Researchers at the French National Research Center (CNRS) and l’Université Claude Bernard Lyon 11 are digging into the secrets of the ‘Zen stone’ phenomenon.
Image credits Nicolas Taberlet, Nicolas Plihon, (2021), PNAS.
It’s not a rare sight to see stones seemingly placed on a pedestal of ice on the surface of frozen lakes. Contrary to all appearances, this is a naturally-occurring phenomenon, referred to as “Zen stones” for its similarities with the Japanese style of garden decoration, or “Baikal Zen”, after the famous Siberian lake.
Although we had theories regarding their formation, ranging from ‘magic’ to ‘wind erosion, we never actually knew for sure why this happened. The study, now, reports that it comes down to sublimation — the process of a solid becoming a gas without turning into a liquid in-between.
Naturally zen
“There’s no direct application to our work. Just the satisfaction of having understood something new,” Dr. Nicholas Taberlet, a physicist at the University of Lyon and first author of the study, told ZME Science in an email. “However our results can be useful for space exploration missions. For example, NASA is planning to send a lander to Europa, which is covered in ice. It’s important to realize that the rover will prevent the ice from sublimating underneath it and to plan the mission accordingly.”
Lake Baikal, in Siberia, is particularly associated with this phenomenon. As the world’s largest, deepest, and perhaps cleanest freshwater lake, as well as one in a very frigid expanse of the globe, conditions here are ripe for ice pedestals to form. The first requirement for this phenomenon is for a stone to become lodged in ice. This isn’t a rare occurrence in places such as Baikal, whose surface is frozen solid for a long period each year (around 5 mo/year). That being said, however, it is a very rare phenomenon on a global scale, which is why ice pedestals are so strongly associated with this lake.
According to the findings, the physical processes that govern this phenomenon can act pretty much anywhere — even, as Dr. Taberlet mentioned, in outer space. All it takes is the right environmental conditions. Ice pedestal formation is relatively rare on the global scale because it requires “thick, flat, snow-free layers of ice” which form only under “longstanding cold and dry weather conditions”. There simply aren’t many places on Earth that satisfy those conditions.
The team explains that the formation of an ice pedestal starts with a stone that initially “rests directly on a flat ice surface”. Over time, however, this ice is gradually eroded to create the final, slender shape. The exact mechanism which protects the ice under the stone, thus forming the pedestal, remained unknown. The goal of this paper was to determine what this mechanism is.
Up to now, our best guess as to what was happening was the direct melting of the ice beneath the stone. But it didn’t really fit in with what we were seeing in the field. Liquid water promotes the melting of ice around it. Since liquids pool, a small puddle forming underneath the stones would not be able to create a pedestal-like shape — it would melt all the ice at the same rate, creating a roughly uniform, concave shape. Furthermore, it didn’t really make sense for ice underneath the stones to melt faster than the rest, since the stone itself should be blocking sunlight, essentially acting as an umbrella providing shade around it. An argument could be made that the stone warmed up under sunlight, which would promote melting, but that would also melt the pedestal, not create the wavy patterns seen in the field.
The main breakthrough in the study came when the team realized that “the ice was indeed sublimating (and not melting)” and that this effect was caused directly by the shade cast by the stone.
Sublimation is the process through which a solid turns directly into a gas, without turning into a liquid in between. Naphthalene (mothballs) are a very good example of sublimation at work. Dry ice and car air fresheners also rely on sublimation.
Through a series of lab experiments and mathematical modeling, the team showed that the stones do indeed act as umbrellas, preventing sunlight from reaching the ice beneath them. At the same time, however, due to the movements of the Sun in the sky and light scattering in the atmosphere, they only provide complete and permanent shade to a small area in the middle — this will become the pedestal.
A very interesting finding is that, although the stones do block infrared energy (i.e. heat) in sunlight from reaching the ice, they are also what causes the dips around them to form. As these stones heat up, they emit infrared radiation in turn (this is known as black-body radiation). In the area around the stone, ice is heated up by infrared waves from the sunlight and the stones at the same time — causing it to melt even faster than exposed ice. But the radiation emitted by the stone isn’t particularly powerful, and not enough to melt through the ice of the pedestal by itself.
Tied into the unique combination of environmental conditions at the site (sustained, very low temperature and humidity levels), which promote sublimation of ice instead of melting, these interactions lead to the creation of the wavy patterns and ice pedestal beneath the stones of the Baikal lake.
The team used both stones and disks of a variety of dimensions and materials (to account for a larger range of physical properties), finding that the ice pedestals form largely independently of these properties.
While the conditions necessary for this process to take place are quite rare on Earth, they would be very, very common on planets lacking an atmosphere, or those with atmospheres that are very dry. As Dr. Taberlet noted, understanding how a rover might promote the sublimation of ice beneath it could prevent some embarrassing — and very dire — situations for our future explorers.
The paper “Sublimation-driven morphogenesis of Zen stones on ice surfaces” has been published in the journal PNAS.
