Lunar Geology Flashcards
Highlands
High albedo, original lunar crust 4.2-4.5 Ga, topographically high, heaviliy cratered
Lunar Basins and Maria
Large meterorite craters larger than 300km.
Largest is the South Pole Aitken Basin on the farside, Largest on the nearside is the Imbrium Basin, Youngest is the Orientale Basin
Effects in lunar crust include: Thrusting up concentric mountain ranges; excavating the crust to 10s of km; covering the moon in thick ejecta deposits; possible rebound of the underlying mantle
The Maria are composed of basaltic lava flows most between 3.1 and 3.9 Ga old but some may have continued erupting till around 1 Ga ago.
Basin =/= Maria -> some basins e.g. far side have no mare basalts and the basalt flows may have started as late as 800 Ma after impact.
Lunar Regolith
Unconsolitated dust and rock fragments fromed by billions of years of meteorite and micrometeorite impacts.
Formed by: Abrasion/Sandblasting of surface by micrometeorites; Fragmentation of surface rocks by meteorites; expantion and contractions of rocks with more than 300oC temerature variations
Small impacts ‘garden’ the regolith by overturning the surface
Older regolith is darker
Lunar Geological Features
1) Impact Craters
2) Rilles - a rima/groove/valley in the lunar surface. 2 types:
a) Sinuous Rilles - Meandering lava channels formed by low viscosity lava flowing downslope, usually in and around maria, most start in regions of depressed areas of previously active volcanic vents. e.g Hadley Rille in Apollo 15
b) Graben / Straight Rilles - Flat floored valley bounded by normal faults, caused by extensional stresses in the lunar surface. 2 types: Arcuate rilles that are curved and concentric usually around maria. or linear rilles that form due to various other forces such as dykes below the surface
3) Wrinkle Ridges - Low-rise long ridges running along the rims of maria. Formed by compression.
4) Volcanic domes
5) Lava tubes and collapse pits
6) Polar Ice - permanently shadowed polar craters
7) Kiputka - Islands of highland materials in mares
Lunar Stratigraphy Column
2 major differences in lunar geochemistry
1) Less iron than earth
2) Deficient in volatiles compared to earth e.g. potassium or sodium
5 Lunar minerals
1) Anorthite-rich Plagioclase - All lunar rocks but especially highlands
2) Clinopyroxene - Dominant in lunar mare basalts
3) Orthopyroxene
4) Olivine - Common in lunar basalts
5) Ilmenite - Opaque Iron-Titanium Oxide in high-Ti Mare Basalts
Lunar Rock Types
Highland rock types
1) Breccias - rock made up of broken fragments of rock (clasts) held together by a matrix, it is a ‘polymict’ if it is made up of broken pieces of several rock types. 2 types are: regolith breccias made up of clasts held together by regolith from impact shockwaves; impact melt breccias when the clasts are held together by solidified impact melt.
2) Ferroan Anorthosites - common type of highland rock made up of >90% Anorthite plagioclase and rich in iron relative to magnesium with small amounts of pyroxene
3) Mg-suite - around 50 plagioclase and can be either: Norite, Plag&OPx; Gabbro, CPx&Plag; Troctolite, Plag & Olivine
4) Alkali Suite - Quartz and Albite/Orthoclase present in very rare circumstances
Mare Basalts
5) 45-70% CPx, 20-45% Plag, 1-25% Ilmenite, 0-20% Olivine. Divided into high and low Ti basalts by >6% Ilmenite.
6) Volcanic glass from fire fountaining - orange beads
Clementine Spacecraft
Lunar orbiter - First laser altimetry data for the moon and multi-spectral imaging
- Anorthite is the most reflective mineral at 60% with olivine and pyroxenes at 25% which consist mare basalts consistent with mares being darker
- the 3 mafic minerals all have an absorption band around a wavelength of 1 micrometre but the wavelength of max absorption is different for each
- Ilmenite has very low reflectivity and is spectrally flat
Using this Clementine was able to map the iron and titanium distribution on both the near and far side.
Lunar Prospector
3 instruments: Gamma-Ray Spectrometer; Neutron Spectrometer; Magnetometer
Gamma rays are emitted from planetary surfaces for 2 reasons:
- Radioactive elements emit gamma rays naturally e.g. uranium and thorium. so gamma rays can be used to map out KREEP rocks and the abundance of radioactive minerals
- Cosmic rays collide with atomic nuclei on the surface as there is no atmosphere. This causes them to release neutrons which are captured by nearby nuclei and emit gamma rays of energy specific to that element. LP used this to map the distribution of major rock-forming elements up to 1 meter under the lunar surface, complementing Clementine’s results.
Neutrons spectrometers can catch neutrons released from interactions with cosmic rays. Neutrons are scattered and attenuated over a characteristic range of energies by the presence of hydrogen molecules in water. Prospector detected this attenuation over the poles, hinting at ice in permanently shadowed craters.
The presence of Ice was confirmed by LCROSS in 2009
Recent missions
Lunar Reconnaisance Orbiter (LRO)
- Best topographic map and highest resolution images of lunar surface
Chandrayaan-1
- First Indian mission to the moon with 2 instruments: X-Ray Spectrometer and Moon Mineralogy Mapper
a) X-Ray Spectrometer - When x-rays from solar flares hit the airless surface of the moon it pushes electrons into higher energy shells. Then these fall back to their orbitals and emit an x-ray equivalent to the difference in energy between the orbitals, specific to that element. This only works on the first few microns of the surface but can give a measurement of the surface composition from x-ray fluorescence.
b) Moon Mineralogy Mapper - Imaging spectroscopy where each pixel records the full spectrum, found hydrated molecules bonded at high latitudes.
LCROSS
- Impacted upper rocket stage into a permanently shadowed crater on the moon to detect water ejected by its impact, it confirmed the presence of water ice in lunar poles
GRAIL
- 2 lunar orbiters mapping the moon’s gravity
Chang-e missions
3 was the first Chinese landing on the lunar surface
4 was the first spacecraft to land on the farside of the moon
5 was the first Chinese Sample return to the moon
Lunar Geophysics
Seisomology - 4 Categories of Moon Quakes
1) Deep Moonquakes - most common and happen at depths of 700-1100km in the lunar mantle and correlate to the moon’s orbit around Earth therefore are most likely due to tidal stresses
2) Shallow Moonquakes - around 100km deep and are the strongest type accounting for most of the energy released and are the rarest. Cause is unkown but could be linked to tectonic activity.
3) Thermal Moonquakes - local to seismometers weak moonquakes occur at sunset and sunrise as the rocks crack due to extreme temperature variations
4) Meteorite and Spacecraft Impacts - useful for probing the structure of the moon
Lunar Interior
- Crust ends at 40km until 1200km where it is possible there is partially molten material as s-speeds are significantly reduced. There is also a crust of 330km with around 90km liquid.
4 main theories on Lunar Origin
1) Rotational Fission
- Material form the moon was spun off a rapidly rotating earth
- This would have required very fast rotation 1.4h/day and 4x the angular momentum of the earth-moon system (angular momentum is conserved)
2) Binary Acceleration
- The Earth would have formed with ring of material around it that accreted into the moon
- This is not consistent with the moon being less iron-rich and volatile rich as they would have formed from the same material as the earth
3) Graviational Capture
- The moon would have been formed elsewhere in the solar system and caught as it passed by Earth
- However it is too big for the energy to dissipate and take it into orbit and would have different iotope ratios to the earth
4) Giant Impact Theory
- Earth was struck by a mars sized object and the moon formed from the collision debris
- Theia (a Mars sized object impacted proto-Earth after it had already differentiated into a crust and mantle.
Debris made up of crust/mantle and impactor material is ejected in the collision.The Debris disk accretes into the moon. Volatiles were lost due to the initially very high temperatures.
Giant Impact Theory
Geological Evolution of the Moon
1) Magma Ocean - The newly accreted impact debris may have fully melted into a magma ocean from released GPE. This caused internal differentiation with anorthite plagioclase floating to the crust and mafic minerals sinking to the bottom.
2) KREEP Layer - May have formed at the end of fractional crystalistaion between the floating plagioclase and sinking mafic. This would explain why they are enriched in the Imbrium region but raises the question of why not on the south pole atkins basin
3) MG Suite of the Moon - May be formed by the excavation of magma chambers by impacts, or could be formed inside impact melts
4) Formation of the impact basins - huge objects colliding and may have caused volcanism fracturing the crust and rebounding the mantle
5) Mare Volcanism - Partial melting of the mantle through fractures in the crust and possible mantle rebound
6) Recent History - the moon may be geologically active due to the presence of shallow moonquakes and possibly volcanism less than 1Gyr old.
