Characteristics and Origins of the Solar System
Lecture
11
October
2, 2000
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 this year.
Picture of Moon. Chapter 8 of
the textbook has pictures of these objects.
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.
- Picture
of Earth from Apollo 17 spacecraft Home Sweet Home
- Pictures
of lunar surface features taken from orbit around the Moon. Apollo
17---picture of crater Copernicus.
- Pictures
from the surface of the Moon.
Apollo 16, people near crater.
- Collection
of rock samples. Total of about
400 kilograms of samples brought back.Astronaut selecting rock sample
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.
- 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.
- 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.
- As
discussed on p173, ages of lunar rocks range from 3.3 to 4.5 billion
years. But there is more to say.
- The
Mare rocks are from 3.2 to 3.8 billion years.Apollo 11, Mare Basalt
- The
Terrae rocks are from 3.8 to 4.5 billion years. Apollo 17, Upland Anorthosite
- 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.
- 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.
- 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.
- We can
use Figure 8.16 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.