Cratering in the Solar System William Bottke Southwest Research Institute Boulder, Colorado

Craters Craters are found on nearly every solid body in the solar system. Craters are found on nearly every solid body in the solar system. If properly interpreted, craters can help us understand how these bodies have evolved over the last 4.5 Gy. If properly interpreted, craters can help us understand how these bodies have evolved over the last 4.5 Gy.

Craters are Formed by Impacting Comets and Asteroids “Killer Asteroid,” National Geographic Television, 2004

The Physics of Impact Cratering Impact parameters. Impact parameters. – Projectile/target composition and porosity, impact energy, etc. Additional effects. Additional effects. – Slumping of crater walls. – Formation of ejecta blankets, secondary craters, spall, etc. – Post-impact effects on the target body (e.g., K-T impact). Impact into “Rubble-Pile” Asteroid Durda, Bottke et al. (2006)

Planetary Chronology from Crater Counts Relative surface ages can be derived from crater counts. Relative surface ages can be derived from crater counts. Absolute ages of various surfaces can be estimated if we understand the impact flux over time (and vice versa). Absolute ages of various surfaces can be estimated if we understand the impact flux over time (and vice versa).

Focus on the Moon The Moon holds the most complete and clear history available of the last 4.5 Gy of Solar System history. The Moon holds the most complete and clear history available of the last 4.5 Gy of Solar System history.

The Lunar Impact Rate Lunar impact rate has been variable with time. Lunar impact rate has been variable with time. Hartmann et al. (1981); Horz et al. (1991)

The Lunar Impact Rate Lunar impact rate has been variable with time. Lunar impact rate has been variable with time. Crater production rates >100 times higher >3.8 Gy ago. Crater production rates >100 times higher >3.8 Gy ago. Hartmann et al. (1981); Horz et al. (1991)

The Lunar Impact Rate Lunar impact rate has been variable with time. Lunar impact rate has been variable with time. Crater production rates >100 times higher >3.8 Gy ago. Crater production rates >100 times higher >3.8 Gy ago. Relatively constant crater rate since ~3.7 Ga. Relatively constant crater rate since ~3.7 Ga. Hartmann et al. (1981); Horz et al. (1991)

More than 40 basins (D > 300 km) formed on the Moon between ~ Gy ago (Wilhelms 1987). More than 40 basins (D > 300 km) formed on the Moon between ~ Gy ago (Wilhelms 1987). Lunar Basins and the Moon’s Early History

The two largest and latest-forming basins with solid age constraints are Imbrium (1160 km) and Orientale (930 km). The two largest and latest-forming basins with solid age constraints are Imbrium (1160 km) and Orientale (930 km). Orientale Basin ( Ga) Imbrium Basin ( Ga) Stoffler and Ryder (2001); Gnos et al. (2004)

Lunar Late Heavy Bombardment Were these two large basins produced by a spike of impactors near ~ 3.8 Ga, creating a terminal cataclysm? Were these two large basins produced by a spike of impactors near ~ 3.8 Ga, creating a terminal cataclysm? Koeberl (2003)

Lunar Late Heavy Bombardment Or were they produced by a declining bombardment of leftover planetesimals from terrestrial planet formation? Or were they produced by a declining bombardment of leftover planetesimals from terrestrial planet formation? Koeberl (2003)

Declining Bombardment Model 0.05 M , M  0.5 M , 5 M , 50 M  Model lunar impact rate for 1 Gy after Moon-forming event. Model lunar impact rate for 1 Gy after Moon-forming event. Bottke et al. (2006), Icarus, in press. Code tracks collisional & dynamical evolution of planetesimals

Declining Bombardment Model 0.05 M , M  0.5 M , 5 M , 50 M  Model lunar impact rate for 1 Gy after Moon-forming event. Model lunar impact rate for 1 Gy after Moon-forming event. Imbrium/Orientale formed between My. Imbrium/Orientale formed between My. Bottke et al. (2006), Icarus, in press. Imbrium & Orientale Formation Time

Declining Bombardment Model 0.05 M , M  0.5 M , 5 M , 50 M  Model lunar impact rate for 1 Gy after Moon-forming event. Model lunar impact rate for 1 Gy after Moon-forming event. Imbrium/Orientale formed between My. Imbrium/Orientale formed between My. Tests indicate we cannot make these basins at the 3σ confidence level! Tests indicate we cannot make these basins at the 3σ confidence level! Bottke et al. (2006), Icarus, in press. Imbrium & Orientale Formation Time

The Terminal Cataclysm If the declining bombardment model cannot work, most lunar basins formed in an impact spike ~3.8 Gy ago. If the declining bombardment model cannot work, most lunar basins formed in an impact spike ~3.8 Gy ago. To produce a system-wide cataclysm, we need to destabilize a large reservoir of asteroids and/or comets. To produce a system-wide cataclysm, we need to destabilize a large reservoir of asteroids and/or comets. The only known way to do this is modify the architecture of the solar system! The only known way to do this is modify the architecture of the solar system!

New Solar System Formation Model Old view. Gas giants/TNOs formed near present locations and reached current orbits ~4.5 Gy ago. Old view. Gas giants/TNOs formed near present locations and reached current orbits ~4.5 Gy ago. TNOs

New Solar System Formation Model Old view. Gas giants/TNOs formed near present locations and reached current orbits ~4.5 Gy ago. Old view. Gas giants/TNOs formed near present locations and reached current orbits ~4.5 Gy ago. New view. Gas giants formed in a more compact configuration between 5-15 AU. Massive TNO population existed between AU. New view. Gas giants formed in a more compact configuration between 5-15 AU. Massive TNO population existed between AU. TNOs Primordial TNOs

Destabilizing the Outer Solar System Watch what happens after 850 My! Tsiganis et al. (2005); Morbidelli et al. (2005); Gomes et al. (2005)

Destabilizing the Outer Solar System Gravitational interactions with planetesimals cause migration. Over time, Jupiter/Saturn enter 1:2 MMR. Gravitational interactions with planetesimals cause migration. Over time, Jupiter/Saturn enter 1:2 MMR. This destabilizes orbits of Uranus and Neptune. This destabilizes orbits of Uranus and Neptune. Tsiganis et al. (2005) Jupiter/Saturn enter 1:2 mean motion resonance

Uranus and Neptune May Switch Positions A “close up” view of the instability. A “close up” view of the instability. Uranus/Neptune: Uranus/Neptune: – Go unstable and scatter off Saturn. – Migrate through disk. Dynamical friction causes orbits to “cool down”. Dynamical friction causes orbits to “cool down”.

Orbits of Giant Planets Model reproduces orbital elements of giant planets. Model reproduces orbital elements of giant planets. Model sensitive to one parameter: disk mass. Model sensitive to one parameter: disk mass. A ~35 Earth mass disk produces long delay and orbits of planets. A ~35 Earth mass disk produces long delay and orbits of planets. Tsiganis et al. (2005)

The Lunar Late Heavy Bombardment The 1:2 MMR crossing causes secular resonances to sweep across the main belt. The 1:2 MMR crossing causes secular resonances to sweep across the main belt. The asteroid belt loses ~90% of its population. The asteroid belt loses ~90% of its population. Comet spikes comes first; asteroids last. Comet spikes comes first; asteroids last. The Moon accretes 6  g, consistent with mass flux estimates from basins. The Moon accretes 6  g, consistent with mass flux estimates from basins. Gomes et al. (2005)

This Model Also Explains… The formation of Uranus and Neptune over reasonable Solar System timescales. The formation of Uranus and Neptune over reasonable Solar System timescales. The formation and orbital distribution of the Kuiper belt. The formation and orbital distribution of the Kuiper belt. The capture and orbital distribution of the Trojan and Hilda populations (which are captured KBOs). The capture and orbital distribution of the Trojan and Hilda populations (which are captured KBOs). The shape of the oldest crater size-frequency distributions on the Moon, Mercury, and Mars. The shape of the oldest crater size-frequency distributions on the Moon, Mercury, and Mars.

Possible Implications for Mars Many buried basins found by MOLA may be ~3.8 Gy old. Many buried basins found by MOLA may be ~3.8 Gy old. Like the Moon, no Martian surface may be older than ~3.8 Gy old! Like the Moon, no Martian surface may be older than ~3.8 Gy old! – No surfaces survived from accretion. – Rocks older than 3.8 Gy can exist and are not a surprise. The earliest Martian events (Early Noachian) may have taken place over a much more compressed timescale than previously thought. The earliest Martian events (Early Noachian) may have taken place over a much more compressed timescale than previously thought.

Conclusions The study of lunar craters has led us to new planet formation models that suggest the Solar System dramatically rearranged itself ~3.8 Gy ago. The study of lunar craters has led us to new planet formation models that suggest the Solar System dramatically rearranged itself ~3.8 Gy ago. Further crater research on the Moon and other bodies may hold the key to properly interpreting the evolution history of many Solar System objects. Further crater research on the Moon and other bodies may hold the key to properly interpreting the evolution history of many Solar System objects.