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Solar System Formation
Also: measuring ages using
Radioactive Decay
Chapter 8
Goals and objectives
• Develop a sense of what scientists know about
the overall universe, its constituents, and our
location
• Understand the link between the composition
and location of the constituents in the solar
system
• Sketch how the planets were formed.
• Compare and contrast the terrestrial, jovian, and
uranian planets.
• Estimate the age of the solar system, given
data on the isotopic composition of meteorites.
Major criteria necessary in a
formation model
• Motions – counterclockwise, almost circles, and
in same disk (flat)
• Two types of planets:
– Terrestrial – small, rocky & metallic, near the Sun &
close together
– Jovian – large, gaseous, far away & far apart
• Asteroids & comets – asteroid belt, and the
comet places: Kuiper belt, Oort cloud
• Must also allow for exceptions to major patterns
Age of Solar System
– Universe is 13.7 Gyr old
– Solar system is “only” 4.6 Gyr old
• What took so long?
– Universe began as H, He
– Earth-like planets need heavy elements
– Heavy elements made in stars
• “Galactic recycling”
• Takes a few billion years to make enough
heavy metals inside stars that later
explode.
Evolution
• Solar nebula began as a BIG low density gas
cloud and it was: (page 237)
– Cold (no sunlight)
– Rotating counterclockwise slowly
– Spherical (almost)
• The gas cloud shrank. (Why?) This results in:
– Faster rotation. Demo. Sports analogy.
– Sun formed.
– Leftovers:
• Planets give off mostly which kind of light?
• Excess IR Light is detected in other star-forming systems
• Suggests ____________ are forming, also.
Collisions in early solar system
• Flattens disk
– Collisions average
out differences
(up & down
cancel)
• Removes
“retrograde” orbits
due to MANY
collisions
• circularizes orbits
• Described on
page 238.
Sun
Top view
Collide & stick!
More on collisions
• Results: objects orbit the same way as rotation of the
original cloud.
– Orbits will tend to be counterclockwise.
– Collisions will tend to make rotations counterclockwise.
• Why would there be more collisions in the early solar
system than there are today?
• Other collision systems look similar (flat, circular
orbits)
– Spiral galaxies, planet rings
• See also page 238.
Forming planets
• Gas cloud shrank
– Most gas went into _____________
– Leftovers can’t easily pull themselves together
– So they collide
– Solid parts form “seeds” – have larger than average
gravity. They are planet precursor.
– Seeds need to be solid for things to stick & stay
• See table 8.1 page 240. NOTES:
– Hydrogen gas = pure hydrogen.
– Hydrogen compounds are molecules such as:
Water (H2O), methane (CH4), ammonia (NH3)
– Demo: what happens to liquid if no atmosphere…
– Work on the “Forming Planets” worksheet.
Terrestrial planets
• “Seeds” are:
– Metallic & rocks
– Small. Why?
• Less stuff for seeds. No solid H-compounds (1.4%).
Only 0.6% metals/rocks.
• MANY collisions/encounters:
– Nearby stuff sticks forms “planetesimals” (pre-planets)
– Collisions can destroy small planetesimals.
– What’s left at the end?
– What’s the temperature like near the Sun?
• Gases won’t stay on the planets
– See figure 8.6 (page 241) to summarize
Frost Line
• “Frost line” = where ices (H compounds) freeze
(condense in book)
– Guess where.
– What would happen to an ice ball that got closer
than the frost line?
– What are ice balls called?
– What happens when they get closer than Jupiter?
– Evaporated ices surround the comet, this region is
called the comet’s “coma”
Jovian Planet formation
• Same idea as Terrestrial planets except:
– Seeds are bigger. Why?
– ALSO: Gas is colder
• Moves slower
• Doesn’t escape as easily
• Planets can hold onto the gas
– Is there a little gas or a lot of gas in solar nebula?
• Which gases?
• Another possibility: Jovian planets formed without
seeds. New theory – needs more testing & details
Planetary rotation
• Planets rotate faster when things hit them in the
direction of their spin.
• Terrestrials form from collisions of big rocks.
– Collision directions are random
– Usually leads to slow rotation unless BIG collision.
• Jovian planets get most mass from
gas falling (accreting*) onto them from orbit
– Gas accretes all in the same direction (dir. of orbit)
– Lots of gas falls down (BIG planets) & is moving fast!
– Usually leads to fast rotation
– *For more information about how accretion works, read about “accretion”
and “accretion disks”, which is also important for star formation & black
holes. See pages 241-242, 554, 589-590, 600-601, 667, 671-672.
Which planets had more collisions?
1. Terrestrial
2. Jovian
3. Same
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Planet forming questions
• Which planet should be mostly metal?
• Which planets should be mostly rock?
• What is there more of: metal/rock or
hydrogen ice-forming compounds?
• Which planets should be bigger (and more
massive): inside or outside the frost line? Why?
Exceptions to planet patterns
• Unusual tilts & rotations  collisions
• Backwards orbiting moons  captured objects
• Earth’s unusually large Moon  BIG collision
– Mars-sized rock hit Earth early on
– Outer layers come off Earth
• Moon forms from debris.
• Looks like Earth’s outside
– Core of impactor stays on Earth
• Moon lacks metal core  density should be __________.
Summary
• See page 247 – excellent summary
• One last topic from chapter 8 – calculating ages
of rocks & solar system stuff.
Calif. Elementary School Science
Standards for solar system
• From California Science Standards, high
school
– Students know how the differences and
similarities among the sun, the terrestrial
planets, and the gas planets may have been
established during the formation of the solar
system
Astronomers would be surprised to find a
Jupiter-like planet at Mercury’s location
around another star.
1. True
2. False
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Radiometric Dating using
radioactive decays
P
Parent
Element
D
Daughter
Element
Half-life – time it takes for ½ of Parent to decay into Daughter.
Examples of radioactive isotopes -
238U
half-life = 4.5 Gyr
232Th half-life = 3.5 Gyr
14C half-life = 6,000 yrs (Carbon dating)
See also pages 248-250 for more on radiometric dating!
Fill this in based on the next slide.
Number of Time Number
of
half lives (years)
Parents
0
1
2
3
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Fraction
parents
still left
Number of
Daughters
# Daughter/
# Parent
Assume we’re doing Iron-60 dating. It has a half-life of 1.5 million years
Radioactive Decay, cont.
At time = 0, the rock formed
1 half-life later…
32 Parent Atoms 0 Daughter
(P)
(D)
______ yrs total
16 Parent 16 Daughter
16 units of heat energy
______ yrs total
______ yrs total
After 2 half-lives
8
24
8 more units of energy, 24 total
______ yrs total
4 half lives
Clicker
question
now
After 3 half-lives
4
28
1
31
______ yrs total
2
30
5 half-lives
After 6 half lives …
How many parents are
left after 2 half lives?
1. 8
2. zero
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Do you think Carbon dating is effective
for a 1 million year old fossil?
1. Yes
2. No
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Why or why not?
Then how do we measure old things’ ages?
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Element P decays into element D with a half-life of 10
million years. You find 3 times as many daughters as
parents. (D/P = 3) How old is the rock?
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5 million years
10 million years
20 million years
30 million years
40 million years
I have no clue
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You find an animal skeleton that has 1/8th as much
carbon-14 in it as living samples have. How old is the
skeleton?
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3,000 years
6,000 years
9,000 years
12,000 years
18,000 years
24,000 years
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Radiometric dating
• Any questions?
• Radiometric dating also called:
– Carbon dating (Carbon-14 dating) if using C.
– Radioactive dating
– Radioisotope dating.
– I won’t call it by these names. These names
won’t be on your test.