An Apollo astronaut collecting rocks on the Moon: they have yielded a bonanza to science for the past 40 years.
Credit: NASA
Twelve men walked on the Moon, collecting 380 kilograms of rock. While they may look much like any other rocks, the truth is the collection brought back to Earth helped to rewrite the history of the early Solar System.
Look at the Moon today and you see a pattern of bright and dark regions, making up the 'face' of the man in the Moon. Through a telescope, the dark regions are revealed as flat plains with few craters, whereas the bright regions are heavily cratered highlands.
The few craters in the smooth plains suggest that these are younger than the highlands. The smoothness also made them the ideal site for the first attempt at a landing. So it was on one of these plains, the Sea of Tranquility, that Apollo 11 commander Neil Armstrong landed the lunar module, Eagle.
Most of the rocks collected at Tranquility were found to be basalts – volcanic rocks – indicating that the plains were formed by flooding with lava. However, the real significance of these rocks emerged when their ages were measured: Tranquility basalts were about 3.6 billion years old.
To put this in context, we have to compare with Earth. Our planet is geologically active, its surface is continually being reshaped. Colliding tectonic plates throw up new mountain ranges that are then worn down by the action of water or ice.
New rocks are formed and then subducted back into the depths. The Earth's surfaces and most of its rocks are geologically young, and there are very few ancient rocks.
When Armstrong and Buzz Aldrin walked on the basalt plain of the Sea of Tranquility, they walked on a surface where nothing much had happened for 3.5 billion years. There is nothing like this on Earth. It turns out that they had landed on one of the younger parts of the Moon, but the rocks they picked up were as old as the oldest rocks known on Earth.
This is the importance of the Moon to science: its geological record covers a period of history that is mostly missing on Earth. Almost all lunar rocks are older than three billion years, many are older than four billion years, and some date back almost to the beginnings of our Solar System around 4.5 billion years ago.
As further Apollo missions explored a range of terrains, new types of rocks began to emerge. Highland rocks were usually found to be breccias, rocks made of fragments of other rocks. On the Moon, the breccias are formed from rock fragments melted together by the asteroid impacts that formed the basins and craters.
Among the rock fragments within these breccias, a light coloured rock called anorthosite was common. This was a surprise. Anorthosite is rare on Earth: it is a rock composed of a mineral called plagioclase feldspar. Some of the lunar anorthosites were found to be ancient, with ages of 4.4 billion years or older.
Another surprise was the discovery of rocks with unusual composition. These rocks were enriched in a number of unexpected chemical elements and were given the name KREEP, an acronym for Potassium (K) Rare-Earth Elements and Phosphorous.
The initially puzzling anorthosite and KREEP were realised to provide the key to understanding the Moon's early history. They were just what would be expected if the Moon's crust formed from the cooling of an originally molten surface: a magma ocean.
As the molten rock cooled, the lighter plagioclase would float to the top, forming a crust of anorthosite. Other heavier minerals would sink. The remaining liquid magma would become enriched in elements that were incompatible with any of the minerals. When this final liquid crystallised, it would be stuck with all the incompatible elements forming the KREEP component.
The hot magma ocean, together with other features of the lunar composition, led to the impact theory for the Moon's origin. In this model, a Mars-sized body hit the early Earth. The core of the impacting body coalesced with that of the Earth, but the material from its mantle was left over to form the Moon. The energy involved would be sufficient to leave the early Moon with its hot magma ocean.
The importance of impacts in the early Solar System is evident from the Moon's cratered surface. By dating Apollo rock samples, we have been able to put a time scale to this impact bombardment. We can then use crater counts to measure the surface ages on other planets and their moons.
But this process has shown up another puzzle: the lunar impact dates all cluster around 3.8 to 4.0 billion years ago, and yet by 3.6 billion years ago, the major impacts had virtually ceased. There was no evidence for impacts earlier than about four billion years.
This has led to the idea of the 'lunar cataclysm', a bombardment of the Moon by asteroids that lasted for around 100 to 200 million years around 3.9 billion years ago, with few impacts either before or after.
The cataclysm hypothesis was controversial when first proposed in the 1970s, and remains so to this day. Critics of the cataclysm idea suggest that we are seeing the tale of a gradual decline of impact rates, and that evidence of the earlier impacts has been erased by more recent events.
However, calculations suggest that the Earth should largely have swept out asteroids in its zone of the Solar System, within a short time after its formation.
There shouldn't have been any left by 3.9 billion years ago – nearly 700 million years after the Solar System formed. If so, where did the impactors responsible for the lunar basins and craters come from?
A recent theory looks to events in the outer parts of the Solar System.
Interactions between the orbits of the giant planets Jupiter, Saturn, Uranus and Neptune could have caused rapid migration of their orbits and might have perturbed smaller bodies such as asteroids and comets throwing them into the inner Solar System and causing the burst of impacts on the Moon, and presumably the Earth.
Is the cataclysm theory correct? Did these dramatic events in the outer Solar System really happen? Scientists continue to study the Apollo lunar samples using increasingly sophisticated techniques, but we really need more samples.
We need to look at different parts of the Moon including its polar-regions and far side. We need to go back and finish the work that Apollo started - this is why further manned missions to the Moon are so vitally important.
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Jeremy Bailey is an associate professor at the Australian Centre for Astrobiology, at the University of New South Wales in Sydney. His research centres on planetary astronomy, including observations of the Solar System, extrasolar planets and the modelling of planetary atmospheres.
