Moon milk (Prehistoric caves)
"Moon milk" is a strange substance that covers the walls of some prehistoric caves, sometimes erasing parietal works. What is it made of ?
This research was carried out on the infrared beamline SMIS in 2009.
Autumn 2024: a look back at this research
In 2009, paleoclimatologist Dominique Genty arrived at the SOLEIL synchrotron with Petri dishes containing a strange white substance reminiscent of Camembert rind. However, this soft, milky material was collected from the walls of a prehistoric cave. It is known as "moon milk," or more commonly "mondmilch." In Lascaux, several thousand years ago, it covered an entire corridor and likely concealed paintings that are now inaccessible. At Chauvet, more than 30,000 years ago, prehistoric humans drew animals with their fingers in the mondmilch on the cave walls, and these perfectly preserved drawings still appear fresh and striking.
According to electron microscopy images, mondmilch seems to be a mixture of calcite and organic matter. At SOLEIL, Dominique Genty chose to analyze his samples using the infrared radiation from the SMIS beamline. The measurements revealed characteristic signatures: those of filamentous bacteria and fungi. Mondmilch is, therefore, a complex assemblage of water, mineral matter, and living organisms—now fossilized. "It's astonishing to see how lively this material, which is tens of thousands of years old, still appears," comments the paleoclimatologist.
These findings supported the thesis defended by Florian Berrouet in December 2009, under the supervision of Jean-Michel Geneste, the curator of the Lascaux cave. He documented an impressive number of caves covered in the whitish substance and regarded mondmilch as a true imprint of archaeological time, as well as a medium in its own right for parietal art. It's as if today's artists were to say they commonly work with charcoal, watercolor... and mondmilch.
Meanwhile, Dominique Genty broadened the scope to include calcite formations known as rimstones or "gours," deposits found in caves that form small dams retaining water. He embarked on an extensive dating campaign of these rimstones using carbon-14 and uranium-thorium methods. The rimstones, structurally related to mondmilch, date back 8,000 years. "With this study, we even made the cover of the journal Radiocarbon," Genty recalls with great pleasure.
Today, he leads a major project supported by the ANR, named DECACLIM, which aims to understand how climate change affects decorated caves. The project also seeks to define appropriate conservation methods. Dominique Genty has access to some of the longest data sets in the world, spanning nearly thirty years. It was in the 1990s that he had an inspired idea: to equip numerous caves with sensors measuring temperature, humidity, and CO2. His data reveals a clear trend, such as an increase of about +1°C in temperature. The project will last four years and will include, among other things, the modeling of air flow and its condensation on the walls within a decorated cave.
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AUDIO TRANSCRIPTION OF THE VIDEO
Dominique Genty
It is brittle, it breaks… it looks a little bit like the rind of camembert cheese, and in fact, it is soft when fresh.
It's full of water, at least it is 2/3 water.
VOICE-OVER
Scientist Dominique Genty took this strange white substance from the walls of a prehistoric cave.
Because of its milky white appearance, it has a very poetic nickname.
Dominique Genty
Moon milk in English, Mond Milch in German, lait de lune in French... it is generally known by its German name.
VOICE-OVER
But despite its pretty name, this white stuff can cause damage. For example, a few thousand years ago, some Mond Milch settled on the walls of a corridor in the Lascaux caves.
Dominique Genty - Research Director - Laboratoire des Sciences du Climat et de l’Environnement (Laboratory of Climate and Environmental Sciences) (CEA/CNRS/UVSQ)
OK, so this occurred after the paintings, so it probably erased some parietal works.
Maybe there are hidden paintings all along the white corridor of the Lascaux cave—who knows?
On the other hand, there is also some older Mond Milch.
That is what we find in the Chauvet cave, where over 30,000 years ago prehistoric men used their fingers to make drawings of animals such as mammoths, and it’s true that when you see drawings that were done on Mond Milch in caves, they look as if they were made just a few days ago, they're really fresh.
So it’s something that is very stable and that probably evolves very slowly.
VOICE-OVER
What is this Mond Milch, with its amazing properties, actually made of?
Mineral deposit, fungus, or bacteria?
Mond Milch is actually a mixture of all three.
Dominique Genty
Here is an image taken with an electron microscope, and as we can see, it consists of needles of calcite, and under these calcite needles there are very fine filaments.
Here, we have spherical shapes.
So they could be filamentary bacteria. It could also be partly fungus... Right now, for this type of Mond Milch, we don't know.
VOICE-OVER
The exact composition of the white substance is still not well known.
It could vary from one cave to another, or even from one wall to another inside a cave.
So to find out more, the scientists took their samples to the SOLEIL synchrotron.
With the infrared beam of the SMIS beamline, they performed a very precise analysis of the composition of samples taken from different prehistoric caves.
Dominique Genty
What interests us is to extract the organic matter from the Mond Milch, to analyse it in this way with the finest possible beam and the most stable beam possible.
In this curve, these peaks are actually characteristic of a particular organic material.
2nd scientist
You see here, this is interesting, because you can see it’s some material that came out of a filament.
Dominique Genty
On the samples we analysed, we found both traces of bacteria and traces of fungi.
VOICE-OVER
The results obtained will be supplemented by biological analyses.
The scientists hope that this work will teach them more about the processes by which Mond Milch is formed.
Dominique Genty
We will try to understand how Mond Milch is formed.
Probably thanks to physico-chemical processes, changes of temperature or pressure of the carbon dioxide in the cave, and possibly thanks to the participation of these organisms, which are either fungi or bacteria.
VOICE-OVER
When they have gained a better understanding of the mechanisms and conditions of development of Mond Milch, scientists will be able to deduce the environmental conditions which prevailed thousands of years ago when Mond Milch developed.
Their work should also make it possible to improve the preservation of prehistoric paintings.
Dominique Genty
For example in a decorated cave, if tourists are allowed inside, they will bring in carbon dioxide, humidity, and heat, which will change the original conditions of the cave.
Those changes can create conditions that are favourable to the development of Mond Milch, and if it starts to develop, it can damage the walls and therefore also the engravings, the paintings, and anything else on the walls.