Researchers are dealing with enigmatic “spiders” from Mars

A picture from NASA’s Martian Scout Orbiter, taken on May 13, 2018 during the winter at the South Pole of Mars, shows an ice cap made of carbon dioxide covering the region and as the sun returns to spring, “spiders” begin to emerge from the landscape. Credit: NASA

Researchers at Trinity College Dublin have shed light on the enigmatic “spiders from March, “Providing the first physical evidence that these unique properties on the planet’s surface can be formed by sublimation of CO2 ice.

Spiders, more formally named araneiforms, are radial systems of negative topography of radial systems of dendritic troughs; patterns resembling tree branches or lightning. These characteristics, which do not exist on Earth, are believed to have been etched into the surface of Mars by dry ice that changes directly from solid to gaseous (sublimating) in the spring. Unlike Earth, Mars ’atmosphere mostly contains CO2, and as winter temperatures decrease, it precipitates on the surface like 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 under Mars atmospheric pressure at the Mars Open University simulation chamber (pictured below) to investigate whether Martian spider-like patterns they could be formed by sublimation of dry ice.

Mars Simulation Chamber

Mars simulation chamber. Credits: Trinity College Dublin

The findings are described in detail in a paper published on March 19, 2021. in the journal Nature Scientific reports: “Formation of Araneiform by carbon dioxide deaeration and strong 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, said:

“This research represents the first in a series of empirical evidence for a surface process thought to modify the polar landscape on Mars. Kieffer’s hypothesis [explained below] has been well received for more than ten years, but has so far been shaped in a purely theoretical context. … Experiments directly show 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’s exciting because we’re starting to understand more about how the surface of Mars is changing seasonally today. ”

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

They used an effect known as the Leidenfrost Effect, whereby if a substance comes in contact with a surface much hotter than the sublimation point, it will form a gaseous layer around itself. When the block reached the sandy surface, CO2 was converted from solid to gas and the material was seen to exit through a plumage-shaped central hole.

In any case, after the block was lifted, the spider gnawed at the gas coming out. Spider patterns were more branched when smaller grain sizes were used, and less branched when larger grain sizes were used.

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

Dr. Mary Bourke, of the Trinity Department of Geography, who supervised the doctorate, said:

“This innovative work supports the new theme that the current climate and weather on Mars have an important impact not only on dynamic surface processes, but also for any future robotic and / or human exploration of the planet.”

The main hypothesis proposed for the formation of spiders (Kieffer’s hypothesis) suggests that in the spring sunlight penetrates this transparent ice and warms the terrain beneath it. The ice will sublimate from its base, which will create pressure and eventually the ice will crack, allowing the pressurized gas to come out through the crack in the ice. The gas runoff trails will leave behind the dendritic patterns recorded on Mars today, and the sandy / dusty material will be deposited on top of the ice in the shape of a plume.

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

In addition, the researchers noted that when the CO2 blocks were released and allowed to sublimate within the sand layer, the sublimation was much stronger than expected and the material was dumped throughout the chamber. This observation will be useful for understanding models of other CO2-related sublimation processes on Mars, such as the formation of lateral repetitive diffuse flows surrounding linear dune ditches on Mars.

The methodology used can be redirected to study the geomorphic role of CO2 sublimation on other active surface Martian features – and indeed, can pave the way for further research into sublimation processes on other planetary bodies without / scarce atmospheres such as Europe or Enceladus.

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