Week 6: Minor bodies of the Solar System

Question 1

Define “Mean Motion Resonance” and give an example of this in the Kuiper Belt.

Answer 1

MMR - The orbital period of the inner body is an integer multiple of the outer body.

Example in the Kuiper Belt:
Many objects in the Kuiper Belt share MMR with Neptune. For example, Pluto with 2:3 resonance. This allows Neptune to scatter many KBOs into unstable orbits, turning them into centaurs, then into short-period comets.

Question 2

Describe the Nice Model

Answer 2

• Neptune, Uranus, and Saturn scatters planetesimals inwards
• Jupiter sends planetesimals into highly eccentric orbits or eject them
• Jupiter and Saturn crosses their 2:1 resonance, increasing their orbital eccentricities, and destabilizing the whole planetary system.
• Jupiter puts Saturn into its current position. This leads to Saturn encountering ice giants such as Uranus and Neptune, which give them a more eccentric orbit.
• Ice giants shift into and scatter planetesimals. This populates the Oort cloud and causes a sudden influx of impacts on terrestrial planets (Late Heavy Bombardment).
• Giant planets then reach their current positions.

Question 3

Define the term “asteroid” and explain what “Kirkwood gaps” are.

Answer 3

Asteroids: A class of minor planet that is:
1 - Orbiting the sun
2 - Not a planet
3 - Not a comet

Kirkwood gaps:
- Density depletions due to resonant perturbations by Jupiter
- This causes asteroids to be ejected from the system, impact another planet, or impact the sun

Question 4

Name 3 types of asteroids and their properties.

Answer 4

1) C-type: Low albedo, carbonaceous

2) S-type: High albedo, Si-rich

3) M-type: Metal-rich, some are made from Fe-Ni alloy

Question 5

State the effects of sunlight on asteroids.

Answer 5

1) Yarkovsky effect: Changes to orbits
- When a warmer day side of the asteroid rotates into the darkness, it emits infrared radiation
- So, as photons carry momentum,
- If dark side is opposite to the direction of motion, asteroid will accelerates
- If dark side is the same as the direction of motion, asteroid will decelerates

2) YORP effect: Changes to spin rate
- As photons carry momentum, photons reflecting off an asteroid will induces a reaction force
- So, if asteroid is asymmetrical, this reaction force will be uneven, which changes the spin rate of the asteroid.

Question 6

State the difference between the term “meteorites” and “meteor”, and give 2 types of stony meteorites.

Answer 6

Meteorites: Rocks that fall from the sky. If discovered on the ground they are called “finds”, if they are observed as meteors they are called “founds”

Meteors: Incandescent object falling from the sky

2 types:
1) Chondrites:
- Have never melted
- Contains chondrules/ melted droplets inside

2) Achondrites:
- Originated in differentiated bodies
- Depleted in siderophile elements such as Fe, but enhanced in lithophile elements such as Al
- Further divided into 3 types: Howardites, Eucrites, Diogenites

Question 7

Define the term “Comets” and state its components

Answer 7

Comets: Icy objects that releases visible gas and dust during its perihelion passage

Components:
1) Nucleus: The solid part which contains most of the mass. Have a very weak gravitational field with irregular shape

2) Coma: Halo of nebula emissions surrounding the nucleus

3) Dust tails: Grains released by the comet

4) Ion tails: Ionized gas carried away by the solar wind

Question 8

Explain how comet tails form

Answer 8

Dust tail:
- Dust accelerated Sunwards by radiation pressure
- Dust grains then experience reduced gravity, causing them to follow a path that lags behind and anti-Sunwards of the nucleus

Ion tails:
- Sublimated gases from coma surrounds the nucleus
- These gases particles are then ionized by UV or electron impact. Some of these glow to form visible ion tails
- Once charged, these particles join the flow of the solar wind.

Question 9

Define the term “Centaurs” and explain how they evolve.

Answer 9

Centaurs:
- Minor bodies that inhabit regions between gas giants.
- They contain frozen volatiles that give it a diverse range of colours

These originates from Kuiper belt as KBOs.
Then turn into short period comets due to giant planets’ gravitational perturbation, which cause them to be captured by or collided with planets, or removed from the region.

Question 10

Describe the Edgeworth-Kuiper belt

Answer 10

• A planetesimal disk extending from Neptune’s orbit
• Dominated by small icy bodies that are remnants of solar system formation
• Many objects in MMR with Neptune. For example, Pluto with 2:3 resonance.
• Example: Triton, Pluto, Arrokoth

Question 11

Describe the Oort cloud

Answer 11

• Spherical clouds of objects where long period comets originate
• Defines the gravitational boundary of the solar system
• 2 distinct regions: Spherical outer cloud, disk-shaped inner clouds
• Largely composed of water, methane, and ammonia ice
• Example: Sednoids