The Earth's Sister Planet
Why we study the
- Learn about:
- a typical large satellite
- the Earth
- the early history of the solar system why?
- The Moon's surface:
- unmodified by atmosphere or water erosion
- Reflects history of impacts and early
The Moon: Basic Facts
- The Moon is in synchronous rotation
because of tidal friction.
- On account of librations, 59% (not 50%) of the
Moons surface can be seen from Earth.
- Mass: 1/80 of Earth's; no gases or water
(ice??) - no atmosphere
- Density is 3.3 gm/cm3 , Earth's
- Two types of surfaces: "seas" or
maria (the lowlands) and the heavily
- Most of our knowledge is from the Apollo
program: 1968 - 72:
- From rocks learn about age, composition, history
of impacts, etc
- Little or no seismic activity: a dead
world - no magnetic field.
The Moon: Highlands and Lowlands
- Moons surface has lighter (higher
elevations) and darker (lower elevations).
- The dark areas were caused by extensive
volcanism early in Moons history. Surface is younger
- Galileo noted that the darker areas are
relatively smooth. He called them "seas" or
Maria (plural). (Mare is singular).
- Maria concentrated on side of moon that
faces the Earth. Most of rest of Moon called
- Maria concentrated on lunar near side
(maria shown in blue/violet).
Clementine Mosaic of the
- The lunar surface covered with dust, most
of it is lunar crust that has been pulverized by impacts.
About 10m thick.
- Lighter color.
- Richer in calcium (Ca) and aluminum (Al).
- Saturated with craters.
- An old surface, not altered since
heavy cratering era of planetary formation.
Age of Lunar Surface
- Apollo samples (some 400 kg of rock
returned by six lunar missions) show:
- Age of highlands: ~ 4 billion years
- Oldest highland rock shown is 4.4 billion
- Age of lowlands or maria: ~3.5 billion years
- Impact at speeds up to 73 km s-1
but not more WHY?
- Deep penetration and vaporization
- A rock (R = 1 km) at 73 km s-1
carries 3 x 1022 joules of kinetic
energy that is converted into heat at impact.
- This energy is equivalent to 10 million Megatons
- Circular crater and ejecta
- The 1 km rock produces a complex crater 100 km
diameter with 5 km high walls.
- This is the size of some of the more prominent
lunar craters such as Copernicus
- Ejecta form walls and a surrounding
- "Rebound" produces a central peak.
- Surface underneath is fractured.
- A "glancing blow" can produce a
chain of secondary craters.
Cratering History of the Moon
- Short period for formation: lunar
highlands formed a few 100 Myr after formation of moon
itself, about 4.6 Byr ago.
- Almost all measured rock samples have very narrow
age range: 3.8-4.0 Byr ago. Older rocks have
been completely pulverized
- Unlike Earth, erosion is inefficient at
- Impact craters are swept away by big lava
flows and new ones form.
- Richer in magnesium (Mg) and iron (Fe)
- Younger surfaces.
- Formed after the highlands by flooding of
- Note comparatively little cratering of maria.
Formation of Maria
- Round shape implies that giant impacts
triggered their formation.
- It took several 100 Myr after impact
before lava welled up from below.
- Ages are 3.8-3.1 Byr: younger than
highlands, but formed over long period of time.
- Relative lack of craters implies
younger age than heavily cratered highlands.
- Lunar samples proved that they were from
- Only a few 100 meters thick
- Lava from 100s of kms below surface,
probably due to radioactive heating
- No evidence for any volcanic activity after
formation of maria: the moon froze not
enough radioactive elements so no heat
- Age of Maria probably 3.8 to 3.1 Byr
- Oldest rocks from other areas are 4.6 Byr
Summary of Steps to Form Maria
- Ejection of large amount of material
- Formation of central peak
- Ringed structure
- Flow of lava into the crater through
cracks in the floor
- Heat from radioactivity in interior melts
- No evidence for volcanoes (in contrast to
Earth, Venus, and Mars).
- But: Rilles, domes, and wrinkled
ridges are evidence of past lava flows.
Key Generalization for Entire
- Can get good estimate for age of planetary
surface (or that of a satellite) from number density of
craters and location within solar system:
- high density of craters - old
- low density - young, surface renewal
- how far from sun?
- how close to giant planets?
Differences Between Lunar and
- All lunar rocks are igneous.
- Lunar rocks do not have a trace of water.
- Earth rocks contain up to 3% water.
- Iron in lunar rocks is not oxidized.
- Lunar rocks are depleted in elements with
low boiling points.
Summary Comparison of Surfaces
of Earth and Moon
- Mountains: No mountains like the earth;
all lunar mountains due to impacts
- No evidence for plate tectonics or erosion
Impact cratering - no volcanoes, but much lava.
- Lower gravity means impacts throw out much
- Moon is differentiated, yet
geologically dead, i.e. inactive.
- How do we know differentiated?
- Know mass and surface density
- pre-Apollo - good guess
- post-Apollo - a rock in the hand is worth
two on the moon.
- Smaller, less massive than Earth, so shorter
cooling time after formation; less radioactivity.
- Moon has virtually no atmosphere
- Retention of atmosphere depends on:
- escape velocity from planets surface
- typical speed of particles in atmosphere
- depends on their mass and temperature
- planets most able to retain atmosphere are thus
massive (high escape velocity) and cold (slow
moving molecules in atmosphere
Origin of Moon
- Probable origin: impact of Mars-sized
protoplanet with differentiated Earth.
- Accounts for composition differences.