Solar System Formation

Question 1

How does a protoplanetary disk form and spin?

Answer 1

Originally from a giant gas cloud, 100s of light years across. Within, there are density variations, which are imbalanced and cause it to spin and collapse towards a centre of gravity.

Question 2

How do minerals form in a protoplanetary disk and what are the 3 classes?

Answer 2

As the gas cools down from cooling to space, minerals precipitate out, in an order based on how compatible they are:
Refractory elements (e.g. Ca) - precipitate at very high T (starting from 1700K), and so are now solid everywhere. They are always present in the same ratio
Moderately volatile elements (e.g. Na) - lower T so may not completely condense
Highly volatile elements (e.g. He) - only condense at very low T, or not at all, hence are depleted as the nebula blows away.

Question 3

How do planets form from the protoplanetary disk?

Answer 3

Nebular gas is lost as solar activity blows it away in strong outflows of atoms. Accretion occurs as particles join together by gravity.

Question 4

Define NUCLEOSYNTHESIS

Answer 4

The process of creating new atomic nuclei from protons, neutrons and pre-existing nuclei. If protons get within 1 trillionth of a centimetre from each other, a strong force binds them together, overcoming repulsion.

Question 5

What forms in Big Bang Nucleosynthesis?

Answer 5

Up to 10^-4 s after the event, quarks and high energy photons formed protons and neutrons. From 10s to 3 mins, protons and neutrons are able to join to get elements up to Lithium

Question 6

What forms in Stellar Nucleosynthesis?

Answer 6

All stars are a balance between gravitational collapse and heat expansion.
H fuses to provide He initially. As heat is lost, the star will eventually partially collapse, causing pressure and temperature to rise. This allows larger and larger elements to form, up to Fe.
Some elements are more common than others since the combination of elments to make them is more likely e.g. just adding He accentuates even elements.

Question 7

C-N-O sequence

Answer 7

12C + H => 13N + γ
13N => 13C + e- + γ
13C + H => 14N + γ
14N + H => 15O + γ
15O => 15N + e- + γ
15N + H => 12C + 4He

Question 8

Why is Iron the last element formed in stellar nucleosynthesis?

Answer 8

Iron is the element with the highest binding energy, and the last element where fusion is exothermic. Past this, elements need energy to be inputted, so are not created by the stars.

Question 9

What 3 processes can make heavier elements?

Answer 9

They need large amounts of neutrons, which can come from supernovae or neutron stars merging.
- Rapid process - rapidly colliding neutrons with stable elements to create unstable elements which decay to more stable elements by converting neutrons to protons.
- Slow process involves colliding neutrons more slowly to create an even greater range in a similar way.
- Proton Process involves colliding protons with an isotope.

Question 10

Name the 3 main moon surface features

Answer 10

Maria - regions with dense igneous basalts
Lunar highlands - low-density igneous anorthosite (plag rich), from fractional crystallisation when it was molten
Craters

Question 11

What is the moons internal structure?

Answer 11

• Small, non-molten iron-rich core (only 1.7% mass compared to Earth’s 33% and not molten), with no magnetic field.
• A silicate mantle and crust.

Question 12

How did the moon form?

Answer 12

• Theia, a smaller planet, collided and caused a stream of vaporized rock, which when cooled, formed the moon. The core ended up mostly in the Earth, and mantle material formed most of the moon.
  
  • The core is only slightly inflated in the Earth since the impact planet was relatively small.

This is supported as the moon is in the same ecliptic plane, the Earth’s axis is tipped and other planets don’t have as big moons.

Question 13

How does a crater form?

Answer 13

The impactor causes a large excavation area, and shock waves. The centre rebounds to form a central uplift, and the weakened rock around faults, often causing rings.

Question 14

What is fractionation?

Answer 14

A process where a mixture is divided into quantities with different compositions, e.g. olivine incorporates Fe and Mg, but Mg more than Fe, so they are both fractionated with respect to each other - Mg rich olivine will be separated from Fe rich olivine.

Question 15

What is the Goldschmidt classification?

Answer 15

Predicts phases elements will enter, different from the fractionation in the nebula:
- Siderophiles - affinity for Fe and other metals
- Chalcophiles - affinity with S
- Lithophiles - affinity with O and Si
- Atmophilic - stay in vapor
An element can be multiple of these.

Question 16

How did the Earth’s core form?

Answer 16

there was initially a magma ocean, and the iron entered a melt that accreted at the bottom in an metal ‘pond’. This slowly percolated down the silicate below, into the core beneath.

Question 17

How did Earth’s atmosphere form?

Answer 17

comets, degassing of mantle.

Question 18

Classes of meteorite:

Answer 18

• Chondrites - relatively unaltered, formed as aggregates of primitive material. They have not been melted and so look like sedimentary rocks. Makes up 86% of examples.
• Achondrites - processed by melting early in the solar system. Made from magma, crust or mantle of the asteroid and has no chondrule
• Iron meteorites - from the asteroid cores
• Stony iron meteorites - from the mantle-core boundary

Question 19

What are some meteorite features?

Answer 19

Carbonaceous chondrites have the same chemical composition as the Sun.
Ca-Al inclusions are some of the oldest, as these are the first elements to condense
Chondrules are spherical inclusions showing these parts were once molten and cooled before compaction, showing they are early.
Presolar grains are formed before reaching the nebula, coming from different stars.

Question 20

What is the structure of gas giants?

Answer 20

They are dominated by H phases, with a molecular phase and a metallic phase below. Below this is an ice of water, methane and ammonia.

Question 21

How could gas giants grow so big?

Answer 21

By the time the nebula was blown as far out as Jupiter, the temperature had passed the ‘snow line’ so water condensed. This promotes dust accumulation, hence it can grow big enough to capture nebula in its gravity.

Question 22

Where are comets present?

Answer 22

The Kuiper belt, past Neptune, which must have accreted when planets launched them to their current positions from further in.
The Oort cloud extends 50000AU out from the Sun and comprises the rest.

Question 23

How does Mercury’s structure differ from Earth’s?

Answer 23

Mercury’s core is 42% of the volume, and the mantle is very small, so perhaps an impact changed this.
No plate tectonics but there are ridges from compression of the crust from cooling.

Question 24

How does Venus’ structure differ from Earth’s?

Answer 24

The whole surface of Venus is ~1bya, when some event covered all previous craters.
The mantle viscosity is much higher due to the lack of water.
There is no subduction, so it is likely that volcanoes create new layers, while the bottom gets transformed back to mantle.
High CO2 levels since there is no water for it to dissolve in.
Water dissociates to O2 and H2 by UV, and H2 escapes to space. Evidence for this is that D/H ratios are high since lower mass H is lost more.

Question 25

What evidence is there for Mars’ past environment?

Answer 25

• There have been cliffs found that look typical of a river delta. Liquid water couldn’t have existed for long since the temperature is too low and atmosphere too thin. however mars was likely warmer in the past, and the denser atmosphere would have allowed liquid water.
• Mount Olympus is the largest volcano in the solar system, evidence of extreme volcanism

Question 26

How can planets be detected?

Answer 26

Eclipses
Blue- and red-shifted light caused by the planet’s mass contributing to the star’s position.