Ten thousand volunteers viewing images of Martian south polar regions have helped identify targets for closer inspection, yielding new insights about seasonal slabs of frozen carbon dioxide and erosional features known as "spiders." From the comfort of home, the volunteers have been exploring the surface of Mars by reviewing images from the Context Camera (CTX) on NASA's Mars Reconnaissance Orbiter and identifying certain types of seasonal terrains near Mars' south pole.

These efforts by volunteers using the "Planet Four: Terrains" website have aided scientists who plan observations with the same orbiter's High Resolution Imaging Science Experiment (HiRISE) camera. HiRISE photographs much less ground but in much greater detail than CTX.

Volunteers have helped identify more than 20 regions in mid-resolution images to investigate with higher resolution. "It's heartwarming to see so many citizens of planet Earth donate their time to help study Mars," said HiRISE Deputy Principal Investigator Candice Hansen, of the Planetary Science Institute, Tucson, Arizona.

"Thanks to the discovery power of so many people, we're using HiRISE to take images of places we might not have studied without this assistance."

Planetary scientist Meg Schwamb, of the Gemini Observatory, Hilo, Hawaii, presented results from the first year of this citizen science project Thursday at the annual meeting of the American Astronomical Society's Division for Planetary Sciences and the European Planetary Science Congress, in Pasadena, California.

The type of terrain called spiders, or "araneiform" (from the Latin word for spiders), is characterized by multiple channels converging at a point, resembling a spider's long legs. Previous studies concluded that this ground texture results from extensive sheets of ice thawing bottom-side first as the ice is warmed by the ground below it.

Thawed carbon dioxide gas builds up pressure, and the gas escapes through vents in the overlying sheet of remaining ice, pulling dust with it. This process carves the channels that resemble legs of a spider.

"The trapped carbon dioxide gas that carves the spiders in the ground also breaks through the thawing ice sheet," Schwamb said.

"It lofts dust and dirt that local winds then sculpt into hundreds of thousands of dark fans that are observed from orbit. For the past decade, HiRISE has been monitoring this process on other parts of the south pole.

"The 20 new regions have been added to this seasonal monitoring campaign. Without the efforts of the public, we wouldn't be able to see how these regions evolve over the spring and summer compared with other regions."

Some of the HiRISE observations guided by the volunteers' input confirmed "spider" terrain in areas not previously associated with carbon dioxide slab ice.

"From what we've learned about spider terrain elsewhere, slab ice must be involved at the locations of these new observations, even though we had no previous indication of it there," Hansen said. "Maybe it's related to the erodability of the terrain."

Some of the new observations targeted with information from the volunteers confirm spiders in areas where the ground surface is made of material ejected from impact craters, blanketing an older surface.

"Crater ejecta blankets are erodible. Perhaps on surfaces that are more erodable, relative to other surfaces, slab ice would not need to be present as long, or as thick, for spiders to form," Hansen said. "We have new findings, and new questions to answer, thanks to all the help from volunteers."

The productive volunteer participation continues, and new CTX images have been added for examining additional areas in Mars' south polar region. Planet Four: Terrains is on a platform released by the Zooniverse, which hosts 48 projects that enlist people worldwide to contribute to discoveries in fields ranging from astronomy to zoology.