Dark spots range in width from 50150 feet (1545 meters), lie hundreds of feet apart, and persist 24 months before disappearing until the next martian spring. Fan-shape features half a mile long often grow days or weeks after dark spots form. Because the fans open downwind, scientists suspected they formed as airborne dust fell back to Mars' surface. Spiders seem to be grooves in the martian surface that converge under a spot. "Once a spider becomes established, it affects the surface so that a vent will form in the same place the following year," says Phil Christensen of Arizona State University.
The scientists found the markings result from seasonal deposits of CO2
frost. In winter, temperatures on Mars dip to 200° Fahrenheit (129° Celsius), which causes the frost to recrystalize and become denser. The frost sinks, taking dust and sand particles down with it. By spring, the frost layer transforms into a 3-foot-thick (1m) ice slab that rests atop the layer of sunken particles. Springtime sunlight heats the dark material, and the ice converts directly into a gas a process called sublimation. Pressure builds beneath the slab until the jets eject CO2
, dust, and sand back into Mars' atmosphere and onto its surface.
The report appears in the August 17 issue of Nature
to view how martian spiders form.