Backyard lunar mineral prospection, Part I
2023 April 6
Knowledge of the lunar surface and its composition has increased enormously in the last six decades thanks to the lunar surface samples returned by the Apollo crewed landings, the automated Soviet Luna missions and, most recently, the Chinese Chang’e 5 probe, quite apart from a fleet of sophisticated spacecraft in lunar orbit. At the same time, lunar science has changed from being almost exclusively the province of amateur astronomers, to a highly specialised professional field in which the role of the amateur appears to have disappeared completely. However, by combining modern CCD cameras coupled to a modest telescope with data from, in particular, the Apollo missions, a computer-savvy observer can investigate lunar geology and even engage in lunar mineral prospection at a fraction of the cost of the Apollo programme. In this paper, we compare standard CCD observations of the Moon taken with a 250mm backyard telescope, using conventional RGB filters, with data from the Indian Chandrayaan-1 probe’s Moon Mineralogy Mapper (M3) experiment. The basic compositional units found in M3 data are clearly identifiable from their colour in the backyard data and correlate well with the compositions found in returned lunar samples.
Introduction
What colour is the Moon? Whether you look at it with the naked eye or through a telescope, your answer is likely to be that the seas are mid-grey and the highlands and ray craters are white. Where significant colour on the lunar surface is observed visually, it is likely to be mainly atmospheric in origin. It is an article of faith that, as the photographs taken of the surface by the Apollo astronauts show a uniformly grey colour, they were obtained mostly using black-and-white film. As we will see, this is not true at all.
The grey areas are the lunar maria – the seas – which we have known for many years 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 impact basins on the visible face of the Moon, smoothing huge areas of the surface and filling in most of the previously existing craters, although a few ghost rings such as Stadius, close to Copernicus, still survive. We know that these maria, the great lava plains, are significantly younger than the highlands, as they have few craters. Previously, the entire lunar surface had been as heavily cratered as the Southern Uplands – the chaotic, massively cratered area in the south of the disc, around the crater Tycho.
Lunar samples showed that not all areas of the Moon were identical chemically. One of the biggest surprises from the Apollo 11 mission was the discovery of an unexpectedly large titanium abundance in the samples returned. All those from Apollo 11 showed around a 7.5% abundance of titanium (expressed as the abundance of TiO2).1 In contrast, Apollo 12’s samples averaged approximately 2.8%, and Luna 16’s about 3.5%, while Apollo 16’s samples show just 0.5% titanium. (We explore some of these figures in more detail later in this paper.) In other words, we know now that there are big differences in the composition of the surface rocks from place to place even if, superficially, the different maria look similar. And, if you know where and how to look, these differences become obvious to the observer, who can turn lunar mineral prospector, armed only with a telescope, a CCD camera and some good software, without spending billions of dollars to go to the Moon.
The Moon as seen from lunar space
Even though the predominant colour on the Moon is, undeniably, grey, there are many shades. One of the favourite features for many observers is the crater Plato. A detailed description of the crater, its topography and the variation of albedo features is found in Marshall & Mobberley (1986).2 What is the distinguishing factor that draws the eye to it from Archimedes, another smooth-floored crater of similar size nearby (see Figure 1)? If we compare them side by side, they seem similar in many respects:
– Archimedes is 81km in diameter, 2.1km in depth and 3.85 billion years old: intermediate in age between the formation of the Imbrium Basin and its flooding with lava.
– Plato is a little larger – 101km diameter – but only 1.5km in depth and of almost identical age to Archimedes.
The greater depth and impressive rim of Archimedes would suggest it is the more spectacular of the two. However, to any lunar observer, what makes Plato stand out so distinctively is the fact that its floor is so dark, while Archimedes is a similar shade to the Imbrium Basin and, at high solar illumination, becomes difficult to pick out, while Plato stands out clearly even at full Moon.
When we enhance the contrast, we find that some areas of the floor of Plato are significantly darker than others, while contrast variations are generally smaller in the floor of Archimedes (Figure 2).
It is an obvious observational fact that the Moon has many albedo and contrast variations, from the darkness of the floor of Plato to the brilliant ray systems of Tycho and Aristarchus. What, though, about colour variations? Some observers describe Aristarchus and its surroundings – an area known as Wood’s Spot, further discussed later – as having a warm cream colour but, to the telescopic observer, there is little in the way of strong colour anywhere on the lunar surface.
Reports of temporary, usually red local colouration were made on numerous occasions between the 1960s and ’90s (e.g. Mills, 1970).3 For example, moonblink devices operating under NASA contract, in which red and blue filters alternated at ~2Hz (NASA Goddard Spaceflight Center, 1965), reported three instances of confirmed temporary red colouration – two in Aristarchus and one in Alphonsus – between 1964 October and 1965 November.4 Such reports have almost disappeared since Sheehan & Dobbins (1999) made a critical re-examination of them,5 although the suspicion is that temporary critical lighting conditions, combined with atmospheric effects, explain most, if not all the reports of transient lunar phenomena.
What, though, about the people who have observed the Moon closer than anyone else? The early Apollo crews had very different impressions of the lunar surface. After Apollo 8 entered lunar orbit, the following exchange occurred with Houston:6,7
069:51:04 Carr: Apollo 8, Houston. What does the ole Moon look like from 60 miles? Over.
[Pause.]
069:51:16 Lovell: Okay, Houston. The Moon is essentially gray, no color; looks like plaster of Paris or sort of a grayish beach sand.
In contrast, the Apollo 10 crew reported something significantly different:8
076:04:14 Young (on board): That’s the weirdest-looking surface – there’s some color in that.
076:04:17 Stafford (on board): There’s the coloring C – it’s a brownish gray.
Both John Young and Tom Stafford repeated several times the description of the surface as distinctly brown. This difference in appreciation of the colour of the lunar surface by different crews caused some consternation within NASA, but was settled finally by Neil Armstrong’s report from the lunar surface itself:9
103:12:44 Armstrong: I’d say the color of the local surface is very comparable to that we observed from orbit at this Sun angle – about 10 degrees Sun angle, or that nature. It’s pretty much without color. It’s gray; and it’s a very white, chalky gray, as you look into the zero-phase line. And it’s considerably darker gray, more like ashen gray as you look out 90 degrees to the Sun.
However, in the post-mission technical debrief, Armstrong gave a somewhat different description, closer to that of the Apollo 10 crew. He reported seeing brown colouration, suggesting that lighting conditions influenced the colour that was observed:10
Probably the most surprising thing to me, even though I guess we suspected a certain amount of this, was the light and color observations of the surface. The down-Sun area was extremely bright. It appeared to be a light tan in color… As you proceeded back toward cross-Sun, brightness diminished, and the (tan) color started to fade, and it began to be more gray. As we looked back as far as we could from the LM windows, the color of the surface was actually a darker gray. I’d say not completely without color, but most of the tan had disappeared as we got back into that area, and we were looking at relatively dark gray.11
Photographs taken on the lunar surface are, save when an astronaut or one of their instruments is in the frame, usually so devoid of colour that it is widely believed that they were almost all taken in black and white, although a number of them do show the light tan hue. In fact, most lunar-surface photography was in full colour, although by no means all. Of the 339 Apollo 11 lunar-surface images, just 107 were taken in monochrome.12 The astronauts on that mission used a 70mm Hasselblad with SO-368, 70mm colour film for much of their surface imaging, with later missions using near-identical configurations. A good example of the effect of colour images appearing at first sight to be black and white is shown in Figure 3.
However, some images of the lunar surface do show clearly the tan tone that Armstrong alluded to, visible at some angles from the Sun. An excellent example is the image that Chang’e 5 took of the Chinese flag (Figure 4).
The only case of strong colour detected on the Moon by astronauts was the famous orange soil near Shorty crater, found by Apollo 17 (Figure 5) and later discovered to be coloured by the presence of microscopic glass beads in the regolith.
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. One of the authors of this paper [MK] has two small lunar meteorite fragments – chunks of rock that have fallen to Earth, blasted off the surface of the Moon by the impact of an asteroid – in his private collection. As Figure 6 shows, one of the fragments is dark grey; the other is slightly lighter grey, with no other colour present. A total of 517 lunar meteorites are known, although many represent multiple fragments of a single fall.14
The darkness of these fragments of the lunar surface 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 7–10% of the light that falls on them (to put this in context, charcoal has an albedo of 4%). The bright lunar highlands reflect 11–18%, making them also a darkish grey in colour, even if they seem bright by contrast with the maria. Some areas of the Moon, though, are much brighter: the crater Aristarchus has an albedo of around 25%.
Why should we expect to see colour on the Moon?
Subtle colour differences do exist on the Moon. These are due to two effects:
– The effects of weathering (see, for example, Elliot et al., 2018) due to cosmic radiation darkening surface materials;15
– The different chemical and mineral content of the rocks.
The former is manifested in the existence of bright craters, which are often the source of ray systems. These craters break through the outer skin of regolith that has been weathered and darkened by radiation, revealing lighter, unweathered material beneath. The surface skin is estimated by Haiken et al. (1991) from Apollo 12 samples to be as deep as 2m.16
The second is manifested by compositional differences from place to place on the lunar surface. On average, just three elements form about three quarters of the mass of the surface. An average Moon rock is about 42% oxygen, 21% silicon and 13% iron. Of the remaining 24% – roughly a quarter of the mass – we have approximately 8% calcium, 7% aluminium and 6% magnesium. Most of the remaining 3% is titanium, with all other elements below 1% in abundance. However, these elements are not evenly distributed and vary widely in abundance.
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