Impact cratering

Question 1

4 classes of craters

Answer 1

Microcraters - mm sized micrometeorites on airless bodies; form the regolith
Simple craters - usually a few km e.g. Meteor crater, Arizona
Complex craters - Flat floor; central peak or ring; terraced walls more than 20km
Multi-ring basins - largest structure, 300km or more; concentric rings of mountains

Question 2

Impact Cratering Process

Answer 2

1) Contact and Compression Stage:
- Immense pressures (more than the core) as shockwaves spread
- Molten material jettied away at high speeds
- Surface is depressed by pressure and impactor is squished, the material melts and lines the crater
2) Excavation Phase:
- Projectile is destroyed by pressures released
- Rock is displaced downwards by the pressure waves, lack of pressure upwards leads to ejected materials
- Crater rim is mechanically displaced up by the pressure
- a ‘Transient Cavity’ is formed (very deep) as shock waves compress the rocks to a maximum
- about 1/3 of the transient cavity depth is ejected by pressure released upwards and falls as an ejecta blanket around the crater
- This continues till all the impact energy is dissipated
3) Modification Phase:
- The transient cavity is unstable once the energy dissipates
- Then transient cavity bottom uplifts with the release of pressure and the crater bottom is filled with infalling excavated material, impact melt and forms a shocked breccia, with fragments of rock cemented by melt
- In a simple crater the steep walls collapse in and the bottom is only flat
- In larger impacts, the walls collapse piece by piece forming terracing
- The rebound of the transient cavity forms a central peak as in water, and if it is large enough may even from a central ring as the rebounded melt spreads out into a ripple before freezing

Question 3

Factors on impacts

Answer 3

• Angle of impacts -> does not affect much unless steeper than 10 degrees as the crater is formed by the shockwaves, not the impactor itself.
• Velocity - as kinetic energy
• Size of projectile, will form a crater 10-20 times as wide

Question 3

Factors on impacts

Answer 3

• Angle of impacts -> does not affect much unless steeper than 10 degrees as the crater is formed by the shockwaves, not the impactor itself.
• Velocity - as kinetic energy
• Size of projectile, will form a crater 10-20 times as wide

Question 4

Impact Ejecta

Answer 4

1) Ejecta blankets
Contains most of the ejecta, falling back on a ballistic trajectory, retaining its horizontal component causing it to slide away from the crater, causing a ground-hugging flow (horizontal avalanche). Continuous ejecta goes on for 1-2 crater radii from the rim, further away it is discontinuous. Deposits are made in an ordered way with the furthest being surface material and closest being deeper material. Ther are also stratigraphically deposited in the opposite direction to originally, useful for remote sensing deep planetary interiors
2) Secondary Craters
Large chunks of excavated material form their own craters as they fall back, max around 4% of impact size
3) Rays
Far bright ejecta blankets from craters