Starship Asterisk*

[img3="This image shows spidery channels eroded into Martian ground. It is a Sept. 12, 2016, example from HiRISE camera high-resolution observations of more than 20 places that were chosen in 2016 on the basis of about 10,000 volunteers' examination of Context Camera lower-resolution views of larger areas. Image credit: NASA/JPL-Caltech/Univ. of Arizona"]http://www.jpl.nasa.gov/images/mro/2016 ... 126-16.jpg[/img3][hr][/hr]

Ten thousand volunteers viewing images of Martian south polar regions have helped identify targets for closer inspection, yielding new insights about seasonal slabs of frozen carbon dioxide and erosional features known as "spiders."

From the comfort of home, the volunteers have been exploring the surface of Mars by reviewing images from the Context Camera (CTX) on NASA's Mars Reconnaissance Orbiter and identifying certain types of seasonal terrains near Mars' south pole. These efforts by volunteers using the "Planet Four: Terrains" website have aided scientists who plan observations with the same orbiter's High Resolution Imaging Science Experiment (HiRISE) camera. HiRISE photographs much less ground but in much greater detail than CTX. ...

The type of terrain called spiders, or "araneiform" (from the Latin word for spiders), is characterized by multiple channels converging at a point, resembling a spider's long legs. Previous studies concluded that this ground texture results from extensive sheets of ice thawing bottom-side first as the ice is warmed by the ground below it. Thawed carbon dioxide gas builds up pressure, and the gas escapes through vents in the overlying sheet of remaining ice, pulling dust with it. This process carves the channels that resemble legs of a spider. ...

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Erosion-carved troughs that grow and branch during multiple Martian years may be infant versions of larger features known as Martian "spiders," which are radially patterned channels found only in the south polar region of Mars.

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 yards (or meters). Multiple channels typically converge at a central pit, resembling the legs and body of a spider. For the past decade, researchers have checked in vain with MRO's High Resolution Imaging Science Experiment (HiRISE) camera to see year-to-year changes in them. ...

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. ...

• Icarus 282:93 (2017 Jan 15) DOI: 10.1016/j.icarus.2016.09.007