Observations

Are colours on Moon photographs real? Harry answers a question about two great images

A picture of the Moon showing colours.

A picture of the Moon showing colours. Image and copyright David Lloyd-Jones ©, all rights reserved

An interesting question about the reality of colours shown on moon photographs was referred to Harry Roberts, who is a frequent contributor to this blog and a member of the Sydney City Skywatchers.

Question from David: Attached are two images of the Moon’s surface. They are LRGB images. Although the RGB channels have been stretched so as to exaggerate the colours each channel has been stretched to the same extent so as to preserve the true balance between them.

I believe that the displayed colours reflect the reality of the Moon’s surface and are not just artefacts of the imaging system. My understanding is that the colours are generated by different minerals in the Moon’s regolith and I would very much like to gain a better understanding of which minerals are indicated by the colours in the two images.

Alternate view of the Moons colours.
A picture of the Moon showing colours. Image and copyright David Lloyd-Jones ©, all rights reserved

To that end I have done some small research on the Internet but so far without much success. There is a wealth of web sites dealing with colour images of the Moon and there is also a wealth of web sites dealing with selenology. The problem is that the colour images are almost invariably of the whole Moon with not much resolution detail and where they do touch upon related chemical composition they are not detailed enough for my purpose. And the volumes of text regarding mineralogy on the Moon do not seem to provide accompanying images that I can compare with my own.

I wonder if some kind person at the Observatory could give me the reference to some source material that will let me relate the colours in my images to the underlying mineral composition.

Answer from Harry: David will find many posts dealing with the “true” colours of lunar terrain on the LPOD site, hosted by Charles Wood the selenologist.

While David’s images may be a bit too red (atmospheric chromatic aberration perhaps) there is much published on this topic. Modern CMOS detectors can capture subtle colour differences that are related to the chemical composition of the surface rocks or emplaced mare basalts. With a good scope and seeing you can see major color differences in many places. Where Mare Serenitatis joins Tranquilitatis is a good place to start.

The colours in David’s photos are real – they represent a range of compositions and lunar “weathering”. See LPOD 31/1/2010.

Also LPOD 13/1/2011 deals with regolith colours. Its a big topic, but David will find much of interest if he searches through the last 5 – 10 yrs of LPOD.

Another Wood article – of course there are many – this one talks of colour and mineralogy on LPOD 3/1/2004.

Apollo era sampling was limited to a few very small areas. Direct sampling yielded pyroclastics at the Apollo 17 site, caused by lunar volcanism. Remote imaging has, I recall, provided some answers to colour/composition questions. It’s one big reason to revisit the Moon – to sort out such questions.

6 responses to “Are colours on Moon photographs real? Harry answers a question about two great images

  • I should clarify that David’s photos were not meant to be representations of the Moon’s surface as seen by eye. The photos were deliberately stretched to bring out any subtle colour differences that are there. Obviously these colour differences would be more obvious with colorimetry as Andrew explains below, but stretching the colour of the images seems like a legitimate technique. Maurice Collins gives a detailed discussion of ‘Supersaturated [colour enhanced] colour images of the Moon’ in the March 2010 issue of Southern Stars. The article can be found here.

  • Honestly, these images are far too rich in colour. The moonlight is of course the light from the sun, and a spectra of the moon shows the same Fraunhofer lines but they are slightly broadened.

    To simply explain the lunar colours is no easy to state in a few sentences, but the process of how we see colour from everyday objects explains it.
    Firstly we can divide material colours as so-called chromophores and non-chromophores – simply a substance seen with colour or that is only seen as white or grey.

    The problem is that the many many minerals on the moon behave like the colour of any object we view here on Earth with the eye, where light is absorbed by the mineral and then reemitted at certain wavelengths either by reflection or. transmission. Hence for some minerals and compounds we do see colour.

    To measure this colour we have to use colorimetry (also spelt colourimetry), which measures certain wavelengths or across the spectra wavelengths (spectrophotometery, often lazily called spectrocolorimetry). In chemistry we can use colorimetry in the laboratory to workout the quantities of one coloured substance or even several.

    So the biggest problem is to identify the many chromophoric minerals on the moon surface, and if you know the proportions, then we can make sense of the variations in composition across the lunar surface. (much of this work was done in the 50s and 60s.)

    What has been found is that different areas of the moon have differing spectra I.e. The example of VIRTIS spectra of the Moon http://hubble.esa.int/science-e/www/object/index.cfm?fobjectid=37182
    Looking at this spectra you can see the three curves, where the peak is at a certain wavelength. I.e. The blue and red line at c.600nm, which corresponds to a temperature of 4830K (yellow orange), and the green is at 800nm being at 3623K (red colour). The mean colour of these three areas are indeed a pale orange. (it would be reasonable to conclude that these regions are rich in lunar minerals.) [More accurately, we should take into account the phase angle of the sun to the reemission, but it makes little difference here.] Mapping the whole region of the moon this way produces a so-called thermal map, very useful for mineralogical understanding of the moon’s composition.

    On the moon there is also the regolith, which is a fine dusty cement-like grey material, just like the Apollo images. Unlike the other coloured minerals, much of the regolith are non-chromophoric substances (not all though). Also the reason colour is no seen is that the light being absorbed is scattered in all directions. (Think of cement powder as grey and mixed and set is usually bright white, an effect of reflectance and scattering.) Hence the albedo of the moon is only 12% (0.12); but the phase angle changes the apparent brightness of the moon. I.e -12.7 at full moon and only −9 to −10 at first or third quarter.

    Together with the coloured minerals in the mare and highlands with the much greater proportion of grey colour, we see only slight coloured variations across the moon. Hence the moon’s colour is mostly grey. (Those who have done chemistry might have seen powered element selenium, named after the moon goddess. It is a dull grey colour.

    Due to much of the moon being covered as regolith, so the moon mostly appears a dullish grey.

    If you have image software I’d suggest you lower the saturation to about 12% to 15%. (Based on the first image you have presented. The image should look on dullish yellow if you 30 years old or under, dullish orange above 30 years old or so. At retirement age, you will see the colouring less pronounced.

    Hope this helps.

    (My Apologies for the length of the reply. My knowledge with this comes from experience in colorimetry.)

  • The topic of colour on the moon is very interesting and certainly capable of being controversial. I was recently viewing a recorded session of the BAA NSW (now The Sydney City Skywatchers). It is of Sir Patrick Moore talking on the moon. One item he shows is from Apollo 17, where Harrison Schmidt is pointing out some orange rock. Of course in the control room on Earth people are saying that he’s gone nuts – everyone knows that moon rocks are just grey. It looks orange to me, and I think it was later confirmed, as Harry points out. The real question is whether this colour can be registered in anything like a large scale photograph of the moon.

    I’m thinking of showing this video of Sir Patrick Moore on ‘The Moon’ at the February Meeting. It’s a curious look at the moon.

    • >Sir Patrick is forgetting that the amount of orange soil to grey soil is a teaspoon in a swimming pool. In fact it was only found by quite accidentally scuffing up the regolith!
      He is a fantastic popularist , but his science is sometimes a little wayward.
      (I’d love to come to this meeting, but I can’t get down the stairs.)
      The colour he sees is a common complaint with the elderly as the vitreous humour, the gel-like substance in the eye, goes yellowish with age. It is the principle cause of colour sensitivity loss, especially ay the blue and red ends of the spectrum. he must certainly having this problem at 87 years old.

      Oh. A good another colour image, showing the slight yellow, is at ;
      http://upload.wikimedia.org/wikipedia/commons/9/98/Apollo_11_2004_scan.jpg

      • Thank you for your informative comments Andrew. The video that Mike Chapman is talking about was a talk recorded at Sydney Observatory about 25 years ago when Patrick Moore was still sprightly and active (though he did complain about the steep stairs in the hotel that I booked him into in the Rocks!) and he was not yet Sir Patrick.

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