page_title: Scalloped topography
section_title: Detection of underground ice
hierarchical_section_title: Scalloped topography / Detection of underground ice
caption_attribution_description: English: Diagram of how the dielectric constant was determined for scalloped terrain on Mars
context_page_description: Scalloped topography is common in the mid-latitudes of Mars, between 45° and 60° north and south. It is particularly prominent in the region of Utopia Planitia, in the northern hemisphere, and in the region of Peneus and Amphitrites Paterae in the southern hemisphere. Such topography consists of shallow, rimless depressions with scalloped edges, commonly referred to as "scalloped depressions" or simply "scallops". Scalloped depressions can be isolated or clustered and sometimes seem to coalesce. A typical scalloped depression displays a gentle equator-facing slope and a steeper pole-facing scarp. This topographic asymmetry is probably due to differences in insolation. Scalloped depressions are believed to form from the removal of subsurface material, possibly interstitial ice, by sublimation. This process may still be happening at present. This topography may be of great importance for future colonization of Mars because it may point to deposits of pure ice.
context_section_description: On November 22, 2016, NASA reported finding a large amount of underground ice in the Utopia Planitia region of Mars. The volume of water detected has been estimated to be equivalent to the volume of water in Lake Superior.
  The calculations for the volume of water ice in the region were based on measurements from the ground-penetrating radar instrument on Mars Reconnaissance Orbiter, called SHARAD.
  From the data obtained from SHARAD, dielectric permittivity, or the dielectric constant was determined. That was found from the amount of penetration of the radar to a reflector at the bottom of the ice-rich layer.  The depth to the reflector was found by examining high resolution photos of the location. Certain places had gaps or windows in the ice rich layer. MOLA topographic maps then told the depth. The top of the ice-rich layer displayed polygons, scalloped depressions, and exhuming craters all of which are believed to indicate ice. At the bottom of the gap was a totally different surface of a different color and full of craters; this was the reflector seen in the radar returns. The dielectric constant, averaged over the entire area, came out to be 2.8. Solid water ice would have a dielectric of 3.0–3.2. Basalt rock which is widespread on Mars would yield 8. So using a ternary diagram from a paper by Ali Bramson et al., the researchers decided the ice-rich layer was a mixture composed of 50–80% water ice, 0–30% rocky content, and 15–50% porosity.
caption: SHARAD finds ice by measuring its radar returns from the surface and from a deeper lower surface. The depth to the lower surface was found from HiRISE images of gaps in the surface.
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