When we gaze up at our beautiful celestial companion in the sky, it seems plain to us that unlike our own varicoloured world, the Moon's surface is restricted to shades of grey. As it rises or sets, or passes into the Earth's shadow during a lunar eclipse, it may appear to take on an orange or red hue, but high overhead on an ordinary night, the Moon has a monotone face. At least, that is how it seems, but there is more than meets the eye. If you have the good fortune of being able to see the Moon up close with a moderately sized or larger telescope, you may just notice the faintest hint of some colour displayed on the Moon's more prominent features.

Perhaps it is better described as the semblance of a hint, since the impression is so subtle, but some of the darker regions are visibly blue. Harder to see still is an extremely dull, rusty yellow tinge to the rugged complexion of the lunar highlands. The effect is just evident in natural colour photographs, as camera sensors don't suffer some of the flawed characteristics inherent to human visual perception. When we see those blues and yellows, we are not fooling ourselves. The Moon is apparently awash with delicate colour.

Why is this so difficult to perceive visually? Our experience of sight is sensitive to contrast. We are less likely to discern low-contrast details than high-contrast details - a bias not common with digital cameras. The Moon's colours are extremely subtle, showing far lower contrast than the various shades of brightness across its face. Fortunately, we can exploit cameras to scrutinise these colours by enhancing their contrast even further. By taking a number of photos and combining them to reduce the influence of digital sensor noise, we can produce an image with showing maximal colour contrast.

The colour intensity in the image is multiplied and its coverage widened to draw out variations that are too similar to be readily noticeable, and the result is a quasi-scientific image illustrating the chemical variance of the lunar surface.

The Moon's surface chemistry is dominated by Silica (SiO₂) and Alumina (Al₂O₃) neither of which are particularly vibrant, but blue and yellow-orange are seen in regions with relatively high concentrations of Titanium Dioxide (TiO₂) and Iron Oxide (FeO) respectively. These two compounds account for the vast majority of the colour revealed in such images, with Titanium Dioxide being abundant in the basalt-rich volcanic plains of the lunar maria and Iron Oxide - rust as we call it on Earth - more common in the rough headlands.

Although our eyes aren't best equipped to reveal it to us, we can safely say that Moon is indeed a colourful world - one whose visage is more nuanced than that of the Earth and most other planets. Observing these hues is a rewarding challenge, and capturing them is a fantastic astrophotography project. Next time you see the Moon, see if you can find its true colours.

Image credits: Tom Kerss