Jeff Taylor

Lunar Science 1

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 2

View of the Earth and Moon Taken from Mars

Jeff Taylor

Lunar Science 3

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 4

Moonstruck

Outline

• 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

Jeff Taylor

Lunar Science 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

Jeff Taylor

Lunar Science 6

Nature of Planetary Accretion

Calculations by Wetherill suggest extensive mixing of planetesimals during planet formation. Recent calculations by Chambers suggest somewhat less mixing, but still a significant amount.

Wetherill (1994)

Chambers (2001)

Jeff Taylor

Lunar Science 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.

Jeff Taylor

Lunar Science 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

Jeff Taylor

Lunar Science 9

Origin of the Moon by a Giant Impact. Painting by Don Davis in The New Solar System

Jeff Taylor

Lunar Science 10

Lunar Formation Processes:

The Giant Impact Hypothesis

Painting and concept by Bill Hartmann

Jeff Taylor

Lunar Science 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

Jeff Taylor

Lunar Science 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

Jeff Taylor

Lunar Science 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.

Jeff Taylor

Lunar Science 14

Some Evidence for the Magma Ocean:Anorthosite Crust

Jeff Taylor

Lunar Science 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

Jeff Taylor

Lunar Science 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 BasinNational Geographic Magazine

Jeff Taylor

Lunar Science 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

Jeff Taylor

Lunar Science 18

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 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)

Jeff Taylor

Lunar Science 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”

Ages of Lunar Sample Impact Melts

0

20

40

60

80

100

120

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)

Nu

mb

er o

f S

amp

les

Warren (2003)

Jeff Taylor

Lunar Science 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

Jeff Taylor

Lunar Science 22

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 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

0.5 mm

Jeff Taylor

Lunar Science 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

Jeff Taylor

Lunar Science 25

Testing the Cataclysm Hypothesis

Jeff Taylor

Lunar Science 26

Testing the Cataclysm Hypothesis

Jeff Taylor

Lunar Science 27

Testing the Cataclysm Hypothesis

Jeff Taylor

Lunar Science 28

Mass Extinctions

Jeff Taylor

Lunar Science 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

TychoTycho

Jeff Taylor

Lunar Science 30

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 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

Jeff Taylor

Lunar Science 32

Jeff Taylor

Lunar Science 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

Jeff Taylor

Lunar Science 34

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 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

Jeff Taylor

Lunar Science 36

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 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

Jeff Taylor

Lunar Science 38

A New Era of Lunar Exploration

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

populations

Jeff Taylor

Lunar Science 39

We are at the beginning of an exciting future…