Scientists have long observed smooth, repetitive patterns of stones and grooves that mark the ground in colder regions in the world. Each intricate design—including circles, orderly rows, and swirls—was created when stones are moved around when tiny, thread-like sprouts of ice emerge from the freezing ground, reports Hannah Hickey for UW News. As these so-called "ice needles" freeze and then melt, they create astonishing patterns similar to those in Japanese zen gardens. However, while researchers speculated that spikes of ice crystals could move soil and rocks to form the patterns, it hasn't been confirmed until now.
Using laboratory experiments and computer modeling, researchers pinpointed how ice needles poking through the ground as they freeze can gradually move rocks and soil into geometric and organic shapes, according to a study published last month in Proceedings of the National Academy of Sciences. The results could help scientists decipher how similar patterns found on planet Mars formed, reports Joanna Thompson for Live Science.
Ice needles develop when the temperature in the air and the temperature of the moist soil are not the same. At night, some types of soil contract when temperatures drop. The water in the soil is then drawn upwards and sticks to the sides of narrow pores in the ground. As the water is drawn out of the Earth, the cold air freezes it, turning it into small sharp, crystal-like structures, per Live Science.
To create the swirls and ridges in a lab setting, the research team placed pebbles on top of a pan filled with wet, fine-grained soil and then froze and thawed the miniature landscape over and over, creating different conditions to see how the pebbles moved as the ice melted, reports Beth Geiger for Science News. Pins of ice poked through the soil like sprouting grass when the wet soil was not frozen, but the air temperature dropped below freezing. The needles grew several centimeters high, raising the pebbles off the ground. When the team increased the temperature, the stones fell off the ice and tumbled to one side.
Over time, researchers could see how the freeze-thaw process cleared patches of exposed soil, and eventually, stones shifted into clusters, creating larger patterns, per Science News. In total, the team exposed the rocks and dirt to 30 freeze cycles during which the ice formed and thawed away. The group noted that stones on the flat ground formed the dizzying array of swirls and loops, while sloped ground formed neat rows of rocks, per Live Science.
"That kind of selective growth involves interesting feedbacks between the size of the stones, the moisture in the soil, and the growth of the ice needles," says study author Bernard Hallet, an Earth sciences expert at the University of Washington, to UW News.
From lab experiments, the researchers built a computer simulator of ice-needled landscapes. The simulator could predict stone movements under various conditions, Science News reports. Using the computer program, the team confirmed that the pattern's rate depended on how many stones there were and how dense the coverage was. Other factors that determined the shapes in nature were how wet the soil was, how tall the ice needles grew and how sloped the ground was.
Some experts suspect that a version of these freeze-thaw cycles may have created patterns observed on the surface of Mars. Soil on the Red Planet has shown evidence of tiny ice crystals, and as the soil on Mars heats up, it may expand only to contract after it begins to cool again, per Live Science. Though the process is more subtle on Mars, it could be enough to shift pebbles and dust over time, according to Live Science.
The study results show that combining laboratory experiments with computer modeling could connect how natural landscapes behave and may eventually lead to more understanding of how these behaviors could change as the climate warms, per Science News.