Sand-laden jets shoot into the polar sky in this view by noted space artist Ron Miller. It shows the Martian south polar icecap as southern spring begins. Credit: Arizona State University/Ron Miller
The dark spots that appear on the South Pole of Mars have been a source of mystery for years. But now, scientists at the US Geological Survey and Arizona State University have announced that these patches result from geysers of soil and gaseous carbon dioxide, in a process unlike any that takes place on Earth.
In the winter, carbon dioxide from Mars's atmosphere freezes on its South Pole, sheathing the pole in dry ice. Scientists have observed for some time that as the weather begins to warm, dark spots appear on this ice.
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Scientists had a handful of theories about what caused these spots, including ice melting to reveal the bare ground below, translucent ice that reveals the ground beneath it, or dirty ice.
A new study published in the Aug. 17 edition of Nature presents compelling evidence for an alternative possibility: that the dark spots are zones when pressurized gas and particles of dirt and sand have erupted, geyser-like, from beneath the ice.
Analyzing high-resolution images of the dark spots taken with the Thermal Emission Imaging System on the Mars Odyssey, the researchers noticed that while the spots begin as circles, as the spring progresses, they form fan-like patterns. Eventually, the markings come to resemble spidery veins across the ice.
It turns out the dry ice covering the Martian South Pole is translucent in many places, allowing sunlight to penetrate to the dark soil underneath. According to the mechanism proposed in the new paper, as the light is absorbed by the soil, it warms the ice from the bottom up. The frozen carbon dioxide then sublimates, forming a layer of gas trapped between the ice and the ground.
The pressure of the gas builds until the gas escapes through a vent in the ice, creating a geyser of carbon dioxide and sediment.
"You've got this high pressure gas that's moving underneath the ice," said Timothy Titus, an astrogeologist at the U.S. Geological Survey. "As it's moving, if there's any loose dust or sand, it brings it along with it."
The sand and dust fall around the vent in a circular pattern, but, over the course of days or weeks, wind carries lighter particles farther from the vent, producing the observed fan-like pattern on the surface of the ice.
Over time, the scientists theorize, the same mechanism produces spidery veins in the ground below the ice.
"This high pressure gas, as it digs up the dirt and sand, is also digging into the ground, forming channels," Titus said. "As these channels form, it becomes the preferred path of the vent."
In subsequent years, the gas tends to follow the same channels, meaning many geysers erupt in the same place year after year. The researchers are still trying to determine how much pressure the geysers release and how high they might shoot.
This study does not mark the first time scientists have speculated about the existence of such geysers on other planets.
In 1989, images showed that Neptune's moon Triton—which is covered by a layer of frozen nitrogen in the winter—is covered in plumes of dark material that could have been the product of geysers. At the time, some planetary scientists, including Robert Brown of the University of Arizona, hypothesized that if geysers were responsible, they might have been driven by a so-called "solid-state greenhouse effect."
"Because the ice is translucent, the sunlight rattles its way down and isn't absorbed until it gets to the dark material below," Brown said. "It's hard for the heat to get out because the over-layer traps it. It literally raises the temperature below the surface, much more than you would expect."