Thought to be encased in a frozen, static crust, the Martian north pole is actually a dynamic place, with sand dunes skidding and sliding in spring.
The dunes were first observed in the 1970s, spotted at edge of Mars's north polar cap. They appeared to be frozen in place. Scientists figured they formed at least 30,000 years ago when Mars's climate was more extreme.
But new images from the sharp-eyed HiRISE camera on NASA's Mars Reconnaissance Orbiter tell a different story.
"In one Mars year, we see really fairly substantial changes on the dunes," said planetary scientist Candice Hansen of the Planetary Science Institute in Tuscon, Arizona. Hansen is lead author on a paper in the Feb. 4 Science reporting the new observations. "That was the surprise."
HiRISE has been snapping high-resolution photos of the Martian surface since March 2006, or about two and a half Martian years. Hansen and colleagues examined images of the same location at different times of year, and found that dark sand streaks and new ravines appeared as the seasons changed.
"Because we had all these years of data where no one saw any changes, people developed theories -- the dunes are cemented by ice, maybe they’re crusted over -- theories for why they were not changing," Hansen said. "In fact, they were probably changing all along, and we just didn’t have instruments that were good enough to see it."
The changes could be forged by a layer of frozen carbon dioxide -- dry ice -- changing directly from solid to steam. "This is a very un-Earthly process," Hansen said.
Every winter, Mars's polar cap is sheathed in a thin blanket of carbon dioxide. In the spring, the warming ice layer sublimates, or shifts directly to gaseous form without bothering to melt first.
This sudden shift destabilizes the dunes and triggers avalanches. In the center panel of the images above, the green or blue stuff is bright fresh frost. The dark streaks are escaping sand.
In another surprise, ravines and gullies seemed to disappear from the dunes from one spring to the next. Models of Martian climate predict that the winds should not be strong enough to shift sand grains, and measurements from the Phoenix lander and the Mars rovers Spirit and Opportunity support that idea.
"Everybody may have to sharpen up their pencils and go back to their climate models," Hansen said, though she points out that they only have two Martian summers to compare. "Is this just an oddball year, or is this something that happens regularly? We’ll need more Mars years to be able to say."
Image: 1) Science/AAAS. 2) NASA/JPL/University of Arizona.
See Also:
