Among the thousands of images sent back by NASA's Mars Global Surveyor are bizarre scenes that look like fields of ferns, trees, and sagebrush. Some Mars enthusiasts are convinced these structures really are plants taking root at the Red Planet's south pole. Sir Arthur C. Clarke--the author of 2001: A Space Odyssey--even wrote to Discover to call attention to them.

A "grove" of Martian frost-ferns.
Photograph courtesy of Malin Space Science Systems
Ah, if only. The smallest of the formations at left is huge, larger than a football field. And Hugh Kieffer, a research geophysicist with the U.S. Geological Survey in Flagstaff, Arizona, notes that they are anything but alive. A spectrometer on the Global Surveyor indicates the fernlike forms are composed of dry ice. When winter sets in on Mars' south pole, temperatures can drop to -200 degrees Fahrenheit, and carbon dioxide, which makes up 95 percent of the atmosphere, freezes onto the surface. "Something causes a small pathway, tortuous or straight, through the ice," says Kieffer. He believes tiny grains of dust, heated by the summer Martian sun, vaporize the surrounding ice and slowly burrow their way downward.

When the grains reach the soil below, they create vents. "Once you have a hole established from top to bottom, gas can get out," Kieffer says. Carbon dioxide converging under the ice toward those holes could move fast enough to erode the soil and produce spiderlike channels around the vents, creating the illusion of an alien garden. Once again, Mars is full of surprises but--so far--no signs of life.