In the beginning….

This

That

Some multidimensional space

Initial condition

Present state

Evolutionary path

EARTH HISTORY

This

That

Some multidimensional space

Initial condition

Present state

Evolutionary path

EARTH HISTORY

Geophysics Focus of this talk

Astronomy, geochemistry, physical modeling

Geochemistry, geology, geobiology

How to think about a Planet (e.g., Earth)?

•  Could discuss provenance- the properties of an apple depend on the environment in which the tree grows

•  Or could discuss it as a machine (cf. Hero[n], 1st century AD)

•  Need to do both

The Way (Most) Geologists think about time

Phanerozoic

Precambrian

Archean

The (logarithmic) way one should think about time if you want to understand processes and their outcome

106 yr 107 108 109 1010 yr

Phanerozoic

The (logarithmic) way one should think about time if you want to understand processes and their outcome

106 yr 107 108 109 1010 yr

Phanerozoic

Earth accretion

What Memory does Earth have?

•  Overall composition (almost a closed system)

•  Isotopic •  Bulk chemistry

(partitioning; provided reservoirs are not fully equilibrated)

•  Thermal if layered

Some Specific issues with Earth 1.  How hot was it? (And does any

of that “signature” remain?) 2. How is the starting state

expressed in the mantle and core composition and layering?

3.  How does this depend on our (imperfect) understanding of planetary accumulation.

4.  What do we learn from the Moon, & from other planets.

5.  What were conditions like on early Earth? What is the origin of atmosphere and ocean.

6.  What about life?

Interstellar medium contains gas & dust that undergoes gravitational collapse

A “solar nebula” forms: A disk of gas and dust from which solid material can aggregate

Terrestrial Planet formation •  Rapid collapse from ISM;

recondensation of dust; high energy processing

•  Small (km) bodies form quickly (<106yr)[observation]. Some of these bodies differentiate ( 26Al heating)

•  Moon & Mars sized bodies may also form as quickly[theory] -will also therefore differentiate (perhaps imperfectly)

•  Orbit crossing limits growth of big bodies: Time ~ 107- 108 yr.

•  Last stages in absence of solar nebula [astronomical obs.]

•  Mixing across ~1AU likely (chemical disequilibrium?)

Rapid formation ofkilometer bodies from dust

Rapid Formation of Moonsized bodies by runaway accretion

Slow (~10 Ma) Formation of Earthlike Planets

The Importance of Giant Impacts •  Simulations indicate that

Mars-sized bodies probably impacted Earth during it’s accumulation.

•  These events are extraordinary… for a thousand years after one, Earth will radiate like a low-mass star!

•  A large oblique impact places material in Earth orbit: Origin of the Moon

In current terrestrial accretion models, the material that goes into making Earth comes from many different regions

It is very unlikely that the Moon-forming projectile would have the same isotopic composition as protoEarth.

Zonation of composition in terrestrial zone is unlikely Results from Chambers,

2003 (Similar results from Morbidelli)

Formation of the Moon

•  Impact “splashes” material into Earth orbit

•  The Moon forms from a disk in perhaps a few 100 years

•  One Moon, nearly equatorial orbit, near Roche limit- tidally evolves outward

Core Formation

Stevenson, 1989

Wood et al, 2006

Core Formation with Giant Impacts

•  Imperfect equilibration⇒ no simple connection between the timing of core formation and the timing of last equilibration

•  No simple connection between composition and a particular T and P.

Molten mantle

Core

Unequilibrated blob

Core Superheat •  This is the excess entropy of

the core relative to the entropy of the same liquid material at melting point & and 1 bar.

•  Corresponds to about 1000K for present Earth, may have been as much as 2000K for early Earth.

•  It is diagnostic of core formation process...it argues against percolation and small diapirs.

T

depth

Core Superheat

Early core

Present mantle and core

Adiabat of core alloy

The “Inevitability” of a Magma Ocean

•  Burial of accretional energy prevents immediate re-radiation - a chill crust can form.

•  In presence of sufficient atmosphere (e.g., steam), the magma ocean is protected.

•  Lower mantle can easily freeze because of pressure - this limits magma ocean depth

surface

Magma ocean

Frozen (but very hot!)

~500km

Steam atmosphere

Differentiation in the Mantle?

CORE

Dense suspension, vigorously convecting. May be well mixed Solomatov & Stevenson(1993)

Much higher viscosity, melt percolative regime. Melt/solid differentiation?

High density material may accumulate at the base.Iron-rich melt may descend?

A Layered Mantle? •  Unlikely to arise in the magma

ocean (suspended crystal stage) •  Could arise from percolative

redistribution (melt migration near the solidus) after magma ocean phase

•  Might (or might not) be eliminated by RT instabilities & thermal convection

•  Could be relevant to D”, or to a thicker layer.

•  Growing evidence for its existence

Kellogg et al, 1999

Cooling times …to decrease mean T by ~1000K

•  From a silicate vapor atmosphere: 103yr •  From a deep magma ocean/steam

atmosphere: 106 yr •  Magma ocean: Up to 108 yr [cold surface!] •  Hot solid state convection : Few x108 yr •  At current rate: >1010 yr

Early Earth* Environment? •  Ocean and atmosphere in

place. •  Ocean may not have been

very different in volume from now. Might be ice-capped.

•  Atmosphere was surely very different… driven to higher CO2 by volcanism, but the recycling is poorly known. When did plate tectonics begin?

•  Uncertain impact flux but consequences of impacts are short lived.

*4.4 to 3.8Ga

Geology, 2002

The End….of the beginning

(but not the beginning of the end)

Other Consequences of Large Impacts

•  Delivery of volatiles- perhaps from Jupiter zone (our water did not come primarily from comets)

•  Impact frustration of the origin of life? Or seeding the origin of life? Maybe both!

Volcanism & Volatile Release

•  Earth’s atmosphere & ocean came in part through outgassing

•  But volatiles are recycled on Earth- the inside of Earth is “wet”

Plate Tectonics & the Role of Water

•  Water lubricates the asthenosphere

•  Water defines the plates

•  Maintenance of water in the mantle depends on subduction; this may not have been possible except on Earth

Water Plate tectonics Magnetic field

These all influence…

LIFE

Conclusions •  Timing of Earth formation still uncertain but

compatible with a few x 107 yr duration. •  Oxygen isotopic similarity of Earth & Moon may

be the legacy of post-giant impact mixing •  High energy origin of Earth ⇒extensive melting

and magma ocean •  Legacy expressed in core superheat &

composition (siderophiles in the mantle, light elements in the core) -but not yet understood. Maybe also in primordial mantle differentiation.

•  Rapid cooling at surface but a “Hadean” world. Impacts may affect onset time of sustained life.

•  Plate tectonics, magnetic field affect life.

That’s All Folks!

•  Thank you for your attention •  Have good trip home •  Feel free to contact me at

djs@gps.caltech.edu