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Astronomy Pic of the Day

Solar System Formation, Quick Review Something perturbs an interstellar gas cloud causing it to begin to rotate and contract due to gravitational forces. As the nebula gets smaller is rotates faster (conservation of angular momentum) and therefore must flatten into a disk shape (due to centrifugal force) along its axis of rotation. It also heats up! Star forms at dense center of Nebula. Accretion of dust particles and atoms occurs – forming planetesimals. Planetesimals accreate and form orbits within plane of disk. Planetesimals which are not accreated into the 8 protoplanets are either trapped in the Asteroid Belt or ejected into either the Kuiper Belt (probably by Neptune) or Oort Cloud (probably by Jupiter and/or Saturn). Protoplanets finish accreting nearby material and settle into their current orbits becoming the Planets as we now know them. Finally an intense period of Solar Wind activity follows which expels the remaining gas from the original Nebula from the Solar System.

But why the differentiation of the Solar system?? Temperature! In the hot central regions, only metals could condense out of the gaseous state to form grains. At greater distances from the central protosun, the temperature was lower, so rocky and icy grains could also form.

The Earth From Apollo 17 on Dec. 7, 1972

General Features Mass: M Earth = 6 x kg Radius: R Earth = 6,378 km Density:  = 5,500 kg/m 3 Age: 4.6 billion years Liquid Water & Atmosphere!

Question Why does the Earth have an atmosphere, while the moon does not? A) The Moon is too close to the Earth to have its own atmosphere B) The Moon is not massive enough C) The Moon rotates too quickly on its axis

Escape Speed α √(mass/radius)‏ Earth: 11 km/s Moon: 2.4 km/s Explains why moon has no atmosphere.

Convection (occurs in Troposphere – the part of atmosphere closest to Earth's surface)‏ Convection also occurs when you boil water, or soup. Think of Earth's surface as a boiling pot. Responsible for all weather & contributes to atmospheric heating Heat competes with gravity to keep the atmosphere buoyant!

Question The Earth should reradiate all of the energy received from the Sun, giving the Earth a surface temperature of 250 K = -23 o C. Why is the average temperature of Earth so much (about 40 o C) higher? A) Gravity B) Greenhouse effect C) Tilt of Earth on its axis

The Greenhouse Effect Main greenhouse gases are H 2 O and CO 2. If no greenhouse effect, surface would be 40 o C cooler!

Questions What are the major layers in the interior of the Earth? How does density change with depth? What about temperature?

Earth's Internal Structure Mantle is semi-solid rock. Cracks allow material to rise => volcanoes. Core temperature is 6000 K. Outer - Molten. Inner - solid. Metallic How do we obtain information about the structure of the Earth's interior?

Like all waves, seismic waves bend or refract. S-waves are unable to travel in liquid. Measurement of seismic wave gives info on density of Earth's interior and which layers are solid/molten.

Earth's Interior Structure Average density Crust Mantle Core 5,500 kg/m 3 3,000 kg/m 3 5,000 kg/m 3 11,000 kg/m 3 Density increases with depth => "differentiation" ‏ Earth must have been molten once, allowing denser material to sink, as it started to cool and solidify. Heating due to (i) bombardment and (ii) radioactivity Bombardment by what? What important result do we see today?

Plates moving at a few cm/year. "Continental drift" or "plate tectonics" Activity on the Earth's Surface

Question What types of geological features or processes would you expect near plate boundaries? What is the driving force behind the motion of the plates?

When plates meet... 1) Head-on collision (Himalayas)‏ 2) "Subduction zone" (one slides under the other)‏ (Andes)‏ 3) "Rift zone" (two plates moving apart)‏ (Mid-Atlantic Ridge)‏ 4) They may just slide past each other (San Andreas Fault)‏ side view top view => mountain ranges, trenches, earthquakes, volcanoes

What causes the drift? Convection! Plates ride on top of convective cells. One cycle takes millions of years => heat loss is extremely slow.

200 million years ago, all the continents were together:

The Moon = 3,300 kg/m 3 (Earth 5,500 kg/m 3 )‏ = 1/6 that of Earth Mass Radius Density Gravity = M Earth  = 0.27 R Earth

Lunar Structure Moon composed of material very similar to Earth's mantle!

The Lunar Surface ● Dark areas: "maria" More recent lava flows. ● Lighter areas at higher elevation: "highlands". ● Many craters (due to meteorite impacts). Only important source of erosion! ● Highlands have 10x the crater density of maria => Highlands are older! maria highlands

Humans First Visit the Moon - Apollo 11 Mission At 10:56 p.m. eastern daylight time on Sunday, July 20, 1969, Neil Armstrong set his left foot on the moon and spoke the words that were to take their place among the great phrases of history: "That's one small step for a man, one giant leap for mankind." Video‏

How did the Moon form? We're not quite sure! Two older theories: 1) "Coformation": The Moon and Earth formed out of the same material at the beginning of the Solar System. Problem: Moon has different density and overall composition... 2) "Capture": The Moon was a stray body captured into orbit around Earth. Problem: an extremely unlikely event & parts of compositions are similar...

So now, Impact theory preferred: Early in Solar System, a Mars-sized object hit the forming Earth, ejecting material from the mantle which coalesced to form Moon. Computer simulations suggest this is plausible.

Tides Tides are due to Moon's gravitational pull being stronger on side of Earth closest to it (Sun also influences tides). Side of Earth closest to Moon feels slightly stronger pull => bulges towards it. Other side feels weaker pull => bulges away compared to rest of Earth. The Earth spins once a day while the bulge always points towards and away from the Moon => two high and two low tides each day.

Tides (cont.)‏