Calculating Ages of Solar System
objects
Radiometric dating
Cratering
Sequencing
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
4
5
Fraction
parents
still left
Number of
Daughters
# Daughter/
# Parent
Assume we’re using an element with a half-life of 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
Clicker
question
now
16 Parent 16 Daughter
16 units of heat energy
How many parents are left
after 2 half lives?
1. 8
2. zero
0
0
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Assume we’re using an element with a half-life of 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 …
Do you think Carbon dating is effective for a 1
million year old fossil?
1. Yes
2. No
0
0
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?
1.
2.
3.
4.
5.
6.
5 million years
10 million years
20 million years
30 million years
40 million years
I have no clue
You find an animal skeleton that has 1/8th as much carbon14 in it as living samples have. How old is the skeleton?
1.
2.
3.
4.
5.
6.
3,000 years
6,000 years
9,000 years
12,000 years
18,000 years
24,000 years
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.
Review: Simplified History
Formation - 4.6 Gyrs ago
Present
Heavy Cratering ~ ½ Gyrs
Cooling, cratering declines ~ 4 Gyrs
Recall: 1 Gyr = 1 billion years
This history was determined by
radiometric dating of rocks ON
THE MOON!
Note: the Sun is ________ years old. It’s 50% burned out.
How much longer will it last?
California Elementary School Science
Standards for solar system
• From California Science Standards, high school
 Students know the evidence from geological studies
of Earth and other planets suggest that the early
Earth was very different from Earth today.
 Students know the evidence for the dramatic effects
that asteroid impacts have had in shaping the surface
of planets and their moons and in mass extinctions of
life on Earth.
Crater Formation
Before
11-70 km/sec! Why is 11 km/sec minimum?
Rocky or icy objects (also called ___ & ___ )
This is going to leave a mark…
Impact site’s ground gets vaporized. So does
the impactor!
Craters
Ejecta
Central Peak
Rim
An object this size
Will make a crater this big!
Diameter of crater is about 10x diameter of rock. Depth of crater is about
1-2x the diameter of rock. See also p. 264-265
Minimum impact speed for craters on
the Moon is ____ than it is on Earth.
1.
2.
3.
4.
0
0
0
0
Faster
Slower
Same
Not enough information
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Crater Rays
Recent impact crater Degas
Located on Mercury
These streaks of ejecta are called “rays”.
Bright initially, but solar wind fades them
over about 100 million years because solar
wind particles chemically alters them.
Most prominent under direct illumination
(at what time, locally?).
So, how can you find the youngest craters on the Moon?
rays
Crater Sizes
Small < 25 km
Big - up to 200 km
Note the central peaks
This puppy is the size of the L.A. basin!
And…
REALLY HUGE!!
Also known as “multi-ring impact basins”
Similar, but a bit
larger crater
formed on
Mercury, and on
the opposite side,
we see “weird
terrain” see page
274.
That just had to hurt.
Big crater on the Moon
How do we measure ages?
1. Relative dating – Principle of Superposition
Ordering of events based on appearance.
2. Crater counts – surfaces with lots of craters are new
or old? [clicker] Why? (see p. 269-271)
3. Radiometric dating of rocks. (see also section 8.5)
Called “Carbon dating” when used on recent fossils.
Lots of craters on the ground. The
ground is:
1. Old
2. New
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Relative Surface Dating – Can you order the events?
A. Volcanic Eruption
B. Earthquake
C. Crater
D. Hills
E. Cratered Terrain
(Hint: old or new?
Created when?)
F. Crater
G. Crater
When did the volcano happen?
1. Before earthquake
2. After earthquake
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A Real Example - Iapetus
Iapetus is one of Saturn’s moons.
Picture taken by Cassini
Scarp (Shrinkage
quake) &
subsequent landslide
There is cratering
on ground. Do
you see many or
few craters in this
red crater?
What does that
tell you about the
circled crater?
Is old or new?
Crater Rim
Smaller
Crater
What can you say
about the age of the
small crater?
Landslide?
Iapetus – entire moon
• Diameter = 1460 km
• Here’s the “big crater” we
just looked at.
• Approx 500 km across
• Landslide is here.
• Ridge all along equator.
Mystery!
• In Fall 2009 – the
light/dark hemispheres
mystery was solved.
Ages
• Now you know how astronomers calculate
ages.
• Study for the last test.
• It is NOT cumulative.