Huge troughs curving outward from the north pole of Mars like the arms of a pinwheel were not carved into the polar ice caps by some mysterious force, researchers have discovered. Instead, the shifting pattern arose from a long process of formation and erosion that gave it the appearance of slowly moving and spiraling inward over time.
A similarly snail-like process gave rise to the Chasma Boreale canyon that cuts into the side of the giant pinwheel pattern, known as the north polar layered deposits (NPLD). The unveiling of the origins of the canyon and NPLD came courtesy of ground-penetrating radar carried by two Mars orbiters.
Scientists had previously favored the idea that a natural force recently carved both the canyon and pinwheel pattern into older geological deposits. But they could not test their theories beyond what they could see on the Martian surface, as if trying to judge a book by its cover.
"Radar is like opening the book; we can read each page now," said Isaac Smith, a planetary scientist at the University of Texas Austin. "People were looking at the outside and thinking they knew what the book was about, but they didn't."
Such technology allowed scientists to take 2-D cross-section images of the troughs and reveal the layers within the walls, like snapshots in time going back through the red planet's history. Radar also helped trace reflective markers that followed the geometry of underground structures to build up a 3-D sense of the layers.
The radar studies do not answer the riddle of what changes in the Martian atmosphere spurred the formation of both the canyon and the younger spiraling troughs. But they do give scientists a new understanding of the timing of the processes that allow the wind and sun to shape the Martian surface over a certain period, and that may lead to more evidence-based climate models for the red planet.
Not built in a day
The Chasma Boreale appears to cut into the side of the ice-rich polar layered deposits which sprawl across 621 miles (1000 kilometers) and are about 1.2 miles (2 kilometers) thick. But the radar studies showed that the massive canyon formed long before the appearance of the shallower troughs which make up the spiraling arms.
Some researchers had suggested that pressure-induced melting or sub-ice volcanic activity caused the canyon to appear. Yet the canyon's birth turned out to result more from the slow workings of climate and time, rather than rapid or catastrophic forces.
"There were many hypotheses about the Chasma Boreale, and all assumed it was a recent feature cut into the polar ice," said Jack Holt, a geophysicist at the University of Texas, Austin. "But now we know it's an old feature, and you can interpret the stratigraphy in that context."
Holt led the radar study on the Chasma Boreale, while his colleague Smith focused on the spiral troughs. Their two studies appear in the May 27 issue of the journal Nature.
Both the Chasma Boreale and the younger troughs formed on top of an older polar ice cap. Layers of water-ice and grit began depositing, and soon an early form of the canyon appeared. But it wasn't alone; a similarly-sized canyon also began to take shape.
Then something in the Martian climate caused the deposits to stop. Erosion then took over, as the wind wore at the surface and the sun caused some ice to sublimate and turn directly into vapor. There was no evidence of water melt from the radar studies, Holt told SPACE.com.
Eventually the layers began depositing again on top of one another, and one of the canyons ended up getting filled in. But natural forces such as the wind somehow spared the Chasma Boreale by preventing deposits from filling it, and helped preserve the canyon that today stretches 311 miles (500 kilometers) long and 62 miles (100 kilometers) wide.
"The [canyon formation] happened for some time with no good age constraints," Holt said. "That was about 75 percent of the way through the history of this, but then the troughs started forming. We don't know why."
Picking up good migrations
The younger, shallower troughs began to form sometime between 2.49 million years and 467,000 years ago. They represented depressions on top of about three quarters of built-up polar layered deposits, but they didn't just sit still.
Instead, a combination of wind and perhaps sun erosion began to wear away at the southern, equator-facing sides of the polar layered deposits. Wind then carried a trickle of eroded material to the northern, polar-facing sides of the deposits.
As a result, the troughs appeared to slowly spiral inward as they crept northward toward the pole. That appearance of movement has strong resemblance to how sand dunes seem to move over time, Smith said.
"Radar shows that three quarters of the ice has been sitting there, but the surface was altered by wind," Smith explained. "Some troughs have moved as much as 65 kilometers [40 miles], and many moved much less."
More material also accumulates on top of the deposits as the spiral pattern tightens, Holt said. That means the deposits get thicker and higher all the time.
More climate mysteries
Understanding how the north polar ice cap patterns appeared may also help scientists understand the global climate of Mars. Holt and Smith hope to continue examining the patterns of accumulation and try to understand why snow or frost built up unevenly to create the polar layered deposits.
"That tells us a story about the wind and possibly the sun," Smith said. "That's the continuing story."
Researchers can plug their evolved understanding of the natural forces into Mars climate models to make the models more realistic. And better models might help reconstruct how water ice transfers between the poles and the mid or lower latitudes of the red planet, through sublimation and frost or snow.
"You can then start placing age constraints on ice deposits at lower to mid latitudes, which are more accessible to robot and human missions," Holt pointed out.
Future work might also solve the mystery of the south polar layered deposits, which also resemble the spiral pattern of their north polar cousins. But unlike in the north, the south polar layered deposits don't appear to move.
Smith speculated that a colder climate and higher elevation at the south polar ice cap may translate into stickier frozen material and weaker winds. Holt also noted that the southern region appears older, so that perhaps the climate simply did not allow for movement during the time in which the deposits formed.
What lies beneath
Part of the reason that the southern polar ice cap remains more mysterious is that radar does not work as well in that region. Reflective markers or structures beneath the north polar layered deposits helped the radar studies trace the geometry and layers underground, but such markers appear less common in the south.
Still, Holt and Smith praised the radar carried by the Mars orbiters as the crucial components that solved at least the origins of the north polar layered deposits. Such equipment has been used in Antarctica since the 1970s, but did not fly out to Mars until the past decade.
The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the Mars Express orbiter can probe deep beneath the surface with less resolution, while the Mars Reconnaissance Orbiter's SHAllow RADar (SHARAD) has both higher frequency and bandwidth with shallower penetration that can still examine the underground layers and structure of the polar layered deposits.
Such powerful tools could begin to make a case for flying even better radar out to Mars someday.
"In the future, we could probably learn even more about the subsurface," Holt said. "There's still more we could learn with a newer, better radar."
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