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Why?
• Since Astronomers find it difficult or impossible to visit most astronomical objects, nearby objects are examined and comparisons/extrapolations made.
Vital Statistics
• 4.5 billion years old,
• 8,000 miles in diameter,
• 6 X 1024 kg (6 million million million million kilograms),
• Orbits the sun in 365.24 days,
• Turns on its axis once every 24 hours.
Crossection:
• Thin (5-30 mile) crust
• 1800 miles of mantle (made of magma, similar to lava)
• Outer core of liquid iron
• Inner core of solid iron.
Interior examined:
• Earthquake waves make good acoustic ‘x-ray’ to probe the layers,
• Density calculations indicate heavy core,
• Magnetic fields denote iron.
Hot as Hell!
• Due to radioactive decay,
• High pressure (lots of weight squeezing in)
• Residual heat from formation, impacts
Heat Escapes
• Heat escapes from the core via convection rolls in the mantle.
• This heat causes eruptions through the crust of volcanoes and rifts.
• This energy drives Plate Tectonics
Continental Drift
• NOT floating on water like lily pads!
• Seven large plates carry the continents (not just landmass) around on the crust, skidding on the mantle.
• Where plates meet (plate boundaries) earthquakes, subduction, volcanoes.
Why Us?
• Because of the surface area to volume ratio of a large rocky planet, its internal heat takes billions of years to escape.
• Compare this with smaller rocky worlds!
Atmosphere
• Any volume of gases that surrounds a planet is called an atmosphere.
• Our mixture of O2 and N2 is called “air”.
• In an atmosphere, weather develops due to differential heating.
• Weather changes the surface of the planet as much as plate tectonics.
Look Out Below!
• Another major producer of surface features is asteroidal/cometary impact.
• This causes small and large craters, and in extreme cases, mass extinctions.
• Evidence of cratering is erased by weather, but the earth has been hit about 20 times as much as the moon.
Robert Duval or Bruce Willis?
• Neither!
• We cannot protect ourselves from impacts unless we know many years in advance.
Vital Statistics
• ¼ the size of Earth;
• 1/80 the mass of Earth;
• 1/6 the gravitational pull of Earth;
• About 28 days to orbit the Earth.• Always keeps one side towards us, therefore its
“day” is 28 days long.• No permanent “dark side of the moon”;• Bone dry, no atmosphere.
Formation and Age
• Almost as old as the Earth.• Formed by two impacts about 4 billion years ago.• An object about ½ the size of Earth hit our
planet, scattering debris from Earth and the impactor that gradually condensed into our moon.
• We know this because of the ratio of isotopes we found in the lunar soil WHEN WE VISITED THERE IN 1969-1973!
Tides
• Caused by differential gravity, the different gravitation pulls from the Moon on opposite sides of the Earth.
• The side that faces the Moon experiences the greatest gravitational force that causes the oceans to bulge.
• The Earth’s center feels a lesser pull that drags it away from the far side, leaving a bulge of water.
Highs and Lows
• As the World Turns [sic] various landmasses move into these bulges and experience high tide.
• Six hours (1/4 turn) later, these same landmasses are out of a bulge and experience low tide.
Spring and Neap Tides
• When the Sun and Moon are lin a line with the Earth, the Solar Tides add to the Lunar Tides and the variation of high and low is great: Spring Tides (no seasonal meaning).
• One week later, the Sun and Moon at at right angles with the Earth, and tidal variation is small: Neap Tides.
• Solar Tides are about 1/3 Lunar Tides.
Near and Far
• The prefix “peri” means near; the prefix “ap” means far.
• Therefore perigee means nearest the Earth in orbit, and aphelion means farthest from the Sun in orbit.
• So, tides are extra big at perigee for the Moon and perihelion for the Earth.