-------Review
For Second Test ------------
You should use the SAMPLE
questions (T&F and fill in the blank) from Chapter Review in text chapters 4, 5, 6, 7, and 8 as your
SAMPLE TEST.
You will not
expected to memorize the names of moons
of other planets except the four Galilean moons of Jupiter, Saturn’s moon
Titan, Mars’ moons.
Highlights For TEST Two:
--------------------------------
T H E EARTH: Atmosphere Mostly Nitrogen About 1/5 Oxygen Traces of other things
Layers of the atmosphere
Troposphere ‑‑‑ bottom (where we live) Clouds, weather
Temperature drops as you go up.
Stratosphere ‑‑‑ 2nd (where jets fly) No clouds, weather Temperature rises as you go up.
Mesosphere ‑‑‑ 3rd
(30 to 60 miles up) Temperature
drops as you go up.
Thermosphere ‑‑‑ 4th (60
to 300 miles up) Temperature rises as
you go up.
Exosphere ‑‑‑ 5th
(where satellites fly) up
to 500 miles.
Effects of the atmosphere
Convection: Caused by heated air rising colder air falling. Cyclones: Earth's rotation converts
vertical convection to rotary motion.
Turbulence makes star images flicker (or twinkle) Atmospheric extinction Absorption of light
by air
More Effects of the atmosphere Scattering of blue light: The sky (scattered light) is blue. Sunsets (transmitted light) are red.
The Greenhouse effect. Sunlight heats the
earth. Heat radiation from the earth is trapped by the atmosphere.
The atmosphere works like a blanket which keeps the earth warm. Carbon dioxide and water vapor are responsible
for trapping the heat.
Climate: The runaway greenhouse effect
(A horror story)
1. Burn the coal and the
trees. More carbon dioxide. More heat is trapped.
2. More heat means higher
average temperatures.
3. Higher average
temperatures melt the ice caps
and evaporate water from the
oceans.
4. Smaller ice caps reflect
less light ‑‑‑ more
sunlight is absorbed and turned into heat.
5. More water vapor in the
air helps to trap more heat. 6. Go back to step 2 and repeat the runaway cycle.
Actually, the earth's climate is probably stable within certain limits.
For example, heating of the earth by the runaway greenhouse effect
would probably be stopped by several "automatic safety
features".
Possible "Automatic Safety
Features"
1. Extra long growing seasons
and fast regrowth in tropical forests would eat up the excess carbon dioxide.
2. Excess water vapor would cause more clouds, and extra snow cover,
reflecting more light back into space.
The heat input from the sun would then drop.
More "Automatic Safety
Features"
3. Excess water vapor would cause more rain which would wash carbon
dioxide out of the air by dissolving it (making soda pop).
However there are limits to the "safety interlocks" built
into our climate.
Limitations of "Safety
Features"
Suppose we burn trees faster than any plants can grow back. Suppose the
water vapor in the air increases to the point where the cloud cover is always
100%.
The climate could run out of responses ‑‑‑ plants
can't grow faster than their natural
rate and the cloud cover cannot increase beyond 100%.
By saturating enough of the climates "defense mechanisms" we
might really get a runaway climate.
Eventually, even a runaway greenhouse effect would reach a new
equilibrium. But it would be nothing
like the present earth. It might be
uninhabitable.
The Concept of Density:
Density = Mass of a unit volume
Example:
Density of water = 1 gram per cubic cm.
Density of iron = 8 grams per
cubic cm.
Average density = (Total mass)/(Volume)
Example: Average
density of earth = 5. 5 grams per cubic
cm.
Structure of the earth's
interior Core Mantle Crust
Core: About half the size of the whole earth. Made of iron and nickel
Inner core is solid Outer
core is liquid.
Mantle: Acts like silly putty. Flows
under steady pressure. Snaps back from
sudden pressure changes.
Lithosphere ‑‑‑ rigid
upper part of the mantle.
Asthenosphere ‑‑‑ flowing
middle part of the mantle.
Mesosphere ‑‑‑ rigid
lower part of the mantle.
Crust 10 to 25 miles thick. Mostly solidified lava from volcanoes.
Granite slabs form continents. Ocean floors are basalt. Granite is a mineral made of silicon,
aluminum oxides.
Basalt is a mineral made of silicon, magnesium oxides. Granite is
lighter than basalt and tends to float on top of it.
Discovery of the Mantle: In 1909 Mohorovicic
noticed that the speed of earthquake waves changed abruptly at a certain level
beneath the earth's surface.
This level was the boundary between
the crust and the mantle. It is now called the Mohorovicic discontinuity or the "Moho".
The Earth's Magnetic Field
Makes magnetic compass needles point more or less North. Traps charged
particles from the sun. Funnels charged particles into the atmosphere over the
North and South poles ‑‑‑ causing the Northern Lights and Southern
Lights.
The Earth's Magnetic Field also
magnetizes iron‑bearing ores as they cool after emerging from volcanoes.
Is probably caused by motions in the earth's liquid iron core. The Earth's
magnetic field has reversed itself repeatedly over
millions of years ‑‑‑ according to the magnetization of
ancient volcanic lava flows.
Puzzle: Why are there
mountains? Rain and wind wear them away at a great rate. We think that the
earth is billions of years old, so they should all be gone by now. Mountains
must be replaced constantly. What makes mountains?
Tectonic Plates:
Another puzzle: Why isn't the
earth one big ocean? The rivers, fed by the rain, wash the substance of the
continents to the sea. The continents are, on the average, several miles higher
than the ocean floors. Why isn't this difference leveled by now?
Yet another puzzle: Why do
earthquakes and volcanoes concentrate in certain regions and not in others?
Francis Bacon (1561‑1626) Noticed still another puzzle, declaring "If the
fit between South America and Africa is not genetic, surely it is a device of
Satan for our frustration. "Francis Bacon was convinced by the
evidence of the map that the continents were once united and had since moved
apart.
Other evidence that the continents have moved: Glacial deposits in South Africa. Similar
fossils on the coasts of widely separated continents.
In 1910, Alfred Wegener
proposed that all the continents were together in one big land mass which broke
up 200 million years ago. He was right.
His only evidence was the map and the similarity of rock formations
where the world‑puzzle seemed to fit the continents together. Not good
enough.
The established picture of an unchanging earth needed to be
contradicted by strong evidence before a new theory could be accepted.
Continental Drift became respectable in the 1960s because of new evidence. On either
side of the mid‑ atlantic
ridge is a series of parallel strips. The rocks in each strip are all
magnetized in the same direction.
Different strips are magnetized in different directions.
\\///\\ / R
/ \\///\\
\\///\\ / I
/ \\///\\
\\///\\ / D
/ \\///\\
\\///\\ / G
/ \\///\\
\\///\\ / E
/ \\///\\
The directions correspond to past directions of the earth's magnetic
field. Volcanic activity is known to be occurring along the ridge.
There was no way to account for this new evidence in the old picture of
an unchanging earth. Evidently, liquid magma is welling up out of the earth,
solidifying, and building new sea floor as the old sea‑floor
spreads.
The sea floor under the Atlantic is spreading at the rate of 2 to 4
centimeters per year. In 400 million years, that comes to 16,000
kilometers. Thus, Africa and the
Americas are drifting apart. The
continents are indeed moving. The
motions have even been detected directly by laser ranging measurements.
Plate Tectonics:The theory of continental drift.
The earth's crust is made up of crustal plates
which float on the fluid part of the mantle (asthenosphere,
remember?). The crustal plates
move around. Crash into each other. Are created at mid‑oceanic ridges. Are
destroyed by sliding under other plates and dissolving in the
asthenosphere.
Plate boundaries are usually the site of earthquakes, volcanoes, and mountain‑building.
The San Andreas Fault in California is where the Pacific Plate is side‑swiping
the North American Plate.
The Appalachian Mountains in North America and the Caledonian Mountains
in Europe are the result of an ancient head‑on collision between the
North American Eurasian plates. The
Indian Plate is even now bashing into the Eurasian plate and building the
Himalayan mountains.
With the possible exception of some of the moons of Jupiter, the
Earth's surface is one of the most
unstable in the Solar System.
Radioactive Dating Look for the products of slow radioactive decay to find out when rocks
became solid. Example: The most common
isotope of uranium (U238) decays to lead (Pb206) with a half‑life of 4. 5 billion years. Half‑life Over
any period of 4. 5 billion years, each individual uranium atom has a 50‑50
chance of changing into a lead atom.
Start with a pure uranium deposit formed while rocks were molten and
could separate easily. After 4. 5 billion years, the deposit will be 50%
uranium and 50% lead.
Decay for longer than a half‑life:
After 9 billion years half of the remaining uranium will change and the deposit will be 25% uranium and
75% lead. Every half‑life (4. 5 billion years), Half of the
remaining uranium changes to lead.
After 13. 5 billion years, the deposit will be 12. 5% uranium and 87. 5%
lead.
The oldest rocks on earth are found to be about three and a half billion years
old. It would take a molten earth
about a billion years to solidify. The
earth seems to be about four and half billion years old. Radioactive dating of meteorites finds that
these objects solidified about 4. 5 billion years ago.
A Quick History of the Earth
Planetesimals collected together to form an object with enough mass to attract
others. As other planetesimals rained
down on the new earth, their energy of
motion melted the rocks so that the early earth was liquid. The heavy elements like iron sank to the
center while lighter elements rose to
the surface.
Atmosphere
Gases and volatile substances such as
water formed an early atmosphere.
The lightest gases such as Hydrogen and Helium were soon lost because
their individual molecules move too fast for earth's gravity to pull them
back. The exact composition of the
earth's first atmosphere is a matter of dispute:
Composition of the early
atmosphere:
Some scientists insist that Hydrogen stayed around for a long time
while others insist that it did not. By
general agreement, there was a lot of nitrogen
(possibly as ammonia) carbon dioxide water
vaporand at least some methane
but no oxygen.
The rains came:
As the earth cooled, it began to rain and there was thunder and lightning. The rain dissolved the Carbon Dioxide to form carbolic acid which
reacted with the rocks to form carbonates.
In this way, the carbon dioxide
was washed out of the atmosphere.
Meanwhile, the lightning bolts fused the ingredients of the early atmosphere to make amino acids ‑‑‑
the molecular building blocks of
life. Amino acids were also
arriving from space ‑‑‑ they occur in meteorites. The warm
seas accumulated nitrated hydro‑carbons
of all sorts, forming what has been called the "primordial soup". Life BeginsWithin the first few hundred
million years, the building blocks of
life became organized into a form
that could reproduce itself. Nobody
knows exactly how this occurred. Most scientists assume that it was a
natural and even inevitable process.
Plants Once life arose, it found a way to tap the energy of sunlight: photosynthesis.
This process releases oxygen
into the atmosphere. The oxygen
reacted with sunlight in the upper
atmosphere to form ozone. The ozone acts as a
barrier to ultra‑violet radiation from the sun.
A Curious Relationship: Life arose from water and requires it.
Life created the ozone layer which protects water molecules from being
broken apart by the sun. Without water, there would be no life on earth. Without life on earth, there would be no
water left here.
The Living Planet:
Life has
created and maintains the conditions which it requires to exist. Our present
earth is inhabitable precisely because it is inhabited.
Origin of the Moon
Fission
Model A single planet formed and
then split apart.
Binary
Accretion Model The Moon formed from a
cloud of debris orbiting the Earth.
Capture‑Collision
Models The moon formed elsewhere
and was captured by the earth.
1/4 size of
earth.
Largest moon
in comparison to its primary in the solar system, except for Pluto-Charon
Craters are due to meteor impacts.
semi‑molten
core
hard mantle
of rock
thin crust (regolith & lithosphere)
The earth's
gravity has stopped the moon's rotation and locked it into it's present earth‑
facing position.
Gravity on
the moon's surface: 1/6 of gravity on
earth.
Notice the difference between weight:
force of gravity
mass: resistance to starting
or stopping
The Maria ‑‑‑
the Lunar Lowlands The face of the "Man in the Moon"
Lava flows.
Craters Round rings of mountains - with radial streaks or rays
Sometimes surrounded by smaller
"secondary craters".
Craters result
from the impacts of objects from space.
Apollo 11, 12, (13), 14, 15, 16, 17 brought back
4500 pounds of rocks.
The
surface: Lunar soil or regolith
Crust about 50 miles thick.
Rocks:
Basalts like on earth-
Anorthosites- Breccias- KREEP
Mercury:
The closest planet to the sun.
Very elliptical orbit
Goes once around the sun in
88 days.
Rotation period = 59 days
2/3 of 88 days.
During one
orbit, mercury spins 1 1/2 times.
Size: About 40% bigger than
our moon.
Moons of
Mercury: none.
Average
density: Same as earth.
Weather on
Mercury 700K at noon and 100K
at night
No atmosphere to speak of.
Length of solar day: 176 earth days ‑‑‑
a "Mercurian day" lasts two "Mercurian years".
Surface
features on Mercury:
Craters
Scarps (i. e. cliffs)
Only one maria‑like basin: Caloris
Planitia
Jumbled terrain opposite the Caloris Planitia.
Maybe from Caloris impact waves.
Mercury has a small magnetic field ‑‑‑
about 1% of earth's.
Venus
Hotter than
Mercury.
Pressure 90 times that at the earth's surface.
Sulfuric acid clouds.
Similar to earth in size and mass.
Orbits the
sun in 225 days.
Rotates backward
once every 243. 01 days.
No moon.
No magnetic
field at all.
The atmosphere of Venus
The main gas
is carbon dioxide.
Venus seems
to have a runaway green‑house effect.
Venus
probably never had liquid surface water to convert the carbon dioxide to
carbonic acid and then to carbonates.
The water
vapor eventually was decomposed by ultraviolet light from the sun. Very little water is left on Venus now.
Without
water to absorb it, the earth's carbon dioxide would form an atmosphere like
that of Venus.
Mars
Rotates on
its axis every 24 Rotation axis tilted at 25o.
About half the size of the earth. Two tiny moons.
Much lower
average density than earth ‑‑‑ probably lacks a large iron
core.
Almost no
magnetic field.
Atmosphere
of Mars
Mostly
carbon dioxide Pressure about 1/200 of earth's.
Temperatures
on Mars
Hottest ‑‑‑
High noon in the Martian Tropics about 98. 6oF.
Coldest ‑‑‑
At night the temperature drops by 180oF. 3v 3
Surface of
Mars
Red sand:
Limonite : no canals.
Polar caps
show water‑ice covered by dry‑ice.
In the
southern hemisphere of Mars: Lots of
craters
In the
northern hemisphere of Mars: Volcanoes, Lava flows, jumbled terrain.
Mariner Valley separates the hemispheres:
Olympus Mons
Tharsis plateau
Covers 25%
of the surface.
About 6 km
above the rest of the surface.
We think
there is a layer of "permafrost" under the soil of Mars.
The Viking
Expedition
Lots of evidence for running water.
Present
pressure is too low for liquid water on the surface.
Life on Mars
‑‑‑ None found.
Life: Evidently
requires a planet at just the right distance from the sun to maintain its
temperature at the temperature of the triple point.
Jupiter: 11 times
the diameter of earth.
mostly Hydrogen and Helium ‑‑
zones ‑‑‑ light
(updrafts)
belts ‑‑‑ dark
(downdrafts)
Rotates once
every 10 hours. The Great Red Spot
‑‑A hurricane
the size of the earth.
The light
cloud tops are ammonia ice.
cold:
150 K
Heated from within.
atmosphere about 1000 km. thick
merges into liquid hydrogen.
About 1/3 of
the way in to the center, the liquid
hydrogen becomes metallic and conducts electricity.
At the very
center, a rocky core about three times the size of the earth. Core temperature about 30,000 K.
The liquid
metallic core and rapid rotation generate a strong magnetic field ‑‑‑
10 times as strong as earth.
Moons of
Jupiter Four Galilean Satellites‑‑
Io larger than our moon
Europa smaller than our moon
Ganymede larger than Mercury
Callisto comparable to Mercury
Io and Europa: basically rock.
Ganymede and Callisto ‑‑‑ giant slush balls.
Io - Covered with active volcanoes
Europa- No craters. A network of dark cracks.
Ocean of
liquid water covered by ice with cracks in it.
Ganymede Largest
moon of Jupiter
Second
largest moon in the solar system.
Cratered
surface like our moon. Shallow
craters ‑‑‑
Callisto -Lots of craters.
Craters are
flattened because ice is not very strong.
One monster
impact feature with many concentric rings of frozen blast waves.
Pip‑squeak
moons of Jupiter
At least 16
more.
The Rings of
Jupiter
Made of dark
material which reflects little light.
Saturn-----------------------------------------------
Slightly
smaller than Jupiter
Lowest
density of any planet
Atmosphere
High haze obscures surface
Quieter than
Jupiter
Made of the
same stuff as Jupiter.
Heated from within‑‑like
Jupiter.
Magnetic field about 1/20 that of
Jupiter.
Interior
probably similar to Jupiter but with a smaller core region.
Moons of
Saturn: Interesting oddities
1. Several
of the small moons are at Lagrange points of the orbits of larger
moons.
2. Two small
moons, Epimethus and Janus, share almost the same orbit. ‑‑‑the "square‑dancing
moons".
3. Prometheus
and Pandora have slightly different orbits and shepherd a ring
between them (the F ring).
Titan, the
Venus of moons:
The only
moon with a dense atmosphere.
About 1. 5
times the pressure at the surface of the earth.
Five times
as deep as ours.
Mostly
Nitrogen (like earth) No oxygen or water but lots of methane.
"Planet"
of the triple point of methane.
The intermediate‑size moons of
Saturn
Mimas ‑‑‑ a crater almost as large as itself.
Enceladus ‑‑‑ polished
like a mirror.
Tethys ‑‑‑ giant fissures indicate past disruption.
Dione ‑‑‑ the ice‑moon.
Rhea ‑‑‑ Extensive light‑ colored "splash" probably made of ice.
Iapetus ‑‑‑ dark on one
side, light on the other.
Hyperion ‑‑‑ large but not
round.
Phoebe: The Odd‑ball moon Far out. Orbits the wrong way.
The Rings of
Saturn
Only a few
kilometers thick.
Old rings
from outer to inner:
"A
ring", Cassini's Division, "B ring", "C ring".
The rings
consist of millions of tiny ice‑covered moonlets.
The Cassini
Division is caused by orbital resonance with Mimas.
The Voyager
missions showed The rings actually consist of thousands of ringlets.
Rotating
spokes in the B ring appear to contradict Kepler's third law.
The F‑ring appears to be "braided".
Uranus ------------------------------
Discovered
by Herschel in 1781. About twice as
far out as Saturn. Orbits the sun once
every 84 years.
Rotation
Period 17 hrs. 14 m
Rotation
axis lies almost in the plane of its orbit.
Its south pole is pointing toward the sun now.
Fifteen
satellites and a series of rings orbit in the plane of the equator.
Composition
of Uranus ‑‑‑ Similar
to Jupiter (and Saturn) in general
Blue‑green
color due to methane in the upper atmosphere.
About 4
times the size of earth
Magnetic
field with Magnetic North 60 degrees away from the true North rotation axis
pole.
It probably
has a rocky core about the size of earth.
Five major
satellites
Oberon ‑‑‑ outermost
Titania ‑‑‑ Cliffs
Umbriel ‑‑‑ Dark
surface
Ariel ‑‑‑ Surface Ice
Miranda ‑‑‑
innermost
Circus Maximus
The "Chevron"
Uranus has
eleven known rings, discovered first by the blocking of starlight.
Neptune
Uranus did
not exactly follow the orbit that Newton's Laws predicted.
With enough
observations of Uranus, the position of the missing planet was pinpointed.
Neptune
orbits about three times as far from the sun as Saturn.
It takes 165
years for Neptune to go around the sun.
In size and
apparent composition, Neptune is a twin
to Uranus.
Rotation
period‑‑‑ 18. 2 hr
Rotation
axis‑‑‑ tilted 29 degrees.
Two major satellites:
Triton ‑‑‑ the largest moon in the solar system.
Orbits
backwards and out of Neptune's equatorial plane. Nereid ‑‑‑ very small.
Very
elongated orbit.
Neptune’s rings are more like thin, dark arcs.
Pluto
Found in
1930 by using a "blinker" to
detect motion.
Extremely
elliptical orbit ‑‑‑ It actually comes closer to the sun than
Neptune.
The orbit is
also steeply inclined to the plane in which the other planets orbit. Its rotation period from brightness
variations ‑‑‑ about 6. 39 days.
Has a moon
named Charon which orbits Pluto once every 6. 39 days and appears to be
about half the
size of
Pluto. About 1/500 the mass of earth.
Pluto and
its moon appear to be ice planets.
Planet X?
The orbit of
Neptune is still not exactly right.
However, an
out‑of‑plane planet orbiting far beyond Pluto would be extremely
difficult to find and could well be out there.
A S T E R O I D
S----------------------------------
Mostly between the orbits of Mars and Jupiter.
Left‑overs from planet formation.
Distributed
in a continuous band with a few
gaps ‑‑‑
Kirkwood's gaps.
Caused by orbital resonance with Jupiter.
The largest
asteroids: Ceres 760 km.
diameter
Pallas 480 km. diameter.
The total
mass of all the
asteroids is about half the mass of our moon.
Nickel‑iron asteroids are
probably fragments of larger
objects ‑‑‑ similar to
Ceres and
Pallas ‑‑‑ that were
originally
molten and differentiated so that iron‑nickel
cores formed.
asteroids
are made of rock and may come from mantle fragments of larger differentiated
objects.
Still other
asteroids contain carbon and volatile
compounds. These were probably never
part of large molten objects.
Trojan
Asteroids at the Lagrange points of Jupiter's orbit.
Apollo
Asteroids have orbits that cross the orbit of the earth.
A major iron
mine in Canada ‑‑‑ the Sudbury Astrobleme may be the remains
of an asteroid that hit the earth long ago.
The asteroid
Eros passes close to earth.
C O M E T S
------------------------------------------
Comets
belong to our solar system.
The home of
comets:
1. Kuiper Belt just beyond Pluto
2. "Oort's
Cloud". thought to extend from
50,000 AU to 150,000 AU.
Slingshot of
a Comet:
If a comet
passes in front of one of the giant planets, the comet
gives up a
little energy to the planet and drops into a path with a fairly short period. This process creates the periodic
comets. If a comet passes behind one
of the giant planets, then it picks up a sling‑shot effect boost ‑‑‑
the same as Voyager.
C O M E
T S T R U C T U R E----------------
Nucleus ‑‑‑
dirty snowball
Coma ‑‑‑ gas cloud
Tail ‑‑‑
pushed by sunlight - The comet's
tail always points away from the sun.
The nucleus
‑‑ the original dirty snowball ‑‑ is covered with dust left
behind when the ice and frozen gases evaporated.
As the
nucleus heats up, it releases jets of gas which can modify the motion of the
comet.
By a
distance of about 2 AU, (around Mars’ orbit) the pressure of the sun's
radiation blows the gas and dust out into a tail.
Actually two
tails:
Straight tail ‑‑‑ Gas ions
Curved tail
‑‑‑ Dust
The light
from a comet: Reflected light from the
nucleus and dust tail shows an absorption spectrum.
Fluorescence
from the coma and ion tail shows an emission line spectrum. Eventually the comet breaks up.
The remains
form annual meteor showers.
M E T E O R
S M E T E O R O I D S
An
interplanetary rock heading toward the earth is a meteoroid.
As the rock
enters our atmosphere and burns up, it is a
meteor. (the flash of
light)
If the rock
makes it all the way down and lands then it is a meteorite.
Meteors:
Most any night that you look, there will be about 5 or 6 meteors every hour.
-----------------------
Meteor
Swarms
Occasionally the earth passes through a
belt of comet debris and there are lots of meteors.
The meteors
in a swarm all seem to come from a particular point in the sky called the radiant.
A meteor
swarm is named after the constellation that its radiant is in.
Most meteors
are very small and fragile and never reach the ground.
25,000 years
ago a big one hit Arizona and left a crater 1. 3 km. across ‑‑‑ the Barringer Meteorite Crater.
Composition
and Origin of Meteorites
The iron meteorites show evidence
of very slow cooling over millions of years ‑‑ in parent meteor
bodies.
The crust
and mantle of a parent meteor body would have fragmented to form
stony and stony‑iron meteoroids.
The iron
core of the parent meteor body would have fragmented to form the iron
meteorites.
Some
meteors, called carbonaceous chondrites contain volatile
compounds These meteorites may have formed
during the first days of the solar system.
Radioactive
dating of meteorites indicates that they were last melted 4. 6 billion years
ago. Other Debris in the Solar System
Interplanetary
dust grains cause the Zodiacal light.
The gegenschein
is seen at the point in the sky opposite the sun. It is due to sun light reflected from the dust particles.
ORIGIN OF
THE SOLAR SYSTEM
Two main
types of models
A. Evolutionary
Solar system
condenses out of a cloud of gas and dust.
Planets are
normal.
B. Catastrophic
Something
pulls material out of the sun. This
material then condenses to form the planets.
Planets are
rare accidents.
The
catastrophic models have not been made to work.
One major
objection to them is the fragility of deuterium.
Another
problem is that hot material pulled from a functioning star tends to disperse
rather than condense.
Evolutionary
or Nebular Theory
A dark
cloud collapsed. The original dark
cloud was turning
slowly in
space. The contraction speeded up this
rotation.
The original
dark cloud contained weak magnetic fields.
The collapse carried these fields with it and intensified them.
Magnetic
fields, frozen into the collapsing cloud transferred angular momentum from the
center to the outer regions.
With its
angular momentum gone, the center of the cloud collapsed and began to heat up
as matter fell in on it.
The outer
regions of the cloud, formed a ring. In the hot inner part of the disk, only
rock and iron could condense and the terrestrial planets formed.
In the cold,
outer part of the disk, water and even some gases could
condense easily and objects similar to the present moons of Jupiter and Saturn
formed.
In the outer
parts of the disk, the newly formed planets swept up much of the remaining
gas from the solar nebula and became gas‑giants.
In the inner
parts of the disk, the gravitational attraction of the nearby proto‑sun
prevented the new planets from sweeping up much gas, so they remained as solid
bodies.
Jupiter
might have gone on collecting gas and increasing its central temperature until
it ignited and became a star.
However, the
proto‑sun got there first. When
its nuclear fire ignited, it became a Tau‑ Tauri star.
Such infant
stars generate strong solar winds.
The Tau‑Tauri
wind blew the remaining solar nebula away and Jupiter lost its chance to be a
star.
Other
Planetary Systems Several dozen have
been discovered by the motion of the star.