The Hadean Eon

The Hadean Eon
4.6 - 3.8 Billion Years Ago
December
November
October
September
August
July
June
May
April
March
February
January
0 Ma
Phanerozoic
C
Modern
Geologic
Time Scale
M
P
540 Ma
Proterozoic
2500 Ma
Four Eons of
Geologic Time
Archean
3850 Ma
oldest surviving rocks
Hadean
4600 Ma
Solar System forms
How can we investigate the formation and
early history of the Solar System?
Meteorites - leftovers from planet formation.
Mercury, Moon - preserve original planetary surface.
Study other solar systems.
• Look at distant interstellar gas clouds (nebulae).
• Look for discs of dust around young stars.
• Look at structure of other solar systems.
Computer Modeling of Planetary Formation
• Must obey the laws of physics.
• Must reproduce the present solar system.
Any rocky material left from the Hadean?
Chondritic Meteorites = Stony - simple mineral composition.
• Remnants of planetary formation.
• age of formation of planets = 4.56 Ga (U-Pb)
Carbonaceous Chondrite
Formation of the Solar System
• Solar nebula collapses due to gravity.
• Proto-sun forms from hydrogen gas.
• Dust particles condense, begin to clump.
• Proto-sun becomes dense enough to begin
hydrogen fusion.
• Jupiter forms early and grows quickly.
• Remaining planets form from leftovers.
Condensing proto-Sun
and solar nebula
The Great Nebula in Orion
Formation of the planets by accretion
completed within 100 million years.
snow line
hot
cold
Gassy and icy planets (volatile)
Metallic and rocky planets (refractory)
Collisions between planetary
embryos led to the growth of
inner solar system planets.
Mercury, Venus and Earth
all have characteristics
attributed to planetary
collisions.
The Moon was formed by a
collision between Earth and
a smaller planet occupying
the same orbit.
http://videos.howstuffworks.com/discovery/29178-assignmentdiscovery-orpheus-impact-and-moon-formation-video.htm
What happened following the formation of
the Moon?
Differentiation due to partial melting of the Earth.
• Dense iron sinks to form the core.
• Lighter, silica-rich magma rises to form a magma
ocean.
Magma ocean solidifies to form a crust.
• First crust was probably basaltic (like modern ocean
floor crust).
• May have remelted repeatedly due to large impacts.
Earth, approximately 4.5 - 4.4 Ga
The Moon - an ancient surface preserving evidence of an
important Hadean geological process - impact cratering.
giant
impact
basins
highlands
Moon Rocks
Oldest moon rocks = 4.5 Ga and 3.9 Ga
• moon formed shortly after the Earth
• lunar surface records events from the Hadean
• intense meteoric bombardment from 4.0 to 3.9 Ga - Late
Heavy Bombardment.
Migration of outer planets during
the early solar system
4.5 - 3.8 Ga
Jupiter
Saturn
Neptune
Uranus
Jupiter
Saturn
Uranus
Neptune
Late Heavy Bombardment
4.0 - 3.9 Ga
Caused by the gravitational disruption of the asteroid belt
and trans-neptunian icy bodies by the rapid migration of the
outer planets.
Earth’s crust, atmosphere, and oceans were likely
completely disrupted by large impacts.
Oldest zircon grains found on Earth - 4.2 and 4.4 Ga (U-Pb)
•Found incorporated into younger metamorphic rocks.
•Zircon forms in granite - continental crust.
•Eroded and incorporated into sedimentary rock.
•Sedimentary rock metamorphosed into gneiss.
Zircon grains imply....
• Formation of continental crust (granite).
• Melting of oceanic crust with water.
• Plate tectonics beneath a liquid ocean.
• Essentially modern rock cycle in operation.
What happened between 4.4 and 4.0 Ga?
Formation of an atmosphere and oceans.
• Outgassing from volcanic eruptions.
• Modern volcanoes: H2O, CO, CO2, N2, H2
• Water vapor condenses to form liquid water.
• Source of Earth’s water?
Earth’s water was probably supplied by a collision
with one or a few icy asteroids during the late phase
of accretion, after the formation of the Moon.
Planetary science: A distinct source for lunar water? François Robert, Nature Geoscience 4, 74–75 (2011)
Formation of oceans from condensing water vapor
How was Earth’s early atmosphere
different from today’s?
No apparent source of O2 gas.
Much higher levels of CO2 (x100?)
• Early sun was only 80% as bright as today.
• Archean Earth did not freeze solid.
Oldest surviving continental crust = 3.8-4.0 Ga
Post-late heavy bombardment
•Acasta Gneiss, Canada
•Metamorphosed sedimentary rock.
•Sedimentary rock implies water and oceans.
•Continental crust produced by plate tectonics.
Acasta Gneiss, Canada
Archean
Hadean
3.8
4.6
Stable crust, atmosphere
and oceans
End of intense bombardment
Oldest surviving continental crust (Acasta Gneiss)
Late Heavy Bombardment
(crust, atmosphere, oceans
disrupted?)
Zircon grains
Plates, atmosphere
and oceans form
Oldest Zircon grain
Water delivered to Earth Crust stabilizes
Formation of the Moon Earth differentiates
Accretion of the Solar System and Earth