A person suffering from arachnophobia might think their fear would stoked on a trip to Mars. However, there is such a thing known colloquially as a Martian “spider”. It is much more innocuous than the eight legged animal that strikes fear into the hearts of millions, but its origins have only been theorized until recently. Now, a team led by a group at Trinity College Dublin has determined that these “spiders” are actually topological troughs formed when dry ice directly sublimates to a gas.
The “spiders”, or to give them their proper name, “araneiforms” have been known for some time. These spider-life features of the Martian terrain form in the spring, but are not known to form at all on Earth. Araneiforms have been captured by various satellites orbiting Mars for the last 20 years. Their transitory nature makes them particularly interesting to scientists looking to better understand Martian seasonality and weather patterns.
Araneiforms are seen on the Martian south polar cap in two high-resolution MOC images taken in southern spring. Each image is about 2 miles wide.
For a long time, there has been a theory about where araneiforms came from. That theory, known as Keiffer’s hypothesis, named after Hugh Kieffer formerly of the US Geological Survey, centered on the idea that the sun would cause the ground under blocks of dry ice to heat up, eventually sublimating the dry ice it is in contact with. Pressure would then build up in the ice block, eventually rupturing it and allowing the gas to escape. The quick escape of the gas then forms the dendritic pattern characteristic of araneiforms in the dust of the Martian surface.
The only problem with this theory, which has been widely accepted in the scientific community, is that it was never demonstrated experimentally. Coverage of the Martian surface is not continuous enough to be able to catch an ice block in the act of sublimating. Therefore, the theory, though widely accepted, was never truly proven.
YouTube video describing the Mars Simulation Chamber.
Credit – Europlanet YouTube Channel
That is where the team from Trinity College Dublin come in. They teamed up with other scientists as Durham University and the Open University, which conveniently had an important piece of kit known as the Mars Simulation Chamber. This experimental setup is able to recreate environments at pressures and temperatures similar to that found on the Martian surface.
However, the Mars Simulation Chamber wasn’t the only interesting piece of experimental equipment the team used. Taking a note from arcades, they used a claw similar to that found in the frustratingly designed games where kids regularly fail to pick up toys. After drilling holes in blocks of dry ice, the team used the claw to suspend them directly over a granular bed. They varied the size of the grains in the granular beds to adjust for particular surface conditions on Mars.
The research team took inspiration from a classic arcade game.
Credit: Wikipedia User Nlan86
Using another well understood process, known as the Leidenfrost Effect, the team was able to get some of the dry ice to directly sublimate when it came in contact with the granular surface, which was heated. The gas thus created quickly escaped through a central hole the team had drilled in each ice block to simulate the fracturing that is believed to take place in the dry ice blocks on the Martian surface.
After each experiment, a very discernible araneiform pattern was visible in the granular bed once the dry ice block was lifted. This provided the first experimental evidence for the creation of these patterns resulting from the sublimation process described in Kieffer’s original theory.
That result is likely the best scientists will be able to do short of observing the actual process directly on Mars. Even once they finally do, arachnophobes can rest assured that any Martian spiders are most likely just terrain patterns caused by rushing CO2 gas. At least as far as we can tell from here.
This picture from MRO taking in May of 2018 shows spiders beginning to forms during the Martian spring.
Credit: NASA/JPL/University of Arizona