Researchers are tackling the enigmatic “spiders” of Mars

An image from NASA’s Mars Reconnaissance Orbiter, acquired on May 13, 2018 during the winter at Mars’ South Pole, shows a carbon dioxide ice sheet covering the region and as the sun returns in the spring, ‘spiders’ begin to emerge from the landscape . Credit: NASA

Researchers from Trinity College Dublin have shed light on the enigmatic ‘spiders of Mars, ”With the first physical evidence that these unique features on Earth’s surface could be formed by the sublimation of CO2 ice

Spiders, more formally referred to as araneiforms, are strange looking negative topography radial systems of dendritic valleys; patterns resembling branches of a tree or fork lightning. These features, which do not exist on Earth, are believed to have been carved into the surface of Mars by dry ice that immediately changes from solid to gas (sublimating) in the spring. Unlike Earth, the atmosphere of Mars contains mostly CO2, and as the temperature drops in winter, it deposits on the surface as CO2 frost and ice.

The Trinity team, along with colleagues from Durham University and the Open University, conducted a series of experiments funded by the Irish Research Council and Europlanet in the Open University Mars Simulation Chamber (pictured below), under atmospheric pressure from Mars, to to investigate whether patterns similar to Martian spiders can be formed by sublimation of dry ice.

Mars simulation chamber

The simulation room of Mars. Credit: Trinity College Dublin

The findings are detailed in a paper published March 19, 2021, in the Nature Journal Scientific reports: “The formation of araneiforms by carbon dioxide venting and powerful sublimation dynamics under atmospheric pressure from Mars.”

Dr. Lauren McKeown, who led this work during her PhD at Trinity and is now at the Open University, said:

“This research provides the first set of empirical evidence for a surface process that is believed to change the polar landscape on Mars. Kieffer’s hypothesis [explained below] has been well accepted for over a decade, but so far it has been placed in a purely theoretical context. … The experiments immediately show that the spin patterns that we observe on Mars from orbit around the Earth can be cut out by the direct conversion of dry ice from solid to gas. It’s exciting because we’re beginning to understand more about how the surface of Mars changes seasonally today. ”

The research team drilled holes in the centers of CO2 ice blocks and hung them with a claw similar to that found in arcades, above granular beds of different grain sizes. They lowered the pressure in a vacuum chamber to the atmospheric pressure of Mars (6 mbar) and then used a lever system to place the CO2 ice block on the surface.

They took advantage of an effect known as the Leidenfrost effect, where when a substance comes into contact with a surface much hotter than the sublimation point, it forms a gaseous layer around itself. When the block reached the sandy surface, CO2 immediately changed from solid to gas and material was seen escaping through the central hole in the shape of a plume

In any case, after the block was lifted, a spin pattern was eroded by the escaping gas. The spin patterns were more branched at finer grain sizes and less branched at coarser grain sizes.

This is the first set of empirical evidence for this existing surface process.

Dr. Mary Bourke, of Trinity’s Department of Geography, who oversaw the doctoral research, said:

“This innovative work supports the emerging theme that the current climate and weather on Mars has a significant impact not only on dynamic surface processes, but also on any future robotic and / or human exploration of the planet.”

The main hypothesis proposed for spider formation (Kieffer’s hypothesis) suggests that in the spring, sunlight penetrates this translucent ice and warms the terrain below. The ice will sublimate from its base, creating pressure, and eventually the ice will rupture, allowing pressurized gas to escape through a crack in the ice. The paths of the escaping gas will leave behind the dendritic patterns seen on Mars today and the sandy / dusty material will be deposited on the ice in the form of a plume.

However, until now it was not known whether such a theoretical process is possible and this process has never been observed directly on Mars.

In addition, the researchers noted that when blocks of CO2 were released and were able to sublimate in the sand bed, the sublimation was much more powerful than expected and material was thrown around the room. This observation will be useful in understanding models of other CO2 sublimation-related processes on Mars, such as the formation of lateral recurring diffusion currents around linear dune channels on Mars.

The methodology used can be reoriented to study the geomorphic role of CO2 sublimation on other active surface features of Mars – and indeed can pave the way for further investigation of sublimation processes on other planetary bodies with no / sparse atmospheres such as Europa or Enceladus.

Reference: “The formation of araneiforms by carbon dioxide venting and powerful sublimation dynamics under atmospheric pressure on Mars” by Lauren Mc Keown, JN McElwaine, MC Bourke, ME Sylvest and MR Patel, March 19, 2021, Scientific reports
DOI: 10.1038 / s41598-021-82763-7

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