A few years ago while playing with dry ice in a lab in the UK, I became obsessed with "The Spiders from Mars" - not Bowie's band - but the squiggly, radial carvings on Mars's south polar regions. Today, I'm thrilled to say we have published our research:
Mc Keown, L., McElwaine, J.N., Bourke, M.C. et al. The formation of araneiforms by carbon dioxide venting and vigorous sublimation dynamics under martian atmospheric pressure. Sci Rep 11, 6445 (2021).
I've been waiting some time to share this work here. Spiders are a type of up-to-1km-wide Martian surface feature. They are so-called because well, they look a bit like spiders. 50m-1km wide spiders. (I know, terrifying. But also pretty unique)
Although their dendritic pattern looks like fork lightning or nerve endings, they are totally unlike any surface feature seen on Earth. Usually with features like this, we tend to consider surface processes that can't really happen on Earth; this is where dry ice comes in!
The Martian atmosphere is 95% CO2 and because like Earth, Mars has an axial tilt, Mars gets seasons! In winter as temperatures cool, CO2 deposits on the surface as frost and ice and in spring when temperatures rise, the ice sublimates away again - it changes from solid to gas.
A while back, researchers Hansen, Kieffer, Piqueux et al. noticed that fans and spots were appearing in spring in the locale of these spiders. They were originally thought to be patches of defrosted ground, but then were found to have the same temperature as CO2 ice.
So Kieffer developed a hypothesis for spider formation; in spring, sunlight penetrates this translucent slab ice and warms it from the base. The ice sublimates, pressure builds and the ice cracks.
Gas rushes to the vent, eroding spiders and depositing the material as fans and spots in the form of a plume. Plumes have been hunted for with HiRISE on Mars but have not yet been directly observed. Check out this visual my friends Featherwax made for me of the whole process.
Now; my experiments! Lots of awesome work has been done to numerically model spider formation but because the process doesn't occur on Earth, it had never been directly observed. We wanted to see if sublimating CO2 in contact with granular material would form spider patterns
So, funded by The Europlanet Society and Irish Research Council we went to the Open University Mars Simulation chamber which is capable of recreating the very low pressures of the Martian atmosphere and also fits a 5'4" woman quite snugly.
I drilled holes in CO2 ice blocks to mimic the "vent" formed when ice cracks on Mars. I enjoyed using power drills very much. Serious excess of beanie hats here too... I was going through a phase.
We then suspended the blocks from a claw like one of those you'd find in an arcade, over sand beds of different grain sizes, and lowered the pressure in the chamber. Once it was at 6mbar we lowered the block onto the surface gently.
Directly condensing CO2 is tricky, so we skipped that step. Instead we made use of the Leidenfrost Effect, where if a substance contacts a surface much hotter than its boiling/sublimation point, it will generate a "cushion" of vapor (like dancing water droplets on a pan).
What we observed was the formation of a plume! These were higher for finer grain sizes and more diffuse for coarser grains. We inferred that gas was rushing towards the central vent, in turn eroding the surface beneath the ice.
When the process was over and we gently lifted the block, guess what we observed? Beautiful spidery greatness. These were more branched for finer grains and less so for coarser ones. This is the first direct observation of spider patterns being eroded by sublimating CO2.
These experiments are a good example of merging the scientific method with a bit of mad creativity. Bonus video: the mess I made when I used dust size grains and let the block drop. This would not happen under Earth pressure. Colleagues have dubbed it the "Laundromat".
Thanks to my supervisors and coauthors Drs Mary Bourke and Jim_McElwaine Matt Sylvest and Manish Patel for all their guidance and hard work on this paper and to colleagues in the field for their encouragement/advice on getting this paper out! Ask me anything!