Lecture #10: Plan The Moon Terrestrial Planets
Both Sides of the Moon
Moon: Direct Exploration
Moon: Direct Exploration
Moon: Direct Exploration Apollo Landing Sites
Moon: Apollo Program Magnificent desolation Buzz Aldrin In the Shadow of the Moon (100 minutes) http://www.intheshadowofthemoon.com
Moon: General Properties Radius ~ ¼ Earth s Mass ~ 1/80 Earth s Density ~ 3.3 kg / liter Distance ~ 384,000 km from Earth
Surface Features Craters: impact features Highlands: low density, light colored rock, heavily cratered, old (4.5 b.y) Maria: high density, dark colored rock, smooth, younger (~3 4 b.y.)
Craters
Craters Large impacts can cause the surface to rebound, forming central peaks Material thrown up onto the crater rim can slide back down into the crater, forming rings
Maria
Formation of Maria Mountains on the edge of maria are formed from debris thrown up during an impact
Differences from Earth No plate tectonics No volcanoes!
Differences from Earth No atmosphere No erosion!
Origin of the Moon 1. Capture Theory Moon was originally a small planet orbiting the Sun that approached the Earth and was captured 2. Twin Formation Theory Moon and Earth were twins forming side by side from a common cloud of dust and gas
Origin of the Moon 3. Fission Theory The Earth initially spun enormously faster than now and formed a bulge that ripped away from the Earth to become the Moon 4. Collision Theory The Moon was formed from debris blasted out of the Earth by the impact of a Mars-sized body at least 4.5 billion years ago
Origin of the Moon 1. Capture Theory 2. Twin Formation Theory 3. Fission Theory 4. Collision Theory
Origin of the Moon: Clues i. Moon s mass relative to Earth is large 1 2 3 4 ii. Moon has little iron, mostly rock, some like material from Earth s crust & mantle, some very different 1 2 3 4 iii. Moon has relatively high abundance of high melting point materials (e.g., gold) but almost no low melting point materials (e.g. water) 1 2 3 4 iv. Moon s orbit is not in Earth s equatorial plane 1 2 3 4 v. Earth s rotation axis has large tilt (23.5 o ) 1 2 3 4
The Birth of the Moon (Collision Theory)
The Origin of Tides The Moon s gravitational attraction creates two tidal bulges on Earth s ocean
High and Low Tides The Earth rotates under water bulges, giving two high tides and two low tides per day at a given location
The Sun creates tides, too! but they are ~ ½ the strength of the lunar tides
Tidal Braking 1. Earth s spin is slowing down (0.002 sec / century) 2. Moon is speeding up in orbit and moving away (4 cm / year, from reflectors on Moon installed by Apollo astronauts)
Synchronous Rotation Moon is keeping same face towards Earth due to tidal braking of the Moon Rotation period = Orbital period ~ 1 month
Mercury Heavily cratered Looks a little like the Moon!
Mercury Density = 5.4 kg / liter.. ~ Earth s Rocky mantle + iron/nickel core Slow spin: 59 days (orbital period = 88 days) No satellites No atmosphere: T day = 800 F / 700 K T night = -280 F / 100 K
Mercury R ~ 0.4 R E Heat loss more efficient than Earth s M ~ 0.06 M E Less radioactive heating than Earth Cooler interior No plate activity few volcanoes in the past
Mercury few volcanoes Virtually no gas production Low escape velocity Gas escapes easily + Proximity to Sun Any gas would be hot & escape easily No Atmosphere!
Venus Density = 5.3 kg / liter.~ Earth s Rocky crust & mantle + iron/nickel core Very thick atmosphere 96.5% CO 2 Clouds of sulfuric acid! T surface = 900 F / 750 K (hot enough to melt lead!) Slow & retrograde spin 243 Earth days tilt of spin axis = 177 o Collision???
Images of Venus from Russian Venera spacecraft The spacecraft only lasted about an hour due to the extreme temperatures and pressure!
Recent radar map of Venus Surface of Venus seems to be relatively young Volcanic activity resurfaces the planet rapidly Few impact craters, due both to the thick atmosphere and volcanic activity
Venus R ~ 0.95 R E Heat retention ~ Earth s M ~ 0.8 M E Radioactive heating ~ Earth s Hot interior limited plate activity Many hot spots Many volcanoes
Venus Many volcanoes Abundant gas (e.g., CO 2 ) production Escape velocity ~ Earth s Gas retention ability ~ Earth s Thick atmosphere! (100 x Earth s atmospheric pressure!) Very strong greenhouse effect Water rain didn t pick up CO 2 from atmosphere because Venus is too close to the Sun (H 2 O is photodissociated)!