Download Impact Age Dating ASTRO 202 Lecture Thursday, February 14, 2008

Review
Impact Age Dating
Carbon-14, Potassium-Argon
isotopic age determination:
ASTRO 202 Lecture
Thursday, February 14,
2008
What is relative
age dating?
(1) Parent decays to daughter
at some predictable rate
(2) How much now?
(3) How much then???
What is relative
age dating?
Before absolute age scales…
scales…
Before absolute age scales…
scales…
… relative age scales
… relative age scales
A terrestrial example: layered
deposits
A terrestrial example: layered
deposits
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What is relative
age dating?
The rationale for using crater counts for relative age dating:
- Assume surfaces form without any craters
Craters are the result of impacts with asteroids
and comets
-Two ways to establish relative scale:
(1) The older a surface, the more craters it
accumulates
(2) Larger impactors are less common than
smaller; the more large craters on a surface, the
older it is
Why aren’
aren’t all surfaces in the solar system
heavily cratered?
Processes that remove craters
from a surface:
Why aren’
aren’t all surfaces in the solar system
heavily cratered?
Result is a variety of cratered surfaces…
surfaces…
Enceladus
Erosion
- Rain/water, ice,
wind
Gravity
- Landslides,
“mass wasting”
wasting”
Resurfacing
- Volcanism, tectonics
Earth
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Why aren’
aren’t all surfaces in the solar system
heavily cratered?
Why aren’
aren’t all surfaces in the solar system
heavily cratered?
Martian dichotomy…
Moderately/sparsely
cratered…
Venus
Southern
Hemisphere
Mars
Why aren’
aren’t all surfaces in the solar system
heavily cratered?
Heavily cratered:
Northern
Hemisphere
Why aren’
aren’t all surfaces in the solar system
heavily cratered?
Mercury…
Mare (young)
Highlands (old)
The Moon…
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Tethys
- saturnian satellite
Diagram courtesy of:
saturn.jpl.nasa.gov
Odysseus Crater
Tethys
Rhea
- saturnian satellite
-large satellite of Saturn
- seems to have
- seems “saturated” with
survived a giant impact
craters
-one of the most heavily
cratered surfaces in the
Solar System
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Counting Craters
Counting Craters
?
The method:
(1) What is a crater?
-Resolution, various stages of erosion can
make consistent identification difficult
??
???
????
http://clickworkers.arc.nasa.gov/top
Counting Craters
Counting Craters
The method:
(1) What is a crater?
-Resolution, various stages of erosion can
make consistent identification difficult
(2) How do we keep track?
-SizeSize-frequency distributions (SDF
(SDF’’s) record
both the number and sizes of craters in a
given area
Cumulative density of craters NH>DL @km−2D
All Craters
Identified
as Secondaries
in the Newton
Area
0.08
0.06
0.04
0.02
0
0
0.5
1
Crater
1.5
diameter , D @km D
2
2.5
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How do we make the
relative scale absolute?
Apollo Lunar Sample
Return
Apollo Program: 1963-1972
The relative scale is only so useful…
useful… For constraining the history of
planets and Solar System formation, we need something better.
11, 12, 14-17 Missions landed
and safely returned humans
from the Moon…
- Combine relative scale with known absolute ages
… and samples!!!
Apollo program to the rescue!
Combining Relative and
Absolute Scales
From the SFD’
SFD’s, we add the
Apollo sample return absolute
ages to create crater
isochrons for the Moon
Extending the Absolute
Scale Beyond the Moon
We still don’
don’t have the ability to retrieve samples from the other
planetary surfaces, so how do we determine absolute ages for these
these
bodies?
-Know something about the distribution of impactors that cause the
the
cratering we see on the Moon
-Determine how this distribution differs at other places in the Solar
Solar
System
Some other factors that affect cratering distribution:
-Atmosphere, gravity, surface strength, erosion and
resurfacing rates
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Extending the Absolute
Scale Beyond the Moon
Extending the Absolute
Scale Beyond the Moon
Example
martian
isochrons
Extending the Absolute
Scale Beyond the Moon
Controversy
There are a number of controversies when it comes
to dating the Solar System…
System…
-The Late Heavy Bombardment
-Secondary Cratering
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Late Heavy
Bombardment
What is the late heavy
bombardment?
Late Heavy
Bombardment
Why does the late heavy bombardment matter?
-Some scientists believe
that the number of
impactors has undergone a
steady exponential decrease
since the formation of the
Solar System
-Did it happen everywhere in the Solar System?
-Are the extrapolations of the absolute scale
to other surfaces correct?
-Others cite the Apollo
sample returns as evidence
that there was a burst of
impacts around 4 Ga known
as the late heavy
bombardment
-What are the implications for models of Solar
System formation?
Secondary Cratering
Shapes of the crater isochrons:
Secondary Cratering
Shapes of the crater isochrons:
Eugene Shoemaker identified two shapes or “branches”
branches” of the
cratering isochrons for the Moon
Fit powerpower-laws (SFD’
(SFD’s) of the form:
N(>D) = c D-b
“Shallow” slope
“Steep” slope
Steep powerpower-law index (b~3,4) dubbed “secondary”
secondary” branch;
shallower (b~2) index called “primary”
primary” branch
Secondaries footprints of ejecta from large primary impacts
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Secondary Cratering
Dominance of secondary and primary branches of SFD
-Some groups claim that secondaries dominate at D < 1 km
-Others claim that secondaries are at most 10% of small
Martian craters
Are secondaries even “secondaries”
secondaries”?
Review
-Even on planets and satellites where we can’
can’t use surface samples
to derive absolute ages, we can establish a relative age scale from
from
crater sizesize-distribution counts
-From lunar samples returned to Earth by the Apollo astronauts,
we can construct a viable absolute age scale for surfaces on the
Moon
-Neukum
Neukum,, Ivanov and others argue that the secondary branch
is formed by a smallsmall-diameter asteroid population
-Using what we know about the surface strength, gravity,
resurfacing and impactor rates on other planets and satellites, the
absolute age scale for the Moon can be extrapolated to a wide
variety of Solar System surfaces
-Controversial issues such as the existence of the late heavy
bombardment and secondary cratering need to be resolved to allow
for more accurate absolute dating of surfaces in our Solar System
System
Example crater isochrons for Mars and the Moon from:
http://www.psi.edu/projects/mgs/chron04b.html
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