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. 
2. 
3. 
4. 
Capture Theory
Twin Formation Theory
Fission Theory
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 (23.5o)
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:
–  Tday = 800 F / 700 K
–  Tnight = -280 F / 100 K
Mercury
•  R ~ 0.4 RE
•  M ~ 0.06 ME
→ Heat loss more
efficient than
Earth’s
→ Less radioactive
heating than Earth
•  Cooler interior
→ No plate activity
→ few volcanoes in
the past
Mercury
•  few volcanoes
•  Low escape velocity
→ Virtually no gas
production
→ 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% CO2
– Clouds of sulfuric acid!
– Tsurface = 900 F / 750 K
(hot enough to melt lead!)
•  Slow & retrograde spin
– 243 Earth days
– tilt of spin axis = 177o
– 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 RE
•  M ~ 0.8 ME
→ Heat retention
~ Earth’s
→ Radioactive heating
~ Earth’s
•  Hot interior
→ limited plate activity
→ Many hot spots
→ Many volcanoes
Venus
•  Many volcanoes
→ Abundant gas
(e.g., CO2)
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 CO2 from
atmosphere because Venus is too close
to the Sun (H2O is photodissociated)!