Today’s APOD
READ Chapter 5 – The EarthHomework 4 Due FridayQuiz on Friday
The Sun Today
A100 Oct. 6 The Earth
Loren Acton (1936 – )U.S. astronaut
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.
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
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)
343
M
R
What’s Inside?
CrustAsthenosphereMantleOuter CoreInner Core
How do we know
this?
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
A sonogram of the Earth
Interior Structure of the Earth A solid, low-density and
thin crust made mainly of silicates
A hot, thick, not-quite-liquid mantle with silicates
A liquid, outer core with a mixture of iron, nickel and perhaps sulfur
A solid, inner core of iron and nickel
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
The densest materials are at the center of the Earth and the least dense materials are at the surface – differentiation
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 temperatureThe 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
Heat is trapped inside the Earth’s interior due to the long time it takes to move to the surface and escape
Pseudo-color infrared image of the Earth from
space. The red areas are warm water during an El
Nino event
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
Earth formed 4.6 billion years ago from the inner solar nebula
Four main stages of evolution:
Differentiation
Bombardment
Cooling
Surface evolution
Most traces of ancient bombardment (impact craters) have been destroyed by later geological activity
Meteorite Impacts Still Occur
Over 150 impact craters found on Earth.
Famous example:
Barringer Crater near
Flagstaff, AZ:
1.2 km diameter
200 m deep
Formed ~ 50,000 years ago by a meteorite of ~ 80 – 100 m diameter
The Chicxulub CraterComet 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
ASSIGNMENTSthis week
Dates to Remember
READ Chapter 5 – The EarthHomework 4 Due FridayQuiz on Friday