• Physics 14, 123
Laboratory experiments reveal the melting course of that generates a generally seen ice characteristic known as a glacier desk.
A glacier desk—a big stone sitting precariously on prime of a slender pedestal of ice—is a typical sight on low-altitude glaciers. A brand new examine gives a proof for a way these options type [1]. Based on experiments with stones of various shapes and supplies, researchers discovered that the floor space and the warmth conduction of the stone are two essential properties controlling whether or not a desk types or not. By incorporating these properties right into a mannequin, the crew estimated the minimal rock dimension required for a desk to type, discovering a price per observations. The mannequin might enable glacier tables to supply a tough gauge of glacier melting in circumstances the place conventional monitoring gear is just not accessible.
Glacier tables sometimes happen at altitudes beneath about 10,000 toes, the place summer season temperatures may be heat sufficient to soften ice. A big rock or boulder mendacity on prime of the glacier can gradual the melting of ice straight beneath it. This protected ice types a pedestal—usually reaching 1–2 meters in top—that helps the rock as the encircling ice melts away. Usually, glacier tables final just a few months, with the rock tipping over as soon as the pedestal diameter turns into too small.
Nicolas Taberlet encountered glacier tables as a mountaineering fanatic. “I came across them a number of times and always thought that they were interesting,” he says. A physicist on the University of Lyon, France, Taberlet puzzled why massive rocks usually type tables, whereas smaller rocks sometimes sink into the ice. So he and his colleagues designed laboratory experiments with 3-cm-thick plates of clear ice that have been inclined at varied angles to resemble miniature glaciers.
The researchers first measured the ice melting within the lab by monitoring the thickness of the plates over time. They confirmed that the ice warming was prompted primarily by radiation from the lab partitions and by convection within the ambient air. Less essential was liquid water runoff. These outcomes have been per observations that the crew had made earlier on actual glaciers.
The crew then examined a wide range of cylindrical stones by inserting them on prime of the ice and monitoring them for a number of hours. Each stone was produced from considered one of six supplies that differ of their warmth conductance, from polystyrene, the weakest conductor, to granite, the strongest. The stones have been between 4 and 14 cm in diameter and between 0.5 and seven cm in top.
Some of the stones fashioned tables, whereas others sank beneath the ice floor. Heat conduction was one issue, as demonstrated by the truth that ice pedestals fashioned beneath polystyrene stones however not beneath granite stones. Polystyrene is a greater “blanket,” shielding the ice from the nice and cozy setting.
Another issue was the form, with thinner stones forming tables extra simply than thicker stones. The researchers described this habits as a geometry-induced amplification of melting: a thicker stone has extra floor space involved with the setting, so it absorbs extra warmth, which causes the ice beneath to soften quicker than the ice beneath a thinner stone. Taberlet explains {that a} related impact (with warmth flowing in the other way) permits high-surface-area “fins” to hurry up the emission of extra warmth from engines and digital parts.
The crew mixed the conductivity and the geometric amplification results right into a generic components for figuring out whether or not ice lined by a stone will soften quicker or slower than uncovered ice. Using this components, they estimated that the minimal dimension for a rock to type a desk is 10–20 cm, which is per observations that almost all tables on glaciers are 1 m or extra in width.
With additional examine of glacier tables, scientists would possibly be capable of use them as glacier benchmarks. “If you don’t have the means to track a glacier continuously, you could simply go there every month of June, say, and measure the height of the glacier tables,” Taberlet says. Such knowledge could possibly be transformed into an estimate of the melting fee. An understanding of glacier tables might additionally turn out to be useful on Jupiter’s moon Europa—a future mission to this icy world might need to fret that its lander would behave like a rock on a glacier, altering the melting fee beneath it.
The experiments and area observations present clarification of the environmental circumstances that result in glacier tables, says Bhanu Pratap, a glaciologist from the Wadia Institute of Himalayan Geology in India. But he believes extra research are wanted to know the consequences of “debris,” resembling rocks, grime, and air pollution, on glacier evolution.
–Michael Schirber
Michael Schirber is a Corresponding Editor for Physics primarily based in Lyon, France.
References
- M. Hénot et al., “Onset of glacier tables,” Phys. Rev. Lett. 127, 108501 (2021).
