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 1
View of the Earth and Moon Taken from Mars Jeff Taylor Lunar Science 2
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 3
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 4
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 5
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) Jeff Taylor Lunar Science 6
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 7
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 8
Origin of the Moon by a Giant Impact. Painting by Don Davis in The New Solar System Jeff Taylor Lunar Science 9
Lunar Formation Processes: The Giant Impact Hypothesis Painting and concept by Bill Hartmann Jeff Taylor Lunar Science 10
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 11
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 12
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 13
Some Evidence for the Magma Ocean: Anorthosite Crust Jeff Taylor Lunar Science 14
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 15
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 16
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 17
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 18
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 19
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 120 Number of Samples 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 20
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 21
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 22
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 0.5 mm Hartmann (2003) Jeff Taylor Lunar Science 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 24
Testing the Cataclysm Hypothesis Jeff Taylor Lunar Science 25
Testing the Cataclysm Hypothesis Jeff Taylor Lunar Science 26
Testing the Cataclysm Hypothesis Jeff Taylor Lunar Science 27
Mass Extinctions Jeff Taylor Lunar Science 28
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 Jeff Taylor Lunar Science 29
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 30
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 31
Jeff Taylor Lunar Science 32
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 33
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 34
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 35
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 36
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 37
A New Era of Lunar Exploration Lunar exploration will require: Robotic orbital missions Landers Rovers Human bases Large human populations Jeff Taylor Lunar Science 38
We are at the beginning of an exciting future Jeff Taylor Lunar Science 39