Moon Lecture Notes
 

Introduction

• most studied object in Solar System (except Earth) because it is closest
• exploration of the Moon
• naked eye
• telescopes
• photography
• rockets and Earth orbiters - artificial satellites
• lunar orbiters and landers - Ranger, Zond, Lunar Orbiter, Clementine
• lunar sample return missions - Apollo, Luna
• politics was fundamental in funding lunar exploration during cold war
• why should we continue to study the Moon?
• Moon formed in same neighbourhood as Earth - same composition
• smaller, so is less "evolved" than Earth
• gives clues to the very early history of the Earth

Moon Observations Summary

• two major provinces on the Moon - maria and terrae (highlands)

• notice a dichotomy between maria and highlands
• why does this dichotomy exist?
• to answer, need to study the history of the Moon

Formation of the Moon

Origin Theories

• co-accretion
• fission
• capture
• impact and re-accretion
• current favourite
• differentiated Mars-sized object impacted into the Earth
• impactor's iron core combined with Earth's iron core
• impactor's mantle and some of Earth's mantle ejected into Earth orbit
• energy involved caused volatile elements (i.e., hydrogen, oxygen) to be lost from ejected material
• ejected material accretes to form Moon in orbit around Earth
• this scenario best fits the data from Apollo samples

Formation of the Highlands

• samples returned from highland areas contained large amounts of anorthosite (a light coloured, low density rock) which was dated at ~4.3 b.y.
• how did this ancient anorthosite form?

• energy from accretion caused the outer 400-600 km of the Moon to melt
• as the melt crystallized, dense particles sank to the bottom, while low density particles floated to the top
• this low density material cooled to form a crust of high albedo anorthosite at the surface of the Moon
• continued bombardment from the last stages of lunar accretion and from passing asteroids and comets created the rugged surface

Impact Craters

• cratering is a major geological process
• not recognized for a long time because on the Earth we have erosion, where wind and water over long periods of time smooth out the surface, erasing evidence of impact craters
• on the Moon , have no atmosphere, no wind, no water, so almost no erosion
• but earliest lunar craters were not recognized because they are so big and because they were filled in by lava flows
• now we recognize that craters range in size from microscopic to basin-sized , depending on the size of the impactor

Cratering Processes

• much of what we understand about cratering processes is learned from experiments
• for simplification, cratering is usually divided into three distinct stages, but in actuality, many parts of the stages overlap and occur concurrently


• Compression Stage
• initial contact between impactor (traveling up to 20 km/s) and target sets up a shock wave
• contact energy produces high velocity jetting
• impactor continues to push into the target
• pressure is so high that target rock and impactor are pulverized
• Excavation Stage
• shock wave causes material to move radially away from point of impact
• weakening of shock wave away from source causes material flow to curve upwards and away, ejecting material above the surface in the shape of an inverted cone
• flow of material away from impact forms a crater cavity
• when shock wave can no longer eject material beyond crater rim, ejecta cone begins to fall to the surface with the bottom of the cone (the last material to be ejected) falling out first, near the crater rim, and the top of the cone (the first material to be ejected) falling out last, furthest from the crater
• Modification Stage
• once crater cavity stops growing, shock wave no longer supports the cavity and gravity takes over
• for small craters, shock wave not strong enough to form gravitationally unstable cavities, so little modification occurs
• for large craters, shock wave pressures form a cavity that collapses under gravity and so larger craters will have modification features such as slump blocks , terraces, central peaks , central rings and basin rings

Crater Dating

• on Earth, can use radiometric dating (decay of radioactive isotopes) to measure the ages of rocks
• on Moon (and other planets) don't always have access to rocks from area you need to date, so need other methods of age dating
• craters can be used to determine relative ages of planetary surfaces
• old surfaces, that have been exposed to cratering processes for long periods of time, are more heavily cratered than young surfaces, which have experienced relatively little cratering
• highlands are older than maria
• large basins tend to be the oldest impact structures, since they obliterate older craters or cover them up with basin ejecta
• confirmed by radiometric dating of available lunar samples

Volcanism on the Moon

• samples returned from mare areas consisted primarily of basalts (dark coloured, medium density rock formed by cooling of lava)
• the age of these rocks ranges from 3.8 - 3.2 b.y.
• how did these basalts form?

• heat produced by the decay of radioactive isotopes causes portions of the Moon's interior to melt, producing molten magma
• magma is less dense than solid basalt, so it is buoyant and wants to rise
• however, the density of the anorthosite crust is even lower than magma, so the magma is prevented from rising to the surface
• eventually, enough magma pools beneath the crust to produce pressures that are great enough to crack the overlying crust and extrude large quantities of lava onto the surface
• lava is liquid and flows downhill, so naturally, it pools in the ancient basins and solidifies, creating smooth flood basalts
• the maria consist of many layers of these kinds of flood basalts
• by ~ 3.0 b.y. the Moon had cooled enough that magma can no longer make it to the surface and volcanism ceases

• what about volcanoes?
• to build volcanoes need a continuous supply of small amounts of lava
• mechanism of getting magma to the surface of the Moon doesn't provide this so see very few volcano structures

• what about the far side?
• don't really know, but some possibilities include...
• the crust on the far side of the Moon is thicker than on the near side
• tidal interaction between the Earth and the Moon

Tectonism on the Moon

• tectonic activity is the sign of a geologically active planet - Earth has a lot of tectonic activity (earthquakes, plate tectonics)
• on the Moon we see evidence for tectonic activity in the past
• basin rings formed by impact
• wrinkle ridges and grabben formed by mare subsidence - large piles of mare layers are heavy and tend to sink causing stretching and cracking of crust at edges of basins ( grabben ) and bunching up of material at centre of basin ( wrinkle ridges )
• don't see evidence for very early tectonism - as the Moon cools it should contract causing the rigid crust to fault and create scarps (~2 km high)
• don't see evidence for plate tectonics
• see very little evidence of current tectonic activity
• several seismic detectors were left on the Moon by the Apollo program
• measured very small moonquakes
• moonquakes coincide with orbital cycles so are caused by tidal forces, not internal processes
• Conclusion: Moon is a dead planet, finished evolving
• However, tectonic studies have revealed clues about the interior of the Moon
• rigid crust, 0 - 60 km
• mantle, 60 -1400 km - may be partially molten below ~1000 km
• core, 1400 - 1740 km - denser than mantle, but not solid iron, FeS?

Summary of Geologic Evolution of the Moon

• formation by accretion of material ejected due to impact of Mars-sized object with the Earth
• melting of the outer layer of the Moon and formation of a low density anorthosite crust
• bombardment of the solidified crust, creating the ancient basins and producing the rugged highland terrain
• radioactive heating of the interior producing magma which rises to the surface, flooding basins and forming the maria
• subsidence of the massive maria, forming wrinkle ridges and grabben
• continued cratering, producing craters on maria and adding to ruggedness of highlands
• minor moonquakes continue, produced by tidal forces with the Earth

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