The Age of Bombardment

Start with a picture of the lunar surface:

http://antwrp.gsfc.nasa.gov/apod/ap020401.html

Last time I discussed the age of rocks from the lunar maria and the terrae. There was a systematic age difference, in that the terrae are about 500 million years older than the maria. We also note that the terrae are heavily cratered, whereas the maria are relatively smooth.

Since the time the Maria formed (about 3.2 billion years) the rate of crater-forming impacts has been relatively low (although they do occur). However, in the approximately 1 billion years up to the end of formation of the Maria, they were occurring at a rate which thoroughly pockmarked the lunar surface. A cartoon version of the history of the lunar surface is given in Fig 9.24 of your book. An even more striking picture is given in Figure 9.25.

The lesson from the Moon indicates that in the early ages of the solar system, the rate of crater-producing impacts was far higher than it has been since.

These results give us insight into the early history of the solar system. The “impactors” were the leftovers from the formation of the planets. Later I will introduce the term planetesimals for these objects.

We can use the cratering rate as a function of time to tell us about the geological history of other solar system objects.
If we see an object with a heavily cratered surface, we can conclude that there have been no geological processes since the age of bombardment.
If we see a smooth surface, on the other hand, this tells us that geological (or hydrological processes) have reformed the surface since the age of bombardment.

Let’s try a couple of examples: See what you think about the geological history of this planet. Think about it, and maybe chat with your fellow students on the bulletin board.

(a) the planet Mercury: http://antwrp.gsfc.nasa.gov/apod/ap030216.html

(b) Jupiter’s moon Europa: http://antwrp.gsfc.nasa.gov/apod/ap961120.html

Throughout the remainder of this course, we will apply these ideas to tell the history of the solar system.

Let’s turn to the Earth. As we have discussed in class a number of times, impact craters are not prominent on Earth because weather and rainfall obliterate them over times of hundreds of thousands to millions of years. Nonetheless, there are some that have been formed sufficiently recently that they are still intact, and in other cases there is a feature left in very old rock layers that can be identified as an impact crater. Many such craters have been identified on Earth, and a web site giving their locations and critical properties is given here.

Big Impact Web Site: http://www.unb.ca/passc/ImpactDatabase

All of these sites are of interest, but one is particularly intriguing..

Last of the Dinosaurs and the Crater of Doom

The Cretaceous-Tertiary Extinctions

Big kill-off 65 million years ago.
60 % of plant and animal species went extinct.
All large land animals.
What happened?

The Crater of Doom

Tell-tale hint was world-wide layer of clay laid down about this time. There was an enhanced abundance of the element Iridium.
Iridium is of enhanced abundance (relative to Earth) in meteorites.
Physicist Luis Alvarez of the University of California suggested that the aftermath of a giant impact had caused these extinctions.

Chicxulub Crater

Probable “killer crater” identified only in last few years.
Chicxulub on coast of Yucutan. Covered at present time.
Crater is about 200 kilometers in diameter, therefore produced by impactor about 20 kilometers in diameter.
Impact dated to 65 Myr ago.
Huge effect; glass beads from rock melt found all over the Caribbean basin.
Probable mechanism of extinction: world-wide blanket of cloud caused by smoke and ash that took years to settle out.

Other Big Extinctions

Recommended book, “The Sixth Extinction” by Richard Leakey.
There have been 5 major extinction episodes since complex life arose on Earth about 580 million years ago. The Cretaceous-Tertiary (K-T) was the last of them. >>>>> Diagram from Leakey’s book.
Most extreme was the “End Permian”, or Permian-Triassic, 250 million years ago.
Quote from Science magazine, September 8 issue: “There was no hiding from the greatest mass extinction of all time. Two hundred and fifty million years ago, at the end of the Permian period and the beginning of the Triassic, 85 % of the species in the sea and 70 % of the vertebrate genera living on the land vanished”.
>>>>>>> Figure from Science magazine showing abruptness of extinctions.
Did an impact cause this too? “For the moment, an impact is just one of several contenders to explain the P-T extinctions”.
It is certainly possible that a big impact, or some other astronomical agent, was responsible for this event. But for now it remains a speculation.
Some scientists have suggested that big impacts, and not natural selection, are the main steering wheel of evolution.

In later lectures, we will discuss what these impactors were, and see that there are still some of them around. (Be sure to check Addendum9 for a gripping picture, and a few more facts about big impacts and mass extinctions).

Comments on the Origin of the Moon

For over a century, it has been realized that there are some odd aspects about the Moon. The oddness became more pronounced in the time of the Apollo landings, when rock samples were returned to Earth. These unusual features of the Moon translate into difficulties in understanding the origin of the Moon.

Some of the attributes of the Moon that enter into consideration are as follows.

It is large relative to the size of the planet it orbits. Mercury and Venus have no moons; Mars has two very small moons.
Its density is significantly less than that of the Earth; about 3.3 rather than 5.5 grams per cubic centimeter.
The material that makes up the Moon rocks is in many ways quite similar to the Earth, and in other ways quite different. The relative abundance of isotopes of oxygen, O16, O17, and O18, is essentially identical to that of the Earth, whereas other objects in the solar system have measureably different oxygen isotopes.
Moon rocks differ from Earth rocks in that they are deficient in volatile materials. Volatiles are materials, such as water, that are driven off when matter is heated. This indicates that that material that formed the Moon was subjected to an intense heating episode.

Prior to the Apollo program, there were three primary theories for the origin of the Moon. These were fission, coformation, and capture from elsewhere in the solar system. I won’t go into a lot of discussion of them because the whole set of information available after the Apollo landings showed that none of them was viable.

The suggestion that was put forth which is, to date, the best possibility is the Giant Impact Theory. According to this viewpoint, the early Earth formed and had enough time for differentiation to occur. The process of differentiation is the process by which dense materials like iron and nickel settle to the center of the planet, leaving the light material in the outer parts of the planet.

At this point (the theory goes) the early Earth was hit by a large planetesimal about the size of the planet Mars, and a large amount of mantle material was thrown out into space, where it later collected to form the Moon. A cartoon representation is shown in Figure 9.29, and shows how it was that the Moon would have formed from lighter rocks, and thus have a lower density than the Earth. It also can explain the low abundance of volatile materials as due to the heating that occurred in the impact event. At the moment, this seems the best suggestion for the origin of the Moon.