture hypothesis.
4. The Giant Impact Hypothesis (favoured)

Question 6

a. Fission Hypothesis – how did the Moon supposedly form? Why was this hypothesis rejected?

Answer 6

Moon broke off from a rapidly spinning Earth.
- Unfortunately, this would require that Earth rotated once every 2.5 hours rather than once every 24 hours, which was difficult to reconcile with the current rotational rate, so this hypothesis was rejected.

Question 7

b. Condensation Hypothesis – how did the Moon supposedly form? What are the two arguments against this hypothesis?

Answer 7

Earth and Moon formed contemporaneously from the same material.

• The first evidence to support this hypothesis would have to be chemical – they must both have the same composition. Of course, they do not.
• The second point needed to support this hypothesis would be to find that the Moon orbits Earth exactly on an equatorial plane. This is not the case, so another hypothesis is killed off.

Question 8

c. Capture hypothesis – how did the Moon supposedly form? Three reasons for rejecting it?

Answer 8

Moon formed as an independent planetary body that was later ‘captured’ by Earth during a close pass.

• the probability for the exact gravitational and dynamic conditions needed for an object the size of the Moon to fall into orbit about Earth is astronomically unlikely.
• some chemicals (most especially oxygen isotopes) were so similar between Earth and the Moon that there had to have been some close ‘genetic’ relationship.
• every single attempt to model a capture via computer program failed miserably.

Question 9

3. Giant Impact hypothesis – what is it? (Why do we like this one?)

Answer 9

• Collision(s) and melting of both bodies.
• The glancing blow(s) gave an increased angular momentum to Earth (i.e., it increased its spin rate).
• The metal core of the impactor (the Mars-size body) separated and dropped into Earth, thus giving Earth a large metal core and its remarkably high density
• the molten mantle material of both bodies mixed, and formed debris in space just above Earth.
• Over a relatively short period of time some of the debris fell back to Earth, but most of it collected into a single mass to become the Moon.

Question 10

a. What is kinetic energy – how are energy and velocity related?

Answer 10

• kinetic energy is that energy a body has by virtue of its motion.
• increases with the square of the velocity, so an object that is moving very fast can release a great deal of energy during a collision.

Question 11

b. What is Theia?

Answer 11

a planet that was somewhat smaller then Earth (this hypothetical planet)

Question 12

c. How did the giant impact affect Earth’s structure?

Answer 12

The metal core of the impactor (the Mars-size body) separated and dropped into Earth, thus giving Earth a large metal core and its remarkably high density

Question 13

d. What material makes up the Moon? Why is its composition similar to Earth?

Answer 13

• The isotopes of oxygen in lunar samples have almost exactly the same proportions as Earth rocks.
• No other object in the Solar System matches Earth composition like this, so we can be quite confident that Earth and Theia compositions must have been well mixed.

Question 14

e. What evidence is used to support that the Moon formed as a result of a large impact?

Answer 14

The isotopes of oxygen in lunar samples have almost exactly the same proportions as Earth rocks.
- so we can be quite confident that Earth and Theia compositions must have been well mixed.

Question 15

f. Was the impactor (Theia) large or small? Do we know?

Answer 15

the only way to get a match for Earth and Moon compositions is to make Theia nearly as massive as Earth

Question 16

g. Relate this hypothesis to the size of the Earth’s iron-rich core (this might also explain the size of Mercury’s core – see chapter 9).

Answer 16

The metal core of the impactor (the Mars-size body) separated and dropped into Earth, thus giving Earth a large metal core