In 2016, 10,000 volunteers searched images of Mars for strange spider-like patterns; they found the formations in places where scientists hadn't seen them before

In 2016, about 10,000 volunteers sat at their computers and studied pictures of Mars. They weren't professional astronomers, just regular citizens looking through images from their own homes. Their job was simple: identify odd spider-like markings on the surface of the red planet. In a statement released by NASA, the volunteers' contribution was said to assist the scientists in finding an additional 20 sites on the surface of Mars to study in greater detail. The results surprised researchers.

So what are Mars’ “spiders”?

These formations are not actual spiders. Scientists call them araneiforms, a word taken from the Latin word for spider. They are channels cut into the Martian ground that radiate away from a small central pit like a set of legs. The formations, the study says, range from tens of meters across to about a kilometer wide, and some connect into a web-like pattern researchers call lace terrain. For years, researchers believed spiders could only grow in one kind of place, a region near Mars' south pole called the South Polar Layered Deposits. This area formed from repeated layers of ice and dust built up over countless seasons. Outside that specific zone, spiders simply hadn't been confirmed.


How does a "spider" form?

During every winter season on Mars, the southern part of the planet gets covered in a layer of frozen carbon dioxide, which is also commonly called dry ice. In spring, sun rays reach the ice layer and heat the surface beneath it. This results in the transformation of the carbon dioxide from a solid form to a gaseous one, and the gas starts breaking out through the weakest spot in the layer. Dust is caught up in its wake and the Martian winds blow that dust into dark, fan-shaped streaks visible from orbit.

It was believed for quite some time that the escaping gas gradually formed the spiders by carving the channels, and this is what was referred to as the carbon dioxide jet mechanism. Previous studies had also found that spiders appeared only when the specific type of transparent ice layer formed on the soil surface in spring.

Why NASA turned to thousands of strangers

Studying this process required going through an enormous number of images, far more than one research team could possibly go through. So NASA scientists teamed up with the citizen-science platform Zooniverse and launched a project called Planet Four: Terrains in 2015. Volunteers worked from a set of 90 full Context Camera images, which were broken down into roughly 20,000 smaller cropped tiles for review on the Zooniverse site, flagging anything that looked like a spider, a crater, or a bumpy, pockmarked stretch dubbed "Swiss cheese terrain," Oxford University reported. The flagged locations then became targets for the High Resolution Imaging Science Experiment camera, or HiRISE, which can see much more detail than the wider survey images the volunteers had examined.

Spiders in strange places

Full results, which were later published in the journal Icarus, showed how extensive this work was. The team, led by Megan Schwamb, a planetary scientist, analyzed 90 photos taken by the Context Camera that covered some 11 percent of the southern region below 75 degrees latitude on Mars. There were 390 definite locations of the spiders identified by the researchers.

About 75 percent of these were in the places where scientists predicted they would be, in the South Polar Layered Deposits. But the remaining 25 percent were not. For the first time, the study confirmed spiders on entirely different types of terrain, including younger polar units and much older highland ground, known to geologists as Early Noachian terrain, some of the oldest surface material found anywhere on Mars. HiRISE imaging confirmed these observations. The same paper also showed that spider locations and Swiss cheese terrain did not coincide in the areas surveyed, indicating that the two features develop under different conditions. In a statement, Oxford University said the finding was an unexpected result, showing that spiders were not as geographically limited as scientists had assumed.

Why this matters

The finding did not contradict the main theory of spider formation, which scientists still think is driven by carbon dioxide jets. What changed is the understanding of where that process can happen. If spiders can carve their way through layers into the ground underneath, then whatever makes terrain prone to such erosion must be more widespread on Mars than researchers previously thought. The underlying rock or soil type doesn't appear to be what limits spider formation; the right seasonal ice conditions do, and those conditions turn out to occur in more places than anyone expected. The authors also noted that crowdsourced mapping such as this might be useful for guiding future missions to Mars since they provide a quick means of identifying any interesting features on the surface of the planet before spacecraft or landers are sent to study them up close.

It's also a reminder of what large groups of curious people can achieve together. Thousands of people working with laptops from their own homes were able to help direct one of NASA's best cameras towards making discoveries that would have been difficult to make otherwise, simply because someone took the time to analyze pictures of a planet several million miles away.