Chapter 5: The Earth + the Moon – Part 2

Clicker Question: The Greenhouse effect would not occur if: A: The Earth had no atmosphere. B: The amount of carbon dioxide doubled. C: We got rid of all the forests. D: The Earth didn’t have an ocean.

Clicker Question: Sunlight absorbed by the Earth’s surface is reemitted in the form of? A: radio waves B: infrared radiation C: visible radiation D: ultraviolet radiation E: X-ray radiation

Clicker Question: When do the largest high tides occur? A: When the Moon is at first quarter B: When the Moon is full. C: When the Earth is at aphelion in its orbit. D: When the Moon is at 3rd quarter.

Clicker Question: The surface gravity of the moon is 1/6 that of Earth. If Matt weighs 120 lbs on Earth, how much does he weigh standing on the moon? A: 120 lbs B: 60 lbs C: 30 lbs D: 20 lbs E: 10 lbs

Clicker Question: Suppose the Moon was half as dense, but the same size. How much would Matt (120 lbs on Earth) weigh? A: 120 lbs B: 60 lbs C: 30 lbs D: 20 lbs E: 10 lbs

General Features Mass: M Earth = 6 x g Radius: R Earth = 6378 km Density: p = 5.5 g/cm 3 Age: 4.6 billion years

Earth's Internal Structure Crust: thin. Much Si and Al (lots of granite). Two-thirds covered by oceans. How do we know? Earthquakes. See later Mantle is mostly solid, mostly basalt (Fe, Mg, Si). Cracks in mantle allow molten material to rise => volcanoes. Core temperature is 6000 K. Metallic - mostly nickel and iron. Outer core molten, inner core solid. Atmosphere very thin

Earth's Atmosphere 78% Nitrogen 21% Oxygen gas is ionized by solar radiation ozone is O 3, which absorbs solar UV efficiently, thus heating stratosphere commercial jet altitudes temperature on a cool day Original gases disappeared. Atmosphere is mostly due to volcanoes and plants!

Earthquakes They are vibrations in the solid Earth, or seismic waves. Two kinds go through Earth, P-waves ("primary") and S-waves ("secondary"):

How do they measure where Earthquakes are centered? * * * seismic stations

Like all waves, seismic waves bend when they encounter changes in density. If density change is gradual, wave path is curved. S-waves are unable to travel in liquid. Thus, measurement of seismic wave gives info on density of Earth's interior and which layers are solid/molten. But faint P waves seen in shadow zone, refracting off dense inner core Curved paths of P and S waves: density must slowly increase with depth Zone with no S waves: must be a liquid core that stops them No P waves too: they must bend sharply at core boundary

Earth's Interior Structure Average density Crust Mantle Core 5.5 g/cm 3 3 g/cm 3 5 g/cm 3 11 g/cm 3 Density increases with depth => "differentiation" Earth must have been molten once, allowing denser material to sink, as it started to cool and solidify.

Earthquakes and volcanoes are related, and also don't occur at random places. They outline plates. Plates moving at a few cm/year. "Continental drift" or "plate tectonics"

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! Mantle slightly fluid and can support convection. Plates ride on top of convective cells. Lava flows through cell boundaries. Earth loses internal heat this way. Cycles take ~10 8 years. Plates form lithosphere (crust and solid upper mantle). Partially melted, circulating part of mantle is asthenosphere.

Pangaea Theory: 200 million years ago, all the continents were together!

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

The Moon = 3.3 g/cm 3 (Earth 5.5 g/cm 3 ) = 1/6 that of Earth Mass Radius Density Gravity = 7.4 x g = M Earth = 1738 km = 0.27 R Earth

This means: period of orbit = period of spin Why? Tidal Locking Earth The tidal bulge in the solid Moon elongates it slightly (2-3 km) along an axis pointing to Earth. Top view of Moon orbiting Earth If orbit period faster than spin period, tidal bulge would have to move around surface of Moon, creating friction, which slows the Moon ’ s spin down until tidal bulge no longer migrates around. We always see the same face of the Moon.

Far side of the moon

Tides A feature of oceans (but solid material has small tides too). Two high and two low tides per day. Tides are due to Moon's gravitational pull being stronger on side of Earth closest to it (Sun causes smaller tides). Earth-Moon gravity keeps them orbiting each other. But side of Earth closest to Moon has slightly stronger pull to Moon => bulges towards it. Other side has 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 => high and low tides.

Tides

The Lunar Surface - Large, dark featureless areas: "maria" or "seas". - Lighter areas at higher elevation: "highlands". maria highlands - Loads of craters (due mostly to meteorite impacts). No winds to erode them away. - Highlands have 10x the crater density of maria.

Lunar Volcanism (long ago) Evidence: - Maria: result of old, widespread lava flows (filled in largest, early impact craters) - Linear chains of craters (not formed by impacts), probably marks ancient fault, collapsed lava domes - "Rilles": ditches indicating old lava flows Remember: volcanism is a way of losing internal heat

Cratering - Impact speeds several km/sec - "Ejecta blanket" of pulverized rock surrounds crater - Impacts => "regolith": ~20 m thick layer of pulverized rock covering Moon.

Cratering Rates Small meteroids common, large ones rare. So same true for craters: Crater size Occurrence 10 km 1 m every 10 million years every month If no other processes (erosion, lava flows) change the surface, the number of craters in an area tells you the age of the surface.

Moon's History Age: 4.5 billion years 3.9 billion years ago: heaviest meteoritic bombardment ended 3.2 billion years -> present no volcanism, cratering continued at lower rate, geologically dead! billion years ago: volcanism created maria. Maria are just the largest craters, filled in.

Lunar Structure And no atmosphere, so no wind or erosion. Surface reflects geologic history well. (from Apollo seismic data and theoretical arguments) Core and asthenosphere take up small fraction of volume compared to Earth case – the Moon is more rigid

How did the Moon form? We're not quite sure! Three older theories: 1) "Fission": The material that would be the Moon was thrown off the Earth and coalesced into a single body. Problem: Earth not spinning fast enough to eject large amount of material. 3) "Capture": The Moon was a stray body captured into orbit around Earth. Problem: an extremely unlikely event, given Moon's size is a substantial fraction of Earth's. 2) "Coformation": The Moon and Earth formed out of the same material at the beginning of the Solar System. Problem: Moon has different density and composition.

So now, Impact theory preferred: Early in Solar System, when many large planetesimals around, a Mars-sized object hit the forming Earth, ejecting material from the upper mantle which went into orbit around Earth and coalesced to form Moon. Computer simulations suggest this is plausible.

So now, Impact theory simulation:

Moon formation Simulations Video links Asteroid Impact on Moon Formation of Moon – Huge Impact Theoryhttp:// eature=relatedhttp:// eature=related

Earth and the Moon in Bulk Density of surface rocks on moon similar to overall density of 3.3 g/cm 3 This + magnetometer data suggests Moon’s iron core is small – 300 km

Earth and the Moon Moon stabilizes Earth’s rotational axis, maintains 23.5 degree tilt => gives us stable climate, seasons, etc. Earth’s rotational period is slowing down – day is getting longer Moon is moving away from earth => 3.8 cm or 1.5 inches per year

Magnetospheres The magnetosphere is the region around Earth where charged particles from the solar wind are trapped.

Magnetospheres These charged particles are trapped in areas called the Van Allen belts, where they spiral around the magnetic field lines.

Magnetospheres Near the poles, the Van Allen belts intersect the atmosphere. The charged particles can escape; when they do, they create glowing light called an aurora.

Aurora over Iceland – March 18, 2012 NASA: Astronomy Picture of the Day NASA: Astronomy Picture of the Day (

Earth's Atmosphere 78% Nitrogen 21% Oxygen gas is ionized by solar radiation ozone is O 3, which absorbs solar UV efficiently, thus heating stratosphere commercial jet altitudes temperature on a cool day Original gases disappeared. Atmosphere is mostly due to volcanoes and plants!

Ozone Hole over Antarctica Led to banning of Chlorofluorocarbons worldwide Had been used in refrigerators/air conditioners, aerosol cans, and as a cleaning solvent