Characteristics and Origins of the Solar System
Lecture
12
September
26, 2001
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.
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/
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.
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.