Scientists recently demonstrated how these 'Spiders' on Mars could form.

 

Araneiforms on Mars. (NASA/JPL/University of Arizona)

We haven't found life on Mars, but we have discovered'spiders'... of a sort.

Araneiforms are dark, spider-like systems of branching, fractal troughs that can only be found in Mars' southern polar regions. Nothing like them exists on Earth, or anywhere else in the Solar System.

That makes it difficult to figure out what causes them, but scientists have just discovered the first physical evidence supporting the most widely accepted model, known as Kieffer's hypothesis. The spider-like forms, according to this theory, are formed by the direct sublimation of frozen carbon dioxide (CO2).

According to planetary scientist Lauren McKeown, this research provides the first set of empirical evidence for a surface process that is believed to change the polar landscape on Mars.

For more than a decade, Kieffer's theory has been widely accepted, but it has always been framed in a purely theoretical context. The experiments show that the spider patterns seen from orbit on Mars can be carved by converting dry ice from solid to gas directly.

Mars is a planet that is both analogous to and dissimilar to Earth. Its axial tilt is very similar to Earth's, so its seasonal temperature differences are also very similar to Earth's (even though the year, and therefore the seasons, are twice as long on Mars). This means substantial temperature drops in the fall and winter, followed by a rebound in the spring and summer.

The Martian atmosphere, on the other hand, is very different from Earth's; it's much thinner and mostly made up of carbon dioxide (around 95 percent). The planet is also far farther away from the Sun than Earth, making it much colder. Carbon dioxide from the atmosphere freezes on the ground when winter arrives, especially at higher latitudes.

Hugh Kieffer and colleagues suggested in 2006 and 2007 that this frozen carbon dioxide sublimates - that is, it transitions from an ice to a gas without melting to liquid - trapped beneath translucent slabs of surface ice in the spring.

Pressure builds as the gas warms and expands, eventually cracking the slab and allowing the gas to escape. The gas carves out a spider-like system of channels in the Martian surface as it flows towards the vent, taking the excavated material with it.

The gas and material are combined and ejected as a high-velocity jet. The araneiform is what remains after the ice slab on top melts.

This hypothetical process, according to Kieffer, is unlike anything seen on Earth; it's also never been seen on Mars - we've only seen araneiforms in satellite imagery - so McKeown and her team devised a laboratory experiment to replicate it.

They took advantage of a phenomenon known as the Leidenfrost effect, which you can see in your kitchen: A droplet of water will levitate if it is placed on a surface that is considerably hotter than the water's vaporization point (this is why water in a very hot frying pan will dance around like mercury).

A CO2 ice slab is lowered onto the glass sand. (Sci Rep, 2021; McKeown et al.)

The team placed a slab of CO2 ice with a single hole drilled through on a surface covered in small grains of glass that simulated dirt, or regolith, in a special chamber with pressure lowered to Mars's atmosphere.

When the ice came into contact with the surface, it began to sublimate, resulting in gas escaping the hole. The team discovered a spider-like fractal system of channels carved in the glass sand where the gas had flowed across it to escape through the hole once the ice was removed.

The process was so intense that material was thrown all over the chamber, implying that sublimation rates on Mars could be an order of magnitude greater than on Earth.

The resulting pattern. (McKeown et al., Sci Rep, 2021)

The researchers repeated the experiment with different grain sizes to see how different regolith textures influenced the results. The more branched the pattern, they discovered, the finer the grain size.

More importantly, the experiment is the first time scientists have actually proved that Kieffer's hypothesized process can occur - a major plus for the araneiforms on Mars.

The findings suggest that geomorphic processes on Mars still have a few secrets up their sleeves, and that carbon dioxide sublimation might explain some of Mars' other odd surface features.

The researchers hope that studying araneiforms over a number of years on Mars will provide more insight into the planet's fascinating seasonal processes.

The research has been published in Scientific Reports.