Russian photographer Olga Zima won in 2016 a photo contest with an image that quickly went viral. In it, a stone appeared to be suspended in midair, only held by a thin ice pedestal in an extremely fragile balance. The photograph was taken at Lake Baikal, in the heart of Siberia, where they appear every winter. Baptized as Zen stones to recall Japanese practice, French researchers have now replicated in the laboratory the process by which they appear to levitate. And it is not because the ice that surrounds them melts.
Nature offers interactions between matter and environmental conditions that, to human eyes, seem as capricious as impossible. This is the case of fairy chimneys, such as those of the Turkish Cappadocia. These columns of rocks that seem to fall at any moment are formed by the presence of a harder rock on a softer base than water or the action of the wind is eroding. Another example is the glacial mesa, in which a large rock rests on a minimal portion of ice. Here, the key element in this particular morphogenesis is that solar radiation melts the frozen part most exposed to the Sun, leaving frozen the part that serves as a pedestal for the stone. There is a third phenomenon, observed in the Andean mountain range or the Himalayas, which forms the penitents, conical icy formations reminiscent of the Capuchins or Nazarenes of the Catholic processions, (see image below). Here, the agent of change is sublimation, the passage of the surrounding ice from solid to gaseous state.
What mechanism creates the Zen stones of Lake Baikal? After Olga Zima’s photography, two camps were created between scientists and amateurs. For some, the stone that began on a flat icy surface ends up resting on a pedestal because the ice melts differently depending on whether it is more or less protected by the shadow of the stone. It is the process observed and studied in the glacial tables. For others, the erosion caused by snow from blizzards would be the cause of the ablation suffered by the ice, as happens with rocks of different hardness. But in Baikal it is very rare for it to snow, it is a very cold area, but also extremely dry. So there is no erosive agent.
To settle the issue, a group of researchers from the French National Center for Scientific Research (CNRS, counterpart of the Spanish CSIC) and the Claude Bernard University of Lyon have verified that Zen stones are formed like penitents, by sublimation and not by erosion or thawing. The process is detailed by Nicolas Taberlet, researcher at the French university and co-author of the study: “Sublimation is a direct transition from solid gas [vapor de agua]. Under normal pressure conditions, it only occurs below zero degrees and if the air is dry enough. It still requires energy [del sol para las piedras zen] but this energy acts as latent heat and the temperature of the ice remains the same. ”
What they did was use metal discs as if they were stones and introduce them into freeze-drying chambers, which dehydrate by freezing. At 40 hours after starting the process, they observed how the ice under the disk disappeared leaving only one thin pedestal that kept it standing. To determine the cause, they used discs made of different metals (copper and aluminum). Although the thermal conductivity of the first is double that of the second, the formation process and final appearance of the different pedestals were similar. So the key is not in the heat that the stone can transmit to the ice, but to the energy of solar radiation.
In Baikal, in the weeks in which floating stones are formed, the average temperature is – 15 º, so solar radiation does not cause de-icing, but there is still an energy contribution which, in its dry and high pressure environment, facilitates the sublimation of ice. What the stone does is act as an umbrella causing uneven impact from this mechanism, giving shape to the pedestal. The whole process lasts from two to four weeks and, despite the fact that the photograph seems to freeze it, it continues until the support of the stone also sublimates and it falls. It won’t rise again until the following winter.
Taberlet acknowledges that his research is of no immediate benefit. “Just the satisfaction of having understood a new phenomenon,” he says. But it also accumulates science to understand other similar phenomena although of different scale, such as that of penitents, which have already been observed in other celestial bodies of the solar system such as Pluto, Saturn, Jupiter satellites and some comets.
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