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Title: Moonstruck: Illuminating Early Planetary History


1
MoonstruckIlluminating Early Planetary History
  • G. Jeffrey Taylor
  • Hawaii Institute of Geophysics and Planetology
  • University of Hawaii at Manoa

2
View of the Earth and Moon Taken from Mars
3
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

4
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

5
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
6
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)
7
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.
8
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

9
Origin of the Moon by a Giant Impact. Painting by
Don Davis in The New Solar System
10
Lunar Formation ProcessesThe Giant Impact
Hypothesis
Painting and concept by Bill Hartmann
11
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

12
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

13
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.

14
Some Evidence for the Magma OceanAnorthosite
Crust
15
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

16
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
17
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

18
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)

19
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)
20
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)
21
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

22
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

23
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
24
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
25
Testing the Cataclysm Hypothesis
26
Testing the Cataclysm Hypothesis
27
Testing the Cataclysm Hypothesis
28
Mass Extinctions
29
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
30
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!

31
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

32
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33
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

34
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

35
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

36
Future Exploration of the Moon
  • Essential to develop and test methods to extract
    resources from extraterrestrial bodies, beginning
    with the Moon

37
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

38
A New Era of Lunar Exploration
  • Lunar exploration will require
  • Robotic orbital missions
  • Landers
  • Rovers
  • Human bases
  • Large human populations

39
We are at the beginning of an exciting future
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