More than two decades ago, scientists took note of strange, spider-like patterns strewn about the dunes of Mars, leaving many baffled and spawning a search for creative explanations. The patterns were visible in satellite images of the Red Planet's south pole. But while they certainly looked creepy — they turned out not to be spiders, at all.
The black and branching spider-like shapes carved across Mars' surface — called araneiforms ("spider-like") — were revealed to be a product of carbon dioxide ice (also called "dry ice"), according to a recent study published in the journal Scientific Reports.
A hypothesis for spider-like shapes on Mars
They measure up to 3,300 ft (1 km) wide, and look nothing like Earth-bound geological anomalies. But scientists successfully recreated the pint-sized version of spiders in a lab, using a machine capable of simulating the Martian atmosphere. Once the cold ice comes into contact with the relatively warmer sediment of Mars' surface, some of the ice instantly changes from a solid into a gas — in a material process called sublimation — taking the shape of spider-like cracks on the surface of the Red Planet, where escaping gas can rise and carve its way up through the ice.
"This research presents the first set of empirical evidence for a surface process that is thought to modify the polar landscape on Mars," said Planetary Scientist Lauren McKeown of the Open University in England, in a statement. "The experiments show directly that the spider patterns we observe on Mars from orbit can be carved by the direct conversion of dry ice from solid to gas."
More than 95% of Mars' atmosphere is composed of the gas carbon dioxide (CO2) — with a great abundance of the ice and frost found in or near the Red Planet's poles in winter likewise composed of CO2. An earlier study in 2003 saw researchers suspecting whether the spiders on Mars might form in the springtime, when sunlight pushes through the translucent layer of CO2 ice, heating the ground below. This heating can force ice to sublimate from its base, increasing pressure below the ice until it cracks into oddly-shaped fractures.
Afterward, the built-up gas would rush through the cracks in a sort of plume, leaving a bizarre spider-leg pattern across the surface of Mars, according to the 2003 team's hypothesis.
Finer sediment grains on Mars make for more 'branchy' spider-like legs
However, planetary scientists just recently acquired the ability to test out the hypothesis on Earth — where the atmosphere is extremely different. This is why the researchers from the recent study crated a small volume of Martian atmosphere on Earth with a device called the Open University Mars Simulation Chamber. Once set to the correct atmospheric conditions, the team inserted sediment grains of disparate size inside, and used a system much like a claw machine we'd expect to find in an arcade to hoist a block of dry ice above the grains.
The team's experiments showed that the spider-sublimation hypothesis was, in fact, accurate. Independent of the sediment grains' size, dry ice always immediately sublimates upon contact with them, pushing the escaping gas upward — carving the oddly spider-shaped cracks vertically through the material. Additionally, the finer the grains, the more "branchy" the spider legs, and vice-versa.
Mars is full of strange geological formations, with a rich, wet history. But from the Valles Marineris system of canyons near its equator to the tiny spider-like cracks strewn about the dunes of the Red Planet, the alien-like material conditions are sure to provide many more examples of bizarre phenomena, and even stranger scientific explanations for how they came to be the way they are.