Planetesimal Collisions as Clues to the Early Dynamic History of - - PowerPoint PPT Presentation

Planetesimal Collisions as Clues to the Early Dynamic History of the Solar System

Fred Ciesla1 Thomas Davison2 Gareth Collins2 David O’Brien3

1University of Chicago 2Imperial College London 3Planetary Science Institute

Bill Hartmann Art

Planetesimals begat planets.

Planetesimals begat planets.

Planetesimals begat planets.

Planetesimals begat planets.

Planetesimals begat planets.

Planetesimals begat planets.

Asteroids are leftover planetesimals and provide clues to the early solar system.

Asteroids are leftover planetesimals and provide clues to the early solar system.

Few meteorites are perfectly pristine samples.

• Meteorites record

significant geophysical processing on their parent bodies

✦

Melting and Differentiation

• f Irons and Achondrites

✦

Metamorphism in Chondritic meteorites

• This alters the physical

and chemistry properties of the bulk meteorite and their individual components.

Radiogenic heating is believed to be largely responsible for planetesimal processing.

• Decay of short-lived

radionuclides provided energy to heat early Solar System bodies

• 26Al - t1/2 = 0.7 Ma
• Favored as the most important

(or only) heat source 26Al 26Mg +Heat

Radiogenic heating is believed to be largely responsible for planetesimal processing.

• Decay of short-lived

radionuclides provided energy to heat early Solar System bodies

• 26Al - t1/2 = 0.7 Ma
• Favored as the most important

(or only) heat source

Hot (Type6

• r Melt)

Cold (Type 3

• r

Crust)

26Al 26Mg +Heat

Models for thermal evolution do fairly well in matching data.

• 8 H chondrites with

chronological constraints on cooling

• Harrison and Grimm (2010)

model matched 7 meteorites

• H-chondrite parent body

constrained to be Rp~100 km and form 2.2 Myr into Solar System evolution

Planetesimal collisions were most frequent and energetic during planetary accretion.

Planetesimal collisions were most frequent and energetic during planetary accretion.

Planetesimal collisions were most frequent and energetic during planetary accretion.

All planetesimals experience collisions throughout the first 100 Myr.

1250 1300 1350 1400 1450 1500

Number of impactors, rimp > 150 m (survivors)

0.00 0.04 0.08 0.12

Probability

µ = 1377.98 σ = 32.11

250 500 750 1000 1250 1500

Number of impactors, rimp > 150 m (disrupted)

0.00 0.01 0.02 0.03 0.04 0.05

Probability

4 8

Number of impactors, rimp > 0.05rt

0.0 0.1 0.2 0.3 0.4

Probability

µ = 1.75

η0.05rt = 84.89%

Survived 100Myrs Disrupted 1 2 3

Number of impactors, rimp > 0.1rt

0.0 0.2 0.4 0.6 0.8 1.0

Probability

µ = 0.27

η0.1rt = 25.21%

1 2

Number of impactors, rimp > 0.2rt

0.0 0.2 0.4 0.6 0.8 1.0

Probability

µ = 0.14

η0.2rt = 14.02%

Davison et al. (2013)

Impacts create localized effects, affecting a small fraction of the body.

Temperature [K] 300 1400 3350 1650 Density [kg/m3]

• iSALE hydrocode

simulations of impact

• 100 km radius dunite

target

• 10 km radius dunite

impactor @ 4 km/s

• Equivalent energy of the

most energetic impact 100% of bodies of this size would experience.

Impacts create localized effects, affecting a small fraction of the body.

Temperature [K] 300 1400 3350 1650 Density [kg/m3]

• iSALE hydrocode

simulations of impact

• 100 km radius dunite

target

• 10 km radius dunite

impactor @ 4 km/s

• Equivalent energy of the

most energetic impact 100% of bodies of this size would experience.

Heat from an impact can persist for same time as radiogenic heat.

• Solve 2D heat

equation

• No radiogenic heat
• Evolution of post-

impact temperature anomaly

• 10 Myrs, Tpeak > 1100K
• 20 Myrs, Tpeak > 900K
• 50 Myrs, Tpeak > 800K
• 100 Myrs, Tpeak > 600K

Heat from an impact can persist for same time as radiogenic heat.

• Solve 2D heat

equation

• No radiogenic heat
• Evolution of post-

impact temperature anomaly

• 10 Myrs, Tpeak > 1100K
• 20 Myrs, Tpeak > 900K
• 50 Myrs, Tpeak > 800K
• 100 Myrs, Tpeak > 600K

Heat from an impact can persist for same time as radiogenic heat.

• Solve 2D heat

equation

• No radiogenic heat
• Evolution of post-

impact temperature anomaly

• 10 Myrs, Tpeak > 1100K
• 20 Myrs, Tpeak > 900K
• 50 Myrs, Tpeak > 800K
• 100 Myrs, Tpeak > 600K

Heat from an impact can persist for same time as radiogenic heat.

• Solve 2D heat

equation

• No radiogenic heat
• Evolution of post-

impact temperature anomaly

• 10 Myrs, Tpeak > 1100K
• 20 Myrs, Tpeak > 900K
• 50 Myrs, Tpeak > 800K
• 100 Myrs, Tpeak > 600K

Heat from an impact can persist for same time as radiogenic heat.

• Solve 2D heat

equation

• No radiogenic heat
• Evolution of post-

impact temperature anomaly

• 10 Myrs, Tpeak > 1100K
• 20 Myrs, Tpeak > 900K
• 50 Myrs, Tpeak > 800K
• 100 Myrs, Tpeak > 600K

Heat from an impact can persist for same time as radiogenic heat.

• Solve 2D heat

equation

• No radiogenic heat
• Evolution of post-

impact temperature anomaly

• 10 Myrs, Tpeak > 1100K
• 20 Myrs, Tpeak > 900K
• 50 Myrs, Tpeak > 800K
• 100 Myrs, Tpeak > 600K

Planetesimals were not cold, dense objects in the early Solar System.

10−1 100 101 102 Time, t [Myrs] 10 20 30 40 50 Impacts per Myrs

Most impacts occur early

Frequency of all impacts >150 m Temperature [K] 300 1400 3350 1650 Density [kg/m3]

Planetesimals were not cold, dense objects in the early Solar System.

10−1 100 101 102 Time, t [Myrs] 10 20 30 40 50 Impacts per Myrs

Most impacts occur early

Frequency of all impacts >150 m Temperature [K] 300 1400 3350 1650 Density [kg/m3]

Impact outcomes strongly depend on state of the target body.

Temperature [K] 170 1300

Ciesla et al. (2013)

Impact outcomes strongly depend on state of the target body.

Temperature [K] 170 1300

Ciesla et al. (2013)

Impacts may explain anomalous meteorites, provided constraints are met.

Temperature [C] Time [yrs]

Impacts may explain anomalous meteorites, provided constraints are met.

Temperature [C] Time [yrs]

Impacts may explain anomalous meteorites, provided constraints are met.

Temperature [C] Time [yrs] Temperature [C] Time [yrs]

Impacts may explain anomalous meteorites, provided constraints are met.

Temperature [C] Time [yrs] Temperature [C] Time [yrs]

• The anomalous

meteorite, Ste. Marguerite, can be explained by impact into radiogenically heated body

✦

Must occur between 2-5 Myr after Solar System formation

✦

Must be energetic enough to liberate materials from depth of ~20 km.

Evidence for/against other impacts exists in meteorite record.

• Thermal alteration of the Iron

IAB/Winonaite meteorites (Schulz

et al. 2009)

✦

Heating to 1000-1100 K at t~14 Myr

• Vaporization and condensation of

CB chondrite metal (Campbell et al.

2001, Krot et al 2005)

✦

Metal vaporization requires lots of energy at ~5 Myr.

• Preservation of CV chondrites in

crust of large planetesimal over ~50 Myr (Elkins-Tanton et al. 2011)

✦

Must avoid impacts almost entirely, but such bodies tend to experience most impacts, and most energetic

• nes.

Krot et al. (2005) Elkins-Tanton et al (2011)

Conclusions/Summary

• Planetesimal collisions were most frequent and energetic during

the first 10-100 Myr of Solar System history.

• Impacts into warm/uncompacted bodies had greater collateral effects than

in previous models. Important for debris disks?

• Meteorites record a number of energetic (large bodies, high

velocity) impacts 3-15 Myr into Solar System history.

• Preservation of pristine materials limits number and scale of

impacts.

• Impacts <1-3 km/s during “compaction phase” of chondrites.
• Preservation of “pristine crust” means some bodies avoided significant

collisions outright.

Gordon Research Conference on Origins of Solar Systems June 28 - July 3, 2015

• Mt. Holyoke College, Massachusetts