The Spiders from Mars! This phrase describes both David Bowie’s backing band in the 1970s as well as a peculiar feature on Mars that has nothing like it on Earth. But we are here to discuss the latter, because scientists have experimental evidence of how they are formed.
In a paper published in Scientific Reports, researchers have simulated the condition of the Martian South Pole and found a process that creates araneiforms, the spider-like feature seen in many orbital observations of the Red Planet. The team tested the so-called Kieffer’s hypothesis for their formation which matched incredible observations from the last decade. And now, it can be recreated faithfully in the lab.
Mars is much cooler than Earth, and its atmosphere is made mostly of carbon dioxide. In winter, at the poles, this freezes from the air onto the ground and in spring it sublimates – it goes straight from solid to gas. What the team saw is that this sublimation is not top-down, but bottom-up.
Sunlight penetrates through the transparent ice and warms the soil beneath it. The ice layer will begin to sublimate closer to the terrain until the outer layer of ice can’t deal with the pressure and cracks, creating the peculiar pattern into which the sandy/dusty material of Mars will be deposited as a plume.
“This research presents the first set of empirical evidence for a surface process that is thought to modify the polar landscape on Mars. Kieffer’s hypothesis has been well-accepted for over a decade, but until now, it has been framed in a purely theoretical context,” lead author Dr Lauren McKeown from the Open University, said 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. It is exciting because we are beginning to understand more about how the surface of Mars is changing seasonally today,” Dr McKeown continues.
The team had an ingenious approach to test this. They used a vacuum chamber to recreate the very low Martian pressure. They hung a perforated block of dry ice (frozen carbon dioxide) to a mechanical arm and lowered it towards simulated soils made of grains of various sizes.
Being near the warmer soil made the block sublimate, with material being lifted into a plume through the block's central hole by the freshly liberated gas. Once the block was removed, an araneiform structure was left on the soil. The finer the soil, the more branched out the pattern was.
“This innovative work supports the emergent theme that the current climate and weather on Mars has an important influence not only on dynamic surface processes, but also for any future robotic and/or human exploration of the planet,” explained co-author Dr Mary Bourke, from Trinity College Dublin.