Download The Earth - Indiana University Astronomy

A100
Oct. 6
The Earth
READ Chapter 5 – The Earth
Homework 4 Due Friday
Quiz on Friday
Today’s APOD
The Sun Today
Looking outward to the blackness of space,
sprinkled with the glory of a universe of lights, I
saw majesty – but no welcome. Below was a
welcoming planet. There, contained in the thin,
moving, incredibly fragile shell of the biosphere
is everything that is dear to you, all the human
drama and comedy. That’s where life is; that’s
where all the good stuff is.
Loren Acton (1936 – )
U.S. astronaut
The Earth
from Space
 Understanding Earth helps us understand other planets
 Understanding other planets helps us understand the
Earth
 How has Earth has changed over time
 Why does it change?
 How does it differ from other planets?
Size and Shape of the Earth
 The Earth is a huge, rocky
sphere spinning in space
and moving around the Sun
at a speed of about 100
miles every few seconds
 Earth also has a blanket of
air and a magnetic field
that protects the surface
from the hazards of
interplanetary space
Earth Data
 Distance from Sun
 1 AU = 1.5 x 108 km
 Orbital velocity
 30 km / second
 Orbital period
 365.256 days
 Rotation period
 23.9345 hours
 Inclination of equator to
orbit:
 23.26 degrees
 Diameter: 12,756 km
 Mass 6 x 1024 kg
 Mean density
 5.5 gm/cm3
 Surface temperature
 -130 F to +140 F (avg.
70F)
An Average
Planet
Our Earth is about average
Earth is the largest and most massive of the
four terrestrial planets, but smaller and less
massive than the four giant, or Jovian, planets.
Earth is third in distance from the Sun among
the four terrestrial planets.
Earth has a moderately dense atmosphere; 90
times less dense than that of Venus but 100
times denser than that of Mars.
Unique Features of Earth
 Plate Tectonics – the only planet with a surface
shaped by this type of tectonics
 Atmospheric Oxygen – the only planet with
significant Oxygen in its atmosphere
 Surface Liquid Water – the only planet where
temperature & pressure allow surface water to be
stable as a liquid
 Climate Stability - differs from Venus & Mars in
having a relatively stable climate
 Life – The only world known to have life; it certainly
has abundant & diverse life forms
 Magnetic Field
 Large moon
Size and Shape of the Earth
The Earth is large enough
for gravity to have shaped
it into a sphere
More precisely, Earth’s spin
makes its equator bulge
into a shape referred to as
an oblate spheroid – a
result of inertia
Composition of the Earth
 The most common elements of
the Earth’s surface rocks are:
 oxygen (45.5% by mass),
 silicon (27.2%),
 aluminum (8.3%),
 iron (6.2%),
 calcium (4.66%), and
 magnesium (2.76%)
 Silicon and oxygen usually
occur together as silicates
 Ordinary sand is the silicate
mineral quartz and is nearly
pure silicon dioxide
Density of the Earth
 Density is a measure of how much material (mass) is packed
into a given volume
 Typical unit of density is grams per cubic centimeter
 Water has a density of 1 g/cm3, ordinary surface rocks are
3 g/cm3, while iron is 8 g/cm3
 For a spherical object of mass M and radius R, its average
density is given by
M
3
4

R
3
 For Earth, this density is found to be 5.5 g/cm3
 Consequently, the Earth’s interior (core) probably is iron
(which is abundant in nature and high in density)
What’s
Inside?
Crust
Asthenosphere
Mantle
Outer Core
Inner Core
How do we
know this?
A sonogram of
the Earth
 Earthquakes generate seismic waves that move
through the Earth with speeds depending on the
properties of the material through which they travel
 These speeds are determined by timing the arrival of
the waves at remote points on the Earth’s surface
 A seismic “picture” is then generated of the Earth’s
interior along the path of the wave
Interior Structure of the Earth
 A solid, low-density and
 A liquid, outer core with
thin crust made mainly of
a mixture of iron, nickel
silicates
and perhaps sulfur
 A hot, thick, not-quite A solid, inner core of
liquid mantle with silicates
iron and nickel
Differentiation
 The densest materials are at the center of the Earth
and the least dense materials are at the surface –
differentiation
 Differentiation occurs in a mixture of heavy and light materials
if these materials are liquid for a long enough time
 The Earth must have been almost entirely liquid in the past
Earth’s
Interior
Earth’s interior
gets hotter
towards the center
(6500K, as hot as
the Sun’s surface)
The Earth’s inner nickel/iron core is solid because the
high pressure (from overlying materials) forces it into a
solid state – even at that high temperature
The outer core is liquid nickel/iron
Why so hot???
The high
temperature in
the Earth’s core
(6500K) is
probably due to
two causes
 Left-over heat from the impact of small
bodies that eventually formed the Earth
The radioactive decay of radioactive
elements that occur naturally in the mix of
materials that made up the Earth
The Earth
cools very
slowly
Pseudo-color infrared image
of the Earth from space.
The red areas are warm
water during an El Nino event
Heat is trapped inside the Earth’s
interior due to the long time it takes to
move to the surface and escape
Age of the Earth
• Radioactive decay used to determine the
Earth’s age
– Radioactive atoms decay into daughter atoms
– The more daughter atoms there are relative to
the original radioactive atoms, the older the
rock is
Age of the Earth
 Radioactive potassium has a half-life of 1.28 billion years
and decays into argon, which is a gas that is trapped in the
rock unless it melts
 Assume rock has no argon when originally formed
 Measuring the ratio of argon atoms to potassium atoms gives the
age of the rock
 This method gives a minimum age of the Earth as 4 billion years
 Other considerations put the age at 4.5 billion years
Early History of
Earth
Four main stages of evolution:
Differentiation
Bombardment
Cooling
Surface evolution
Earth formed 4.6 billion
years ago from the inner
solar nebula
Most traces of ancient bombardment
(impact craters) have been destroyed by
later geological activity
Meteorite Impacts Still Occur
Famous
example:
Barringer
Crater near
Flagstaff,
AZ:
Over 150 impact craters
found on Earth.
1.2 km
diameter
200 m deep
Formed ~ 50,000 years ago by a
meteorite of ~ 80 – 100 m diameter
The Chicxulub Crater
Comet nucleus impact producing the Chicxulub crater ~ 65
million years ago may have caused major climate change,
leading to the extinction of many species, including dinosaurs.
The impact of a large body
formed a crater ~ 180 –
300 km in diameter in the
Yucatán peninsula, ~ 65
million years ago
Evidence of the event is
seen in rock layers around
the world
Dates
to
ASSIGNMENTS
Remember
this week
READ Chapter 5 – The Earth
Homework 4 Due Friday
Quiz on Friday