Geology of the Moon

This article arose as the result of comments from several people at the September 2004 Workshop that they don't observe the Moon very often and don't know much about it, despite the fact that it is our nearest neighbour in space - light takes just over a second to get there. It has fascinated the curious since the earliest times in human history. So why do we not observe it more often? What exactly do we know about the Moon?

Everyone is familiar with the "onion skin" model of the interior of the Earth: a thin rocky crust, a think mantle, and a nickel-iron core comprising of a solid outer and liquid inner core. So one question we might ask is whether the Moon has a similar structure, or whether it is a fairly homogeneous sphere. Just as drilling to the centre of the Earth was not necessary to determine its structure, we do not need to do it on the Moon either.

But before diving into geology, let's see what we can determine from just observing the surface from the Earth with binoculars or a small telescope.

When looking at the Moon, the most obvious feature you notice is the contrast between the light and dark areas. The dark regions (Mare Imbrium, Mare Serenitatis, etc.) have long been known not to be "seas" as thought by the astronomers who first named them. Looking closely it is possible to detect a few craters within the mare and, with higher magnification, large mountain ranges surrounding some of them. In contrast, the lighter regions tend to be much more heavily cratered and are known as the lunar highlands.

We know the dark areas are not really seas or oceans, but what we now know to be craters were not always thought to be so either. Before the age of space exploration there was quite a debate as to their nature. One group claimed them to be impact features caused by lumps of rock flying around in the early solar system, while the other (including Sir Patrick himself) suggested that they were more likely to be volcanic features, the calderas remaining after eruptions when the Moon's surface was forming. We now know them to be craters of course, but in the days before space travel and high resolution imaging this was not obvious. The study of our nearest neighbour has come a long way in the past century.

What can we tell about craters from simple observations? By examining them for signs of erosion we can estimate relative ages: the more eroded and the less distinct a crater is, the older it probably is. Younger craters often have a central peak where the meteorite (known as the "impactor") hit. Over time, this central peak disappears. On the Earth this is largely due to erosion and weathering, but this cannot be the case on the Moon as there is no atmosphere.

We can also tell something about the size and energy of the impactor from measuring the size of the crater and the extent of the ejector blanket (the stuff that got blasted from the surface during the impact and eventually settled back to the ground). It is the re-settling of ejecta that is largely responsible for making older craters less distinct.

Cratering counts
Guessing relative ages from the amount of erosion works to some extent, but only for individual craters. In order to date an area we need another method. In the early solar system as the planets were forming there was a lot of debris around. As these small rocks orbited the young Sun they would eventually hit larger bodies and produce a crater. Over time the number of these rocks decreased as they crashed into proto-planets and hence the cratering rate - the number of craters produced in a given time period - reduced. If the surface of the larger body was also changing with time through its own geological processes such as volcanism or plate tectonics, then older parts of the surface would be the most heavily cratered while those which had been formed later (due to lava flows from volcanic eruptions for example) would have fewer craters. From this we can already say, just by simple observation, that the lunar highlands are older than the mare because they are more heavily cratered.

The principle of superposition
Age estimates of individual craters (and other features) can be made more systematic through the use of the principle of superposition. As well as surfaces being renewed by geological processes, newer impacts can partially or wholly obscure older ones. Recent fault lines can split craters down the middle, and ejecta from nearby impacts can make them less distinct. This principle is a very useful one in geology and is just as applicable on the Earth and other planets as it is on the Moon.

Close up of Mare Imbrium to illustrate the principle of superposition

A good example of the principle of superposition can be seen in the Mare Imbrium area of the Moon. The first event that occurred was the major impact which formed the Imbrium basin itself (image 1). Further impacts then occurred as more lumps of rock hit the surface creating craters such as Archimedes within the basin (image 2). Then a major lava flow filled the Imbrium basin and flooded or "embayed" other features within the basin (image 3). The last stage in this process is further impacts on the cooled lava basin (image 4).

This is a simple and very useful technique which can be applied to the rest of the Moon and any of the rocky planets or moons in the solar system. It does, however, only give relative ages of features. To date features specifically we need to use other forms of measurement.

Dating the events
This was one of the scientific outcomes of the Apollo missions. Over the six missions, several different areas of the surface were visited and a total of 382 kg of rock was returned to Earth for analysis. As is well known, Apollo 11 landed in Mare Tranquillitatis (or the Sea of Tranquillity), but the other missions went to other parts of the surface. Apollo 12 landed in Oceanus Procellarum, and Apollos 16 and 17 landed in different areas of the lunar highlands.

Two missions landed in areas important to the example of the Imbrium basin above: Apollo 14 landed in the Fra Mauro region and was able to sample ejecta from the Imbrium impact, while Apollo 15 landed in the Apennines, the mountain range stretching around part of the Imbrium basin, and was able to sample the Imbrium infill - the lava that filled the basin after the Archimedes impact occurred.

By measuring the relative abundances of specific compounds within the samples it was possible to date the areas where they were taken, giving us a timescale to put on our sequence of events. The Apollo 14 samples showed that the Imbrium basin was formed by an impact which occurred about 3.85 billion years ago, and the samples taken by the crew of Apollo 15 showed that the subsequent lava flow which flooded the basin took place about 3.5 billion years ago. From this we can also say that the lunar highlands formed before this, so they must be older than 3.85 billions years, and that craters within the Imbrium basin which have not been embayed must be younger than 3.5 billion years.

Going further back
We now have a sequence of events and some dates for events on the surface of the Moon, but the observations so far tell us very little about the formation of the Moon in the first place. The standard theory for the formation of the Moon is that a Mars-size impactor collided with the young Earth and the mantle of both Earth and the impactor were blasted into orbit. The Moon formed as the orbiting material began to condense. This explains why the Moon is there at all, but not the surface features we see today.

A simple model that explains the major observations of the surface structure is known to geologists as the magma ocean theory. In this model, the young Moon gathers material through impacts, a process known as accretion. The energy from these impacts melts the top few hundred kilometres of the lunar surface creating a "magma ocean".

As more material is swept up in the formation of the planets, the frequency of the impacts begins to decrease and the magma ocean begins to cool. As it cools, different minerals crystallise and begin to sink or float, depending on their relative density. Heavy minerals such as olivine and pyroxene sink towards the solid lunar interior while lighter minerals such as anorthosite rise to the surface forming giant "rock bergs".

As the cooling progresses, a crust of light anorthosite forms at the surface as well as an olivine/pyroxene-rich lower layer. Sandwiched between these two layers is a "residual melt", a layer rich in potassium, rare earth elements and phosphorous, which generates heat by radioactive decay on timescales of 0.1 to 1 billion years.

As the formation of the solar system progresses, large impacts, such as those that caused Imbrium, occur. These events fracture the outer anorthosite crust. The residual melt, not quite solid due to the heat from the radioactive decay, can seep through the fractures and fill in the basins. This is the process that is thought to have filled in the Imbrium basin: the basaltic magma from the residual melt migrated upwards through the fractures in the surface caused by the impact, filling in the impact basin to form the mare we see today. As the magma is made up of different minerals than the largely anorthosite crust, the flooded mare appear darker than the surrounding lunar highlands.

Problem solved?
So we have an age scale for surface features, and a workable theory for the formation of the outer layers of the Moon. Is that all the the questions of lunar geology solved? Luckily not! There are variations in the chemical make-up of different bits of the lunar surface: there are iron anomalies, concentrations of mass in particular places and numerous other features not touched on in this article. There is plenty more to explore on the Moon, and even more when you start to compare lunar features to those we see on Mars, Venus and the other rocky bodies in the Solar System.

Further reading
If you are interested in reading more about this subject, a very good book on comparative planetology throughout the solar system is "The New Solar System" by J. Kelly Beatty, Carolyn Collins Petersen and Andrew Chaikin, published by Sky Publishing and Cambridge University Press. As well as the basic facts and figures for each body in the Solar System, the book summarises what we have learned from planetary explorations of the last few decades and contains some introductory planetary science.

Last updated: Tuesday, 03-Mar-2009 02:50:21 GMT