Moonstruck: Illuminating Early Planetary History

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MoonstruckIlluminating Early Planetary History

• G. Jeffrey Taylor
• Hawaii Institute of Geophysics and Planetology
• University of Hawaii at Manoa

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View of the Earth and Moon Taken from Mars
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The Moon Keystone for Understanding Planetary
History and Processes

• Natural laboratory for studying planetary
  processes
• Preserves a record of its earliest historygreat
  implications for unraveling the histories of the
  terrestrial planets
• Preserves a record of its bombardment historythe
  only existing record of Earths bombardment
  history
• Moons origin and evolution is inexorably
  intertwined with that of Earth
• Only body from which we have samples of known
  geologic context
• History known well enough to allow us to ask
  sophisticated questions
• Readily accessible

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MoonstruckOutline

• Fundamental problems
• The Dynamics of Planetary Accretion
• Chemical and Physical Processes of Lunar
  Formation
• Impact History of the Early Solar System
• Phanerozoic Bombardment History of the Inner
  Solar System
• Early Planetary Melting to Form Primary Crust,
  Mantle, and Core
• Lunar Regolith and History of the Sun
• Future Exploration

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Planetary Accretion
The rocky planets formed by accretion of small
objects to make larger and larger bodies. This
took place in the cloud of gas and dust
surrounding the primitive Sun.
Painting by Don Davis in The New Solar System
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Nature of Planetary Accretion
Wetherill (1994)
Calculations by Wetherill suggest extensive
mixing of planetesimals during planet formation.
Recent calculations by Chambers suggest somewhat
less mixing, but still a significant amount.
Chambers (2001)
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Nature of Planetary Accretion
On the other hand, our current view of the
compositions of the inner planets suggests that a
compositional gradient is preserved.
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Planetary Accretion

• Bulk composition of the Moon important for
  understanding planetary accretion
• Role of nebular gradients
• Extent of mixing of planetesimals
• Needed Additional lunar samples from places far
  from the Apollo-Luna zone and geophysical
  measurements to determine
• Composition of lowermost crust and upper mantle
• Thickness of the crust on the far side
• Composition and compositional heterogeneity of
  the mantle

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Origin of the Moon by a Giant Impact. Painting by
Don Davis in The New Solar System
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Lunar Formation ProcessesThe Giant Impact
Hypothesis
Painting and concept by Bill Hartmann
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Lunar Formation Processes

• Giant impact firmly entrenched in our thinking
• Models suggest Moon made mostly of projectile, so
  we can test extent of mixing and determine which
  elements were affected by the moon-forming event
• If Earth and Moon have the same composition, then
  elemental fractionation during giant impact was
  limited
• Needed
• Improved estimate of the bulk composition of the
  Moon
• Improved understanding of the timing of formation
  of the Earth and Moon

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Accretion, Lunar Formation, and Astrobiology

• Testing models of planetary accretion allows us
  to assess source of materials, including
  volatiles, to the Earth
• Energetic large impact might have substantially
  devolatilized growing Earth, implying that water
  and other volatiles came after the Moon formed
• Lunar studies essential part of the puzzle to
  understand formation and earliest history of the
  planets

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Early Planetary Melting

• A central tenet in lunar science is that the Moon
  melted substantially when it formed.
• This is called the magma ocean
• Many lines of evidence support the idea, but
  details of the processes that operated in it are
  obscure.

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Some Evidence for the Magma OceanAnorthosite
Crust
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Early Planetary Melting

• Outer layers of Moon provide information about
  formation of primitive crust and crystallization
  of magma ocean.
• Provides insight into differentiation of other
  planets.
• Need samples from wide variety of settings on the
  Moon e.g., farside highlands, SPA basin,
  central peaks of craters to determine
• Composition and variation of the deep interior of
  the Moon
• Provide evidence on the duration of the magma
  ocean epoch

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Impact History of Early Solar System

• Ages of impact melt rocks from the lunar
  highlands suggest that there was a peak in the
  impact rate of planetesimals between 3.8 and 3.95
  billion years
• Was there a spike in the impact rate?

Formation of the Imbrium Basin National
Geographic Magazine
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Importance of the Concept Dynamics

• Numerous imaginative ideas to explain early
  bombardment and cataclysm (if it happened)
• Left over debris from formation of terrestrial
  planets
• Late formation of Uranus and Neptune, which
  scatters nearby planetesimals
• Break-up of a large main-belt asteroid
• Asteroid scattering by 2-3 planets in the region
  that is now the asteroid belt
• Comet shower caused by close passage of a star
• Cataclysm confined to Earth-Moon system

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Importance of the Concept Astrobiology

• Earth
• Bombardment history
• Supply of volatiles and organics to prebiotic
  Earth
• Habitability of Earths surface for the first 600
  My after formation
• Episodic catastrophic impacts?
• Effect of these on life (Episodic origin and
  extinctions? Creation of suitable hydrothermal
  environments for life?)
• Relevance to other planets (Mars, Venus)

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The Evidence

• Comes from studies of impact melts
• Identify melt groups
• Determine ages
• Try to associate them with basins
• Only impact melts provide reliable ages for
  impact events

Dalrymple and Ryder (1996)
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The Evidence

• Appears to be a clustering of ages of impact
  melts around 3.8 to 4 Ga
• Has led to the idea of a lunar cataclysm

Warren (2003)
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The Evidence

• Associated with basins on basis of where Apollo
  missions and Luna 20 mission landed
• Apollo 14 Imbrium ejecta
• Apollo 15 Imbrium ring
• Apollo 16 Nectaris ejecta
• Apollo 17 Serentatis ring
• Luna 20 Crisium ejecta

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Problems with the Evidence

• Everything is from Imbriumwe are dating only one
  event
• Samples all from near side, all sites within
  reach of Imbrium ejecta
• Imbrium area focus of high Th (hence REE etc.),
  characteristic of most basaltic impact melts
  (most dates on these)
• Counter argument
• Melts have different chemical compositions and
  compositional clusters But maybe basin-sized
  impact melts vary in composition more than
  smaller terrestrial craters that have been
  studied
• Melts groups have different ages but maybe
  trapped Ar in some

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Problems with the Evidence

• Stonewall (Hartmann, 1975 2003) Early,
  declining bombardment continuously
• Resets ages
• Comminutes rocks so they are too small to
  recognize
• But there are mare basalts 3.85 4.23 Gy, yet
  they survived
• There are also pristine rocks older than 4 Gy,
  but Hartmann says there are excavated by events
  that dig beneath the pulverized zone

Hartmann (2003)
14053, 3.95 Ga
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Testing the Cataclysm Hypothesis

• Date basins that are
• Far from Imbrium
• Have compositionally distinct impact melt sheets
• Are stratigraphically older
• Good place South Pole Aitken Basin on lunar
  farside
• Oldest basin, with others superimposed on it
• Must return samples ages need to be measured to
  0.01 Gy
• Testing cataclysm idea was a major driving force
  for a SPA sample return mission being recommended
  by the Decadal Survey

Topo
Fe
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Testing the Cataclysm Hypothesis
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Testing the Cataclysm Hypothesis
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Testing the Cataclysm Hypothesis
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Mass Extinctions
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Phanerozoic Bombardment

• The Moon preserves an exquisite record of
  bombardment since 3.5 Ga, including the last 0.5
  Ga (the Phanerozoic), in the form of isotopically
  dateable crater ejecta and impact melt rocks.
  This record is largely unexplored
• Big implications for impact history of Earth
• Impacts as drivers of mass extinctions and
  evolutionary radiations
• The modern impact hazard to civilization

South Ray Crater
Tycho
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Phanerozoic BombardmentDating Techniques

• Samples from specific impact craters
• Crater ejecta (cosmic ray exposure ages, up to
  200 million years old
• Impact melt rocks (some ejected, most on floors
  of craters)
• Accuracy of 1 of age (i.e., 0.6 My for crater
  formed 65 My ago)
• Large range of crater sizes (1 to 25 km)
• Implies sample return missions and human field
  work
• Orbital methods optical maturity, rock
  populations, morphology
• Calibrated by craters dated directly
• Only way to date hundreds of craters in a
  reasonable time
• Lots of development needed to do this!

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Lunar Regolith and History of the Sun

• Dave McKay (JSC) The Moon is a solar telescope
  with a tape recorder.
• Sun affects climate on Earth
• Can understand solar physics better by obtaining
  data on solar evolution
• Key problems
• We do not have regolith samples of known age and
  solar exposure
• We do not fully understand regolith dynamics

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Lunar Regolith and History of the Sun

• Needed Find and make detailed studies of
  regolith layers between basalt flows of different
  ages
• Borders of flows
• Rilles that cut down into underlying flows
• Flows exposed by uplift
• Stagnant regolith layers
• Requires human field work and sample returns
  possible role for rovers

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Future Exploration of the Moon

• Context
• President Bushs initiative states, Use lunar
  exploration activities to further science, and to
  develop and test new approaches, technologies,
  and systems, including use of lunar and other
  space resources, to support sustained human space
  exploration to Mars and other destinations
• This clearly calls for an active program in lunar
  science, resource utilization, technology
  development, development of a permanent
  infrastructure in cis-lunar space, and initial
  space settlement

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Future Exploration of the Moon

• We need to use orbiting spacecraft and robotic
  landers to address lunar science/astrobiology
  problems and to assay potential resources

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Future Exploration of the Moon

• Essential to develop and test methods to extract
  resources from extraterrestrial bodies, beginning
  with the Moon

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Future Exploration of the Moon

• Essential to learn to use robot-human
  partnerships to conduct field work and other
  activities outside a shielded habitat, e.g.,
• Teleoperators that make use of human brain for
  observing and making decisions
• Autonomous robots for simple tasks

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A New Era of Lunar Exploration

• Lunar exploration will require
• Robotic orbital missions
• Landers
• Rovers
• Human bases
• Large human populations

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We are at the beginning of an exciting future