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"Spiders" from Mars? NASA spots them forming

Mars is home to many geological phenomena that we'd never see on our home planet, some of which are the peculiar networks of veiny troughs found around its South Pole. These "spiders" are usually thousands of years old, but now for the first time scientists seem to have caught some in their early stages, giving the researchers a new foothold in the mission to unlock the icy secrets of the arachnid-like formations.

The reason we'd never see these spiders popping up on the surface of the Earth is because they are formed by carbon dioxide ice, which doesn't occur naturally on our planet. On Mars in the wintertime however, sheets of "dry ice" cover and seal away the ground near the poles.

Then when spring rolls around, the sun warms the ground beneath and begins to thaw out the bottom of the sheets, converting the carbon dioxide ice into gas which builds pressure until the sheet is cracked, creating a geyser of gas, sand and dust that is pulled from beneath. The cracks can measure tens of meters to hundreds of meters and usually the channels converge on a central point, giving them the resemblance of a spider and its legs.

For the last ten years, scientists have kept a watchful eye on the planet's surface through NASA's Mars Reconnaissance Orbiter (MRO), hoping to catch one of these spiders in their formative stages. And now they believe they have pinpointed some near the south pole, which have grown and sprouted branches throughout three Mars years. The rate of growth observed in these infant spiders suggests that the fully formed variants take more than 1,000 Martian years to grow (one Mars year is equal to around 1.9 Earth years), according to the scientists.

"We have seen for the first time these smaller features that survive and extend from year to year, and this is how the larger spiders get started," said Ganna Portyankina of the University of Colorado, Boulder. "These are in sand-dune areas, so we don't know whether they will keep getting bigger or will disappear under moving sand."

The influence of the sand dunes in the spider formation is a bit unclear at this stage. While they appear to play some sort of role, the scientists say they could also prevent many infant spiders like these from going on to form full-scale geologic features. Six years ago, researchers located similar spring-time troughs near the north pole, though these were found to fill with sand and disappear within a year. The scientists say one possibility is that the spiders need the right conditions to grow: ground soft enough to allow them to form, but not so soft that the channels are quickly filled.

"There are dunes where we see these dendritic (or branching) troughs in the south, but in this area, there is less sand than around the north pole," Portyankina said. "I think the sand is what jump starts the process of carving a channel in the ground."

The research was published in the journal Icarus.



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