What is the colour of the Moon?
Introduction to an investigation by members of the MAS digital lunar imaging group.

At an MAS meeting in the Godlee observatory, before we closed for the Christmas break in December 2005, Anthony Jennings showed some new digital pictures he had taken of the moon. I was intrigued to notice slight colour differences between certain lunar formations and I asked him to digitally increase the colour saturation, using image-processing software. The result was startling. It showed a picture of the moon that closely matched that in the Larousse Encyclopaedia of Astronomy (1967) drawn by the French astronomer L. Rudaux.[1] This is to my knowledge the only map of the moon that attempts to show colour on the lunar surface; colour so subtle that very few observers have attempted to describe it.

 
General Map of the Moon drawn by L. Rudaux from the Larrouse Encyclopeida of Astronomy 1967

Two years ago, I discussed the visual detection of lunar colouration with expert moon observer, Nigel Longshaw. That evening we spent some time looking at the full moon with the 8-inch Godlee refractor and slowly we started to see that certain areas of the moon did indeed have some colouration. The technique is straightforward and can be done by anyone with a small telescope…but bigger, the better. Simply ask yourself how the moon could be painted or drawn in colour. Is the lunar surface really monochrome; shades of grey, black and white, or are there regions where browns, blues, yellows, greens and even purple tints are detectable? Digitally enhanced imaging answered the question in one go.

Digital imaging is new to me but compared with 'normal' 35mm photography the results are much quicker and better. The basic images apparently contain more readily extractible information than photochemical pictures. Whole-disc images, and some with a x2 telenegative lens, taken in December 2005, using a Canon 350D and a 127mm Mak-Cass, were processed using Adobe Photoshop 5.5. One-button, Autolevels, adjusted the contrast and brightness and I increased the colour saturation to +70 (whatever that means). The results exactly confirmed Rudaux. The moon is indeed colourful.

This image was made on 17 December 2005 with a Canon 350D DSLR via
a 127mm Mak-Cass with x2 tele-extender lens to give 3000mm focal length at
f23.6. 1/80 second at ISO 100. Image processed on Photoshop 5.5 with
one-click 'autolevels' then by increasing the colour saturation to +70. Kevin Kilburn

Other digital images provided by others confirmed exactly what had already been learned. The lunar Maria show shades of brown and blue. They are definitely not grey. M. Tranquillitatis is blue and this overflows into a band of colour flowing into and along the eastern edge of M. Serenitatis, whose central area is a yellow-brown. The eastern corner of M. Imbrium is brown and there are patches of brown south of S. Iridium on an otherwise bluish floor to the Imbrian basin. Remote sensing and narrow-band colour imaging from NASAs Galileo spacecraft confirms this. The blue colouration of the maria basalts is put down to their high titanium content. The brown coloration is due to the later out-flowing of iron-rich lava. This is asserted by comparing remotely sensed surface colourimetry with ground truth Apollo mineral samples and laboratory calibrated mineral colourimetry.

The most colourful region is Wood's spot, an area adjacent to the crater Aristarchus. Visually it is vaguely yellowish. In 1922, Wood recorded it as having a spectral reflectivity similar to the sulphurous deposits around volcanic regions on Earth. Colour enhanced digital images show it as a dirty yellow, by far the strongest colour shade on the earthward face of the moon.

Between Aristarchus and Herodotus, the Portuguese observer, Filipe Alves, distinctly shows a bluish area. He published an article on lunar colour and digital remote sensing in Sky&Telescope, July 2005.

Lunar surface colour and TLPs: In the mid 1960s, during the immediate pre-Apollo years, the Lunar Section of the British Astronomical Association, under the direction of Patrick Moore, collected a great deal of observational reports that suggested that certain areas of the lunar surface were prone to transient colour events, localised short-term brightening, or surface detail obscuration. These transient lunar phenomena (TLPs) had been reported for decades by British and American lunar observers, although it is true to say that even now (2006) they remain a mystery. Nevertheless, those observers who have recorded them remain convinced that what was seen was real. I have personal experience of witnessing three TLP events. In January 1967, Patrick Moore, Terrance Moseley, Mike Duckworth, Peter Sartory and I reported a small red abnormality on the outer wall of Gassendi. This remains one of the few independently corroborated colour anomalies seen by multiple observers and is recorded in a NASA Technical Report.

When I say it appeared red, this is strictly incorrect. To draw attention to colour anomalies, there had been research in the USA using monochrome TV cameras to observe specific lunar features through alternating red and blue filters. A red surface anomaly then appeared light in the red filter and darker in the blue. A blue anomaly gave the opposite effect, dark through the red filter and light in the blue. The name 'moon-blink' was coined and the BAA Lunar Section took this up by recommending the visual observation of suspect lunar features via alternating red and blue filters in the Moon-blink Project. What observers actually detected was a 'blink' effect due to the contrasting red-blue comparison of lunar surface colour seen through alternating filters. It did work, after a fashion. The visual shock of seeing alternating red and blue lunar detail took a lot of getting used to and it was a matter of individual interpretation as to what colours were being detected. Nevertheless, there was some consistency between observers. Patrick Moore drew up a list of suspect TLP areas that we were encouraged to monitor using moon-blink filters and we received and communicated observations via the monthly BAA Lunar Section Circular. On the basis of observations made in the USA and UK, and others spanning two centuries of lunar observation. NASA Technical Report TR R-277. was published in July 1968 as a Chronological Catalog of Reported Lunar Events http://www.mufor.org/tlp/lunar.html


In late December 1969, I also recorded two uncorroborated observations in the area between Aristarchus and Herodotus. These were 'blue' events. The first was on the morning of 26 December and the second,

Observation made of possible TLP activity between the craters
Aristarchus and Herodotus on 26 December 1969. The moon-blink filters
suggested a blue enhancement in the areas shown. Kevin Kilburn

 

Observation made of the Aristarchus area, 28 December 1969 with similar
interpretation as that of the observation two days earlier. Kevin Kilburn

The Portuguese observer, Filipe Alves, showed that digital imaging of the moon can enhance colour detail barely detectable to the naked eye http://www.colormoon.pt.to/

His pictures of Aristarchus show that this is a region with highly differentiated colour regimes that perhaps can be detected by the visual observer. The prominent yellow area is known as Wood's Spot and is possibly the most obvious patch of colour seen on the lunar surface. The blue colouration around Aristarchus and Herodotus is perhaps what I saw. Its colour is far less readily observable.

Images by Alberto Fernando, José Ribeiro & Filipe Alves . (Taken from: http://www.colormoon.pt.to/)

Transient lunar phenomena are apparently confined to the edges of the maria and are most often seen at or shortly after sunrise on that part of the moon. One suggestion has been that they are associated with out-gassing from dawn-warmed regions where subsurface gas has accumulated during the cold lunar night. This has never been substantiated. Here is another possible explanation for TLPs: They may be mineral rich coloured areas of the moon that brighten under certain, specific, solar illumination angles and become just detectable with the naked eye or through simple alternating red and blue filters, as were employed in the 1960s under the 'moon-blink' project. Visual colour detection is a function of the brightness and intrinsic colour saturation of the target. If this is at a low level, colour cannot be sensed with the naked eye. The red and blue filters employed in moon blink apparatus may have made surface colour more detectable by 'blinking' the colour contrast effect. Digital image enhancement may now bring lunar surface colour above that otherwise just-detectable threshold and hold the key to explaining TLP.

The MAS is now putting together a programme of digital photographic observation. Whole-disc digital imaging has already confirmed the visual appearance seen by colour-acute observers and narrow-band remote sensing technology. We also need to collect high-resolution digital images of the moon. Target areas are the Maria and basalt filled craters. Historically documented TLP targets will be imaged and compared with previously recorded observations. We also need to know how the coloured areas of the moon react to increasing and decreasing sun angles.

This new project may provide answers to questions perhaps still to be asked… moon colour is visually subjective, yet instrumentally demonstrable. How it relates to the vexed question of TLP still needs to be addressed.

Kevin J Kilburn, FRAS
Manchester Astronomical Society and the Society for the History of Astronomy

[1.] As pointed out by Chester astronomer, Richard Baum, the original French version of the Larousse Encyclopaedia of Astronomy was published in 1948 and also included this coloured moon map by Rudaux.. The idea of telescopically visible widespread colour on the lunar surface is therefore at least sixty years old. Thanks go to Richard for showing this 1948 illustration to Nigel Longshaw and Kevin Kilburn in May 2006.


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