Characteristics and Origins of the Solar System

Lecture 12

September 26, 2001

What We Learned From the Surface of the Moon

 

 

Last time we talked about the major surface features of the Moon: Terrae, Maria, Craters, Crater Rays, and Impact Basins.  For good picture of these objects, look at p178 or the Astronomy Picture of the Day  for January 13 of  2000. Chapter 8 of the textbook has pictures of these objects. 

For a picture from one of the Apollo spaceships, check out

http://www.solarviews.com/r/moon/moon1sm.gif

            A computer simulation of the moon rotating in front of you, which could only be seen by a spaceship, is given by

http://www.solarviews.com/cap/moon/vmoon1.htm

 

 

Before the actual landing on the Moon, scientists had figured out that the lunar craters were impact craters, i.e. holes excavated by the energy released when an impacting object (meteor) strikes the surface of an object.  The process is illustrated in Figure 8.14 of your book, which has a nice discussion of the whole business. The book also has a handy-dandy relation that the size of the crater is in the ballpark of 10 to 15 times the diameter of the projectile. 

 

At the time this was figured out, the Moon was the only astronomical object known to possess craters (with the exception of a couple of examples here on Earth).  A great deal has changed in the last few decades. 

 

The Moon as Seen by the Apollo Spacecraft

Let’s look at some pictures of the Moon and its characteristics as seen by the Apollo spacecraft.

 

http://ilewg.jsc.nasa.gov/expmoon/Apollo17/A17_sampact.html

http://ilewg.jsc.nasa.gov/expmoon/Apollo11/

 

Characteristics of Lunar Rock Samples

The most valuable aspect of the Apollo mission was analysis of lunar rock samples.  What was particularly important was dating the age of formation of the rocks via the technique of radioisotope dating, as we have discussed. 

 

            The following results were obtained.

  1. All lunar rocks are igneous; no sedimentary, no metamorphic.  The rocks of the lunar highlands are of a sort called anorthosite, the rocks in the maria are basalts.
  2. Radioisotope dating showed the lunar rocks (normal ones you would pick up on the surface of the Moon) to be extremely old, older than all rocks on the Earth. The ages of the lunar rocks are discussed on p173 of book, but the discussion of the topic is more brief than I like. So here goes.
  3. As discussed on p173, ages of lunar rocks range from 3.3 to 4.5 billion years. But there is more to say. 
  4. The Mare rocks are from 3.2 to 3.8 billion years.
  5. The Terrae rocks are from 3.8 to 4.5 billion years.
  6. There is therefore a difference in the age of formation of the terrae and the maria.  The difference is small compared to the age of the Moon, but significant.
  7. Combining the realization that craters were caused by impacts, and the fact that the terrae are heavily cratered while the maria are quite smooth, leads to a fascinating conclusion about the early history of the solar system

 

The Age of Bombardment

 

            Look on p177 and read the section entitled “Using Crater Counts”.  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 highlands. 

            This indicates that in the first billion years of the history of the solar system, the rate of crater-producing impacts was far higher than it has been since.

            This has been quantitatively expressed in the form of Figure 8.16.  I have a better one.

>>>>>> Space Commander version of cratering rate versus time.

            This plot has two consequences.

  1. It gives 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.
  2. We can use Figure 8.16 to tell us about the geological history of other solar system objects.
  3. 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.
  4. 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.