Moon

+273 F to -273 F 1/6 g @ surface

No atmosphere. 0.27 diameter of Earth - 3476 km.

Naked eye can see 120 mi features Þ see maria only

Maria, Mare = sea - flat, relatively smooth.

Highlands - mountains and craters - oldest terrain > 4 billon yrs (lunar surface 4.5 billion yrs since differentiation)

Craters: all sizes - mainly nearly all <100 km, 95% impact, 5% volcanic

Crater density: # craters / million km²

Cratering rates:

Probably similar on all inner planets and moon for last 3.8 billion yrs

Earlier, first 700 million yrs (3.8 - 4.5 billion yrs ago) rate much greater - 1000x grater 4.0 than 3.8 billion yrs ago - scant evidence from before 4.0 billion yrs ago

3.9 billion yrs ago called late heavy bombardment -

Either unique disruption of asteroid or end of accretion phase.

Even so, only about one 1 km crater every 2,500 yrs during heavy bombardment

Maria: smooth, darker areas seen on near side only

Due to more recent flooding by lava 3.2 - 3.9 billion years ago, based on solidification ages of lunar rocks from Apollo Project

A few huge impacts released molten material from below partially solid crust Þ Deposition of denser material on surface

Lunar Basins: large (over 300 km diameter) impact craters - about 30 known

Date to 3.9 - 4.1 billion yrs ago

Largest: 2200 km near south pole

Youngest: Mare Imbrium 3.9 billion yrs ago

Synchronous rotation - 1 : 1 relationship between rotational and orbital periods. Tidal bulge otherwise moves around moon creating friction Þ rotation slows to equal orbital period.

Rays: Lighter ejecta (cast off material) radiating from certain large impact craters. Has not yet turned dark by radiation and mixing

Rills: Winding, sharply defined valleys resembling dry riverbeds. Volcanic origin - like lava tubes.

Surface features sharply defined on large scale (no weather, low gravity), but rounded on smaller scale - constant micrometeorite ( < < mm) bombardment

Soil called regolith - rock fragments and glassy minerals small (1 mm or less) spherules from hot impacts rapid cooling without grain formation - abundant in soil Þdark

Moon rocks 843 lb. by Apollo, some volcanic; pitted by micrometeorite impacts

Highlands: crustal rock anorthosites - mineral oxides of silicon, aluminum, calcium, and magnesium + some basalts from impacts penetrating crust -

earliest 4.2 billion yrs ago

Surface material 3.5 - >4 billion years old

No magnetic field and seismic measurements imply no molten core

Almost no seismic activity

5 ALSEPs (Apollo Lunar Surface Experiment Package) measured solar wind, heat flow from interior, gas particles, and seismic waves (seismometer) - very sensitive.

Þ network to analyze seismic waves from impacts and moonquakes - measured seismic response of entire moon Þ moon very quiet, may have partially molten (rocky) center

Powered by nuclear batteries

No tectonic activity

Lunar Origin:

Giant Impact Theory: Mars-sized object impacts earth.
Debris from impact ejected into orbit about earth.
Debris then accretes to form moon.

Þ Average density like Earth's mantle (3.3 g/cc), no metallic core, no magnetic field

Impact Energies

E = 1/2 mv² (in joules = kg m²/sec²)

metric ton = 1000 kg

megaton = 10⁶ x 10³ kg = 10⁹ kg

kg TNT = 4 x 10⁶ joules

megaton TNT = (10⁹ kg) 4 x 10⁶ joules / kg = 4 x 10¹⁵ joules

10 km asteroid:

density = mass/volume = 2.5 g/cc or 2,500 kg/m³

V = 4/3 pi R³ = 5.22 x 10¹¹ m³

V x M / V = M = 5.22 x 10¹¹ m³ x 2.5 x 10³ kg/m³

Þ mass = 1.3 x 10¹⁵ kg

Earth: velocity = escape velocity (11 km/sec) +

typical relative velocity (9 km/sec)

400 x 10⁶ m²/sec²

Þ velocity = 20 km/sec

km / sec = 10³ m/sec

E = 1/2 m v² = 1/2 (1.3 x 10¹⁵ kg) (20 x 10³ m/sec)² = 2.6 x 10²³ joules

2.6 x 10²³ joules / 4 x 10¹⁵ joules/megaton = 6.5 x 10⁷ megatons

Largest bomb ever tested: 53 megatons yield by Soviet Union

Hiroshima fission bomb: 0.02 megatons

Moon: v = approx. 1/2 v for Earth Þ 1/4 energy Þ 6.5 / 4 x 10⁷ megatons

= 1.625 x 10⁷ megatons

Each kg striking Earth equals 33 kg of TNT!