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Tests on Earth appear to confirm how the Red Planet\u2019s spider-shaped geologic formations are carved by carbon dioxide.<\/em><\/p>\n Since discovering them in 2003 via images from orbiters, scientists have marveled at spider-like shapes sprawled across the southern hemisphere of Mars. No one is entirely sure how these geologic features are created. Each branched formation can stretch more than a half-mile (1 kilometer) from end to end and include hundreds of spindly \u201clegs.\u201d Called araneiform terrain, these features are often found in clusters, giving the surface a wrinkled appearance.<\/p>\n The leading theory is that the spiders are created by processes involving carbon dioxide ice, which doesn\u2019t occur naturally on Earth. Thanks to experiments detailed in a new paper published in The Planetary Science Journal, scientists have, for the first time, re-created those formation processes in simulated Martian temperatures and air pressure.<\/p>\n \u201cThe spiders are strange, beautiful geologic features in their own right,\u201d said Lauren Mc Keown of NASA\u2019s Jet Propulsion Laboratory in Southern California. \u201cThese experiments will help tune our models for how they form.\u201d<\/p>\n The study confirms several formation processes described by what\u2019s called the Kieffer model: Sunlight heats the soil when it shines through transparent slabs of carbon dioxide ice that built up on the Martian surface each winter. Being darker than the ice above it, the soil absorbs the heat and causes the ice closest to it to turn directly into carbon dioxide gas \u2014 without turning to liquid first \u2014 in a process called sublimation (the same process that sends clouds of \u201csmoke\u201d billowing up from dry ice). As the gas builds in pressure, the Martian ice cracks, allowing the gas to escape. As it seeps upward, the gas takes with it a stream of dark dust and sand from the soil that lands on the surface of the ice.<\/p>\n When winter turns to spring and the remaining ice sublimates, according to the theory, the spiderlike scars from those small eruptions are what\u2019s left behind.<\/p>\n For Mc Keown and her co-authors, the hardest part of conducting these experiments was re-creating conditions found on the Martian polar surface: extremely low air pressure and temperatures as low as minus 301 degrees Fahrenheit (minus 185 degrees Celsius). To do that, Mc Keown used a liquid-nitrogen-cooled test chamber at JPL, the Dirty Under-vacuum Simulation Testbed for Icy Environments, or DUSTIE.<\/p>\n \u201cI love DUSTIE. It\u2019s historic,\u201d Mc Keown said, noting that the wine barrel-size chamber was used to test a prototype of a rasping tool designed for NASA\u2019s Mars Phoenix lander. The tool was used to break water ice, which the spacecraft scooped up and analyzed near the planet\u2019s north pole.<\/p>\n For this experiment, the researchers chilled Martian soil simulant in a container submerged within a liquid nitrogen bath. They placed it in the DUSTIE chamber, where the air pressure was reduced to be similar to that of Mars\u2019 southern hemisphere. Carbon dioxide gas then flowed into the chamber and condensed from gas to ice over the course of three to five hours. It took many tries before Mc Keown found just the right conditions for the ice to become thick and translucent enough for the experiments to work.<\/p>\n Once they got ice with the right properties, they placed a heater inside the chamber below the simulant to warm it up and crack the ice. Mc Keown was ecstatic when she finally saw a plume of carbon dioxide gas erupting from within the powdery simulant.<\/p>\n \u201cIt was late on a Friday evening and the lab manager burst in after hearing me shrieking,\u201d said Mc Keown, who had been working to make a plume like this for five years. \u201cShe thought there had been an accident.\u201d<\/p>\n The dark plumes opened holes in the simulant as they streamed out, spewing simulant for as long as 10 minutes before all the pressurized gas was expelled.<\/p>\n The experiments included a surprise that wasn\u2019t reflected in the Kieffer model: Ice formed between the grains of the simulant, then cracked it open. This alternative process might explain why spiders have a more \u201ccracked\u201d appearance. Whether this happens or not seems dependent on the size of soil grains and how embedded water ice is underground.<\/p>\n \u201cIt\u2019s one of those details that show that nature is a little messier than the textbook image,\u201d said Serina Diniega of JPL, a co-author of the paper.<\/p>\n Now that the conditions have been found for plumes to form, the next step is to try the same experiments with simulated sunlight from above, rather than using a heater below. That could help scientists narrow down the range of conditions under which the plumes and ejection of soil might occur.<\/p>\n There are still many questions about the spiders that can\u2019t be answered in a lab. Why have they formed in some places on Mars but not others? Since they appear to result from seasonal changes that are still occurring, why don\u2019t they seem to be growing in number or size over time? It\u2019s possible that they\u2019re left over from long ago, when the climate was different on Mars\u2014 and could therefore provide a unique window into the planet\u2019s past.<\/p>\n For the time being, lab experiments will be as close to the spiders as scientists can get. Both the Curiosity and Perseverance rovers are exploring the Red Planet far from the southern hemisphere, which is where these formations appear (and where no spacecraft has ever landed). The Phoenix mission, which landed in the northern hemisphere, lasted only a few months before succumbing to the intense polar cold and limited sunlight.<\/p>\n Andrew Good Karen Fox \/ Molly Wasser 2024-122<\/p>\n<\/div>\n
Jet Propulsion Laboratory, Pasadena, Calif.
818-393-2433
andrew.c.good@jpl.nasa.gov<\/p>\n
Headquarters, Washington
202-358-1600
karen.c.fox@nasa.gov\u00a0\/ molly.l.wasser@nasa.gov<\/p>\n