Dry ice gone wild: araneiform on Mars
Every year, Mars’ polar regions are covered by a seasonal layer of CO 2 ice (dry ice). We are just beginning to understand the important role this volatile plays as an active agent of geomorphic change on Mars. The HiRISE camera on the Mars Reconnaissance Orbiter has been used to study sublimation activity in the spring for 3 Mars years. C hannel features often organised in radial patterns were noted and known informally as “spiders”, more formally as “araneiform terrain” (Image 1). They are tens to hundreds of m wide, with individual channels measuring several meters wide. Estimates of depth are in the order of ~ 2 m, decreasing with distance from the center of the araneform. Thin channels widen and deepen as they converge. Where they drape pre-existing topography, the channels are larger in the uphill direction suggesting they were eroded by pressurised fluid (Hansen et al, 2010).
Image 1: Subset of HiRISE Image ESP_011420_0930 locatedf at 87.0°S / 127.27°E. A variety of patterns of channels have been carved in the surface and are conformally-coated with seasonal ice. At the time this image was taken, L s = 184.3 (southern spring), the sun had just started peeking above the horizon and the scene is covered with the seasonal ice cap, ~1m thick. Araneiform channels in this image are 1-2 m deep and ~3-5 m wide. The image is 1 km across.
Satellite images of Mars show dark albedo fans forming over ice in the southern hemisphere (see similar fans in Image 1). These were interpreted as having formed from plumes of dust escaping from the subsurface. Kieffer et al (2006) proposed a model to explain this process: Over the course of the winter the ice anneals into a translucent slab. In the spring sunlight penetrates through the ice. This energy, along with stored heat, warms the ground below, causing the slab of ice to sublimate from the bottom. Gas is trapped between the layer of ice and the ground. When a rupture in the ice occurs the gas flows to the vent entraining fine material from the surface. The escaping gas carries the entrained material out above the ice where it falls out downwind in fan-shaped deposits (Kieffer et al, 2006) (Image 2 and 3).
Image 2: Model for the formation of polar spots, fans and spiders. After Figure 4 from Kieffer et al, 2006
Image 3: An Artist's conception of sediment-laden jets that shoot into the polar sky from the south polar ice cap as southern spring begins. CREDIT: Ron Miller/Arizona State University
The deposition of this volatile seasonally on the surface of Mars and the energy released during sublimation make it a potentially important geomorphic agent on Mars. Recently, CO 2 sublimation was shown to trigger avalanche face flows on Mars dunes (Hansen et al, 2011).
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