Spiral troughs in Mars polar caps are born in ‘cyclic steps’

Both Martian polar caps have deep spiral troughs that slice into them for dozens or hundreds of kilometers, but their origin and development has been much debated by scientists. New work by a team of researchers led by Isaac Smith (University of Texas) combines theoretical insights with data from the Shallow Radar (SHARAD) instrument on NASA’s Mars Reconnaissance Orbiter to suggest an origin for the features. The work is published in the Journal of Geophysical Research.

SUBTRACT, THEN ADD. As katabatic winds descend over the polar cap, they sublimate the top layer of ice (red-line zone) on the high side and load up on water vapor. Then as the winds rise when they hit the katabatic jump (KJ), they cool (blue-line zone) and deposit ice and snow, thus building up the surface. Over time, the asymmetric erosion and deposition process drives the trough, termed a cyclic step, to move upwind — towards the left in the diagram. (Image taken from Figure 16 in the paper.)

Each trough is a step in a repeating cycle that involves both ice sublimation and deposition, the team says. Unlike what happens with a dune, for example, which migrates downwind with the sand grains that comprise it, a cyclic step is a feature that moves upwind. This is a critical detail needed to match the subsurface profiles within the polar cap as detected by SHARAD, an ice-penetrating radar. The profiles show a steady deposition of new layers in an upwind direction, while preserving a marked trough in the ice cap’s surface.

Two ingredients — winds blowing off the polar cap and the ice cap itself — work in combination to produce the troughs, says the team. Chilled air descends over a polar cap, then flows downward and outward at the cap’s surface. (Coriolis force also deflects the air flow into a spiral pattern.) This outflow, termed a katabatic wind, often blows strongly. The wind also warms through compression as it descends.

When a warmed katabatic wind approaches a topographic trough, it first erodes the ice cap by sublimation, becoming saturated with water vapor. Then, as the wind hits the “katabatic jump” on the far side of the trough, it rises, cools, and drops ice and snow, building up the surface. The removal of ice on one side and deposition on the other results in a depression that migrates upwind.

“The katabatic jumps repeat because the winds become fast enough several times,” says Smith. “The ice caps are huge.” In addition, he says, steeper slopes produce katabatic jumps at shorter intervals because steeper slopes accelerate the winds more quickly. It is this repetition that makes the steps cyclic.

Smith notes, “The troughs spiral because Coriolis force steers the winds into spiral paths. Since katabatic jumps stand perpendicular to the winds, the jumps also spiral outward, giving the troughs their characteristic beautiful pattern.”

“We propose that the effects of this jump are sufficient to make trough clouds, which in turn precipitate as snow or directly deposit the water vapor to the surface,” the scientists write. “Clouds typically form when either pressure increases or temperature decreases.”

Prior to trough formation, the north polar topography was smoother than today, the team notes, adding that this appears not to be a difficulty in getting troughs started initially. “In experiments, cyclic steps and hydraulic jumps have been created on a smooth surface, much as the troughs in the north polar layered deposits may have begun.

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