This may seem like a pretty trivial question: save during an eclipse, or when the Moon is low on the horizon and seen through a thick layer of dirty atmosphere, 99.999% of people will answer “grey”, although some romantics will offer “silver”. When you look up at the Moon in the night sky, the predominant colour will be white, with grey patches. These grey areas are the lunar maria – the seas – which, for many years we have known to be relatively flat plains of lava. About three and a half billion years ago, magma from the lunar interior flooded a series of giant basins on the visible face of the Moon, smoothing huge areas of the surface. Previously, the whole lunar surface had been as heavily cratered as what are usually referred to as the Southern Uplands, the chaotic, massively cratered area in the south of the disk, around the crater Tycho.
We know that the maria, the lava plains, are younger than the highlands as they have few craters. The rule works anywhere in the solar system: where there are many craters, it is old terrain; where there are few craters, the surface is much younger. Just by counting the number of craters in an area of the lunar surface we can give a pretty accurate estimate of how old that area of the Moon is.
A curiosity is that these lava plains are, with some minor exceptions, limited to the side of the Moon that we can see. The reason seems to be related to the fact that this side of the Moon is at a much lower elevation than the far side. Impacts have blasted away the top few kilometres of the surface on the Earth-facing side, making it much easier for the underlying magma to get through and flood the surface, simply because the lunar crust on the Earth-facing side was so much thinner when the maria were flooded.
Early Apollo crews gave conflicting descriptions of the colour of the lunar surface. Some crews observing the Moon from lunar orbit, described the colour as brownish, others could only see grey. What was evident was that the changing angle of illumination was changing the way that the astronauts perceived the colour of the lunar surface. However, images taken on the lunar surface are so colourless that many people believe that they were mainly shot with black and white film when, in fact, colour film was always used.
Take a look at the famous image of Neil Armstrong reflected in the visor of Buzz Aldrin. It is in full colour, as is attested by the gold of the footpad of the lunar module and of the tube lying on the ground in front of Aldrin. Similarly, the red, white and blue of the Stars and Stripes patch on his shoulder is clearly visible. The ground though is just a mid-grey colour.
What of this image of Ed Mitchell holding the thumper, which was connected to a series of geophones to obtain data about the lunar interior by seeing how vibrations passed through the surface layers? It looks to be a black and white image until you notice that the cable to the geophones is actually gold-coloured.
Unlike the surface of the Earth and the surface of Mars, in which weathering of surface minerals leads to a wide range of colours, colour is unusual on the Moon. Given that much of the surface is basaltic lava, this is not very surprising: as the good citizens of the island of La Palma will tell you, when it cools, lava is dark grey and, without wind and water to weather it, would stay that way. Back in 1969, I queued, along with thousands of other people, to see the sample of moonrock – actually moondust – at Bristol University. After two hours in the cold waiting on Park Street, I entered, accompanied by my mother (I was only 9 at the time). The centrepiece of the exhibition was a small petri dish containing a few grams of dark material, not unlike charcoal, subject of reverential looks by the visiting public. My mother was not impressed and suggested that she could have collected some soot from our chimney, placed it in the display case and no one would have spotted the difference.
Very few people who are not researchers have seen actual lunar rocks except in pictures. The samples returned from the Moon are tightly controlled and private possession is illegal; the FBI takes a very close interest in any tip-offs about moon rocks that may be in private hands. There is though one completely legal way to own a piece of the Moon: I have two, small lunar meteorite fragments – chunks of moonrock that have fallen to Earth, blasted off the surface of the Moon by the impact of an asteroid – in my private collection. A total of 517 lunar meteorites are known, although many represent multiple fragments of a single fall. As you can see from the image below, one of the fragments that I own is dark grey, the other is slightly lighter grey.
This is not a coincidence. Of major bodies in the solar system, only Mercury is darker than the Moon. The dark lunar plains, the maria, reflect from 7-10% of the light that falls on them (to put this in context, a material with 4% albedo is generally reckoned to be black). The bright highlands reflect 11-18%, making them also a darkish grey in colour, but they seem bright by contrast with the maria. Some areas of the Moon, though, are much brighter: the crater Aristachus has an albedo of around 25% and is often reckoned to show a warmish, cream-yellow colour; the rays that emanate from young craters such as Tycho are also much brighter than average – these are material excavated when the crater formed and that have been sprayed over the surface by the explosion. Aristachus, itself a centre of rays, is so bright that it can be distinguished even when in sunlight, illuminated just by reflected earthlight. In fact, Aristachus has even been mistaken for an erupting volcano shining in the lunar night.
Subtle colour differences do exist on the Moon. These are due to the different chemical and mineral content of the rocks. On average, just three elements form about three quarters of the mass of the surface. An average moonrock is about:
- 42% oxygen
- 21% silicon
- 13% iron
Of the remaining 24% – roughly a quarter of the mass – we have approximately
- 8% calcium
- 7% aluminium
- 6% magnesium
Most of the rest is titanium. However, these minerals are not evenly distributed.
Titanium is quite rare both on Earth and in the lunar highlands; in contrast, it is abundant in the lava plains of the maria. On average, there is more than six times as much titanium in these plains as there is on Earth or in the lunar highlands. The Apollo 11 samples surprised scientists with large amounts of titanium that they contained but, in some Apollo 17 samples, the quantity was truly astonishing, equalling the amount of aluminium.
Similarly, the amount of iron in the lunar highlands is similar to the amount on Earth, but it is about twice as common in the lunar maria. In contrast, there is about the same amount of aluminium in the lunar plains as on Earth, but aluminium is much more abundant in the lunar highlands. It is thus not surprising to discover that there is a strong anti-correlation between the two elements in moonrock: where the samples are rich in iron (for example, the Luna 24 landing site), they are poor in aluminium and where they are rich in aluminium (as in the Apollo 16 landing site), there is much less iron.
What this adds up to is the fact that different areas of the Moon are composed of quite different minerals and that leads to differences of tone.
Take a look at this colour photograph of the north-eastern quadrant of the Moon by Spanish amateur astronomer Juan Jose Godoy. In some of the lava plains there is a clear, brownish shade, while others are most definitely grey.
Now, let’s stretch the colour a little and add the names of some of the features to make identification easier and see what happens:
Forgetting the border area that is in heavy shadow, where the colour balance appears to have gone wrong due to the darkness of the image, an astonishing range of tonalities start to appear among the greys from blues, through browns and orange.
Below is the same image without the names of features superimposed. Mare Frigoris and Mare Serenitatis show some amazing colour variations with browns, orange, mid and dark greys in different regions. These show that what appears to be a single, flat plain is actually made up of many different lava flows of different compositions that flowed out and filled the Serenitatis Basin. Elsewhere, the crater Aristillus and the mountains to the right, towards the North Pole of the Moon, both have a strongly bluish cast. There are even some patches of green in the lunar Alps, between Plato and Cassini.
Different people describe the colours in different ways, but as a general rule, one can say that the brown or orange tone of Mare Serenitatis represents lave flows that are poor in iron and rich in aluminium. In contrast, although not shown in this photograph because it is out of the field of view off to the left, the neighbouring Mare Tranquilitatis, where Apollo 11 landed, has large areas of much darker, titanium-rich basalt that has a distinctly dark blue tint: we can see a small patch of such titanium-rich lava in Mare Frigoris.
The dominating factor in the colour of different areas of the Moon thus usually comes down to how much iron and titanium they contain. Iron oxide is a dark mineral and, when mixed with titanium oxide, becomes darker still. Where iron dominates, the colour is dark blue, where titanium is abundant, it is much redder and observed as brown or orange.
Colour enhancement techniques can be used to exaggerate slight differences in grey tones, providing a powerful tool for mapping the geology of the lunar surface. With this method, the greyish maria transform from greys into delicate shades of navy blue or brown, corresponding to varying levels of iron and titanium in the basalt that fills them. The highland areas are lighter in colour, and show typically shades of yellow, pink and pale blue. Once again the differences are due to geological composition, but there may also be ageing effects due to bombardment by sub-atomic particles from the Sun.
The cream-yellow colour of the area around Aristachus is intriguing. Where have we seen this colour, or a similar one before? One of the most emblematic images from the science missions that closed the Apollo programme was the one showing orange soil at Shorty crater. This colour produced enormous excitement as it seemed to be evidence of recent volcanic activity on the Moon. The orange colour was found to be due to large numbers of glass beads in the soil but, to the disappointment of Jack Schmidt who had discovered it, although it was evidence of a fire fountain, it was one that had existed three and a half billion years ago.
We know from observations from orbiting satellites that the soil around Aristachus has a high proportion of glass. One suggestion is that that the soil around Aristachus is rich in these bright, orange glass beads.
So, when you contemplate the light of the silvery Moon, do not swoon (as the song suggests), try to appreciate the subtleties of colour from place to place on the surface that tell us about the geology and the history of the lunar surface.
One day, these colour differences may guide prospectors as to which areas of the Moon to explore in search of mineral riches that could one day replace mining on Earth. When man goes back to the Moon, it will not be for a quick visit: even the initial visits will at least double the longest Apollo stay on the Moon and the aim of these first return flights will be to set up a permanent moonbase. This time we are returning to the Moon to stay and part of the reason for staying will be to exploit its riches. In space, it is raining soup and we have not yet learnt to hold out a bowl!
The fate of mankind may well hang on how well we learn to stop depending on the Earth’s scarce resources and start to exploit effectively the resources that the Moon and the planets can offer.
 Typical prices are 200-400 Euros per gram for lunar meteorites, compared to the approximately 1500 Euros per gram for Martian meteorites.