Department of Earth Sciences

The HRSC image shows the northern part of the outflow channel Shalbatana Vallis, located at the Martian dichotomy (see context map), a region separating the heavily cratered southern highlands from the northern lowlands. Outflow channels are wide valleys eroded by vast amounts of water, most likely formed by catastrophic flooding events triggered by the rapid release of subsurface groundwater. Shalbatana Vallis formed approximately 3.5 billion years ago during the Hesperian time period. It carved its way for over 1,300 kilometers along the highlands of Xanthe Terra, all the way down to the lowlands of Chryse Planitia (see context map).

Shalbatana is not the only outflow channel in this region. In fact, the southern circumferential region of the Chryse Planitia basin is characterized by the outlets of Mars’ largest outflow channels, which converge radially into the lowlands. Outflow channels are always found in combination with chaotic terrain, a labyrinth of irregular mesas and hills, like Chryse Chaos and Hydraotes Chaos (see context map). Shalbatana Vallis’ source region is the approximately 100-kilometer large impact crater named Orson Wells in the Xanthe Terra highland region (see context map) which is also characterized by chaotic terrain.

The valley winds its way from the left side (south) to the right (north) side of the HRSC image (see annotated image). Shalbatana is up to approximately 10 kilometers wide and about 500 meters deep. The low depth-to-width ratio is characteristic of outflow channels because they are carved rapidly which widens their path. However, compared to other outflow channels in this region, such as Ares Vallis, Shalbatana is relative narrow. Over time, the valley was infilled by different materials and was probably deeper in the past. On the left side of the image there is a roundish depression formed by the water floods. This depression is characterized by chaotic terrain. A dark layer can also be spotted, which could be volcanic ash deposited by wind (see perspective view). In October last year, the “Flight around Xanthe Terra” was published, capturing the broader context of the region. This film takes the viewer from the highlands to the lowlands following the course of Shalbatana Vallis.

Not only was the valley infilled over time, but large impact craters on the surrounding plains were also almost entirely buried (see annotated image). Only small hills in a circular shape, which were once part of the crater rim, indicate the existence of these craters. In the annotated image only one of them is noted – can you spot the others? The plains appear very smooth and are partly dotted with rounded hills and mesas (flat plateaus). These resistant rock formations are remnants of a formerly more continuous surface layer that eroded over time. The infilling material could consist of lava flows. In some areas, the surface appears crumpled, which is a common feature on volcanic plains known as wrinkle ridges (see annotated image). These form due to the contraction of cooling material at the surface, causing it to compress and fold slightly. Although many large impact craters are buried in this scene, some younger craters formed after the infilling occurred. These craters show a prominent ejecta blanket, which may indicate the presence of subsurface ice (see annotated images).