Input from about 10,000 volunteers viewing images from Martian south polar regions has identified targets for closer inspection, yielding new insights about seasonal slabs of frozen carbon dioxide (dry ice) and erosional features called “spiders.”
The volunteers from around the world have been exploring the surface of Mars by examining images from the Context Camera (CTX) on NASA’s Mars Reconnaissance Orbiter (MRO) and identifying certain types of terrain around Mars’ South Pole. The collected information is used by scientists planning observations of Mars by the orbiter’s High Resolution Imaging Science Experiment (HiRISE) camera, which photographs much less ground but in much greater detail, compared to CTX.

“It’s heartwarming to see so many citizens of Planet Earth jump in to help study Mars,” said HiRISE Deputy Principal Investigator Candice Hansen, of the Planetary Science Institute. “Thanks to the discovery power of people, we’re taking pictures of features of Mars with HiRISE of places we would not have imaged without this assistance.”

“In the spring the dry ice turns to gas and carves unusual features in the Mars surface, resulting in exotic terrains described informally as ‘spiders,’ ‘Swiss cheese’ and ‘channel networks,’ – this is what we asked our citizen scientists to find in the CTX images,” said PSI Senior Scientist Hansen.

The terrain type called spiders or “araneiform” (from the Latin word for spiders) is characterized by multiple channels converging at a point, resembling long legs of a spider. Previous studies concluded this ground texture results from thawing of extensive sheets of ice bottom-side first as the ice is warmed by the ground underneath. Thawed carbon dioxide gas builds up pressure underneath. Wherever it finds a place to escape through the overlying sheet of remaining ice, a rapid flow out through that vent pulls dust with it. Gas flowing under the ice toward the escape point picks up that dust as it carves the channels that resemble spiders’ legs.

Results from the first year of this citizen-science project, named “Planet Four: Terrains,” were presented today by Meg Schwamb, Planetary Scientist at Gemini Observatory, Hilo, Hawaii, at the the American Astronomical Society (AAS) Division for Planetary Sciences (DPS) and 11th European Planetary Science Congress (EPSC) in Pasadena, Calif. Hansen is the Principal Investigator for the Planet Four citizen science project.

“This project was inspired by the success of earlier citizen-science activities,” said Schwamb. “Mars’ south pole is sculpted by a never-ending cycle of freezing and thawing of exposed carbon dioxide ice. As carbon dioxide thaws into gas it carves distinctive channels and pits in the ground. Volunteers have helped identify more than 20 regions in mid-resolution images to investigate with higher resolution.”

Planet Four: Terrains is on a platform released by the Zooniverse, an organization that hosts 48 projects that enlist people worldwide to contribute to discoveries in fields ranging from astronomy to zoology.

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.

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 erodible, compared 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.”

Visit to see an image of spidery channels eroded into Martian ground.

A grant from NASA’s Mars Reconnaissance Orbiter mission funds Hansen’s work on the project.