The HRSC scene captured the lowlands of Utopia Planitia located in the Martian northern hemisphere (see context map). Utopia Planitia is one of the three largest impact basins with a diameter of approximately 3,300 kilometers, which formed during the early stages of the planet's history. Since the Viking 2 lander in 1976 revealed its unique icy features, Utopia has become of great interest to Martian researchers. The plains contain a large subsurface ice reservoir, which was spectrally confirmed by the Phoenix lander in 2008. Not only spectral measurements but also a variety of landforms resemble those found in terrestrial periglacial landscapes, indicating the presence of substantial water ice beneath the surface. This HRSC view provides an ideal landscape to observe those features.

The play of colors in the new HRSC image is indeed worth a look: bright surface deposits on the left side merge in the middle of the image with dark surface deposits from the right (see color image). The left side is composed of typical Martian regolith, while the dark areas could represent volcanic ash deposits made up of mafic minerals such as olivine and pyroxene. Interestingly, when the Viking Orbiter captured Mars in 1976, the dark material was not as widespread in Utopia Planitia as it is today. Aeolian Processes must have deposited it over the last 50 years (see right image). The lower left side shows brighter colors than the surroundings (see color image). Conversely, this material appears darker in nighttime thermal imagery, leading to the conclusion that it could be loose, fine-grained sand that loses the heat obtained during the day more quickly at night and therefore appears darker. On the right side, an approximately 15-kilometer-wide impact crater displays its prominent ejecta blanket (see annotated image). The ejecta blanket represents excavated material from the subsurface emplaced during the impact event. In this case, the ejecta shows a layered appearance, which may indicate subsurface ice melted during the impact. The impact crater is also covered by the dark material mentioned above. Besides the large crater there are numerous smaller craters with distinct ejecta blankets (see annotated image). The large crater itself shows a characteristic pattern on its floor, which is interpreted as concentric crater fill (see annotated image). Concentric crater fill is a common feature found in the mid-latitudes of Mars and is mostly composed of ice-rich material showing some “creep” movements. Similar deposits also occur in valleys, where they are referred to as lineated valley fill. Together, these features are considered analogous to terrestrial debris-covered glaciers.

The left side of the image shows a prominent feature of the Utopia basin: depressions up to two kilometers wide and 20 kilometers long intersect to form a polygonal pattern. These depressions are interpreted as grabens that formed a large polygonal structure (see annotated image). The grabens are also visible toward the middle of the image, but they appear to “fade away” and do not show clear borders like those on the left side. This could be because they were infilled by other materials or were not very deep from the beginning. The origin of these grabens is complex and potentially related to the presence of large amounts of water. As part of the northern lowlands, Utopia Planitia was long time hypothesized to have hosted a body of water, possibly a lake or even an ocean, early in Martian history. Utopia Planitia could have accumulated sediments and acted as a sink for sedimentary deposits. Wet, fine-grained sediments would then become compacted and displaced downslope along bedrock slopes of the buried topography. Tectonic processes may also have contributed to the formation of these graben structures. On Earth, seismic surveys have found polygonal graben networks in layers of mudrocks beneath oceans in sedimentary basins. Some graben show a dark layer along their upper edges. These could represent emerging dark, fine-grained ash layers. The depressions continue to the surroundings of the HRSC image and compromise a large area of Utopia Planitia. Read more about the giant graben structure in the press release from 2016: Big, bigger, giant – A former ocean within Utopia Planitia?.

The dark unit on the right side of the image shows signs of erosion in some parts, as it is no longer a continuous layer anymore and appears to have been more extensive in the past. Between the dark, resistant mounds, depressions can be observed. These circular to elliptical depressions are known as scalloped depressions. With higher-resolution imagery, such as HiRISE data, the naming of these depressions becomes clear: the morphology of the edges resembles the shape of scallops (see HiRISE image). Scalloped depressions are very common in the mid-latitudes, where subsurface ice is present, and are interpreted as a type of periglacial feature. They likely form through the removal of ice-rich subsurface material, either by melting or by sublimation (direct phase change to gas), followed by the collapse of the overlying terrain. The resulting subsidence of the surface due to ice loss in the subsurface is called Thermokarst. Scalloped depressions are characterized by a steep, pole-facing scarp and a gentler, equator-facing slope. This asymmetry is thought to result from differences in solar heating: greater solar insolation on the equator-facing slopes leads to more sublimation of ice than on the pole-facing slopes. These depressions do not occur in isolation but rather merge into larger areas, providing a perfect example of how Mars’s surface is constantly changing.