PYTS/ASTR 206 – The Moon 1 l Announcements n Homework #2 due now w50% credit if handed in next Tuesday n Mid-term #1 in 1 week wBased on the first 10 lectures wMultiple choice - bring pencils wLasts 1 hour and starts on time

PYTS/ASTR 206 – The Moon 2 PTYS/ASTR 206 – The Golden Age of Planetary Exploration Shane Byrne – The Moon

PYTS/ASTR 206 – The Moon 3 In this lecture… l Two types of terrain n Highlands n Maria l Geologic features on the Moon n Craters and Volcanoes l Formation of the Moon n Giant impacts & Magma Oceans l The late heavy bombardment l Formation of the Maria l The recent years

PYTS/ASTR 206 – The Moon 4 l The moon is very close in comparison to other solar system objects n It’s 30 Earth diameters away n Mars (when closest) is ~6000 Earth diameters away n Other planets are even further

PYTS/ASTR 206 – The Moon 5 l 5 th largest satellite in the solar system l Rock-like density l No atmosphere n So no wind action e.g. sand dune n No river channels or rainfall etc… l Critical body for planetary science n Much of what we do in studying solid planets started with work on the Moon n Simpler to understand than most planets

PYTS/ASTR 206 – The Moon 6 l Easy to get to – heavily visited by spacecraft l Late 1950s to early 1970s n Robotic craft from the USA & USSR wRanger wLunar Orbiter wSurveyor wLuna n Robotic Rovers (USSR) wLunokhod n Manned missions (USA) wApollo l 1990s n Clementine n Lunar Prospector

PYTS/ASTR 206 – The Moon 7 l More missions l Chandrayaan (India) l Chang’E (China) l Kaguya (Japan) l Smart 1 (Europe) l LRO (USA) – launching soon

PYTS/ASTR 206 – The Moon 8 l Maria n Dark material – once thought to be seas l Terrae n Highlands n Light material – once thought to be the dry land Two terrain types

PYTS/ASTR 206 – The Moon 9 l No one knew what the far side of the Moon looked like until 1959 n Soviet Union launched Luna 3 l Far-side looks nothing like the near side n No Maria Near side Far side

PYTS/ASTR 206 – The Moon 10 MariaHighlands (Terrae) Low elevationsHigh elevations DarkerBrighter Few cratersMany craters (saturated) SmoothRough Volcanic featuresFew volcanic features

PYTS/ASTR 206 – The Moon 11 l The Moon is a really simple planet l Craters n Simple craters <18km diameter n Complex craters >18km diameter n Multi-ring basins l Pervasive ‘gardening’ from micrometeroites n Upper few km of the crust is fractured n Upper few meters has been turned into regolith Geologic features on the Moon Moltke – 1km Euler – 28km Orientale – 970km

PYTS/ASTR 206 – The Moon 12 l Volcanic features on the Mare n Lava flow fronts n Sinuous rilles (Collapsed lave tubes) n Vents and domes

PYTS/ASTR 206 – The Moon 13 l Mare basalt l Highland Anorthosite l Volcanic features on the Mare l Apollo samples l Tell us that the mare are sheets of volcanic rock

PYTS/ASTR 206 – The Moon 14 l Most of the lunar rocks look like this n Breccia n Fragments of rock fused together

PYTS/ASTR 206 – The Moon 15 l Maria n Overlapping volcanic flows and impacts n Samples show a volcanic composition wBasalt l Highlands n Just overlapping impacts l How did the Moon get this way?

PYTS/ASTR 206 – The Moon 16 l Facts to consider n Moon depleted in iron & volatile substances wLike light elements and water n Oxygen isotope ratios similar to Earth n Moon doesn’t orbit in Earth’s equatorial plane l Possible theories (that didn’t work) n Earth and Moon co-accreted wExplains oxygen isotopes wDoesn’t explain iron and volatile depletion n Earth split into two pieces wSpinning so fast that it broke apart (fission) w…but the Moon doesn’t orbit in Earth’s equatorial plane n Capture of passing body wEarth captures an independently formed moon as it passes nearby wPretty much a dynamical miracle wDoesn’t explain oxygen isotope similarity to Earth Formation of the Moon

PYTS/ASTR 206 – The Moon 17 l Current paradigm is G iant impact n Earth close to final size n Mars-sized impactor n Both bodies already differentiated n Both bodies formed at ~1 AU

PYTS/ASTR 206 – The Moon 18 l How does that explain the iron depletion n Bodies were already differentiated n All the iron sticks around in Earth’s core n Moon rock comes from Earth’s mantle – explains Oxygen isotope similarity n Red = iron n Yellow = rock From Robin Canup, SWRI Boulder

PYTS/ASTR 206 – The Moon 19 l What does that mean for the temperature of the two bodies? n Both very hot… n Magma oceans 100s of kilometers deep n Explains the Moon’s lack of volatile elements From Robin Canup, SWRI Boulder

PYTS/ASTR 206 – The Moon 20 l Accretion of lunar material into the Moon within a few years! l High-accretion rates mean surface is molten n Magma ocean probably a few hundred km thick n Apollo 11 returned highland fragments, first suggestion of Magma ocean n Idea since extended to other terrestrial planets Core? Light minerals float ANORTHOSITE Dense minerals sink

PYTS/ASTR 206 – The Moon 21 l When the magma ocean freezes it seals in a lot of heat n Will become important later… l Crustal Thickness Asymmetry n Average crust 54-62km thick (45km at Apollo sites) n Far-side crust is much (about 15km) thicker n Crustal asymmetry is one of the central unanswered questions in lunar science

PYTS/ASTR 206 – The Moon 22 l Does the Moon have a core? n Remnant magnetism shows there was once a liquid core n Seismic experiments from Apollo – inconclusive n Another major unanswered question l Better seismic experiments would answer this  Apollo seismometers were all close together  Didn’t probe very deeply  Switched off in the 1970s to save $$

PYTS/ASTR 206 – The Moon 23 l Once the crust is solid – craters start to form l Some of these are still very large n 1000s of km across l Oldest rock fragments n From highlands ~4.5 Gyr ago l Spike in cratering rate n Gyr ago n Late heavy bombardment n All the big basins we see today date from this period l All the inner planets suffered this bombardment Impact basins and the late heavy bombardment Earliest certain evidence for life on Earth – 3.5 Gyr ago Possible evidence for life– 3.8 Gyr ago

PYTS/ASTR 206 – The Moon 24 l Bombardment movie played here n Research by LPL graduate student Dave Minton

PYTS/ASTR 206 – The Moon 25 l Big basins used to divide up the lunar timeline n Nectaris n Serenitatis n Imbrium n Orientale n etc…

PYTS/ASTR 206 – The Moon 26 l What does the Moon look like at that point? n Heavily cratered n All bright material n Orbits close to the Earth n Dotted with huge basins l The cratering rate dies off rapidly l Meanwhile…. n Things have been heating up in the subsurface 3.8 Gyr ago

PYTS/ASTR 206 – The Moon 27 l Still need to add the dark patches n Huge amounts of volcanic material erupted onto the surface n Fills the existing big basins Formation of the Maria 3.8 Gyr ago3.1 Gyr ago

PYTS/ASTR 206 – The Moon 28 l Maria start forming as the heavy bombardment era ends. n Maria crater density is much lower than the highlands n Regolith is shallower than highlands probably a few meters deep l Craters continue to accrue at a relatively slow rate until present day Mare formation period Known from Apollo samples Maria Highlands Cratering rate low after these lava flows form

PYTS/ASTR 206 – The Moon 29 l No volcanism on the far side n Thicker crust n Lava floods the pre-existing basins

PYTS/ASTR 206 – The Moon 30 l Mare material originates deep in the crust n Maria lava fill pre-existing depressions (impact basins) n Very similar to terrestrial basalt wExcept that it is completely devolatilized n Darker color due to higher Fe content l Amounts are small… n Most Maria 1-2km thick n 5km in Imbrium, 0.6km in Orientale n Individual flows ~10-40m thick n VERY low viscosity l Flood basalts

PYTS/ASTR 206 – The Moon 31 l A few kilometers of volcanic rock is heavy… even on the Moon. n Subsidence of crust under basin n Compression in the center n Extension at edges Wrinkle Ridges Graben

PYTS/ASTR 206 – The Moon 32 l So how reliable is this? n Everything is based on Apollo samples n Some of these sites were later found to be geochemically ‘unusual’ l Lunar meteorites back up the Apollo results n Ejected from the Moon by impacts n Random sampling from around the Moon

PYTS/ASTR 206 – The Moon 33 l What happens after that? n Not much…. n More craters form over the last 3.1 Gyr, but a much lower rate. n The Moon has been pretty much dead for three billion years n Craters form bright rays that get darkened over time by micrometeorites wE.g. Tycho

PYTS/ASTR 206 – The Moon 34 l Giant impact between Earth and Mars sized body forms Moon l Magma ocean n Olivine rich rocks crystallize (rocks that sink) n Anorthosite highland formation (rocks that float) l Late Heavy bombardment n Homogenizes regolith up to 20 km n Large basins form l Impact rate declines significantly n Life forms on the Earth ? l Maria erupt onto surface n Mare material fills in preexisting basins l Lighter cratering continues l Recent craters still have bright rays Copernican 1.0 – 0 Ga Pre-Nectarian 4.5 – 3.92 Ga Nectarian 3.92 – 3.85 Ga Imbrian 3.85 – 3.15 Ga Eratosthenian 3.15 – 1.0 Ga Lunar Timeline

PYTS/ASTR 206 – The Moon 35 In this lecture… Next: Craters l Reading l Chapter 10 to revise this lecture l Chapter 11-1,11-2 & 11-3 for next lecture l Two types of terrain n Highlands n Maria l Geologic features on the Moon n Craters and Volcanoes l Formation of the Moon n Giant impacts & Magma Oceans l The late heavy bombardment l Formation of the Maria l The recent years