1. Lecture: Planetology Continued (cratering) Part II: Solar System
Updated: Feb 14, 2011

2. 2
Planetology
D. Cratering

3. Contrasting regions on the Moon3
Contrasting regions on the Moon
A lightly cratered region, showing
smooth regions and only a few
mountains.
A heavily cratered region,
showing craters and mountains,
and much topography

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A region of the Moon, showing heavily and lightly cratered parts of the surface Photo from Lick Observatory

5. This is true for other moons, and planets such as Mercury5
Lower Crater Density
Higher Crater Density

6. Dating of Surfaces using Craters6
How can we use crater density (the number of
craters per unit area of the surface) to estimate
the relative ages of these different surfaces
on the Moon or a planet?
Consider a beach on planet Earth …

7. The Beach: A “young” surface7
The Beach: A “young” surface

8. The Beach: An “old” surface8
The Beach: An “old” surface

9. A mix of old and young surfaces9
A mix of old and young surfaces

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Youngest
Intermediate
Oldest

12. Estimate the age of the beach surface *12
Estimate the age of the beach surface *
Observations: Three areas of the beach
2 prints/m2
12 prints/m2
Saturated
(>20 prints/m2)
* Age means the time since the last wave came
along and wiped away the footprints.

13. Estimate the age of the beach surface13
Estimate the age of the beach surface
Observations: Three areas of the beach
2 prints/m2
12 prints/m2
Saturated
(>20 prints/m2)
Assumption: One person (2 prints) walks by every hour.
and no waves have washed away prints for at least a day

14. Estimate the age of the beach surface14
Estimate the age of the beach surface
Observations: Three areas of the beach
2 prints/m2
12 prints/m2
Saturated
(>20 prints/m2)
Assumption: One person (2 prints) walks by every hour.
and no waves have washed away prints for at least a day
Conclusions: a) The first area is about 1 hour old
b) The second area is about 6 hours old
c) The third area is 10 or more hours old

15. The same principle is applied to planetary surfaces that show different crater densities15
It is assumed that
craters are caused
by impacting objects;
that is, they are
impact craters
Saturn’s satellite
Enceladus

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It is further assumed that the rate of impacts has changed greatly over the age of the Solar System
In the earliest few hundred
million years after the
origin of the Earth, Moon,
and planets, the rate of
impacts was very high, but
rapidly reduced to a very
small rate that continues
to the present day.
We will return to this in
another lecture.

17. Summary of Cratering Studies17
Summary of Cratering Studies
Most of the craters were formed in the earliest years of the Solar System’s history.
With certain assumptions, we can use crater densities on planetary surfaces to estimate the relative ages of those surface regions.
Some Planetary surfaces are extremely old (lots of craters), others are very young.

18. Radioactive Dating 18
How we measure the absolute ages of rocks from Earth, the Moon, Mars, and the asteroids
Rocks contain small quantities of radioactive elements and their “daughter” elements.
The original radioactive elements (formed in stars) were incorporated into the rocky planets as they formed.
Over the years, the radioactive elements spontaneously decay to make other elements.
This process continues to the present time.

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Table 6-3, p.145

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How it works…
Figure 6.11 Radioactive Decay This graph shows (in red) the amount of a radioactive sample that remains after several half-lives have passed. After one half-life, half the sample is left; after two half-lives, one half of the remainder (or 1/4) is left; and after three half-lives, one half of that (or 1/8) is left. Note that in reality, the decay of radioactive materials in a rock sample would not cause any visible change in the appearance of the rock; the splashes of color seen here are shown for educational purposes only.
Fig 6-11, p.145

21. We can measure the amounts of Uranium, Lead, Rubidium, 21
We can measure the amounts of
Uranium, Lead, Rubidium,
Strontium, and other parent-daughter
combinations of elements in
meteorites and in lunar rocks.
Knowing the half-life of each of
these transmutations allows us to
calculate the “age” of the meteorite.
The “age” of a rock is the time since it last solidified
from the molten state.
Parts of the Allende meteorite solidified billion years ago

22. The Age of the Moon’s Surface22
The Apollo astronauts brought rocks back from the Moon (late 1960s to early 1970s)
Radioactive dating shows these rocks to be very old, but not as old as the meteorites
This “calibrates” the crater-density method of dating surfaces.

23. Meteor Crater in Arizona Diameter ~0.8 mile
Craters: The result of the impact of
asteroids and comets on the planets 23
Diameter ~0.8 mile Age ~50,000 years
Meteor Crater in Arizona Diameter ~0.8 mile
Meteor Crater
In Arizona

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Objects from space, entering Earth’s atmosphere and impacting on the surface

25. Manicouagan Crater in Canada25
Manicouagan Crater in Canada
An impact crater
preserved in the
ancient rocks of
the Canadian
Shield.
Diameter 70 km
(45 miles)
Age 200 million
years

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Heavy Bombardment
The early solar system was a violent place with many impacts & collisions of planetesimals.
MARS
MOON