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Stop 7 at Hellas

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View of the Reull Vallis region on Mars
View of the Reull Vallis region on Mars
Image: Portion of a THEMIS day infrared mosaic (diagonal gray areas are data gaps)
Location: Southern highlands northeast of the Hellas impact structure
Scale: Image width is approximately 1050 kilometers

Reull Vallis is a ~1500 km long complex canyon system consisting of three main reaches: upper, middle, and lower (see above image). The canyon system winds its way from the ridged plains of Hesperia Planum through the resurfaced highlands of Promethei Terra and the smooth/channeled plains near the head of Harmakhis Vallis. Reull and Harmakhis Vallis may have been once connected, but their intersection is now covered by a large lobate debris apron.

What is a lobate debris apron? Planetary scientists who have studied the shape, size, and surface characteristics of the aprons believe they are similar to rock glaciers or debris-covered glaciers on Earth. The key aspect of their intpretations is that the aprons likely contained water ice that allowed them to flow downslope and away from the source areas of canyons, massifs, and crater walls. What is not clear from these studies is how much ice was in the aprons and is there still any there right now. The amount of ice is important for scientists attempting to model how a Martian glacial feature may form on the surface. In 2009, analysis of data returned from a subsurface shallow radar (SHARAD) instrument onboard the Mars Reconnaissance Orbiter (MRO) spacecraft orbiting Mars indicates that ice may exist just a few hundred meters below the surface of debris aprons in this region [research article by Plaut et al, 2009, Geophysical Research Letters]. Scientists believe the structure of the aprons is similar to a debris-covered glacier and that the ice is relatively clean (not chemically different from pure water). Although the age of the ice is uncertain, the potential of shallow subsurface ice on Mars today is tantalizing to think of as a water source if humans were to set foot and live on the planet!

These large aprons are common throughout the area and are shown in more detail in the next stop.

The upper reaches of Reull are very different from its middle and lower reaches. The upper reaches have streamlined islands, scour marks, sinuous narrow channels, and lateral terraces or benches. The images below show examples of the terraces (t) and scour marks (s) in the upper reaches of Reull. As described in the previous stop at Dao and Harmakhis Valles, these features are indicative of fluvial processes (water erosion). Thus, it seems that Reull Vallis at some point in its past was formed by fluid flow across the surface, perhaps like a river on Earth. The middle and lower reaches have more similarities to Dao and Harmakhis Valles. No terraces or streamlined islands are observed, the channel walls are steep, and it is evident that there was collapse of the adjacent plains into the channel. These observations suggest that in the evolution of Reull Vallis, there was a transition from a fluvial-dominated (perhaps water) system to a collapse-dominated system.

Terraces (t) and scour marks (s) along the channel floor of Reull Vallis
Terraces (t) and scour marks (s) along the channel floor of Reull Vallis
Image: modified from figure 7 in Crown et al., 2005, Journal of Geophysical Research
Location: Along the upper reaches of Reull Vallis
Scale: Image width is approximately 65 kilometers

Terraces (t) and deflection of Reull Vallis around a crater
Terraces (t) and deflection of Reull Vallis around a crater
Image: modified from figure 7 in Crown et al., 2005, Journal of Geophysical Research
Location: Along the upper reaches of Reull Vallis
Scale: Image width (excluding black no-data areas) is approximately 17.4 kilometers

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