Geyser (Mars)

Martian geysers are putative sites of small gas and dust eruptions that occur in the south polar region of Mars during the spring thaw. "Dark dune spots" and "spiders" - or araneiforms - are the two most visible types of features ascribed to these eruptions.

They are unlike any terrestrial geological phenomenon. The reflectance (albedo), shapes and unusual spider appearance of these features have stimulated a variety of hypotheses about their origin, ranging from differences in frosting reflectance, to explanations involving biological processes. However, all current geophysical models assume some sort of geyser-like activity on Mars. Their characteristics, and the process of their formation, are still a matter of debate.

These features are unique to the south polar region of Mars in an area informally called the 'cryptic region', at latitudes 60° to 80° south and longitudes 150°W to 310°W; this 1 meter deep carbon dioxide (CO₂) ice transition area—between the scarps of the thick polar ice layer and the permafrost—is where clusters of the apparent geyser systems are located.

The seasonal frosting and defrosting of carbon dioxide ice results in the appearance of a number of features, such dark dune spots with spider-like rilles or channels below the ice, where spider-like radial channels are carved between the ground and the carbon dioxide ice, giving it an appearance of spider webs, then, pressure accumulating in their interior ejects gas and dark basaltic sand or dust, which is deposited on the ice surface and thus, forming dark dune spots. This process is rapid, observed happening in the space of a few days, weeks or months, a growth rate rather unusual in geology – especially for Mars. However, it would seem that multiple years would be required to carve the larger spider-like channels. There is no direct data on these features other than images taken in the visible and infrared spectra.

The geological features informally called dark dune spots and spiders were separately discovered on images acquired by the MOC camera on board the Mars Global Surveyor during 1998–1999. At first it was generally thought they were unrelated features because of their appearance, so from 1998 through 2000 they were reported separately on different research publications ( and -respectively), with the first "jet" or "geyser" models starting to be proposed and refined from 2000 onwards.

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