Mike Mellon wrote:Utopia Planitia's Surface (ESP_032108_2240) (HiClip)
The Martian landscape often owes its existence to the influences of liquid water and ice. This observation shows a couple of landforms that may result from the loss of large amounts of ice from subsurface deposits: polygonal patterns of troughs and large scallop-shaped depressions. Collectively, such landforms are referred to as "thermokarst."
Cold ice is generally strong and supports the weight of overlying soil. But when ice is lost through melting or sublimation, the supported surface can subside or collapse into the gradually growing cavity left behind by the lost ice. The shapes of the resulting depressions can offer us with clues (and lingering questions) to the origin of the ice.
Under the proper climate conditions ice may form and seasonally accumulate in a honeycomb network of vertical fractures that appear when ice-rich soil contracts each winter. On Earth this form of subsurface ice is called an "ice wedge." Special conditions are needed for this ice to accumulate and develop into a large wedge, namely warm temperature and abundant surface water. A thick layer of thawed wet soil forms allowing water to percolate into the open contraction cracks within the permafrost beneath. Later, loss of this wedge ice, by for example sublimation, results in deep depressions marking the honeycomb network.
Likewise, the larger scallop depressions might point to a past climate of frozen ponds or local patches of windblown snow collected in hollows. These surface ice deposits could later be covered by the ever-shifting soils and dust. In either case, the currently bitter cold and dry climate of Mars is not conducive to forming either of these buried-ice forms. Therefore, these landforms point to a warmer, but still cold, climate in the geologic past.
Alfred McEwen wrote:A Unique View of Curiosity Rover in Yellowknife Bay (ESP_032436_1755) (HiClip)
This image was acquired with a large spacecraft roll to the east when the subsolar latitude was -7.26 degrees, close to the latitude of MSL (-4.6 degrees), resulting in an image with the sun, the MRO spacecraft, and the MSL Curiosity rover on the surface all aligned in nearly a straight line (phase angle of just 5.47 degrees).
This geometry hides shadows and better reveals subtle color variations. With enhanced colors, we can view the region around the landing site and Yellowknife Bay. The rover is the very bright spot near the lower right. The rover tracks stand out clearly in this view, extending west to the landing site where two bright, relatively blue spots indicate where MSL's landing jets cleared off the redder surface dust.
The rover is now driving south towards the large mound in Gale Crater officially named Aeolis Mons and also called "Mount Sharp."
Patrick Russell wrote:Ice Deposition and Loss in an Impact Crater in Utopia Basin (ESP_032569_2225) (HiClip)
This image spans from wall to wall across the center area of an impact crater. From what we see, a lot has happened to modify the appearance of the crater since it was formed, and this subsequent activity is the main interest of this observation.
First, the crater is no longer deep and bowl like: it is shallow and generally flat across its interior, indicating it has been filled with material. The small-scale relief features of this filled surface give clues as to what has happened. The parallel wavy ridges suggest that the material was able to move and flow, perhaps in several successive stages, and likely due to the presence of ice in the ground.
The fine scale pits and larger scale depressions suggest that more recently some of this ice may have disappeared by sublimating (changing from a solid directly to a gas) into the atmosphere, therefore deflating the surface. This story of deposition and loss of ice-rich material, possibly occurring over several cycles over the recent part of Mars' history (or longer, and possibly continuing today), is consistent with similar features in the broader region of the Utopia Basin.
Credit: NASA/JPL/University of Arizona
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