Ch 6: Earth and Moon

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Ch 6 Earth and Moon
Summary Constellation Scorpius Tides,
Precession Earth as a Planet (lab 5) Parameters
of a planet How to measure density of a planet
Small Angle Approximation (Done again for
stars and in Lab1) Interior composition Seismology
Convection Motion of Crust Moon Highlands and
Maria Craters Age Origin
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Earth, Moon and Terrestrial Planets
Four Terrestrial Planets Mercury, Venus, Earth,
Mars Interiors hot, denser elements, layered,
often liquid Surfaces very thin, rocky, lower
density elements Atmospheres Comparative
planetology the similarities and
differences Origin - Evolution Tides
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Constellation Scorpius
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Scorpius short and long exposure
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When are Tides High?
There are 2 high and 2 low tides every day. Moon
makes 2/3 of the tides on the Earth with the sun
the remaining 1/3. The highest tides happen when
sun-Earth and moon are in a line new and full
moon. Moon orbits Earth in 29.5 days (relative
to sun, since we use solar time). Each day, the
moon travels 1/29.5 of its orbit 12 degrees to
the east. Earth rotates 12 degrees in 50 minutes.
Hence tides are 50 minutes later each day.
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Tides come from Gravity
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Tides are caused by the Difference in Gravity
Tides are caused by the difference in the pull of
gravity across a body. The difference decreases
as 1/r3. Tides are largest for close (small r)
objects. Tides are important for Mercurys
orbit, Moons spin, Earths spin, they heat
Jupiters moon Io.
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Precession of the Earths Pole
Because of the pulling of the Moon and Sun, the
Earths pole does not remain pointing near
Polaris.
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The Earth as a Planet
Basic Properties size, mass, density,
composition, age Major Parts Interior (most
of the mass) Surface Atmosphere
Magnetosphere Formation Evolution
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How to find the Mass
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How to find the Radius
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The Small Angle Approximation
If know distance L and angle A, can find distance
D.
When angles A are small, which is nearly always
true for astronomically distant objects, then we
can ignore trigonometry, giving a simpler
equation for distance D in terms of L and A
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Radians give a Simpler Equation
Radians simplify the equation 2p radians 360
degrees 1 radian 360 degrees/2p
57.3deg 2pA/360 a angle A in radians. The
equation becomes L a D. In the First Virtual
Lab this is written Sra, where SL, rD and aa.
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How to find the Mean Density
We find that the mean (bulk) density of the Earth
is much higher than that of surface rocks. It
must have something denser than rock inside.
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Four parts of the Earths Interior
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The Earths Interior dense, hot and fluid (Lab 5)
Seismology. The arrival time of sound waves
generated by earthquakes gives the mean sound
speed and density for various paths through the
Earth. Lack of S-waves on other side of Earth
shows liquid outer core.
solid inner core liquid outer core plastic
mantle crust Differentiation the densest
elements (Fe, Ni) sank to the core, while lighter
floated.
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Earths Composition is not that of the surface.
Continental Crust is 0-40km thick (lt egg
shell) 0.4 of Earths mass atypical in
composition elements in red prefer
mantle core.
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Astro Quiz
Which 4 elements make up 94 of the Earths
mass? Which elements prefer the crust?
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How was Earths Interior Heated?
The heat was generated by collision of
planetisimals differentiation Fe (He on
Saturn) sinking to form core in lt100Myr decay
of radioactive elements Potassium, Uranium and
Thorium
The radioactive decay continues to heat the
interior of rocky planets today.
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Heat flows from Interior to Surface by Convection
convection moving hot rock carries heat out
from center. Hot rock rises to near the
surface. When near surface move
horizontally. Surface rocks cool by emitting IR
radiation (they radiate) into space. Cool rock
is denser and sinks. Motion causes continents to
move.
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Convection moves Plates
Red lines show plates regions of surface that
move together. Check Japan there is a question
in Lab 5 on this.
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Creation and Destruction of Crust
Continental crust is less dense and hence
floats on top of oceanic crust. The oceanic
crust is subducted (goes under.).
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Carbon Dioxide is a critical factor
The carbonate-silicate cycle negative feedback
Without subduction and water, the atmosphere and
climate on the Earth would have been very
different. Life also plays an important role.
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Moon
Heavily cratered, light colored mountainous
highlands. Darker, younger smoother maria.
Composition similar to Earths mantle, with
little or no iron core.
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The Highlands
Craters of all sizes. Overlapping. Look best
when sun is low - long shadows exaggerate
elevations. Standing in these craters you would
not see their rims. Large craters have central
mounds and slumped walls
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Maria
Maria are flat, lava flooded, younger, lacking in
largest craters. We also see lava flooded
craters.
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Craters on the Moon
almost impact craters not volcanoes. Most
craters, especially the rare large ones, formed
4B yr ago, in the first 500Myr after moon formed.
The whole solar system shows similar heavy
early bombardment.
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Ages from Radioactive Half-lives
We know the initial composition of elements and
isotopes. Composition changes as nuclei naturally
decay. Half-life time it takes for half of it
to change. Get age from graph below, from amount
of original left.
Start
1 half-life
2 half-lives
3 half-lives
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The Formation of the Moon A Mars-sized Impact
Protoplanet
Earth