THE MOON and MERCURY, Part 2

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SPIN--ORBIT RESONANCES or TIDAL LOCKING

THE MOON

The BODY TIDES the EARTH raises in the MOON have forced it into
SYNCHRONOUS ROTATION:

  • Its rotational period = its orbital period
  • We see only one face of the Moon (approximately)
  • The Moon's bulge is larger than that the Earth could currently induce:
    it probably `froze in' billions of years ago when the Moon was only ~250,000 km away
    and just solidifying.
  • There is a wobbling, mostly due to the Sun's differential force, called
    LIBRATION, which means that over the entire year we see 59% of the Moon's surface.
  • Most moons in the solar system are similarly TIDALLY LOCKED to their planets.
  • Pluto and Charon are mutually SYNCHRONIZED.

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    MERCURY

  • MERCURY has been TRAPPED into a slightly more complicated
    3:2 SPIN--ORBIT RESONANCE due to body tides raised by the Sun
  • $P_{spin}$ = 58.6 days; $P_{orbit}$ = 88.0 days;
  • One Mercury Day (noon to noon) = 2 Mercury Years = 176.0 days
  • Mercury's highly eccentric orbit means that it can never be synchronized
    at all times, but tidal forces are strongest near PERIHELION and the 3 spins = 2 orbits
    closely correspond to synchronicity at perihelion.

    Similar RESONANCES explain the gaps in Saturn's rings, and gaps
    in the ASTEROID BELT.

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    PLANETARY AGING

  • Both the Moon and Mercury were once tectonically active, but no longer:
    they are charter members of the DEAD PLANETS SOCIETY.
  • Heavy cratering during the first 500 Myr or so after crusts cooled.

    Plenty of heat of formation plus radioactivity meant early volcanism.

  • Heat loss is proportional to planet's area.
    L ~ 4 pi R^2 sigma T^4
  • Heat stored is proportional to planet's volume.
    E ~ (4/3) pi R^3
  • Therefore 'lifetime' proportional to planet's radius.
    t ~ (E/L) ~ R^3/R^2 ~ R

    The lack of atmosphere means that the Moon and Mercury are continually
    bombarded with meteoroids of all sizes.

    The most heavily cratered areas are the oldest parts of the surface.
    Less heavily cratered areas (MARIA on Moon) solidified later.

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    LUNAR MISSIONS

    RACE TO THE MOON: more political than scientific.
    JFK pledged to get there by the end of the 60's.

  • The Soviet LUNA's were the first to pass, crash-land and
    photograph the backside in 1959.
  • Unmanned RANGER missions -- crash landed in 1963/64.
  • LUNAR ORBITERS sent detailed photos: 1966/67.
  • SURVEYORs made soft landings and didn't sink into a REGOLITH
    (pulverized surface layers) in '66--'68
  • Manned MERCURY and GEMINI series. Debate on how to best get
    people there and BACK.
  • APOLLO 11: Landed on 20 July 1969; Armstrong, Aldrin in LUNAR MODULE;
    Collins in ORBITER.
  • Apollos 12, 14, 15, 16 and 17 also landed with the last man on the moon,
    Schmitt on 14 December 1972.

    Recent unmanned missions:

  • CLEMENTINE (1994) sent back wonderfully detailed photos in 11 bands, from
    IR through UV. It also had extremely accurate altimeters.
  • LUNAR PROSPECTOR (1998) probed layers with radars and confirmed the
    existence of ICE CRYSTALS mixed with REGOLITH in craters in the
    POLAR REGIONS where they are permanently shaded from sunlight.

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    SURFACE AND INTERIOR OF THE DEAD ONES

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    MOON

    LUNAR ROCK compositions: same ELEMENTS and simlar types.

  • An important test that the universe is not too weird.

    But, fewer VOLATILE (more easily evaporated, like Aluminum) compounds
    and relatively more REFRACTORY (very hard to evaporate, like Titanium)
    ones are found on the Moon.

  • MARIA are LAVA FLOWS, mainly basaltic.
  • TERRAE or highlands comprise most of the CRUST.

    SEISMIC monitors revealed minute moonquakes (or moonquivers)

  • CRUST (60 km thick on Earth side, 150 km on back -- fewer maria on the rear)
  • RIGID LITHOSPHERE --- extends another 900 km or so.
  • SOFT ATHENOSPHERE --- inner 700 km or so.
    The "moonquakes" are produced at the lithosphere/athenosphere boundary.
  • Central 200 km is a partially metallic CORE, perhaps still partially molten.

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    MERCURY

    Only MARINER 10 took close-up images of Mercury -- and these are
    only good on one side.
    So our knowledge of Mercury remains poor.

  • From earth it is usually within 17 degrees of the Sun,
    rarely up to to 28 degrees, so it's never seen for more than
    two hours after sunset or before sunrise.

    Large scarps (up to 3 km high), due to compression after cooling.

    Caloris basin --- 1400 km across, seismic shocks produced
    "weird terrain" on the opposite side of the planet.

    Many fewer MARIA than on the Moon: thicker crust.

    Its craters are typically smaller than those on the Moon since
    its stronger gravity didn't let ejecta fly as far.

    Mercury's density is almost as high as the Earth's yet its
    mass is much less. Therefore it is less compressed by gravity.

  • Together these facts imply its CORE is larger:
    about 75% of its radius, or 1800 km.

    Mercury's very small MAGNETIC FIELD may be a remnant of a
    DYNAMO when it was FASTER SPINNING and MORE LIQUID inside.

    BUT BOTH MERCURY AND THE MOON HAVE LOST ALMOST ALL THEIR
    HEAT OF FORMATION AND ARE NOW TECTONICALLY DEAD.