Terrestrial%20Planetary%20Geology:%20Moon,%20Mercury,%20Venus%20

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Terrestrial Planetary GeologyMoon, Mercury,
Venus Mars
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MOON and MERCURY

• Small, rocky and no longer active.
• So similar, its worth studying them together
  (even though ones a moon and ones a planet).

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Key Similarities

• Small masses Moon 0.012 Mercury 0.055
  MEarth Small sizes 0.27 and 0.38
  REarth Therefore, low escape speeds 2.4 and 4.2
  km/s
• Highly cratered surfaces
• No atmospheres (other than temporarily attracted
  solar wind), SO there are 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
  (11 and 32)
• No (Moon) or extremely weak (Mercury) magnetic
  field

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Surfaces of Moon and Mercury
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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 (similar to Earth and Venus).
• Formation Mercury probably similar to Earth,
  Venus and Mars. 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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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 OR the
  Moons day its year 1 month
• 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.

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The Synchronized Moon

• 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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Tidally Locked Mercury

• MERCURY has been TRAPPED into a slightly more
  complicated 32 SPIN--ORBIT RESONANCE due to body
  tides raised by the Sun Pspin 58.6 days
  Porbit 88.0 days 1 Mercury Day (noon-noon)
  2 Mercury Yrs 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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Mercurys 32 Spin-Orbit Resonance
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Mercury Is Hard to See Tight Orbit Means its
w/in 28 deg (lt 2 hr) of Sun
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Determining Planetary Rotation via Radar
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Craters and Surface Dating

• No 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 History
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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.
• US 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.

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Apollo Missions

• APOLLO 11 Landed on 20 July 1969 N.
  Armstrong, B. Aldrin in LUNAR MODULE M.
  Collins in ORBITER.
• Apollos 12, 14, 15, 16 and 17 also landed
  successfully, with the last man on the moon, H.
  Schmitt leaving on 14 December 1972.

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Some More Recent Unmanned Missions

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

Huge Aitken basin 2000 km across 10 km
deep! Near lunar S pole.
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SURFACES AND INTERIORS OF THE DEAD ONES

• MOON
• LUNAR ROCK compositions same ELEMENTS and
  similar types as Earth. Key test that the
  universe is not too weird.
• But, fewer VOLATILE (more easily evaporated, like
  Aluminum) compounds and more REFRACTORY (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.

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Cratered and Lava Covered Lunar Surface
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Lunar Interior

• 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 The
  "moonquakes" are produced at the
  lithosphere/athenosphere boundary.
• Central 300 km is a partially metallic CORE,
  perhaps still partially molten.

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Cutaway View of Lunar Interior
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MERCURY

• Only MARINER 10 took close-up images of Mercury
  -- and these were only good on one side. So our
  knowledge of Mercury was poor until current
  Messenger Mission flybys turning to orbiter.
• Large cliffs (a.k.a. scarps) up to 3 km high, due
  to compression after cooling.
• Caloris basin --- 1400 km across, seismic shocks
  from impact 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 its stronger gravity didn't let ejecta
  fly as far.

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Impact Craters Mercurys Scarps
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Caloris Basin and Weird Terrain Huge Impact and
Focused Seismic Waves
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Mercurys Interior

• Mercury's density is almost as high as the
  Earth's yet its mass is much less. Therefore its
  less compressed by gravity.
• Together these facts imply its CORE is larger
  about 75 of its radius, or 1800 km.

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Magnetic Fields

• The Moon has no large scale magnetic field.
• Mercury's very small MAGNETIC FIELD may be a
  remnant of a DYNAMO when it was FASTER SPINNING
  and MORE LIQUID inside. Tidally locked so
  it spins SLOWLY and since its small the liquid
  solidified quickly.
• BUT BOTH MERCURY AND THE MOON HAVE LOST ALMOST
  ALL THEIR HEAT OF FORMATION AND ARE NOW
  TECTONICALLY DEAD.