THE MOON AND MERCURY, Part 1

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BUT FIRST

THE ACTIVE EARTH

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Let's finish our discussion of Seismology first:

Seismographic measurements have located:

  • boundaries between inner/outer core;
  • outer core/mantle;
  • composition changes in mantle (600 km);
  • oil/gas deposits in crust.

    Seismic measurements can even detect excess rotation of
    inner core, upwellings and sideways motions in the mantle.

    MODERN SEISMOLOGY CAN GIVE A 3-D PICTURE, CHANGING IN TIME,
    OF THE EARTH'S INTERIOR!

  • There is a good analogy with 3-D images of people's interiors from
    MRI (Magnetic Resonance Imaging) or CAT (Computer Aided Tomography) scans.

    MAGNETIC FIELDS

    To create a magnetic field one ALWAYS NEEDS a CURRENT of some sort.
    In planets and stars we get this from:

  • MAGNETIC (ELECTRICALLY CONDUCTING) MATERIAL (Fe/Ni for Earth)
  • FLUID (liquid outer core for Earth)
  • ROTATION

    The Earth's field is about 0.5 Gauss, N magnetic pole about 80 deg N;
    S magnetic pole about 60 deg S.

    It extends into space: THE MAGNETOSPHERE, protects us from most
    COSMIC RAYS and SOLAR WIND.

  • Charged particles trapped in the Van Allen belts; when they penetrate
    the atmosphere, yielding the AURORA BOREALIS & AURORA AUSTRALIS
    (recombination lines of Nitrogen and Oxygen)
  • Solar storms: disrupt communications, power grids.

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    TECTONIC ACTIVITY

    The Earth's CRUST is divided into PLATES.

    CONTINENTAL DRIFT: Africa and S. America had similar shapes and rock formations.

    CONVECTION in the MANTLE drags the plates.

    SEAFLOOR SPREADING at OCEANIC RIDGES: where new crust solidifies over hot-spots.

  • REVERSALS in Magnetic Polarity are frozen into newly solidified rocks;
    this allows dating of rocks and plotting of drift back some 500 Myr.

    Where plates COLLIDE:

  • Earthquakes; either stick, then slip (TRANSFORM FAULTS);
    or Oceanic is SUBDUCTED under Continental (melts and rejoins the mantle).
  • MOUNTAINS built from folding in COLLISIONS.
  • VOLCANOS most likely at plate boundaries too.
  • Maps of the current plate positions and motions and likely future positions were shown,
    as were illustrations of where most volcanoes and earthquakes are seen, as were sketches of how
    subduction, sea floor spreading and island chain production via a hot spot (Hawai'i).
    See Figs. 7.9 -- 7.16 for similar ones.

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    SMALL DEAD `PLANETS': MOON and MERCURY

    Key similarities:

  • Small masses: 0.012 and 0.055 M_earth
  • Small sizes: 0.27 and 0.38 R_earth
  • Therefore, low escape speeds: 2.4 and 4.2 km/s
  • Highly cratered surfaces
  • No atmospheres (other than temporarily attracted solar wind)
  • Big temperature swings (100--400 K for Moon,
    100--700 K for Mercury, the largest variation of all planets)
  • Rotation and revolution locked in RESONANCES (1:1 and 3:2)
  • No (Moon) or extremely weak (Mercury) magnetic field

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    Key differences:

  • Density: 3.3 g cm^-3 for Moon (lower than any terrestrial planet)
    vs. 5.4 g cm^-3 for Mercury (very similar to Earth and Venus).
  • Formation scenario: Mercury probably condensed in a fashion similar to Earth,
    Venus and Mars. The moon was probably formed by a collision of a large planetesimal
    (maybe Mars size) with the proto-earth after much of the metals had differentiated and sunk.
  • Somewhat higher crater density on the Moon
    more impacts further out in Solar System.
  • Maria --- dark lava flows --- clearly seen on the Moon;
    Mercury has intercrater plains that are not clearly volcanic in origin,
    but it probably did have extensive volcanism early in its history.

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    ATMOSPHERIC COMPOSITIONS

    The structure and composition of an atmosphere is determined by a
    battle between pressure gradients and gravity.

    This is true for ALL PLANETS and STARS (and for their interiors too).

    A stable situation requires HYDROSTATIC EQUILIBRIUM: the difference between
    pressures pushing out and in are exactly balanced by gravity.

    At a given temperature, a molecule will have an average random velocity of:
    v_thermal = (3 k T / m_{molecule})^{1/2}
    where k = 1.38 x 10^-23 J K^-1 is Boltzmann's constant.

    Example: For the earth, T_atm = 270 K,
    m_N_2 = 2 x 14 x (1.67 x 10^-27 kg) = 4.68 x 10^-26 kg
    So, v_th,N_2 = 0.488 km/s
    And, v_th,H_2 = v_th,N_2 (m_N_2/m_H_2)^1/2 =
    = 0.488 km/s (28/2)^1/2 = 1.83 km/s

    But, there is a wide MAXWELL--BOLTZMANN DISTRIBUTION
    in velocities at a given T so a particular type of molecule will
    EVENTUALLY ESCAPE if

    v_thermal > 0.16 v_esc = 0.16 (2 G M / R)^{1/2}

    Example: for earth, v_esc = 11.2 km/s so N_2 stays, H_2 leaves.

  • for Mercury, T_max = 700 K so
    v_th,N_2,Merc = (700 K/270 K)^1/2 v_th,N_2,earth = 0.786 km/s
    But v_esc,Merc = 4.2 km/s < 6 x 0.79 km/s
    so even N_2 leaves Mercury

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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.