Researchers tackle the enigmatic “spiders” of Mars

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

Researchers at Trinity College Dublin have been shedding light on the enigmatic “spiders of Mars, ”Providing the first physical evidence that these unique features on the planet’s surface can be formed by sublimation of CO2 ice..

Spiders, more formally known as araneiforms, are odd-looking radial negative topography systems of dendritic valleys; patterns that resemble the branches of a tree or a fork beam. These features, which are not found on Earth, are believed to have been carved into the Martian surface by the change of dry ice directly from solid to gas (sublimation) in the spring. Unlike Earth, the atmosphere of Mars primarily comprises CO2, and as temperatures drop in winter, it is deposited on the surface in the form of CO2 frost and ice.

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

Mars simulation chamber

The simulation chamber of Mars. Credit: Trinity College Dublin

Their findings are detailed in an article published on March 19, 2021, in the Nature Journal Scientific reports: “The formation of Araneiformes by carbon dioxide ventilation and vigorous sublimation dynamics under Martian atmospheric pressure”.

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

“This research presents the first set of empirical evidence for a surface process believed to modify the polar landscape on Mars. Kieffer hypothesis [explained below] it has been well accepted for more than a decade, but so far, it has been framed in a purely theoretical context. … Experiments show directly that the spider patterns we observe on Mars from orbit can be carved out by the direct conversion of dry ice from a solid to a gas. It’s exciting because we are beginning to understand more about how the surface of Mars is changing seasonally today. ”

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

They made use of an effect known as the Leidenfrost Effect, whereby if a substance comes into contact with a surface much hotter than its point of sublimation, it will form a gaseous layer around itself. When the block reached the sandy surface, the CO2 went directly from solid to gas and material was seen escaping through the central hole in a plume shape.

In each case, once the block was lifted, the escaping gas had eroded a spider pattern. Spider patterns were more branched when using finer grain sizes and less branched when using coarser grain sizes.

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

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

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

The main hypothesis proposed for the formation of spiders (Kieffer hypothesis) suggests that in spring, sunlight penetrates this translucent ice and warms the ground below it. The ice will sublimate from its base, causing pressure to build up, and eventually the ice will break apart, allowing pressurized gas to escape through a crack in the ice. The paths of the escaping gas will leave behind the dendritic patterns observed on Mars today and the sandy / dusty material will settle on the ice in the form of a plume.

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

In addition, the researchers observed that when CO2 blocks were released and allowed to sublimate within the sand bed, the sublimation was much more vigorous than expected and material was thrown throughout the chamber. This observation will be useful to understand models of other processes related to CO2 sublimation on Mars, such as the formation of lateral recurrent diffusive flows that surround linear dune ravines on Mars.

The methodology used can be refocused to study the geomorphic role of CO2 sublimation in the formation of other features of the active Martian surface and, in fact, can pave the way for future research on sublimation processes in other planetary bodies with sparse atmospheres. or null like Europa or Enceladus.

Reference: “Araneiform Formation by Carbon Dioxide Ventilation and Vigorous Sublimation Dynamics Under Martian Atmospheric Pressure” 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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