Meteorite Shock Ages, Early Bombardment, and the Age of the Moon

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Hawaii), A. Jackson (ASU) John Weirich (U. West.

1 Meteorite Shock Ages, Early Bombardment, and the Age of the Moon William Bottke 1, David Vokrouhlicky 1, Simone Marchi 1, Tim Swindle (U. Arizona), Ed Scott (U. Hawaii), A. Jackson (ASU) John Weirich (U. West. Ontario), & Hal Levison 1 1 Institute for the Science of Exploration Targets Southwest Research Institute, Boulder, CO

Moon/Mars craters and samples tell us early history, but lots of unknowns (e.g.

2 Early Bombardment Constraints D > 150 km Craters on the Moon and Mars Early bombardment tells us about the endgame of planet formation. Key issue is finding/interpreting constraints. Moon/Mars craters and samples tell us early history, but lots of unknowns (e.g., resurfacing times, sample biases). Asteroids (and meteorites) are ancient enough to probe earliest days of planet formation.

3 Part 1: Late Bombardment Signatures In Meteorites Impact melt from Chelyabinsk bolide Thanks to Simone Marchi, Barb Cohen, Dave Kring, and Kai Wunnemann

4 Problem: Asteroids Hit Each Other! Asteroids hit one another over ~4.56 Ga of solar system history! Using K.E. = ½ MV 2, the most energetic impacts should be from main belt. Main belt impacts should dominate signals from other bombardment sources.

5 Asteroid Collisional Histories Should be Dominated By Main Belt Impacts Short-Lived Population Vesta in the Asteroid Belt For 4.6 Ga Evidence for outside impactors should be hard to find, especially if impact energy is all that counts.

6 Meteorite Impact Heating Ages # of Impact Heating Events Impact melt from Chelyabinsk bolide Elapsed Time (Gyr) Expect impacts to make random pattern of heating events. Meteorites from different parent bodies should have very different shock degassing age distributions.

7 Ages from Lunar Rocks and Meteorites HED meteorites from Vesta Relative Lull? Sum probability H-chondrite meteorites from D > 200 km body Relative Lull? Samples from ancient lunar terrain via Apollo 16 Age (Gyr Ago) Many impact-shocked lunar rocks and meteorites have ~ Ga ages after a relative lull between ~ Ga. To zeroth order, patterns similar to ancient lunar ages. Bogard (2006; 2011); Marchi et al. (2013) Swindle et al. (2014)

8 How Do We Make Ar-Ar Reset Ages? Ar-Ar reset ages are likely produced by crater formation. Crater debris must be hot enough long enough to strongly heat material in the breccia lens or ejecta blanket. What kind of impacts can do this to asteroids like Vesta?

Simulations from Kai Wunnemann Impact Simulations on

9 High velocity impacts produce higher temperatures and heat a larger volume of material. Simulations from Kai Wunnemann Impact Simulations on Vesta Big Projectile V = 5 km/s Temp (K) > Small Projectile V = 18 km/s <

10 Impact Heating Trends Volume shock / Volume proj How Much Heated Material Leaves Crater Pierazzo et al. (1997) V < 5 km/s: Relatively little heating takes place. V > 10 km/s: Volume of heated material increases! V > 15 km/s: Heated material scales with impact energy. Pierazzo et al. (1997); Wunnemann et al. (2008); Barr and Citron (2011); Marchi, Bottke et al. (2013)

11 Impact Velocities on Vesta V = 14 km/s V = 10 km/s V = 6 km/s Volume shock / Volume proj Main Belt Most main belt asteroids strike Vesta at V < 6 km/s. These events produce relatively little heating. Bottke et al. (1994); Marchi, Bottke et al. (2013)

12 Impact Velocities on Vesta V = 14 km/s V = 10 km/s V = 6 km/s Volume shock / Volume proj Main Belt High V comes from high e orbits, which can still hit Vesta! These impacts may produce ~1000 times more heating! Bottke et al. (1994); Marchi, Bottke et al. (2013)

13 Impact Velocities on Vesta V = 14 km/s V = 10 km/s V = 6 km/s Main Belt High V comes from high e orbits, which can still hit Vesta! These impacts may produce ~1000 times more heating! Bottke et al. (1994); Marchi, Bottke et al. (2013) (1994)

14 The Nice Model Nesvorny and Morbidelli (2013) Jupiter Saturn Uranus Neptune Nice model describes how Jupiter-Neptune migrated to their current orbits after possible delay of many 100 s Myr.

15 Asteroids Lost Between AU Bottke et al. (2012); Nesvorny et al. (2017) Asteroids ejected from inner main belt About a main belt s mass is lost. ~0.5% hit Mars. Mars/Moon impact ratio of 10:1 Long bombardment tail! Main Belt Evolution

16 The LHB and Impact Heating on Vesta Bottke et al. (2012); Marchi, Bottke, Kring, et al. (2013)

17 Late Bombardment Summary HED meteorites from Vesta Late Heavy Bombardment Leftover Planetesimals? Sum probability H-chondrite meteorites from D > 200 km body Samples from ancient lunar terrain via Apollo 16 Age (Gyr Ago) Lots of Impacts Ga! What About 4.5 Ga? What About In-Between? Bogard (2006; 2011); Marchi et al. (2013); Swindle et al. (2014)

18 Part 2: Evolution of Giant Impact Ejecta and the Age of the Moon

Earth/Moon evolution (e.g., magma ocean, etc.). The precise nature of planet formation processes.

19 Sample references: Yin et al. (2002); Touboul et al. (2007); Borg et al. (2011); Marty et al. (2013) Finding the Age of Moon-Forming Impact Age of Moon-forming impact can help us understand: Starting point of Earth/Moon evolution (e.g., magma ocean, etc.). The precise nature of planet formation processes. Unfortunately, lunar/terrestrial samples yield wide range of ages (~10 My to > 200 My after first solids at Ga). Our goal: Find the age of giant impact in another way.

20 Pebble Moving Without Gas Drag Orbit perturbed by protoplanet, but no accretion.

21 Gas Drag and Pebble Accretion Pebbles feel headwind, become captured by large bodies.

22 Making Planets Using Pebbles Planetary Mass (Earth masses) Semimajor axis (AU) Planets grow from tiny pebbles affected by gas drag. Few smaller planetesimals, so early bombardment may be limited to large rare impactors. Bottke et al. (2010); Levison et al. (2015); Levison, Kretke, Walsh, and Bottke (2015)

23 Pebble Accretion, Planets, Asteroid Belt Planetary Mass (Earth masses) Terrestrial Planet Region Asteroid Belt Jupiter Saturn Ice Giants Giant Planet Region Primordial Kuiper Belt Semimajor axis (AU) Model naturally produces low mass Mars and asteroid belt. Most of the action is done in < 100 Myr (~4.45 Ga). Bottke et al. (2010); Levison et al. (2015); Levison, Kretke, Walsh, and Bottke (2015)

24 Lunar Impact Rate from Leftover Planetesimals Imbrium Basin ( Ga) Imbrium & Orientale Formation Time Model Dynamical evolution Collisional evolution Assumed population had a range of starting masses. Goal: Reproduce Imbrium and Orientale at their inferred ages. Bottke et al. (2007)

25 Lunar Impact Rate from Leftover Planetesimals Imbrium & Orientale Formation Time LHB population self-destructs! We find an impact rate of 10-4 basins / My 200 My = 0.02 basins. We see 2 basins! Imbrium Basin ( Ga) Bottke et al. (2007)

26 Ar-Ar and U-Pb Impact Ages in Meteorites Top are Ar-Ar ages, all from shock/melt rocks Bottom are U-Pb. Ages > 60 My from impact. Both show features My after CAIs Sample references: Gopel et all. (1994); Bogard (2011); Swindle et al. (2014); Hopp et al. (2014)

27 Impacts on Vesta at Same Time Bogard (2011); Cohen et al. (2013); Marchi et al. (2013) (4) Vesta, with colors set to mineral compositions Unshocked eucrites show Ar-Ar age spike at ~4.48 Ga. Hot material excavated by impact that quickly cooled.

28 Moon-Forming Impact: Likely the Largest Youngest Collision in Terrestrial Planet Region Only ~0.5% of Earth mass added to Earth afterwards (HSE abundances: Ir, Pt, Au) Bottke et al. (2010); Canup (2008; 2012); Cuk and Stewart (2012)

29 What Happens to Giant Impact Ejecta? Giant impact ejects ~1-5% Earth masses out of Earth-Moon system ( main belt masses). Ejecta input into inner solar system for planet formation endgame! Red ejecta escapes Canup (2012); Jackson and Wyatt (2012)

30 GI Ejecta May Blast Nearby Bodies! As Erik Asphaug showed, impact debris may go far and hit other things, including objects in the asteroid belt!

31 GI Ejecta May Blast Nearby Bodies! Ejecta Moon-Forming Impact Asteroid Belt As Erik Asphaug showed, impact debris may go far and hit other things, including objects in the asteroid belt!

Ejecta Evolution From Giant Impact Evolution of Giant Impact Ejecta Consider giant impact debris ejected from Earth-Moon system. Encounters and resonances spread it around!

32 Ejecta Evolution From Giant Impact Evolution of Giant Impact Ejecta Consider giant impact debris ejected from Earth-Moon system. Encounters and resonances spread it around! Some ejecta reaches orbits where it can hit asteroids (e.g., Flora, Vesta). Key point: Ejecta will hit asteroids at very high velocities (> 10 km/s)! See also Jackson and Wyatt (2012)

33 Choosing a Size Distribution for GI Ejecta Not much known. Insights gleaned from asteroid collision models and the Vesta family created by Rheasilvia basin. Vesta Family Rheasilvia (500 km) 100 Vesta Family Factor of ~ Diameter (km)

34 Possible GI Ejecta Size Distribution 12, 700 km We assume GI ejecta size distribution has simple shape. By mass balance, ~1-2% Earth masses can become ~10 10 D > 1 km bodies.

35 Collisonal Evolution of GI Ejecta GI ejecta loses factor of ~100 in mass in < few My. The wave-like shape is diagnostic of collisional evolution.

36 Modeling Ar-Ar Formation on Asteroids V = 1 km/s V = 3 km/s V = 5 km/s V = 7 km/s V = 9 km/s We predict giant impact ejecta should produce Ar-Ar ages on asteroids that have (i) an early peak and (ii) a slow fade.

Modeling Ar-Ar: Variables Moon-Forming Impact Ejecta Leftover Planetesimals Ratio of events from Moon forming ejecta to leftover planetesimals Time of

37 Modeling Ar-Ar: Variables Moon-Forming Impact Ejecta Leftover Planetesimals Ratio of events from Moon forming ejecta to leftover planetesimals Time of Moonforming event Ar-Ar sources: giant impact ejecta and leftover planetesimals. Variables: Time of giant impact, ratio of Ar-Ar production from sources

38 Modeling Ar-Ar: Variables Leftover Planetesimals Ratio of events from Moon forming ejecta to leftover planetesimals Time of Moonforming event Moon-Forming Impact Ejecta Ar-Ar sources: giant impact ejecta and leftover planetesimals. Variables: Time of giant impact, ratio of Ar-Ar production from sources

Modeling Ar-Ar: Variables Leftover Planetesimals Ratio of events from Moon forming ejecta to leftover planetesimals Moon-Forming Impact Ejecta Time of

39 Modeling Ar-Ar: Variables Leftover Planetesimals Ratio of events from Moon forming ejecta to leftover planetesimals Moon-Forming Impact Ejecta Time of Moonforming event Ar-Ar sources: giant impact ejecta and leftover planetesimals. Variables: Time of giant impact, ratio of Ar-Ar production from sources

40 Modeling Ar-Ar: Variables No Ar-Ar Data Combined Normalized Signature Combined signature compared to Ar-Ar data. Get best fit for variables.

41 Modeling Ar-Ar: Variables No Ar-Ar Data Combined Normalized Signature Combined signature compared to Ar-Ar data. Get best fit for variables.

42 Modeling Ar-Ar: Variables No Ar-Ar Data Combined Normalized Signature Combined signature compared to Ar-Ar data. Get best fit for variables.

43 Bottke et al. (2015) Results for Shocked Stony Meteorites Leftover planetesimals and GI ejecta hit asteroids Ar-Ar reset ages made in D > 10 km craters. Excellent fits between model and data! Predict giant impact at 105 ± 25 My after CAIs

44 Comparison with Oldest Lunar Ages Our Moon formation age agrees with the oldest known ages of lunar rocks and many model predictions. Plot modified from Borg et al. (2011). Data compiled from literature

45 Age data from Barboni et al. (2017). New 40K constants from Vogel & Renne (2008); Treiloff et al. (2003) Moon Formation Age From Zircons New work suggests that Moon formed > 4.51 Ga? Most 40 Ar- 39 Ar ages use 40 K decay constant from Steiger & Jager (1977), but new ones make ages Myr older.

Conclusions Animation from GSFC Ar-Ar age profiles suggest two bombardment components: 1. Early impactors: From leftovers of accretion and Moon-forming impact ejecta (< 4.45 Ga). Moon forms ~4.5 Ga. 2.

46 Conclusions Animation from GSFC Ar-Ar age profiles suggest two bombardment components: 1. Early impactors: From leftovers of accretion and Moon-forming impact ejecta (< 4.45 Ga). Moon forms ~4.5 Ga. 2. Late impactors: From late giant planet migration at ~4 Ga that liberated asteroids and comets from stable regions. Marchi, Bottke et al. (2012; 2013); Morbidelli et al. (2012); Bottke et al. (2012; 2015)

PLANETARY FORMATION 5) THE SOLAR SYSTEM : GRAND TAK & NICE MODEL. Aurélien CRIDA

PLANETARY FORMATION 5) THE SOLAR SYSTEM : GRAND TAK & NICE MODEL Aurélien CRIDA Has the Solar System always been as now? So far, you know that Mercury ia at 0,4 AU from the Sun Venus at 0,7 the Earth at