Making polar spiders in the lab

The formation of “spiders” during Martian south polar spring is one of the most spectacular natural phenomena on the Red Planet. Briefly, what happens is that every winter carbon dioxide ice forms a translucent layer above the sandy ground. Then as spring sunlight passes through the layer to heat the ground, it causes CO2 gas to accumulate beneath the layer. When the pressurized gas finally bursts through in several places, geysers of gas and dust erupt violently, eroding converging spider-shaped channels under the ice and scattering dust fans on top of it.

LABORATORY SPIDERS. Real Martian polar spiders (top) and their made-in-the-laboratory cousins (below) have similar shapes and branching patterns. (Image taken from Figure 1 in the paper.)

Recently, a group of researchers led by Simon de Villiers (University of Oslo) recreated (on a much smaller scale) the formation of these “spiders” in a terrestrial laboratory. Their report appears in Geophysical Research Letters.

Dubbing the spider-like features “araneiforms,” the team found they could create very similar patterns inside a cell filled with air and unconsolidated granular material (spherical silicate glass beads).

The researchers drilled a small hole in the top cover (made of thin glass) of the cell. Then they slowly pulled the cell top upward, allowing air to enter the cell without disturbing the grains. Finally they let the cell cover drop quickly back to its original position.

This drove air and entrained particles out of the cell through the small hole in the top. After working the cell through several cycles, the team found they could produce converging dendritic patterns similar to those seen on Mars. Also, as they flexed the cell cover, they saw the formation of straight, braided, and quasiperiodic oscillating channels, unlike meandering channels on Earth.

“The experiments demonstrate that the erosion of granular material caused by gas flow and venting produces patterns similar to Martian araneiforms,” the team concludes. “And thus supports the hypothesis that these features are produced by the venting of CO2 gas during the Martian spring.”

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