Water, water, everywhere…?

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.

lobesOfDifferentKinds_IAG_IOTM

Image 1: Lobes and levees, scale bars are 50 m in all cases. Wet debris flow deposits in Svalbard, image credit DLR HRSC-AX campaign. Lava flows on Tenerife, aerial image courtesy of IGN, Plan Nacional de Ortofotografía Aérea de España. Self-channelling pyroclastic deposits at Lascar volcano, Chile, Pleiades image. Depositional lobes in Istok crater on Mars, HiRISE image PSP_007127_1345.

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).

SA_templates_colour

Image 2: Slope-area plot from Montgomery & Foufoula-Georgiou (1993) with the additional debris flow domain of Brardinoni & Hassan (2006) indicated with a dashed line. The dotted line indicates the adjustment to the alluvial domain boundary considering the gravitational acceleration of Mars. The thin lines delimit the domains and the thick line shows the typical trend of the data points from a well-developed fluvial system.

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).

GasaHiRise_IAG_IOTM

Image 3: Gullies in Gasa crater on Mars, HiRISE ortho-image DT1EB_014081_1440_014147_1440_A01 and the “wetness index” below calculated from the publically available elevation model DTEED_014081_1440_014147_1440_A01. Blue shades indicate “wetter” terrain, i.e. higher drainage areas and lower local slopes.

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).

slopeAreaPlotsPubli2

Image 4: Process domains derived from statistical analysis of data from the Earth (talus, debris flow and fluvial) and the Moon (talus), with superposed data from martian gullies in red. Purple points are fresh impact crater slopes without gullies on Mars.

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).

Further Reading:

Brardinoni, F., Hassan, M.A., 2006. Glacial erosion, evolution of river long profiles, and the organization of process domains in mountain drainage basins of coastal British Columbia. J. Geophys. Res. F Earth Surf. 111, doi:10.1029/2005JF000358.

Brusnikin, E.S., Kreslavsky, M.A., Zubarev, A.E., Patratiy, V.D., Krasilnikov, S.S., Head, J.W., Karachevtseva, I.P., 2016. Topographic measurements of slope streaks on Mars. Icarus 278, 52–61. doi:10.1016/j.icarus.2016.06.005

Cedillo-Flores, Y., Treiman, A.H., Lasue, J., Clifford, S.M., 2011. CO2 gas fluidization in the initiation and formation of Martian polar gullies. Geophys. Res. Lett. 38, doi:10.1029/2011GL049403.

Conway, S.J., Balme, M.R., 2016. A novel topographic parameterization scheme indicates that martian gullies display the signature of liquid water. Earth Planet. Sci. Lett. in press. doi:10.1016/j.epsl.2016.08.031

Conway, S.J., Balme, M.R., Murray, J.B., Towner, M.C., Okubo, C.H., Grindrod, P.M., 2011. The indication of Martian gully formation processes by slope–area analysis. Geol. Soc. Lond. Spec. Publ. 356, 171–201. doi:10.1144/SP356.10

Costard, F., Forget, F., Mangold, N., Peulvast, J.P., 2002. Formation of recent Martian debris flows by melting of near-surface ground ice at high obliquity. Science 295, 110–113. doi:10.1126/science.1066698

de Haas, T., Hauber, E., Conway, S.J., van Steijn, H., Johnsson, A., Kleinhans, M.G., 2015. Earth-like aqueous debris-flow activity on Mars at high orbital obliquity in the last million years. Nat. Commun. 6.

Dundas, C.M., Diniega, S., McEwen, A.S., 2015. Long-Term Monitoring of Martian Gully Formation and Evolution with MRO/HiRISE. Icarus 251, 244–263. doi:10.1016/j.icarus.2014.05.013

Gallagher, C., Balme, M.R., Conway, S.J., Grindrod, P.M., 2011. Sorted clastic stripes, lobes and associated gullies in high-latitude craters on Mars: Landforms indicative of very recent, polycyclic ground-ice thaw and liquid flows. Icarus 211, 458–471. doi:10.1016/j.icarus.2010.09.010

Hartmann, W.K., Thorsteinsson, T., Sigurdsson, F., 2003. Martian hillside gullies and Icelandic analogs. Icarus 162, 259–277, doi: 10.1016/S0019-1035(02)00065-9.

Hauber, E., Reiss, D., Ulrich, M., Preusker, F., Trauthan, F., Zanetti, M., Hiesinger, H., Jaumann, R., Johansson, L., Johnsson, A., Olvmo, M., Carlsson, E., Johansson, H.A.B., McDaniel, S., 2011. Periglacial landscapes on Svalbard: Terrestrial analogs for cold-climate landforms on Mars. Geol. Soc. Am. Spec. Pap. 483, 177–201. doi:10.1130/2011.2483(12)

Hoffman, N., 2002. Active Polar Gullies on Mars and the Role of Carbon Dioxide. Astrobiology 2, 313–323. doi:10.1089/153110702762027899

Johnsson, A., Reiss, D., Hauber, E., Hiesinger, H., Zanetti, M., 2014. Evidence for very recent melt-water and debris flow activity in gullies in a young mid-latitude crater on Mars. Icarus 235, 37–54. doi:10.1016/j.icarus.2014.03.005

Johnsson, A., Reiss, D., Hauber, E., Zanetti, M., Hiesinger, H., Johansson, L., Olvmo, M., 2012. Periglacial mass-wasting landforms on Mars suggestive of transient liquid water in the recent past: Insights from solifluction lobes on Svalbard. Icarus 218, 489–505. doi:10.1016/j.icarus.2011.12.021

May, C.L., Gresswell, R.E., 2004. Spatial and temporal patterns of debris-flow deposition in the Oregon Coast Range, USA. Geomorphology 57, 135–149, doi: 10.1016/S0169-555X(03)00086-2.

Montgomery, D.R., Foufoula-Georgiou, E., 1993. Channel network source representation using digital elevation models. Water Resour. Res. 29, 3925–3934, doi: 10.1029/93WR02463.

Pilorget, C., Forget, F., 2016. Formation of gullies on Mars by debris flows triggered by CO2 sublimation. Nat. Geosci 9, 65–69, doi: 10.1038/ngeo2619.

Raack, J., Reiss, D., Appéré, T., Vincendon, M., Ruesch, O., Hiesinger, H., 2015. Present-Day Seasonal Gully Activity in a South Polar Pit (Sisyphi Cavi) on Mars. Icarus 251, 226–243. doi:j.icarus.2014.03.040

Soare, R.J., Conway, S.J., Dohm, J.M., 2014. Possible ice-wedge polygons and recent landscape modification by “wet” periglacial processes in and around the Argyre impact basin, Mars. Icarus 233, 214–228. doi:10.1016/j.icarus.2014.01.034

Whipple, K.X., Dunne, T., 1992. The influence of debris-flow rheology on fan morphology, Owens Valley, California. Geol. Soc. Am. Bull. 104, 887–900, doi: 10.1130/0016-7606(1992)104<0887:TIODFR>2.3.CO;2

Wilson, J.P., Gallant, J.C., 2000. Terrain Analysis: Principles and Applications. John Wiley and Sons.

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