The Moon
A Scorched and Battered World

April 4, 2011, jeb

Outline THE MOON General Properties The Moon's Orbit around the Earth Moon's Gravity on its Surface The Moon's Surface Features and Character The Moon's Internal Features Origin of the Moon Evolutionary History of the Moon Phases of the Moon Lunar eclipses Other Sources on the Moon

The Moon - General Properties

• Radius = 1738 kilometers (about 1/4 of Earth)
• Mass = 7.4 x 10²² kilograms (about 1/80 of Earth)
• Average density = 3340 kg/m³
  • => fewer heavy metals (like iron) than Earth
• Atmosphere (None)
  • => very weak gravity
  • => low escape speed (2.4 km/s, while earth is 11 km/s)
• Temperature
  • 400K (daytime)
  • 100K (nightime)
• Earth-moon distance = 384,000 km = 1.28 light-seconds

The Moon's Orbit around the Earth

• Orbits Earth in 27.3 days
• Rotates synchronously with orbit (same side always faces Earth)
  • From the Earth, we see the same side of the Moon all the time
  • From the Moon, the Earth appears to hang fixed in one place in the sky, slowly rotating on its axis
• Elongated shape of the Moon results in the synchronous orbit
  • Presumably, the Moon's elongated shape was created long ago by the Earth's tidal influence when the Moon was young, and the moon is now "tidally locked"

Moon's Gravity on its Surface

So the force pulling on you (suppose you are M₂), on the surface of the Moon, would be proportional to M₁ / R², or the mass of the Moon divided by the distance to the center of the Moon, squared.
Since the Moon has about 1/80 of the mass of the Earth, and it has about 1/4 of the radius of the Earth, gravity on the surface would appear about (1/80)/(1/4)² = 1/5 as great as on the Earth. (a more accurate estimate comes out to 1/6.)

The Moon's Surface Features and Character

• Maria - large flat areas resulting from lava flows
  • Fourteen (14) named maria, the largest being Mare Imbrium (1100 km diameter)
  • more iron than other parts of surface
  • greater density (3300 kg/m³)
  • material originated from mantle
• Highlands - lighter appearing, elevated areas
  • rich in aluminum
  • somewhat lower density (2900 kg/m³)
  • material originated in crust
• Craters - very old
  • Image from Apollo 8 orbiter shows craters as small as 500 m across
    • Craters range in size from 50 km to 500 m
    • diameter about 10 times the size of the incoming meteoroid and depth twice its size.
  • mostly from meteoritic impact - very old
  • Orientale Basin
    • large lunar crater with concentric rings of cliffs
  • Copernicus and secondary cratering of Eddington and Reinhold
  • Steady "rain" of micrometeoroids falls on Moon's surface
    • Beads formed during explosive impacts show subsequent miniature craters
• Lunar Dust
  • about 20 meters deep
  • pulverized by meteoroids
  • but erosion is slow -
    • about 5 m per billion years, 1/10,000 of rate on Earth due to Earth's weather
    • Barringer Crater in Arizona, only 25,000 years old, already shows erosion
    • Barringer crater would likely survive on Moon for several billion years
• Lunar Ice
  • Two 1990's lunar missions (Clementine and Prospector) found evidence for ice at the Moon's poles
  • 2009 Lunar CRater Observation and Sensing Satellite (LCROSS) measured water
    • 1 part in 100,000 - less than desert sand on Earth -but NOT ZERO!
• Volcanism
  • explains some craters
    • some lava found in maria
      • Hadely Rille
    • Crater-chain probably due to cracking of surface
  • but volcanism ended early in the Moon's history
• Far side surprise
  • no major maria => due to asymmetric crust
  • almost entirely highlands
• Moon's surface is frozen in time: very little surface erosion
  • no wind or running water, to cause erosion
  • no plates on surface to cause volcanism or seismic activity
  • some erosion from micrometeroids
    • cumulative effect of micrometeroids => dust

The Moon's Internal Features

• Moon's average density (3340 kg/m³) is similar to surface rock
  • No dense core
• No magnetic field => iron deficiency
• Cooler (1500K) interior than Earth
• Crust is thicker on the lunar far side
  • resulted from tidal forces
  • and explains absence of maria on far side

Origin of the Moon

• Likely origin from collision of large object (Mars-sized) with young Earth
  • Moon is similar to Earth's mantle

Evolutionary History of the Moon

• Moon formed about 4.6 billion years ago
  • Oldest rocks discovered on Moon about 4.4 billions years old
• During first 0.5 billion years, surface was heated and remelted by frequent meteoritic bombardment
• Then, volcanism filled basins with lava, which became the maria
  • This phase lasted from 3.9 to 3.2 billion years ago
• Last 3 billion years has seen heavy erosion of the surface from constant meteoritic bombardment

Phases of the Moon

The Moon's cycle of phases takes a little over 29 days to complete
although, one sidereal month is 27.3 days

Lunar eclipses

Earth's shadow sweeps across the Moon, temporarily blocking the Sun's light and darkening the Moon in a lunar eclipse

When the Moon passes directly in front of the Sun, daylight is briefly turned into night in a solar eclipse

Other Sources on the Moon

StarDate Guide to the Moon
Views of the Solar System - the Moon
The Nine Planets - the Moon