PTYS/ASTR 206Earth 2/27/07 The Earth as a Planet.

PTYS/ASTR 206 Earth2/27/07

The Earth as a Planet


Announcements

• Reading Assignment– Chapter 18: sections 18-1 through 18-9 (pp. 378-399)

• 3rd Homework due on Thursday.

• Public Lecture tonight at 7:30PM in this auditorium– Prof. Bob Strom: “Global Warming”– Look for sign-up sheet (5 pts e.c. added to total activities score)

• Next study-group session is tomorrow from 10:30AM-12:00Noon – in room 330.

• I will not be able to make office hours today.


Earth as a Planet• Densest planet in the solar

system• Largest terrestrial planet• Only planet (currently) to have

– Plate tectonics– Liquid water on the surface– Life– Oxygen-rich atmosphere

– Other planets may have some of these (though not as much) – looking for them is important !


Sources of Earth’s Energy


The Earth’s Internal Structure

• The Earth’s inner and outer cores are composed of almost pure iron with some nickel mixed in

• The mantle is composed of iron-rich minerals

• Both temperature and pressure steadily increase with depth inside the Earth


Earthquake Distribution


Drift of Continental Plates over time

• About a few centimeters per year– Ranging from about the rate

at which fingernails grow, to that of hair growth

• The continents were all together around 250 million years ago

– The Earth is 4.56 Billion years old

– That is only 0.4% of the lifetime of the Earth

• In terms of a human lifespan, that’s about 4 months !


Plate tectonics • Rigid plates move along the top of

convective motions in the ashenosphere

• Molten material wells up at oceanic rifts, producing seafloor spreading, and is returned to the asthenosphere in subduction zones

• As one end of a plate is subducted back into the asthenosphere, it helps to pull the rest of the plate along

Mid-Atlantic ridge


Plate tectonics is responsible for most of the major features of the Earth’s surface, including mountain ranges, volcanoes, and the shapes of the continents and oceans


• Plate tectonics is involved in the formation of the three major categories of rocks:

• igneous rocks – cooled from molten

material• sedimentary rocks

– formed by the action of wind, water, and ice

• metamorphic rocks – altered in the solid

state by extreme heat and pressure


• The Earth’s atmosphere is divided into layers called the troposphere, stratosphere, mesosphere, and thermosphere

• In the troposphere, temperature decrease with altitude

• Ozone molecules in the stratosphere absorb ultraviolet light; thus temperature rises with altitude

The Structure of Earth’s Atmosphere


The Importance of Earth’s Atmosphere on the Surface Temperature

• By balancing the heat input from the Sun, with the heat radiated by the Earth, it is possible to determine the temperature of the Earth

• However, this gives a temperature that is much smaller than the actual one– The Earth should be well below freezing

• Why is the Earth so warm?– The Greenhouse effect !

PTYS/ASTR 206 Earth2/27/07298K 293 K 288K 283K 278K

Temperature

• The temperature of the soil just a few feet below Earth’s surface fluctuates on a daily cycle (this is the diurnal variation) as Sun warms the surface during the day, and the Earth cools off at night.

• The fact that the Earth’s surface is heated by the Sun is well known by those who walk barefoot in the summer!


The Greenhouse Effect: Basic processes

• Energy coming from the Sun is absorbed by Earth’s surface and heats the Earth. – The wavelength of peak emission of solar radiation is in the visible

part of the spectrum which is not significantly absorbed by Earth’s atmosphere

• The Earth radiates energy (like all heated objects), with a peak that is in the infrared part of the spectrum– Earth’s atmosphere contains certain gases that absorb infrared

radiation (called greenhouse gases)– Because of the presence of these gases, the radiated energy from

the Earth cannot escape!– The Earth is heated more

• until the right balance outward-directed energy equals the inward-directed energy


Atmospheric Transparency

• Earth’s emitted infrared light cannot escape because the atmosphere blocks it – thus it heats the Earth until a balance is reached.



Greenhouse gases

• Molecules that absorb infrared radiation

• Natural– Water Vapor H2O (most abundant)– Carbon Dioxide, CO2

– Methane, CH4

– Nitrous oxide, NO2

• Increased during industrial revolution– carbon dioxide (CO2) – methane (CH4) – nitrous oxide (N2O) – chlorofluorocarbons


Greenhouse effect -- results

• Earth is warmed up– With no Greenhouse Effect, average

temperature would be below freezing

• Changing amount of Greenhouse Effect changes Earth’s temperature, weather– Changes not uncommon during Earth history– Usually changes what species are present– Details almost impossible to predict


Greenhouse Effect: misconceptions

• Greenhouse Effect has something to do with ozone depletion – it doesn’t!

• Greenhouse Effect involves reflected sunlight – it doesn’t!

• Greenhouse Effect is new phenomenon on Earth – it isnt!

• Greenhouse Effect is harmful to life – it isnt!

UV blocking part

Solar Spectrum

Ozone hole


Atmospheric Circulation

• Air flows from High pressure to Low pressure– High pressure – moist warm air

(equator)– Low pressure – cool dry air (poles)

• If the Earth were not spinning on its axis, then there would be the classic “Hadley cells” which are large-scale atmospheric convection cells

Hadley Cells for a Non-Rotating Earth


The Coriolis force

• The atmospheric circulation patters are influenced by the Earth’s rotation.

• In particular, there is a an additional force on the atmosphere (it exists only for rotating objects)– It is called as the Coriolis

force.– This force is responsible for

altering the path of an object moving on a rotating object


Circulation in our atmosphere results from atmospheric convection

and the Earth’s rotation

Because of the Earth’s rapid rotation, the circulation in its atmosphere is complex, with three circulation cells in each hemisphere


Global Circulation


The Solar Wind / Earth’s Magnetic Field Interaction and Earth’s Magnetosphere


Earth’s Magnetosphere

• Earth’s magnetic field shields the planet from the Solar Wind

• It carves out a cavern in the solar wind called the magnetosphere

• The Van Allen radiation belts are part of Earth’s magnetosphere huge intensities of charged particles can be trapped


How do magnetic fields shield us from charged

particles?

• Magnetic fields influence the motion of charged particles

• Depending on the energy of the incident particle, it may be deflected by Earth’s magnetic field

• Solar wind and other high-energy solar particles are shielded by Earth’s magnetic field

• High-energy cosmic rays suffer only minor deflection by Earth’s field.

• Earth’s magnetic field can be used as a giant laboratory experiment to determine the properties of cosmic rays


Origin of Aurorae• Occasionally, a small amount of

solar wind penetrates into Earth’s upper atmosphere.

• When charged particles strike molecules in the atmosphere, the molecules are excited and emit light with various colors (blue, green, and red, mostly)

• This happens at the poles because plasma moves mostly along lines of magnetic force that originate in Earth’s polar regions


Aurora The pattern of auroral light around the north and south magnetic poles is called the auroral oval. It expands and contracts over a period of hours and days, depending on geomagnetic activity.


Van Allen Radiation Belts

• James Van Allen insisted that the first spacecraft to leave Earth’s atmosphere be equipped with a Geiger counter to measure radiation

• They discovered the Van-Allen Radiation Belts


What are the Van Allen belts?

• Radiation trapped in Earth’s magnetic field– Inner Belt: Cosmic-Ray

Albedo Neutron Decay– Outer Belt: Trapped Solar

Wind

• Particles can remain trapped for very long periods of times (years!)– An atomic bomb blast test

in the upper atmosphere in 1962 “Starfish Prime” (now banned by treaty) filled the region with particles

• 5 years later, many of them were still there


Starfish Prime test: skyglow as seen in Honolulu, 1962

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PTYS/ASTR 206Earth 2/27/07 The Earth as a Planet.

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