NASA orbiter detects infant versions of Martian ‘spiders’

Researchers using NASA's Mars Reconnaissance Orbiter (MRO) report the first detection of cumulative growth, from one Martian spring to another of channels resulting from the same thawing-carbon-dioxide process believed to form the spider-like features.

The spiders range in size from tens to hundreds of metres. Multiple channels typically converge at a central pit, resembling the legs and body of a spider. 

"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 in the US. 

"These are in sand-dune areas, so we don't know whether they will keep getting bigger or will disappear under moving sand," Ganna said. 

Dunes appear to be a factor in how the baby spiders form, but they may also keep many from persisting through the centuries needed to become full-scale spiders. 

The amount of erosion needed to sculpt a typical spider, at the rate determined from observing active growth of these smaller troughs, would require more than a thousand Martian years. One Martian year lasts about 1.9 Earth years. 

Carbon-dioxide ice, better known as "dry ice," does not occur naturally on Earth. 

On Mars, sheets of it cover the ground during winter in areas near both poles, including the south-polar regions with spidery terrain. Dark fans appear in these areas each spring. 

Hugh Kieffer of the Space Science Institute in the US put those factors together in 2007 to deduce the process linking them: Spring sunshine penetrates the ice to warm the ground underneath, causing some carbon dioxide on the bottom of the sheet to thaw into gas. 

The trapped gas builds pressure until a crack forms in the ice sheet. Gas erupts out, and gas beneath the ice rushes toward the vent, picking up particles of sand and dust. 

This erodes the ground and also supplies the geyser with particles that fall back to the surface, downwind and appear as the dark spring fans. 

This explanation has been well accepted, but actually seeing a ground-erosion process that could eventually yield the spider shapes proved elusive. 

Six years ago, researchers using MRO's High Resolution Imaging Science Experiment (HiRISE) camera reported small furrows appearing on sand dunes near Mars' north pole at sites where eruptions through dry ice had deposited spring fans. 

However, those furrows in the far north disappear within a year, apparently refilled with sand. 

The newly reported troughs near the south pole are also at spring-fan sites. They have not only persisted and grown through three Martian years so far, but they also formed branches as they extended. The branching pattern resembles the spidery terrain.