From many decades before the space age, devoted amateurs did observe the Moon. Its surface was described and mapped in painstaking detail, crater after crater, mountain after mountain. This provides up to today very interesting guides for observers but it is, to be honest, dreadfully boring reading. You get lost in detail and it becomes impossible to see the larger picture, the context in which for instance a crater you observe must be seen. It all changed with the beginning of the space age. To put men on the Moon, we needed to learn a lot of our celestial neighbour. The occasion was grasped by some inspired scientist to study the Moon and change our view of it completely. It went for instance into the hands of geologists and they showed us a Moon full of excitement and mystery.

Stratigraphy or the study of the occurrence of layers in relation to each other is a very powerful tool in geology. An older layer is for instance always found below a younger layer (if no overturning of layers has occurred which can in most cases be deduced). Another principle is that if a layer is intersected by another, the former is older than the latter. Thus by simply looking very well to sequences of deposition, relative ages can be deduced. Stratigraphy was and still is much studied on Earth. In 1962 Shoemaker and Hackman realised its potential for Moon exploration. Craters, impact basins and especially their ejecta deposits were excellent markers to distinguish between older and younger deposits and in this way unravelling the sequence of events making up the Moon’s geological history.

Theophilus, Cyrillus and Catharina

Let us for instance look at the crater trio Theophilus, Cyrillus and Catharina. Theophilus is superposed on Cyrillus meaning that the former is younger than the latter. Notice also that the older crater is more degraded since it is eroded by other impacts for much longer. The relation of Catharina with the other two craters is less certain at first sight since it is not covered or does not cover one of them. Here the degree of degradation comes in again. It is more degraded and has more small craters superposed on it than Cyrillus and must therefore be older. Thus besides the superposition principal (which structure is found above the other) also the degree of degradation is a powerful tool to put structures in a relative time frame.

Mare Imbrium region with indication of Copernicus (1), Eratosthenes (2), mare lava (3),Archimedes (4), Montes Apenninus (5) and the three main rings of the Imbrium impactbasin.

If you get lost between craters, impact basins, etc. take a look at Mare Imbrium and its surroundings. It is a classic area to begin study lunar stratigraphy. Copernicus is a young crater with a beautiful and impressive ray system. Nearby Eratosthenes, however, shows no rays. Taking a detailed look, the ray system of Copernicus is superposed on Eratosthenes. This means that Copernicus is younger than Eratosthenes. Both craters are younger than the mare on which they are located. Crater Archimedes for instance is filled and surrounded by lava, meaning that it is older than the surface lava floods of Mare Imbrium. Finally, the lava forming Mare Imbrium is found inside the Imbrium impact basin. This impact basin can be seen in the form of the remnants of its mountainous rings. One of the most obvious rings is formed by the Montes Alpes, Montes Apenninus and Montes Carpatus. An inner is ring is formed by smaller mountain ranges (Montes Recti, Montes Teneriffe, Montes Spitzbergensis) and even isolated mountain peaks (Piton, Pico). A third ring is suggested by a complex of mare ridges. So from these easily made observations, the next sequence of events in the Imbrium region is derived (from old to young):

• Formation of the Imbrium impact basin
• Extended period of volcanism, filling the basin. Impact craters formed during this time, such as for instance Archimedes, are filled and engulfed by mare basalts.
• Eratostenes as an example of large craters which are formed after the period of major lava extrusion but which are old enough to have their ray system faded away.
• Copernicus as an example of large craters with a ray system.

It also gives us a synthesis of the Moon’s geological history. Right after the formation of the Moon, our natural satellite was battered by numerous impacts. The testimony of that can be found in the southern highlands. The largest impacts gave rise to the giant impact basins. Lava extrusion must have also occurred in these days but only few examples can be seen on today’s surface because they were destroyed or buried under more recent deposits. Later on the rate of impacts on the Moon decreased and no more impact basins were formed. In fact the Orientale impact basin must have been one of the last. Lave kept flowing out on the surface, filling the natural depressions: the impact basin. Cratering continued but with a decreasing rate. With time the outflow of lava also decreased because the Moon’s internal geothermal machinery gradually extinguished. Thereafter there were only occasional large impacts forming the youngest craters such as Copernicus and Tycho.

Just as we have a geological time scale of the Earth, a similar time scale was constructed for the Moon. The events we can deduce in the Imbrium region were vital for the construction of this table. Five so called systems were defined in this geological time table of the Moon mainly based on the observations in and around the Imbrium impact basin: Pre-Nectarian System, Nectarian System, Imbrium System, Eratosthenian System and Copernican System. Main characteristics are summarised in the table below.

By trying to deduce which deposit is younger than the other, a relative sequence of events can be deduced. No absolute dates can be assigned. One of the many fine scientific outcomes of the space program was that it provided scientists with rock samples which could be dated. Therefore the main events in the Moon’s geological history could be situated in time.

Geological time table of the Moon

System	Age (year)	Examples		Events
Copernican Eratosthenian Imbrian Nectorian Pre - Nectorian	1.10 10exp9 3.20 10exp9 3.85 10exp9 3.95 10exp9 4.55 10exp9	-Very few large craters	-Tycho -Aristarchus -Copernicus	-Craters with bright ray systems -craters with degraded bright ray systems
		-few large craters -Apollo 12 lava -Apollo 15 lava	-Eratosthenes -Imbrium lava	-Copernican like craters but without bright ray systems
		-Luna 16 rocks -Mare lava -Apollo 11 lava -Apollo 17 lava	-Mare basalts -Mare Orientale -Mare Imbrium	-extensive volcanism
		-Apollo 14 rocks -large impact frequency -Apollo 16 rocks	-Mare Crisium -Mare Humorum -Mare Nectaris	-Many big craters -Impact basins and their ejecta blankets
		-very large impact frequency	-Mare Serenitatis -Mare Smythii -Mare Tranquillitatis -Mare Nubium -Formation of the Moon

Based on this time table a geological map of the Moon was constructed. This map shows the well know topographic map of the Moon, but is overlain by colours. These colours refer to the age of the structures. It provides thus information about the age of the structure and it surroundings you are observing. In this way it is an invaluable companion beside the topographic maps for every serious lunar observer.

Geological map of the Moon (Wilhelms, 1987).

This geological maps can be found in the book ‘The Geologic History of the Moon’ by Wilhelms (1987) (also downloadable at http://cps.earth.northwestern.edu/GHM/ or on
http://astrogeology.usgs.gov/Projects/webgis .

Like the examples of Theophilus, Cyrillus and Catharina and the Imbrium region, every area on the Moon has its own story to tell. By simply looking at the relations between different structures and units, a lot can be learned about it and this can be placed in a time frame by using the geological map. It simply gives observing the Moon an extra dimension. Try it and the Moon will never get dull!