Nature creates art on the frozen surface at Lake Baikal, Siberia.
On the frozen lake, you will find stones balancing on narrow pedestals of thin ice in shallow cavities. It looks as though they were carefully crafted and placed. Scientists have now reproduced the phenomenon in the laboratory, and these stones are known as 'Zen stone'.
Scientists discovered that the stone acts as an umbrella by shading a small amount of ice from sunlight and preventing it sublimating. This results in a thin structure supporting the stone.
The shallow depression around the base creates by far-infrared black body irradiance.
"Zen stones, which are amazing natural structures consisting a stone standing on an icy pedestal, whose origin was long misunderstood," French National Centre for Scientific Research in France physicists Nicolas Taberlet & Nicolas Plihon explain in their new study.
"We show that they are caused in part by variations in the sublimation rates of the surrounding ice. This leads to slow formation of a platform, adding to the few reports of sublimation driven pattern formation.
Understanding this process helps to understand other differential ablation processes that occur on ice surfaces such as the debris-covered glaciers whose existence is under threat by global warming and icy bodies in outer space.
Zen stones in nature and the team's experiments. (Taberlet & Plihon, PNAS, 2021)
Zen stones are stunningly beautiful. This has led to viral sharing and image posts on popular sites attributions to wind erosion.
Taberlet and Plihon point out that these formations don't show signs of mechanical wear associated with wind ablation. Zen stones can be found even in caves which offer some protection against the elements.
Researchers wanted to investigate the formation mechanisms of these ice pedestals. They recreated the process in a laboratory vacuum chamber. The stone was represented by an aluminum disk that was placed on top of a block made of ice. It then sublimated infrared black body radiation from the chamber's walls.
The absence of an aluminum disc caused the ice to sublimate isotropically (uniformly) mimicking the isotropy that diffuse sunlight can cause on a cloudy day. Researchers observed that the pedestal formed when the disk was placed on top of the ice. However, this can happen at a very slow rate in nature.
The lab produces a 'Zen Stone'. (Taberlet & Plihon, PNAS, 2021)
They then used numerical modeling to understand the process. It was discovered that diffuse solar radiation sublimates the ice, and transforms it into vapor directly, bypassing the liquid stage.
The rock can cause uneven sublimation rates. The rock acts as an umbrella and protects the thinnest column from radiation that can cause sublimation to occur last.
Scientists found the cavity at the bottom of the column to be more complicated than expected. Taberlet and Plihon proved that the cavities at the base of the column were formed by far-infrared radiation from the stone. This radiation emits temperatures ranging between -20 to 0 Celsius.
They write that while the umbrella effect dominates Zen stone dynamics, the far-infrared emission by the stone is a secondary effect which creates the depression around the pedestal.
Researchers said that this is very different to the formation of natural structures glacier tables with a similar shape and size. They form when the ice surrounding a large rock melts, creating an object that looks very similar to a table.
Glacier tables are formed when the rock acts as a thermal insulation. Therefore, their thermal properties (conductivity, specific heat) can be crucial. Our experiments have shown that Zen stones are exactly the opposite," researchers conclude.
"Moreover, heat conduction within glacier table is critical. There is a minimum thickness, which depends on the material, below which the stone sinks into the ice and forms a table. This regime is not possible to observe in conditions where Zen stones form. Even a thin stone can lead to an ice pedestal.
The research was published in PNAS.