Counterrotating Ice Pinwheel On Mars' North Polar Cap

Caption: Alan Howard of the University of Virginia first suggested the ice trough migration model based on Viking spacecraft data back in 1982. His theory, that wind erosion and sunlight shape and move the troughs, was never widely accepted, but the new data supports it.

Caption: Chasma Boreale is indicated by an arrow in this modern image of the Martian north pole.


Interesting how the accumulation and dispersal of ice over billions of years can create these large land forms! I had to think about the following explanation a bit to make sense of it. Since winds streaming off the ice cap will spiral to the right due to Coriolis force as they descend south, that means the prevailing winds will be perpendicular to the troughs. In addition, the troughs will inch east over the eons.

Interesting too is how this process apparently doesn't occur at Mars' South Pole (the South Pole is mostly dry ice, as opposed to the mostly water ice of the North Pole, so there is a compositional difference). Interesting too is that this process doesn't occur in Antarctica, even though Antarctica experiences the same kind of winds as Mars' North Polar Ice Cap does.

I don't quite buy the explanation below that local topography inhibits their formation in Antarctica. I'm sure the Martian North Polar Ice Cap has local topographical variations too, but the spirals developed anyway. The Coriolis Force is just as influential on Earth as it is on Mars, so why no spirals in Antarctica? There must be some kind of extra factor to set these spirals in motion. Perhaps the initial ice cap has to be remarkably smooth to get the process going, and maybe Antarctica and the Martian South Polar Cap just aren't smooth enough. And maybe there's no going back either: once the spirals start, that smoothness is gone - permanently. So, maybe the transition into the spiral phase isn't a climatic change at all, but more like a phase change - a nucleation event, if you will. So, maybe one normal, humdrum day eons ago, out of nowhere - they just started!:

Jack Holt of the University of Texas and his graduate student Isaac Smith used radar data from MRO's Shallow Subsurface Radar to crack the case. Examining the details of this new data set has laid open the ice cap's internal structure, revealing clues to the massive ice troughs' formation.

Apparently, the wind did it.

"Radar cross sections reveal layers of ice deposited throughout the ice cap's history," says Holt. "The size and shape of those layers indicate that wind has played a key role in creating and shaping the spiral troughs."

Not only does wind shape the spirals, but also it causes them to move. They rotate around the north pole, turning like an excruciatingly slow pinwheel, curiously enough, against the wind.

Smith explains the process: "Cold air from the top of the ice cap sweeps down the slope, gaining speed and picking up water vapor and ice particles along the way. As this wind blows across the trough and starts up the other slope (the cooler side, facing away from the sun), it slows and precipitates the ice it holds. All of this ice is deposited on this cool slope, building it up, so the trough actually grows and migrates, over time, against the wind."

The Coriolis force generated by Mars' rotation twists the winds sweeping down from the ice cap.

"That explains the troughs' spiral design," says Smith.

Similar formations can be found in Antarctic regions of Earth, but without the spiral shape.

Icy megadunes in Antarctica do not spiral like the ice troughs of Mars. "You don't see spirals in Earth's Antarctic ice sheet because local topography there prevents the winds from being steered by the Coriolis force."

The radar data have solved another icy mystery, too--the origin of Chasma Boreale.

Chasma Boreale is a Grand Canyon-sized chasm that slashes through the midst of the spiraled troughs. Theories to date suggested that either wind erosion or a single melt event excavated Chasma Boreale within the past 5 to 10 million years.

"Not so," says Holt. "The MRO data clearly show the chasm formed [long before the spirals did] in a much older ice sheet dating back billions of years. Due to the shape of that ancient sheet, the chasm grew deeper as newer ice deposits built up around it. Winds sweeping across the ice cap likely prevented new ice from building up inside the chasm [so it never filled up]."

The radar data also revealed a second chasm matching Boreale in size.

"This chasm's never been seen before -- unlike Boreale, it did fill up with ice, probably because it's in a different location. Boreale is closer to the highest points of the ancient ice cap, where the winds are stronger and more consistent."

By discovering that both Chasma Boreale and the ice troughs were shaped by similar processes over different timescales, Holt and Smith answer some questions about Martian climate history. But they're also sparking new ones.

"For a long stretch of Martian history the ice layers were regular and uniform, then there was a distinct period when the spiral ice troughs got started," says Smith. "Something changed. There must have been a very fast (relatively speaking) and powerful change in climate. We still don't know what that change was."