EAS 206 Quiz 2 (Moon)

Why is the moon important?

• only other planet on which humans have set foot

• Provides a record of the first 1 Ga of Solar System history

• Provides insights into the basic mechanics of planetary evolution

Moon period of rotation/revolution

27.3 days on its axis

Role of the moon for life on Earth

• Gravitational attraction between Earth and Moon causes slightly elliptical shape

• Tides on Earth and the Moon

• Moon stabilizes the Earth’s tilt

• Tidal flats

• Early evolution of life

Maria

Any of the relatively smooth, low, dark areas on the Moon, made of basalt

Terrae

A densely cratered highland on the Moon, made of anorthosite

Lunar Time Scale

Established by Shoemaker and Hackman, using relative age determinations and Apollo samples, uses the principle of cross-cutting relationships/superposition (lava must be younger then rock in order to fill a crater

Causes in drops of cratering rate

1. Sweeping up of plantesimals

2. Late heavy bombardment (lunar cataclysm), pulse of impactors from asteroid belt

Simple Crater

• bowl shaped

• Circular with well defined rim

• Eject blanket

• Less than 20k diameter

Complex craters

• central peak

• Has terraces (inner rim)

• Has central uplift

• 20-200km diameter

Craters greater than 300km

• have multi-ring basins

• Have peak-ring basins

• More significant modifications in larger craters

How Crater Degradation can occur

1. Later impacts that strike the original crater

2. Crater may be covered by ejecta

3. Crater may be buried by lava flows or sediments (not in the moon)

4. Erosion due to wind or water

5. Tectonic modification by folding or faulting

6. Tectonic modification due to isostatic adjustment (ongoing “modification stage”)

Volcanic Features on the Moon

• impact craters

• Flood basalts (= maria)

• Lava flows

• Sinuous rilles

• Low domes

Flood basalts

• Occurs on the moon

• filled impact basins between 3.8 and 3.1 Ga (or younger)

• Melting within interior, erupted lava

• Lava flowed long distances

• Erupted from fissures

• There are no large shield volcanoes, only small, low domes

Hadley rille

sinuous rille visited by the Apollo 15 astronauts, 135 km long, 1.2 km wide and 370 m deep

Sinuous rilles

On the moon, meandering channels formed by lava; they are thought to be either collapsed lava tubes or lava channels

How to determine the Internal Structure of the Moon

• Seismic data (Astronauts set up seismometers to listen for very small “Moonquakes”)

• Details of crust (Change in composition to peridotite (olivine-rich) at base of crust)

• Paleomagnetism (Suggest presence of a core)

Internal Structure of the Moon

• S-waves suggest asthenosphere

• Fluid outer core

• Solid inner core - will get smaller when moon gets cooler over time

• Partially molten layer surrounding core

Moon formation

• Formed from a mixture of Earth’s outer layers and a Mars-sized body

• struck the Earth ~4.5 billion years ago

• terrestrial material was less dense than Earth’s deeper interior

• core is small relative to those of other terrestrial planets because the Moon formed with fewer dense elements

Mineral on the moon

Silicates:

• Olivine

• Pyroxene

• Plagioclase feldspar (with anorthite or albite)

Oxides:

• ilmenite

Anorthosite

• Most abundant and oldest rock type on the moon crust

• Igneous

• Composed of plagioclase anthorite feldspar

• Breaks into small pieces on impact

Lunar Basalts

• Composed of: plagioclase feldspar, pyroxene, olivine, ilmenite

• Have titanium, potassium

• More titanium, zirconium, chromium, no H2O, lower sodium than Earth basalts

• Low viscosity (high fluidity)

• Include “KREEP basalts”

KREEP basalts

Component of lunar basalt

• K (potassium)

• REE (rare earth elements)

• P (phosphorous)

  • Also rich in Th (thorium)

Lunar Glass Beads

• Collected by Apollo 17 astronauts

• Mostly spherical

• Result of eruption – fire fountaining (dissolved gases in pressurized magma)

• Shows there used to be water in the moon

Breccia

• rock made up of angular fragments of other rocks (mare basalts, anorthosite, microbreccias)

• Formed during moon impact process

Regolith (soil)

• Loose, unconsolidated fragments

• Mixture of rock fragments

• Micrometeorite impacts

• Agglutinates

Agglutinates

• Breaks up and/or melts particles in regolith

• Formed by small particle melt from Micrometeorite impact

• Occurs because no atmosphere on moon so particles from space hit surface directly

Minerals only found on the moon

Oxides: Armalcolite, Tranquillityite

Apollo Next Generation Sample Analysis (ANGSA) Program

Goal is to examine sealed Apollo 17 moon regolith samples with current, state-of-the-art analytical tools (cosmic ray exposure age)

Chang’e 5 (China robotic moon sample mission)

• found mare KREEP basalt

• New mineral found – Changesite

• Found magmas come from very dry mantle source

Chang’e 6 (China robotic moon sample mission)

• Landing site within Apollo Crater

• Got samples of ejected lunar mantle

• Found mixture of fragments of local mare basalt, breccia, agglutinate, and glasses

Importance of the Apollo 11 mission

First human lunar landing and lunar samples, sampled old mare basalts and found: water not important, maria are very old, maria are volcanic

Importance of the Apollo 12 mission

Retrieved parts of Surveyor 3, sampled young mare basalts and found: “young” maria are very old, Copernicus may have formed approximately 0.9 Ga ago, granite exists on the Moon

Importance of the Apollo 14 mission

Used hand cart for rocks, sampled Fra Mauro Imbrium ejecta breccia and found: region is ejecta blanket of Imbrium basin, imbrium basin formed approximately 3.9 to 4.0 Ga ago, trace-element-enriched rocks very abundant

Importance of the Apollo 16 mission

First study and samples of the highland plains, found: Most flat highland areas formed by pooled impact ejecta; probably related to major multi-ringed basins, highlands are anorthositic

Importance of the Apollo 17 mission

First scientist-astronaut (geologist) on Moon, sampled massifs from older basin (Serenitatis), flat valley plains between mountains and dark mantling (young volcanics?) (basalts, breccias and volcanic glass), found: very young volcanism not evident, vriety of breccias may represent older events

The Luna Missions

Done by the Soviet Union, including Luna 16, 20, 24

Lunar Reconnaissance Orbiter (LRO)

Satellite that orbits the moon and takes pictures with lasers, detected ice regolith in the South Pole by illumination pics of craters, finds caves to hide from detectable radiation

Observations that a moon origin hypothesis must explain

• relatively large size compared to the earth

• Small iron core/low bulk density

• Composition similar to Earth’s mantle

• Lack of volatiles

• Moon orbit not in Earth’s orbital plane and moving away from Earth

• Similar oxygen isotope composition

TFL

Terrestrial Fractionation Line, all rocks from Earth and some moon rocks fall on this line, shows Earth and moon are made of the same stuff

Oxygen moon isotopes

Same element of different atomic masses, analysis of oxygen is from rocks NOT atmosphere

Moon Giant Impact Hypothesis

1. Mars-sized body impactor Theia and Earth are already differentiated (have their own cores)

2. Theia collides with Earth, mantles are mixed together at high temperatures and the cores of both coalesce

3. Debris ring forms the moon fast

Happens 50ma after creation of the solar system

Moon Capture Hypothesis

The Moon formed somewhere else in the Solar System, this hypothesis does not explain how the Earth and moon have the same oxygen isotopes

Moon Fission Hypothesis

The Earth spun so fast that the Moon split off from it

Moon multiple impact hypothesis

Many moonlets impact the Earth and debris disk forms moon

Pre-Imbrian Period (Pre-Nectarian and Nectarian Periods) and the moon

• Accretion of debris ring around Earth, coalesces rapidly to form the Moon

• Lunar Magma Ocean

• Crystallization of the magma ocean

• Early volcanism (KREEP basalts)

• Heavy bombardment, formation of impact basins

Process of crystallization of the lunar magma ocean

• Uses Bowen’s Reaction Series

• Anorthosite crust formed by plagioclase feldspar floating

• Olivine and pyroxene sink to the interior

• KREEP is the last material to crystallize

Imbrium Period and the moon

• Marked by formation of Imbrium Basin

• Extrusion of the mare basalts

• Melting at 400km depth

• Cooling, lithosphere becomes thicker

Eratosthenian Period and the moon

• Post-Volcanic

• Cratering is dominant

• No rays, dark ejecta, subdued craters

• Limited mare basalt volcanism

Copernican Period and the moon

• Post-Volcanic

• Cratering only

• Bright, rayed craters

• 1.1GA to the present

• Less bombardments