Last week we learned how to date rocks once we get them back to Earth, so that we can analyze them in our high-tech Earth-based labs. Of course, we’ve only visited a very small proportion of the surface of the Moon. How do we decide on relative ages of the places we haven’t visited?

Actually, the Apollo landing sites were very carefully selected to allow the astronauts to sample from a variety of different geologic units. So we do have some idea of the ages of various flows. However, most of the “Geological Map of the Moon” is based on mapping that was done using satellite images like the one shown here. In this exercise, you will determine the relative ages of the various craters and other geological formations in this area.

Examine these images carefully, and then prepare a chronology that includes all the formations and features listed below. The chronology can be set up as a simple list starting with the oldest formation or feature and ending with the youngest. You will quickly learn, however, that it may not be possible to define a single, unique sequence.

Once you have created your chronology, justify it by writing brief descriptions of the evidence used to place each formation or feature where it is in the sequence. Where age ambiguities occur, justify the permissible range of relative ages for the ambiguous features.

An example of how you might justify a relative age sequence would be:

“Small crater D must be younger than large crater A because it occurs within the large crater and could not have survived formation of the large crater had it been there first.” In geological parlance, we would say that crater D is superposed on crater A. This is an example of a rather obvious relationship; others are more subtle.

Here’s the list of formations and features to be ranked in time:

brighter plains material (Pb)

bright rays

graben structure

large crater A

large crater B

large crater C

small crater D

small crater E

small crater F

small crater G