Post contributed by Dr. Susan J. Conway CNRS and LPG Nantes, France
The similarity of water-formed landforms on Earth is often used as a key argument for the involvement of liquid water in shaping the surfaces of other planets. The major drawback of the argument is “equifinality” whereby very similar looking landforms can be produced by entirely different processes. A good illustration is leveed channels with lobate deposits (Image 1). Such landforms can be built on Earth by wet debris flow, lava flow, pyroclastic flows and they are also found on Mars (de Haas et al., 2015; Johnsson et al., 2014) where the formation process is debated.
Equifinality is a particular problem for gullies on Mars (see previous post) where recent movements, including erosion of new channels, movement of debris and boulders within channels and the formations of new depositional lobes and fans, tend to occur in winter when temperatures are too low for liquid water (Dundas et al., 2015; Raack et al., 2015). Therefore an alternate mechanism has been presented: CO2-sublimation driven processes, whereby gas released by sublimation fluidises the sediment creating a gas supported granular flow (Cedillo-Flores et al., 2011; Hoffman, 2002; Pilorget and Forget, 2016). The only Earth analogue for such flows are pyroclastic flows, which do have leveed channels and lobate termini, as seen in fresh martian gullies (Image 1). Wet debris flow was originally proposed to have formed martian gullies in the past (Costard et al., 2002; Hartmann et al., 2003), but considering the similarity of the landforms produced by wet debris flow and gas-supported flows, this assumption is now being questioned (Dundas et al., 2015; Pilorget and Forget, 2016).
A potential solution could be found in the analysis of 3D data. Although landforms can appear visually similar, the positions of their various components can be differently organised within the landscape. For example, the lobate termni of granular flows rarely lie on slopes lower than the dynamic angle of repose (Brusnikin et al., 2016), yet debris flow lobes can be found on almost any slope (May and Gresswell, 2004; Whipple and Dunne, 1992). By considering the different physical processes that shape a landscape, this principle has been taken further. Two types of movement compete to form a landscape: those driven by local effects (e.g. soil creep) and those driven by cumulative effects (e.g. river networks, where channel formation is controlled by the upstream drainage area). Image 2 shows a plot of local slope against drainage area (which can be calculated from a digital elevation model) and the different process domains that have been found on Earth – both the position within this plot and trends in the data give information about process. Elevation models at 1 m/pix are now being made on Mars, allowing us to apply this analysis to Mars – Image 3 shows the “wetness index” (Wilson and Gallant, 2000) calculated for gullies in Gasa crater Mars – the log of the ratio between the drainage area and local slope (Conway et al., 2011).
Conway and Balme (2016) performed a statistical analysis “slope-area” plots (Image 2) and other 3D plots using terrestrial fluvial gullies, wet debris-flow gullies, talus slopes and lunar dry flows as input end-members. Plotting martian gullies along with these end-members they found that martian gullies are more similar to the “wet” terrestrial end-members (Image 4).
Another potential solution is the use of the concept of a landform assemblage. Inferring process for a single landform can be ambiguous, yet if we find a group of landforms together, in the same arrangement as they occur in a landscape formed by a particular process on Earth then it becomes unlikely that a different process could be responsible for the whole assemblage. For example, the location of stone garlands on slopes adjacent to martian gullies and sorted patterned ground on the flat terrain is consistent with a periglacial (or freeze-thaw) landscape on Earth (Gallagher et al., 2011). Gullies have also often been noted in the presence of lobes (resembling solifluction lobes) and polygonally patterned ground (Johnsson et al., 2012; Soare et al., 2014) an arrangement common in periglacial environments, e.g. Svalbard (Hauber et al., 2011).
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