Impact Cratering Dating

1
Impact Cratering Dating

• Nathan Marsh

2
Relative Dating

• Simple but not as informative
• Measures the crater densities (craters per square
  kilometer)
• Generally a less cratered area is younger than a
  more cratered area

3

• Lab analysis of Apollo samples of the Mare
  provided data on absolute dates when lava
  solidified
• Provided data on the relationship between age
  and number of craters

4
Reference points in dating

• The Apollo and Luna missions analyzed specific
  dates on certain areas of the moon
• Late Heavy Bombardment craters 3.9 Gy
• Light-toned Terrae (highlands) plagioclase
  feldspar 4.5 Ga
• Dark-toned Mare volcanic basalts
• Maria have 200 times fewer craters
• Mare ages 3.1-3.8 Ga
• Terrae basin ages all 3.8-4.0 Ga

5
Absolute DatingProduction Function

• A more exact way to date surfaces
• This is the ratio of smaller craters created for
  every large one
• Plotted in log-log space
• Vertical position is related to age

6
Absolute Dating

• This can be estimated if the rate of formation of
  craters is known
• The crater size/frequency distribution is
  displayed on a relative (R) plot
• D- the geometric mean diameter of the size
• N- Number of crates in the size
• A is area counted
• Bu is the upper limit of the size
• Bl is the lower limit

7

• Absolute ages allow comparison of events on
  different planets
• The impact crater size distributions for Mercury
  and other inner solar system objects seem to be
  consistent with an early cataclysmic heavy
  bombardment.
• The comparison of the R plots on the highland
  cratering records on Mercury, the Moon and Mars
  describes the nature of the objects that were
  impacting during the period of heavy bombardment
  within the inner solar system

8
Crater Saturation

• Eventually a point is reached when the craters
  are so crowded that new impacts destroy craters
  as fast as they make new ones
• The surface reaches a phase known as steady state
  cratering, or crater saturation
• During this period, no new information can be
  deciphered about the age other than the surface
  is at least 4 billion years because only during
  an early intense bombardment was a surface able
  to acquire enough hits to become saturated

9
(No Transcript)
10
The Late Heavy Bombardment

• Crater dating is used to estimate the relative
  age of a planetary object based on crater
  statistics
• The manned Apollo missions returned iron rich
  lunar rocks that dated back to 3.9 billion years
• Same time as the Late Heavy Bombardment

11

• Thought that the bombardment came from the
  asteroid belt and effected all inner solar system
  objects including Mercury
• May have been ejected from the asteroid belt by
  the combined gravitational fluctuations of
  Jupiter
• Or a a giant collision in the asteroid belt
• Late heavy bombardment is used as a reference
  point
• Not many large craters (gt100km) are made, mostly
  smaller impacts

12
Crater Retention Ages

• Erosion, dust movement and lava flows can
  completely bury craters
• Smaller craters may only last 1 Myr while larger
  ones will last 100 Myrs on Mars
• Lunar craters are eroded away at a much slower
  rate due to smaller impacts and the walls slump
  into the cavity and fill it up

13
Conclusion

• Relative and Absolute dating
• Crater count can date a surface
• Crater ages are linked relative to position on
  other craters
• Apollo missions convert the relative data to
  absolute ages
• Crater Saturation
• Craters become saturated to a point beyond
  ability to date past 4 Ga
• Late Heavy Bombardment
• May have come from the asteroid belt
• Caused by collision of gravity of Jupiter
• Dating Difficulty
• Craters are eroded by other impacts and lava
  flows