Martian Geyser - Morphology

Morphology

The geysers' two most prominent features (dark dune spots and spider channels) appear at the beginning of the Martian spring on dune fields covered with carbon dioxide (CO₂ or 'dry ice'), mainly at the ridges and slopes of the dunes; by the beginning of winter, they disappear. Dark spots' shape is generally round, on the slopes it is usually elongated, sometimes with streams -possibly of water- that accumulate in pools at the bottom of the dunes. Dark dune spots are typically 15 to 46 metres (50 to 150 feet) wide and spaced several hundred feet apart. The size of spots varies, and some are as small as 20 m across, —however, the smaller size seen is limited by imaging resolution— and can grow and coalesce into formations several kilometres wide.

Spider features, when viewed individually, form a round lobed structure reminiscent of a spider web radiating outward in lobes from a central point. Its radial patterns represent shallow channels or ducts in the ice formed by the flow of the sublimation gas toward the vents. The entire spider channel network is typically 160–300 m across, although there are large variations.

Each geyser's characteristic form appears to depend on a combination of such factors as local fluid or gas composition and pressure, ice thickness, underlying gravel type, local climate and meteorological conditions. The geysers' boundary does not seem to correlate with any other properties of the surface such as elevation, geological structure, slope, chemical composition or thermal properties. The geyser-like system produce low albedo (reflectivity) spots, fans and blotches, with small radial spider-like channel networks most often associated with their location. At first, the spots seem to be grey, but later their centres darken because they gradually get covered with dark ejecta, thought to be mainly basaltic sand. It should be noted that not all dark spots observed in early spring are associated with spider landforms, however, a preponderance of dark spots and streaks on the cryptic terrain are associated with the appearance of spiders later in the season.

Time-lapsed imagery performed by NASA confirms the apparent ejection of dark material following the radial growth of spider channels in the ice. Time-lapsed imaging of a single area of interest also shows that small dark spots generally indicate the position of spider features not yet visible; it also shows that spots expand significantly, including dark fans emanating from some of the spots, which increase in prominence and develop clear directionality indicative of wind action.

Some branching ravines modify, some destroy and others create crust in a dynamic near-surface process that extensively reworks the terrain creating and destroying surface layers. Thus, Mars seems to have a dynamic process of recycling of its near surface crust of carbon dioxide. Growth process is rapid, happening in the space of a few days, weeks or months, a growth rate rather unusual in geology – especially for Mars. A number of geophysical models have been investigated to explain the various colors and shapes' development of these geysers on the southern polar ice cap of Mars.