New Views on the Lunar Late Heavy Bombardment William Bottke Southwest Research Institute Boulder, Colorado

Craters 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.

Impact History of the Moon The Moon has the most complete and clear impact history available of the last 4.5 Gy of Solar System evolution.

Question: What is the Lunar Late Heavy Bombardment? In this talk I will show you that Southwest Research Institute is at the forefront of understanding the solar wind that fills the space around our solar system and its interaction at the galactic frontier!

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

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

Ages of Lunar Samples Very few lunar samples are older than 4.0 Ga! Dalrymple and Ryder (1991; 1993; 1996) Poikilitic impact melt rocks from Apollo 17, presumably from the Serenitatis melt sheet (Dalrymple and Ryder 1996) Sample references: Tera et al. (1974); Hartung et al. (1974); Hartmann et al. (2000)

Lunar Basins and the Moon’s Early History More than 40 basins (D > 300 km) formed on the Moon between ~3.8-4.6 Gy ago (Wilhelms 1987).

Lunar Basins and the Moon’s Early History Imbrium Basin (3.91-3.82 Ga) Orientale Basin (3.82-3.75 Ga) Stoffler and Ryder (2001); Gnos et al. (2004) The two largest and latest-forming basins with solid age constraints are Imbrium (1160 km) and Orientale (930 km).

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

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

Question: Can Leftovers from Terrestrial Planet Accretion Produce the Late Heavy Bombardment? In this talk I will show you that Southwest Research Institute is at the forefront of understanding the solar wind that fills the space around our solar system and its interaction at the galactic frontier!

Planet Formation in the Inner Solar System The planet formation population was originally made up of planetesimals and Moon- to Mars-sized planetary embryos.

Celestial Ellipses A planet orbits the Sun in an ellipse with the Sun at one focus (Johannes Kepler 1571-1630)

The Earth’s Orbit Focus Ellipse

Semimajor axis: a in AU (Astronomical Units) Eccentricity: e (=0.0 for circle, 0.0<e<1.0 for ellipse) Aphelion Perihelion ae a e = 0.0 e = 0.3 e = 0.8

Inclination: i is angle between the orbital and reference planes z orbit y i reference plane x

Track Leftover Planetesimals in Declining Bombardment Model We need impactors that are leftover from terrestrial planets formation. Dynamical excitation takes place over few My-100 My Jupiter/embryos excite surviving planetesimals. Embryos collide to create planets. Embryos Planetesimals Petit et al. (2001); Bottke et al. (2005)

Compute Impact Rate on Moon Imbrium Basin (3.91-3.82 Ga) 0.05 M, 0.005 M 0.5 M, 5 M, 50 M Imbrium & Orientale Formation Time Bottke et al. (2007) LHB Model Dynamical evolution Collisional evolution Assumed population had a range of starting masses. Goal: Reproduce Imbrium and Orientale at their inferred ages.

Compute Impact Rate on Moon Imbrium Basin (3.91-3.82 Ga) 0.05 M, 0.005 M 0.5 M, 5 M, 50 M Imbrium & Orientale Formation Time Bottke et al. (2007) LHB population self-destructs! We find an impact rate of 10-4 basins / My × 200 My = 0.02 basins. We see 2 basins!

Conclusions (Part 1) The declining bombardment model cannot produce Imbrium and Orientale (as well as other Nectaris-era basins). Existing lunar basin constraints may be more consistent with a terminal cataclysm.

Question: How Does One Create a Terminal Cataclysm? In this talk I will show you that Southwest Research Institute is at the forefront of understanding the solar wind that fills the space around our solar system and its interaction at the galactic frontier!

The Terminal Cataclysm 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. The only known way to do this is modify the architecture of the solar system!

The “Nice” Model of the Lunar Late Heavy Bombardment Much of this work is found in 3 Nature papers: Tsiganis et al. (2005); Morbidelli et al. (2005); Gomes et al. (2005)

Motivation: Planet Formation Problems Problem 1: Standard accretion models cannot make Uranus and Neptune in the age of the Solar System (if they formed near current locations). Problem 2: Jupiter and Saturn have non-trivial eccentricities and inclinations. Gas accretion should reduce these values to zero! Problem 3: Current Kuiper belt only contains ~0.1 Earth masses of material, too small to make Pluto et al.

Motivation: Planet Formation Problems Koeberl (2003) Problem 4. The lunar basins were most likely formed by a terminal cataclysm near ~ 3.8 Ga.

Making the Jovian Planets Formation of Planetary Embryos To speed up planet formation, assume Jovian planet cores formed closer to Sun! Objects then need to move to current locations. Thommes et al. (1999)

Making the Jovian Planets Formation of Planetary Embryos To speed up planet formation, assume Jovian planet cores formed closer to Sun! Objects then need to move to current locations. Thommes et al. (1999)

Can Giant Planet Migration Solve These Problems? The giant planets migrate when embedded in a disk. Migration basics: Neptune goes outward because it is easier to feed things to Uranus than to kick them out. Jupiter very good at throwing things out of Solar System Result: Saturn, Uranus, and Neptune move outward.

Mean Motion Resonances The ratio of the rate of motions of two bodies around the Sun (i.e., 1 / revolution period) is a simple fraction. This is an example of the 2:1 mean motion resonance

More on Giant Planet Migration Malhotra (1995) argued Kuiper belt resonant population was due to this migration. Problem: Jupiter/Saturn’s eccentricities too small at the end.

Migration and Resonance Crossing This problem may be solved if Jupiter and Saturn crossed the 1:2 mean motion resonance with one another.

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

New Solar System Formation Model TNOs Old view. Gas giants/TNOs formed near present locations and reached current orbits ~4.5 Gy ago. Primordial TNOs New view. Gas giants formed in a more compact configuration between 5-15 AU. Massive TNO population existed between 15-30 AU.

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

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

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

Some of the Predictions Made by the “Nice” Model In this talk I will show you that Southwest Research Institute is at the forefront of understanding the solar wind that fills the space around our solar system and its interaction at the galactic frontier!

Orbits of Giant Planets Nice model reproduces orbital elements of giant planets. Model sensitive to one parameter: disk mass. A ~35 Earth mass disk produces long delay and orbits of planets. Condition: The disk must end at 30-35 AU (or Neptune would continue to migrate)

“Push Out” Scenario for the Kuiper Belt Levison and Morbidelli (2003) Objects in primordial disk move out with Neptune’s migration. Some were released when Neptune encountered large objects in disk.

“Push Out” Scenario for the Kuiper Belt Outer border of Kuiper belt resides at 2:1 mean motion resonance with Neptune.

Capture of the Trojan Asteroids

Capture of the Trojan Asteroids Trojans are Kuiper Belt Objects! Problem: No existing model can reproduce Trojan inclination distribution (e.g., capture by gas drag). Solution: Trojans captured during LHB.

The Lunar Late Heavy Bombardment The 1:2 MMR crossing causes secular resonances to sweep across the main belt. The asteroid belt loses ~90% of its population. Comet spikes comes first; asteroids last. The Moon accretes 61021 g, consistent with mass flux estimates from basins.

Comets in the Asteroid Belt? Nice model predicts some comets may be embedded in main belt. D-type asteroids are distinctive and have similar orbital properties! Captured KBOs D-Type Asteroids D-type spectra from D. Tholen and S. J. Bus databases Comets in main belt data from Hsieh and Jewitt (2006)

Evidence: Shape of Lunar Crater SFD Main belt size distribution has had a wavy shape for 4 Gy. NEO population is more subdued. Fireball Detections (Halliday et al. 1996) NEOs from LINEAR (Stuart and Binzel 2004; Harris 2002) Bolide Detections (Brown et al. 2002) Main Belt NEOs From Bottke et al. (2005a,b)

q = -3.0 differential (-2.0 cumulative) Log [Cumulative Number / km2] Relative “R” Plots q = -3.0 differential (-2.0 cumulative) Log [Cumulative Number / km2] Log [Diameter (km)] Bob Strom quote from my first planetary course: “Everything looks good on an cumulative plot!”

q = -3.0 differential (-2.0 cumulative) Log [Cumulative Number / km2] Relative “R” Plots q = -3.0 differential (-2.0 cumulative) Log [Cumulative Number / km2] Relative (R) Log [Diameter (km)] Log [Diameter (km)] Divide crater data by q = -3 differential slope.

Log [Cumulative Number / km2] Relative “R” Plots q = -2.0 Log [Cumulative Number / km2] Relative (R) Log [Diameter (km)] Log [Diameter (km)] Divide crater data by q = -3 differential slope.

Log [Cumulative Number / km2] Relative “R” Plots q = -4.0 Log [Cumulative Number / km2] Relative (R) Log [Diameter (km)] Log [Diameter (km)] Divide crater data by q = -3 differential slope.

Evidence: Shape of Lunar Crater SFD Main belt size distribution has had a wavy shape for 4 Gy. NEO population is more subdued. Main Belt NEOs From Bottke et al. (2005a,b)

Evidence: Shape of Lunar Crater SFD LHB crater SFDs are wavy like main belt SFD. Post-LHB craters are consistent with NEOs. LHB process needed to deplete main belt in size-independent manner. Since asteroids strike Moon last, their SFD is observed. Strom et al. (2005)

Hadean Zircons Trail et al. (2005) Zircons (ZiSiO4) are diamond-like objects found in Earth’s crust. Some are the oldest minerals on Earth, with one formed 4.4 Ga. Hadean zircons are the only dated material on Earth to have gone through the LHB. Most found in Jack Hills, Western Australia. The cores are igneous. The overgrowths record high temperature events taking place all over the Earth.

Hadean Zircons Core ages are generally 4.2 Ga, while overgrowths are at ~3.95 Ga. Nothing is found in between. If overgrowths were produced by the LHB, the LHB was most likely a terminal cataclysm.

Possible Implications for Mars Many buried basins found by MOLA or MGS data may be ~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 took place over a much more compressed timescale than previously thought.

Evidence of LHB in Other Systems? FIR Outer Disks (1-10 AU) vs. Time Classical Evolution or Punctuated Equilibrium? Question: Is the far-IR excess seen in other systems evidence for their own LHB events? Habing et al. (1999); Meyer et al. (2000); Habing et al. (2001); Spangler et al. (2001)

Conclusions The lunar basins with solid age constraints were formed from a terminal cataclysm rather than a declining bombardment scenario. The Nice model assume the giant planets formed within 18 AU and migrated to their current positions at 3.8 Ga. This model explains, among other things: The formation of Uranus and Neptune over reasonable timescales. The orbits of the Jovian planets The inclination distribution of the Trojans. The terminal cataclysm at ~3.8 Ga. The wavy shape of the impactor population. The edge and shape of the Kuiper belt. The irregular satellites orbiting the Jovian planets