If you visit the Small Sea of Lake Baikal in Russia in winter, you are likely to see an unusual phenomenon: a flat rock balanced on a thin ice pedestal, similar to the stacking of zen stones common in Japanese gardens. The phenomenon is sometimes referred to as the Baikal Zen Formation. The typical explanation for the formation of these formations is that the rock captures light (and heat) from the sun, thereby melting the ice below until there is only a thin base to support it. The water under the rock freezes again at night, and it has been suggested that wind may also play a role.
According to a new paper published in the Proceedings of the National Academy of Sciences, two French physicists now believe they have solved the mystery of how these structures came about – and their solution has nothing to do with the thermal conduction of the stone. Rather, they attribute the formation to a phenomenon known as sublimation, in which snow or ice evaporates directly into vapor without going through a water phase. In particular, the shadow that the stone provides hinders the rates of sublimation of the surrounding ice in its surroundings, while the more distant ice sublimates more quickly.
Many similar formations occur naturally in nature, such as Hoodies (high, spindly structures that form in the sedimentary rock over millions of years), Mushroom rocks or rock pedestal (the base was eroded by strong dusty winds) and glacier tables (a large stone that sits precariously on a narrow ice pedestal). But the underlying mechanisms by which they arise can vary widely.
As we reported last year, for example, a team of applied mathematicians from New York University examined the so-called “stone forests” that are common in certain regions of China and Madagascar. These pointed rock formations, like the famous ones Stone forest in China’s Yunnan Province, are the result of solids dissolving in liquids in the presence of gravity, creating natural convection currents.
On the surface these stone forests look pretty much like “Penitent“: snow-capped ice columns that form in the very dry air found high in the Andean glaciers. Charles Darwin described Penitentes in 1839 during an excursion in March 1835 en route from Santiago, Chile Penitentes covered snowfields forced the Argentine city of Mendoza Create new artificial versions of penitentes in the laboratory. But penitentes and stone forests differ in terms of the mechanisms of their formation. The prongs of a stone forest are carved by rivers that do not play a major role in the formation of penitents.
Some physicists have suggested these penitents form if Sunlight evaporates the snow directly into steam (sublimation). Tiny ridges and hollows form, trapping sunlight in them, creating extra heat that creates even deeper hollows, and these curved surfaces in turn act like a lens that speeds up the sublimation process. A Alternative proposal adds an additional mechanism to account for the oddly periodic fixed spacing of penitents: a combination of vapor diffusion and heat transport that creates a steep temperature gradient and thus a higher rate of sublimation.
In the case of the Baikal Zen stone formations, the process appears to be similar to the sublimation hypothesis for Penitentes, according to co-authors Nicolas Taberlet and Nicolas Plihon of CNRS in Lyon, France. Earlier this month they did published a somewhat related study in Physical Review Letters on the natural formation of glacier tablets (a rock supported by a slender column of ice). They were able to make small-scale artificial glacier tablets in a controlled environment and found two competing effects that control the onset of glacier tablet formation.
In the case of smaller stone caps with higher thermal conductivity, a geometric increase in the heat flow causes the cap to sink into the ice. In the case of a larger cap with a lower thermal conductivity, a reduction in the heat flow occurs because the cap has a higher temperature than the surrounding ice and forms a table.
For this latest study, Taberlet and Plihon set out to explore the underlying mechanisms behind the natural formation of Baikal Zen structures. “The rarity of the phenomenon stems from the rarity of thick, flat, snow-free ice layers that require long-term cold and dry weather conditions,” the authors write. “Weather records show that the melting of the ice is practically impossible and instead the weather conditions (wind, temperature and relative humidity) favor the sublimation that has long been characteristic of the Lake Baikal area.”
So the researchers set out to reproduce the phenomenon in the laboratory to test their hypothesis. They used metal disks as experimental analogs of the stones and placed the disks on the surface of blocks of ice in a commercial lyophilizer. The device freezes material, then reduces the pressure and adds heat, causing the frozen water to sublime. The higher reflectivity of the metal disks compared to stones prevented overheating of the disks in the chambers of the lyophilizer.
Both aluminum and copper disks created the Baikal Zen formations, although copper has almost twice the thermal conductivity of aluminum. The authors concluded that the thermal properties of stones were therefore not a decisive factor in the process. “Far from the stone, the rate of sublimation is determined by the diffuse sunlight, while the shadow it creates in its surroundings inhibits the sublimation process,” the authors write. “We show that the stone only functions as a screen, the shade of which hinders the sublimation and thus protects the underlying ice, which leads to the formation of the base.”
This was then confirmed by numerical model simulations. Taberlet and Plihon also found that the depression or depression surrounding the plinth is the result of far-infrared radiation emitted by the stone (or disc) itself, increasing the overall rate of sublimation in its surroundings.
It’s quite different from the process that leads to glacial tablets, despite the similar shape of the two formations. In the case of glacier tables, the umbrella effect is only a secondary factor in the underlying mechanism. “Glacier tablets appear on low-lying glaciers when weather conditions melt the ice instead of sublimating,” the authors write. “They form in warm air while the ice stays at 0 degrees Celsius, while zen stones form in air that is colder than the ice.”
Understanding how these formations occur naturally could help us to learn more about other objects in the universe, since the sublimation of the ice brought Penitentes on Pluto and the landscape formation on Mars, Pluto, Ceres, the moons of Jupiter, the moons of Saturn and has influenced several comets. “In fact, NASA’s Europa-Lander project aims to look for biosignatures on Jupiter’s ice-covered moon, on the surface of which different sublimation can threaten the stability of the landers, and this needs to be fully understood,” the authors concluded.
DOI: PNAS, 2021. 10.1073 / pnas.2109107118 (Via DOIs).
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