Chapter 8 The Moon and Mercury

  • Soviets had first contact with Moon:

    • First spacecraft to fly past Moon: January 1959

    • First spacecraft to (crash) land on Moon: September 1959

    • First pictures of far side of Moon: October 1959

    • The United States is (so far) the only country to send people to the Moon

    • First person on Moon: July 1969

    • Last person on Moon: December 1972

8.1 Orbital Properties

  • Distance between Earth and Moon has been measured to accuracy of a few centimeters using lasers

  • Viewed from Earth, Mercury is never far from the Sun

8.3 Surface Features on the Moon and Mercury

  • Moon has large dark flat areas, due to lava flow, called maria (early observers thought they were oceans)

  • Moon also has many craters ( from meteorite impacts)

  • Far side of Moon has some craters but no maria

  • Mercury can not be imaged well from Earth; best pictures are from Mariner 10

  • Cratering on Mercury is similar to that on Moon

8.4 Rotation Rates

  • Moon is tidally locked to Earth— its rotation rate is the same as the time it takes to make one revolution, so the same side of the Moon always faces Earth

  • Mercury was long thought to be tidally locked to the Sun; measurements in 1965 showed this to be false

  • Rather, Mercury’s day and year are in a 3:2 resonance; Mercury rotates three times while going around the Sun twice

More Precisely 8-1: Why Air Sticks Around

  • Air molecules have high speeds due to thermal motion. If the average molecular speed is well below the escape velocity, few molecules will escape.

  • Escape becomes more probable

    • For lighter molecules ( higher speeds for same kinetic energy)

    • At higher temperatures

    • For smaller planets (escape speed is less)

  • Molecules in a gas have a range of speeds; the fastest (and those that are headed in the right direction) will escape

  • Both Mercury and Moon have no atmosphere. So both have large day-night temperature excursions

8.7 Interiors

  • Moon’s density is relatively low, and it has no magnetic field— can not have sizable iron/nickel core

  • Mercury is much denser than the Moon and has a weak magnetic field— not well understood!

8.8 The Origin of the Moon

  • Current theory of Moon’s origin: Glancing impact of Mars-sized body on the still-liquid Earth caused enough material, mostly from the mantle, to be ejected to form the Moon