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

2 Planetology D. Cratering

Contrasting regions on the Moon 3 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

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

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

Dating of Surfaces using Craters 6 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 …

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

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

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

10

11 Youngest Intermediate Oldest

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.

Estimate the age of the beach surface 13 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

Estimate the age of the beach surface 14 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

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

16 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.

Summary of Cratering Studies 17 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.

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.

19 Table 6-3, p.145

20 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

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 4.566 billion years ago

The Age of the Moon’s Surface 22 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.

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

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

Manicouagan Crater in Canada 25 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

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