Astronomy Notes

Part 4: Earth and Moon

John P. Pratt

The Earth's Age

The earth appears to be 4.6 billion years old.
Radioactive measurement support estimates made from assumed rates of sedimentation.
Radioactive measurements are based on several assumptions and are not always reliable.
In any case, the earth appears to be very old, just from looking at the rock strata.

The Earth's internal structure

The earth's gravitational force on the moon shows it has a heavy core, twice as heavy as rock.
We think the core is iron-nickel because that is right mass and such meteorites fall from the sky.
Seismic waves from earthquakes show that the outer core is liquid, the inner is solid.
The thick rock outer half of the earth is called the mantle.
The asthenosphere is the partially melted layer just under the crust.
The crust, or lithosphere, is very thin (a few km) and floats on top of the asthenosphere.

The Earth's interior is like a smelter.

Plate Tectonics

The continents are on large plates that float on the asthenosphere.
As the asthenosphere moves, it drags the plates with it, causing earthquakes.
The mid-Atlantic ridge is spreading, causing the U.S. to move westward, over the San Andreas fault in California.

Volcanoes

Underground molten rock is called magma, when it reaches the surface it is called lava.
Entire mountains are volcanic.
Many planets have huge volcanic areas.

The Earth's Magnetic Field

The earth is like a huge, but weak, magnet which causes compasses to point approximately north.
The earth's core is belived to be too hot to be a magnet.
The earth's magnetism is thought to come from currents in the liquid nickel-iron core.

The Earth's Atmosphere

The earth's early atmosphere is thought to have been steam (H₂O) and carbon dioxide (CO₂).
The steam condensed into the oceans and the CO₂ became limestone rocks.
The atmosphere's 23% oxygen content apparently comes entirely from plants.

Coriolis Drift

Because the earth is rotating, anything moving in the northern hemisphere tend to drift right.
That causes low pressure areas to circle counter-clockwise, high pressure counter-clockwise.

Impacts on the Earth

Until recently, geologists ignored most catastropic events in earth history
Huge meteorites have made large craters, like the Winslow crater in Arizona
It is now believed that the extinction of the dinosaurs were caused by such an impact.
After geologists witnessed a comet impact Jupiter, they have found many more craters on earth.

Physical Characteristics

The moon is about 1/4 the radius of the earth.
The moon's density is about half that of the earth, like one big rock (no iron core).
The moon had no atmosphere, so stars are visible in the daytime.
There is much less earthquake activity than on earth.

Surface Features

The moon's surface has thousands of huge impact craters.
The side facing the earth also has huge plains called mares ("seas") covered with dark lava flows.
Bright rays extend out from large craters.
It is believed that the craters were mostly formed early in the formation of the solar system.

Tidal Forces

Tidal forces are caused by gravity pulling harder on the near side of a body than the far side.
Tidal forces have caused the moon to be in a synchronous orbit, so it always faces the earth.
The tides on earth are caused by these tidal forces, with the moon pulling harder on near side.
The reaction of the tides on the moon is to accelerate it into a slightly more distant orbit.
The moon also slows the earth's rotation very slightly.
Roche's limit is the distance from a planet at which a small body will be torn apart by tidal forces.

The Origin of the Moon

It is still a mystery where the moon came from.
Its composition is without iron, but much like the mantle rocks of the earth.
The best theory is the unlikely giant impact hypothesis, that something broke it from the earth's mantle.