in the beginning….early earth* environment?! • ocean and atmosphere in place.! • ocean may not...
TRANSCRIPT
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