29:50 Modern Astronomy
Fall 1999
Lecture 36 ...November 22, 1999
The Moon- II: Ages of Moon Rocks and the Geological History of the Moon

Watch the skies! Watch the skies!
Aliens among us; the movie The Day the Earth Stood Still
Full Moon tonight at 1 AM; Moon also at perigee; 56 . It is as far as 63 . in its orbit. Next week an exciting development in astronomy; December 3 is landing of Mars Polar Lander at Martian coordinates: 195 West, 76 S, about 800 kilometers from the Martian south pole.

Last time I was talking about radioisotope dating of Moon rocks. I explained what isotopes were, and what radioactive decay is. Today, I will continue, in particular giving the results from dating of the moon rocks. Recall that we use the radioactive decay of ,

Since this process occurs with a precisely known half life, we can use the ratio of Rb to Sr atoms to determine the age of the rock.
diagram showing Rb and Sr density in a rock.

Here's a question for the curious to think about. How can we insure that there was no Sr in the rock to begin with? Clearly if there were (and there has to be some) it would distort the inferred age of the rock. There is an answer to this which I would be happy to explain to anyone.

What are the results when you make these measurements with the Moon rocks?
Table with ages of lunar rocks.
Slide of ``Genesis Rock'' from Apollo 15.
From this table of ages we can deduce three things.

1. All moon rock are real old, all 3-5 billion years old. The face of the moon has changed very little since the earliest days of the solar system. Most terrestrial rocks are far younger.
2. All rocks igneous, formed in volcanic-like processes. No metamorphic or sedimentary rocks. Not only does the Moon not have water now, it apparently never did.
3. Most interesting, Mare rocks formed 0.5 to 1.0 billion years after the formation of the Terrae.

This last fact allows us to draw a fascinating picture of the early history of the Moon, and in fact, of the Solar System. These data lead us to conclude that there must have been an early ``Age of Bombardment''.

The rocks from the heavily cratered Terrae are 4-4.5 Billion years old. The impacts must have occurred during this period of time, or afterward.
The rocks from the Maria formed from 3 -3.7 or so billion years ago. There are few craters on the Maria, so the crater-forming impacts must have been largely completed by that time.
To be sure, impacts did continue and continue to the present day; the crater Copernicus is estimated to have been formed 1.1 Billion years ago. However, the bulk of the craters were formed long before that.

People have made quantitative measurements of the density of craters as on various places on the Moon for which we have radioisotope dates, and from this inferred the crater-forming rate as a function of time. This is illustrated in the following Figure.
Figure with cratering rate as function of time (from Christianson and Hamblin). This figure shows the cratering rate, or number of impacts per unit area per unit time, from the present going backwards.

We see that at the time of the formation of the crater Copernicus, 1.1 Billion years ago this rate was only a few times greater than now.
Reality Check: An idea of how long ago 1.1 Billion years ago was. I will ask questions about subjects you all know better than I do.

• How long ago did the first humanoids appear (Australopithecus Afarensis)? million
• How long ago was the famous Cretaceous Tertiary extinctions? 65 million years ago.
• Permian period (first dinosaurs) 250 million
• When was first life on land? 400 - 500 million
• When was Cambrian period ( 500 - 600 million)

The net effect of this is that all of this geological and biological history of Earth fits well within the time since the crater Copernicus formed. And it sits up there on the Moon fresh as a daisy!

Most of the mare formation occurred between roughly 3 -4 billion years ago. Over this period the bombardment rate increased drastically from about 7 times the current rate to perhaps 30 times the current rate. Before 4 billion years ago, there were no Maria, and the cratering rate was higher by a factor of several hundred. An intriguing artist's conception of this epoch is shown on the painting on the introduction to Chapter 9.

All of these data allow us to piece together the following picture of the geological history of the Moon. It is illustrated in Figure 9-24 of your book.
The Moon formed at the same time as the rest of the Solar System objects, 4.50 - 4.60 billion years ago.
The Moon's surface was heavily bombarded by meteors, forming the craters, many of which we see today. Large, asteroid-sized objects produced huge impact basins. This occurred 4.6 to 3.8 billion years ago.
From roughly 4 - 3 billion years ago, we had the Mare era, in which lava flowed from the interior of the Moon and filled in some of the impact basins, forming the Maria.
This lava hardened, and formed the Moon as it pretty much is today. An occasional meteor has dug out a new crater, but pretty much the lunar surface is as it was 3 billion years ago.

A fantastic representation of this is given in Figure 9-25 of your book, on p188. If you have it with you, I would suggest opening it up. The panel at the extreme left shows the Moon as it would have appeared 4.0 billion years ago, right at the beginning of the Mare-forming period. The entire lunar surface is pockmarked with craters, including some huge impact basins caused by the largest of these. The Moon at this time would have been visibly different from the present Moon, even for naked eye observations.

By about 3 billion years ago, we would have had the middle panel. Lava would have flowed from the lunar interior and filled the impact basins. The moon of this period is practically indistinguishable from that of today (far right panel), the only difference being that the Maria have been targets for cratering the past several billion years.

The fascinating conclusion for this is that when you look at the surface of the Moon, you are looking at a landscape which has changed very little in the last 3.2 billion years. This differs radically from the case with the Earth's surface, for which major changes were introduced by glaciers only 12,000 years ago.

The lesson we can learn about the rest of the Solar System. We can use the information we have learned from the Moon. It allows us to make important inferences about astronomical objects just from photographs of the surface. The Moon data shows clearly an intense ``age of bombardment'' in the first several hundred million years of the Solar System's history. For most of the history of the Solar System, the crater-producing-impact rate has been relatively small.

This means if we see a planetary surface which is heavily cratered, it means that there has been relatively little surface activity since 3.0 to 3.5 billion years ago. If the surface shows few craters, we can conclude that geological processes have modified the surface in the past 3.0 billion years. An obvious illustration of this is the different surfaces of the Earth and the Moon. Another example is the different surfaces of the Jovian moons Europa and Callisto (you can see them through binoculars these evenings). We shall encounter many more examples in the coming weeks.

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