Moonstruck:
Illuminating Early Planetary History
G. Jeffrey Taylor
Hawai`i Institute of Geophysics and Planetology
University of Hawai`i at Manoa
Jeff Taylor
Lunar Science
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View of the Earth and Moon Taken from Mars
Jeff Taylor
Lunar Science
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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 history—great
implications for unraveling the histories of the
terrestrial planets
• Preserves a record of its bombardment history—the
only existing record of Earth’s bombardment history
• Moon’s 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
Jeff Taylor
Lunar Science
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Moonstruck
Outline
• Fundamental problems:
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–
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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
Jeff Taylor
Lunar Science
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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
Jeff Taylor
Lunar Science
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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.
Jeff Taylor
Lunar Science
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.
Jeff Taylor
Lunar Science
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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
Jeff Taylor
Lunar Science
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Origin of the Moon by a Giant
Impact. Painting by Don Davis in The
New Solar System
Jeff Taylor
Lunar Science
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Lunar Formation Processes:
The Giant Impact Hypothesis
Painting and concept by Bill Hartmann
Jeff Taylor
Lunar Science
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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 moonforming 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
Jeff Taylor
Lunar Science
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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
Jeff Taylor
Lunar Science
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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.
Jeff Taylor
Lunar Science
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Some Evidence for the Magma Ocean:
Anorthosite Crust
Jeff Taylor
Lunar Science
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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
Jeff Taylor
Lunar Science
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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
Jeff Taylor
Lunar Science
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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
Jeff Taylor
Lunar Science
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Importance of the Concept: Astrobiology
• Earth:
– Bombardment history
– Supply of volatiles and organics to prebiotic Earth
– Habitability of Earth’s 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)
Jeff Taylor
Lunar Science
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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
Jeff Taylor
Lunar Science
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”
Ages of Lunar Sample Impact Melts
Number of Samples
120
100
80
60
40
20
0
2
2.2
2.4
2.6
2.8
3
3.2
3.4
3.6
3.8
4
4.2
4.4
Age (Ga)
Warren (2003)
Jeff Taylor
Lunar Science
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The Evidence
• Associated with
basins on basis of
where Apollo missions
and Luna 20 mission
landed:
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–
–
–
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Apollo 14: Imbrium ejecta
Apollo 15: Imbrium ring
Apollo 16: Nectaris ejecta
Apollo 17: Serentatis ring
Luna 20: Crisium ejecta
Jeff Taylor
Lunar Science
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Problems with the Evidence
• Everything is from Imbrium—we 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]
Jeff Taylor
Lunar Science
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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
Jeff Taylor
Lunar Science
Hartmann (2003)
0.5 mm
14053, 3.95 Ga 23
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
Jeff Taylor
Lunar Science
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Testing the Cataclysm Hypothesis
Jeff Taylor
Lunar Science
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Testing the Cataclysm Hypothesis
Jeff Taylor
Lunar Science
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Testing the Cataclysm Hypothesis
Jeff Taylor
Lunar Science
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Mass Extinctions
Jeff Taylor
Lunar Science
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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
Jeff Taylor
Lunar Science
South Ray Crater
Tycho
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Phanerozoic Bombardment:
Dating 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!
Jeff Taylor
Lunar Science
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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
Jeff Taylor
Lunar Science
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Jeff Taylor
Lunar Science
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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
Jeff Taylor
Lunar Science
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Future Exploration of the Moon
• Context:
– President Bush’s 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
Jeff Taylor
Lunar Science
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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
Jeff Taylor
Lunar Science
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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
Jeff Taylor
Lunar Science
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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
Jeff Taylor
Lunar Science
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A New Era of Lunar Exploration
• Lunar exploration will
require:
– Robotic orbital
missions
– Landers
– Rovers
– Human bases
– Large human
populations
Jeff Taylor
Lunar Science
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We are at the beginning of an exciting future…
Jeff Taylor
Lunar Science
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