Transcript ASTR178-Week3

ASTR178
Other Worlds
A/Prof. Orsola De Marco
9850 4241
[email protected]
http://www.physics.mq.edu.au/current/undergraduate/uni
ts/ASTR178/
Announcements
• Use the undergraduate computer laboratories.
• I have posted pdf files of all the lecture
notes (weeks 1 and 2) – 4 slides to a page.
• This is the website of the book, which contains a lot more
than the book: http://bcs.whfreeman.com/universe9e/
Help questions (For this week)
• 1-28, page 232, 233.
• Most of the questions in this chapter are relevant
since I covered this chapter pretty thoroughly.
Planet conjunction!
• August 13th, look to the
West to see a great planet
conjunction. You will also
see the Moon. This can be
the first data point for
your practical.
• You should be able to draw
the ecliptic on this picture.
• Where is the Sun in this
picture?
In last class
• A few words on the Sun to start a tour of
the Solar System.
• What is light.
• Comparing the planets.
• The density of the planets.
• The largest “satellites”.
• What are planets made of.
• The planetary atmospheres
• The outer solar system.
• Craters, volcanos and magnetic fields.
In this class
• A quick wrap up of the magnetic fields in the Solar System.
• Earth’s energy sources
• The Greenhouse effect
• Earth’s layers: crust, mantle and core
Planetary magnetic fields
(from last week)
Magnetic fields
•Mercury, 1% that of Earth’s
•Venus, none
•Mars, only in the highlands
•Jupiter, 14 times Earth’s
•Saturn, a little less than Jupiter
•Uranus and Neptune, have magnetic field
• WHY?
Earth
Earth’s sources of energy
•Internal heat
•The Sun
Dating rocks: radioactive dating.
• Some elements spontaneously change to other
elements, taking a known time to do so.
• If you know the original amount of the parent
and child element in a rock, you can measure the
current amount of child isotope to determine the
age of the rock, or how long since it solidified.
• Example: 235U decays to
704 million years.
207Pb
with a half life of
The green-house effect
The green-house effect: in depth
• Sun hits Earth, some energy reflected, some energy is absorbed and then
re-radiated, balance is reached – we know that the balance is reached
because the temperature on Earth is stable.
• NOTE: the energy that reaches Earth, heats Earth; warm Earth radiates
(all warm bodies radiate). Earth radiates in the infrared.
• We know how much energy reaches us from the Sun and we know how much
is reflected from clouds etc., so we know how much Earth radiates.
• Earth’s radiation allows you to calculate Earth’s average temperature.
• It urns out that if Earth really radiated all
the energy it receives, it would have an average
temperature of -19 C.
• Earth’s actual average temperature is 14 C.
• What is wrong with this calculation?
• Green-house gases, H2O and CO2 trap infrared
radiation from Earth, keeping the atmosphere
warmer than it would be otherwise.
Richard Dixon Oldham
(Irish) 1858-1936
Hypothesized the existence of a molten core.
Inge Lehmann (Danish)
1888-1993
Hypothesized the existence
of the inner core
Key Ideas
• Magnetic Fields and Planetary Interiors: Planetary magnetic
fields are produced by the motion of electrically conducting
liquids inside the planet. This mechanism is called a dynamo.
If a planet has no magnetic field, that is evidence that there is
little such liquid material in the planet’s interior or that the
liquid is not in a state of motion.
• The magnetic fields of terrestrial planets are produced by
metals such as iron in the liquid state. The stronger fields of
the Jovian planets are generated by liquid metallic hydrogen
or by water with ionized molecules dissolved in it.
Key Ideas
• The Earth’s Energy Sources: All activity in the Earth’s
atmosphere, oceans, and surface is powered by three sources
of energy.
• Solar energy is the energy source for the atmosphere. In the
greenhouse effect, some of this energy is trapped by infrared
absorbing gases in the atmosphere, raising the Earth’s surface
temperature.
• Tidal forces from the Moon and Sun help to power the motion
of the oceans.
• The internal heat of the Earth is the energy source for
geologic activity.
Key Ideas
• The green house effect: green-house gases (e.g., CO2 and
H2O) are those that let visible radiation through but not
infrared. This means that they trap heat into Earth’s
atmosphere, keeping the air warmer than it would be
otherwise.
• The Earth’s Interior: Studies of seismic waves (vibrations
produced by earthquakes) show that the Earth has a small,
solid inner core surrounded by a liquid outer core. The outer
core is surrounded by the dense mantle, which in turn is
surrounded by the thin low-density crust.
• Seismologists deduce the Earth’s interior structure by studying
how longitudinal P waves and transverse S waves travel
through the Earth’s interior.
• 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.