Moon & Mercury Overview • Both Small • Mercury: Densest Planet! • Moon: Density less than Earth
Mercury Venus Earth Moon Mars Distance (Sun) 0.39 AU 1 AU 1 AU Orbital Period 88 days 1 year 27.3 days** Rotation Period 59 days 23.9 hours 27.3 days Radius (km) 6378 km
Radius (R⊕) 0.38 R⊕ 1 R⊕ 0.25 R⊕ Mass (kg) 6.0x1024 kg
Mass (M⊕) 0.055 M⊕ 1 M⊕ 0.012 M⊕ Avg. Surface T -290 to 800 F̊ 59 F̊ -240 to 240 F̊ ** around Earth Lunar Surface • Cratered – 30,000 big (> 1km)
– millions of smaller craters MercurianMercury Surface - Surface Features
Messenger Images Era of Heavy Bombardment Period of intense impacts in the inner S.S. by leftover planetesimals from outer S.S.
4.1-3.8 billion yrs ago
affects all Terr. planets Lunar Surface • Dark Maria (“Seas”)
• Lighter Highlands (“Lands”, Terrae) Lunar Surface • Maria
What’s different?
Why? • Highlands Apollo Lunar Surface Maria: smooth, flat, fewer craters
Highlands: VERY cratered
Maria have been resurfaced! Maria Formation After Heavy Bombardment
Volcanic Activity caused by heat from radioactive decay flooded old craters Dating the Lunar Surface Crater Counting!
Which is oldest? The Moon - Space Exploration • Luna 3 (Russia), 1959 Luna 3 farside • NASA Rangers (Impactors), 1961-1965 • Orbiters (99% mapped), 1966-1967 • Surveyors (landers), 1966-1968 • Apollo (manned), 1968-1972 • Clementine (Multi-wavel. mapping), 1994 • Prospector (polar orbiter), 1998 • SMART-1 (ESA Impactor), 2003-2006 • Kaguya (Japan), 2007- Prospector • Chang-e 1 (China), 2007- • Chandrayaan (India), 2008- Orbiter • Lunar Recon. Orbiter, 2009- – LCross Impactor, 10/2009 Apollo Missions - 6 Landed • Apollo 11 (July 20, 1969), 12, 14, 15, 16, 17 Apollo Missions - 6 Landed • Both Maria and Highlands Discoveries from the Apollo Missions • Lifeless – no fossils – no organic material • No significant atmosphere – surface pulverized by impacts, covered in dust (regolith) – doused with cosmic rays & UV radiation Discoveries from the Apollo Missions • Moon is Moving Away • Apollo Lasers • motion is 1.5cm/yr • Moon was once much Closer • 50,000 miles (2x around Earth) • 5x as large in sky Apollo Discoveries: Internal Structure • Moon is differentiated & SOLID (A) rocky crust • 60-100 km thick
(B) thick mantle • ~800 km Lithosphere • Mantle ~1200 km
(C) small (solid) iron core • ~700 km thick Apollo Discoveries: Composition • Igneous Rock (Ignis - fire) Moon was Volcanic – Maria Maria • Basalt • Heavier Elements: Iron, Manganese, Titanium • Young (3.1-3.8 bil. yrs)
– Highlands • Anorthosite • Lighter Elements/Minerals: Silicon, Calcium, Aluminum Highlands • Older (4.0 - 4.3 bil yrs) Discoveries from the Apollo Missions • Moon has same isotopes of elements as Earth –formed from same material • No Iron in moon rocks –Avg. density matches Earth’s MANTLE
⇒ New theory of Lunar formation... Discoveries from the Apollo Missions • Surface rocks are OLD but YOUNGER than EARTH (4.5 billion yrs) Maria – Maria: Younger • 3.1-3.8 billion years
– Highlands: Older • 4.0-4.3 billion years
Moon formed LATER Highlands Lunar Formation: Post-Apollo Earth was hit by a large planetesimal (size of MARS!!) Ejected Material formed Moon
When in Earth’s History? How do we know? Puzzle of the Day:
•What’s wrong with this picture? The Moon: Synchronous Rotation • Orbital Period = Rotation Period (27.3 days)
• We always see the same face
• “Dark Side” of the moon - DARK or NOT?
Apollo 16 Moon: Tides Tidal Force: Non-Uniform force across diameter of an object • Recall Gravity...
➡ Moon’s gravity stronger on Near Side ➡ creates 2 Tidal bulges ‣ 2 high tides/day ➡ Stabilizes Earth’s Axis Tilt (Seasons!) Moon: Gravity & Tides • Earth exerts force on the Moon
•Tidal Friction slowed Moon’s rotation...
➡ Synchronous Rotation Water on the Moon? • Clementine (1994) – flew over pole – at pole some craters are always in shadow – radar measurements detect ice in the bottoms of these dark craters • LCROSS (2009) – finds WATER!! LCROSS: Water on the Moon!