A novel topographic parameterization scheme indicates that martian gullies display the signature of liquid water
Introduction
Gullies on Mars (Malin and Edgett, 2000) are widespread: they are concentrated in the mid-latitudes and can be found on steep slopes polewards of about 30° (Dickson et al., 2007). Global and hemispheric studies have revealed that mid-latitude gullies are located on slopes oriented towards the pole (Balme et al., 2006, Bridges and Lackner, 2006, Dickson et al., 2007, Harrison et al., 2015, Heldmann et al., 2007, Heldmann and Mellon, 2004, Kneissl et al., 2010, Marquez et al., 2005) while higher latitude examples have little, or no preferred orientation. The distribution and orientation of gullies are consistent with their formation at high obliquity, when pole-facing slopes receive maximum summer insolation. Together, this evidence led to the conclusion that gullies formed as water-rich debris flows (Costard et al., 2002).
However, increased insolation can also trigger dry mass wasting or destabilization of solid CO2. Narrow channels observed on the Moon (Bart, 2007, Senthil Kumar et al., 2013, Xiao et al., 2013) and on the asteroid Vesta (Krohn et al., 2014, Scully et al., 2015) have been identified as analogues to martian gullies by some authors, yet these exist on airless bodies where erosion by traditional low-viscosity fluids is unlikely and whose surfaces are almost certainly completely dry. Hence, dry mass-wasting has been considered a potential formation mechanism for martian gullies. Some of the recent modifications observed in martian gullies, including new deposits and channel formation, have been found to occur at the time of year when CO2 frost is subliming (Dundas et al., 2015, Dundas et al., 2012, Dundas et al., 2010; Raack et al., 2015, Vincendon, 2015). Therefore mechanisms involving gas release triggering granular flow (Cedillo-Flores et al., 2011, Pilorget and Forget, 2016), have been suggested for gully-formation. Theoretical modelling (Cedillo-Flores et al., 2011) predicts that sand-sized or smaller grains can be mobilized by CO2 gas-sublimation under martian conditions but, unless there is a confining “lid” (Pilorget and Forget, 2016) on the flow, it rapidly converts from a gas-supported to a simple granular flow. Hence, we consider the visually-similar, gully-like granular flows observed on the Moon as suitable analogues for this process. We also consider mass-wasting deposits on Earth, in which water likely played a very minor role, as possible analogues for this process.
Here we go beyond plan-view comparisons of morphology, such as those illustrated in Fig. 1, by examining the three-dimensional properties of terrestrial, lunar and martian gullies. The inspiration for this study came from the delimitation of process-domains from digital elevation models of fluvial catchments on Earth. Montgomery and Foufoula-Georgiou (1993) calculated upslope drainage area and local slope for elevation data-pixels within fluvial catchments and showed that these properties follow a specific pattern in log–log space that depends on which processes were active in the catchment. They included process domains for fluvial and debris flow processes. We have further developed this approach by including other terrain attributes that can discriminate between processes such as cumulative area distribution, area distribution and 25 m downslope index, and by including dry granular flows (rockfalls, ravel and dry mass wasting) as an end-member process. Such hydrological analyses are not typically performed at the scale of the martian gullies (i.e. <5 km) because the data were not historically available. In an earlier study (Conway et al., 2011b) though, we showed that a qualitative comparison of slope-area and cumulative area distribution plots could discriminate between terrains dominated by debris flow, rockfall and fluvial processes on Earth at this scale. In the current study, we find that differences are also apparent in area distribution, and 25 m downslope index plots, as illustrated in Fig. 2. We extend our previous work by analyzing additional sites, including the Moon, and, more importantly, by performing a statistical analysis of the data.
Section snippets
Hydrological analysis
The datasets used are fully described in the Supplementary material, summarized in Tables S1 and S2. We followed the same approach as Conway et al. (2011b) in generating the terrain attributes necessary for these analyses and a visual summary of these calculations is shown in Fig. S3. In brief, we used the multi-direction flow algorithm “dinf” which partitions flow into downslope neighbours in any direction (Tarboton, 1997). From these non-integer flow directions we calculated the (fractional)
Interpretations and discussion
Our earlier study (Conway et al., 2011b) showed that some martian gullies qualitatively resembled terrestrial debris flows in terrain analysis data, and that this was not due to crater-wall topography producing spurious debris-flow like results. For the first time, our new analysis demonstrates quantitatively that, when using terrain parameters that best separate granular flow landforms from fluvial or debris flow landforms, martian gullies overlap the parameter space for both debris flow and
Conclusions
Our results support the interpretation that liquid water was inherent to the process that formed martian gullies. This conclusion is based upon a new method, yet is in agreement with many other studies that examine the topographic profiles (Conway et al., 2014), morphology (Gallagher et al., 2011; e.g., Johnsson et al., 2014, Levy et al., 2010), and geological and physiographic settings (e.g., Costard et al., 2002, Dickson et al., 2015, Head et al., 2008) of martian gullies. Liquid water must
Acknowledgments
We thank Jay Dickson and one anonymous reviewer for their helpful comments, which greatly improved the manuscript. We acknowledge funding from the Leverhulme Trust in support of this work (grant number RPG-397). This study would not have been possible without data from the UK's Natural Environment Research Council (NERC) Geophysical Equipment Facility loan numbers 977 and 1006, NERC Airborne Research and Survey Facility surveys IPY07-04, EUFAR12-02, European Facility for Airborne Research
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