Elsevier

Earth and Planetary Science Letters

Volume 454, 15 November 2016, Pages 36-45
Earth and Planetary Science Letters

A novel topographic parameterization scheme indicates that martian gullies display the signature of liquid water

https://doi.org/10.1016/j.epsl.2016.08.031Get rights and content

Highlights

  • We present new terrain analyses from high-resolution DEMs of martian gullies.

  • We find that liquid water was involved in the formation of martian gullies.

  • Dry processes do not explain gullies topographic signatures.

  • Process-level interpretation from 2D images can be unreliable.

  • Statistical analysis of 3D data provides a better way to determine process.

Abstract

Martian gullies resemble gullies carved by water on Earth, yet are thought to have formed in an extremely cold (<50°C) and dry (humidity <100 precipitable micrometers) surface environment (cf. Mellon et al., 2004). Despite more than a decade of observations, no consensus has emerged as to whether liquid water is required to form martian gullies, with some recent studies favouring dry CO2-driven processes. That this argument persists demonstrates the limitations of morphological interpretations made from 2D images, especially when similar-looking landforms can form by very different processes. To overcome this we have devised a parameterization scheme, based on statistical discriminant analysis and hydrological terrain analysis of meter-scale digital topography data, which can distinguish between dry and wet surface processes acting on a landscape. Applying this approach to new meter-scale topographic datasets of Earth, the Moon and Mars, we demonstrate that martian gullied slopes are dissimilar to dry, gullied slopes on Earth and the Moon, but are similar to both terrestrial debris flows and fluvial gullies. We conclude that liquid water was integral to the process by which martian gullies formed. Finally, our work shows that quantitative 3D analyses of landscape have great potential as a tool in planetary science, enabling remote assessment of processes acting on planetary surfaces.

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

References (85)

  • T.N. Harrison et al.

    Global documentation of gullies with the Mars Reconnaissance Orbiter Context Camera and implications for their formation

    Icarus

    (2015)
  • M.H. Hecht

    Metastability of liquid water on Mars

    Icarus

    (2002)
  • J.L. Heldmann et al.

    Observations of martian gullies and constraints on potential formation mechanisms II. The northern hemisphere

    Icarus

    (2007)
  • J.L. Heldmann et al.

    Observations of martian gullies and constraints on potential formation mechanisms

    Icarus

    (2004)
  • S.W. Hobbs et al.

    A comparison of semiarid and subhumid terrestrial gullies with gullies on Mars: implications for Martian gully erosion

    Geomorphology

    (2014)
  • A. Johnsson et al.

    Evidence for very recent melt-water and debris flow activity in gullies in a young mid-latitude crater on Mars

    Icarus

    (2014)
  • G. Jouannic et al.

    Laboratory simulation of debris flows over sand dunes: insights into gully-formation (Mars)

    Geomorphology

    (2015)
  • G. Kminek et al.

    Report of the COSPAR Mars special regions colloquium

    Adv. Space Res.

    (2010)
  • T. Kneissl et al.

    Distribution and orientation of northern-hemisphere gullies on Mars from the evaluation of HRSC and MOC-NA data

    Earth Planet. Sci. Lett.

    (2010)
  • K. Krohn et al.

    Mass movement on Vesta at steep scarps and crater rims

    Icarus

    (2014)
  • N.L. Lanza et al.

    Evidence for debris flow gully formation initiated by shallow subsurface water on Mars

    Icarus

    (2010)
  • J.S. Levy et al.

    Identification of gully debris flow deposits in Protonilus Mensae, Mars: characterization of a water-bearing, energetic gully-forming process

    Mars Express after 6 Years in Orbit: Mars Geology from Three-Dimensional Mapping by the High Resolution Stereo Camera (HRSC) Experiment

    Earth Planet. Sci. Lett.

    (2010)
  • A. Marquez et al.

    Evidence of gully formation by regional groundwater flow in the Gorgonum-Newton region (Mars)

    Icarus

    (2005)
  • J.P. McNamara et al.

    Channel head locations with respect to geomorphologic thresholds derived from a digital elevation model: a case study in northern Thailand

    For. Ecol. Manag.

    (2006)
  • M.T. Mellon et al.

    The presence and stability of ground ice in the southern hemisphere of Mars

    Icarus

    (2004)
  • J.F. O'Callaghan et al.

    The extraction of drainage networks from digital elevation data

    Comput. Vis. Graph. Image Process.

    (1984)
  • C.H. Okubo et al.

    Constraints on mechanisms for the growth of gully alcoves in Gasa crater, Mars, from two-dimensional stability assessments of rock slopes

    Icarus

    (2011)
  • K. Pasquon et al.

    Present-day formation and seasonal evolution of linear dune gullies on Mars

    Icarus

    (2016)
  • J. Raack et al.

    Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars

    Icarus

    (2015)
  • S.C. Schon et al.

    Keys to gully formation processes on Mars: relation to climate cycles and sources of meltwater

    Icarus

    (2011)
  • J.E.C. Scully et al.

    Geomorphological evidence for transient water flow on Vesta

    Earth Planet. Sci. Lett.

    (2015)
  • T.K. Tesfa et al.

    Extraction of hydrological proximity measures from DEMs using parallel processing

    Environ. Model. Softw.

    (2011)
  • Z. Xiao et al.

    Mass wasting features on the Moon – how active is the lunar surface?

    Earth Planet. Sci. Lett.

    (2013)
  • A.H. Aston et al.

    Identifying Martian gully evolution

  • C. Atwood-Stone et al.

    Avalanche slope angles in low-gravity environments from active Martian sand dunes

    Geophys. Res. Lett.

    (2013)
  • M. Balme et al.

    Orientation and distribution of recent gullies in the southern hemisphere of Mars: observations from High Resolution Stereo Camera/Mars Express (HRSC/MEX) and Mars Orbiter Camera/Mars Global Surveyor (MOC/MGS) data

    J. Geophys. Res., Planets

    (2006)
  • F. Brardinoni et al.

    Glacial erosion, evolution of river long profiles, and the organization of process domains in mountain drainage basins of coastal British Columbia

    J. Geophys. Res., Earth Surf.

    (2006)
  • N.T. Bridges et al.

    Northern hemisphere Martian gullies and mantled terrain: implications for near-surface water migration in Mars' recent past

    J. Geophys. Res., Planets

    (2006)
  • N.A. Cabrol et al.

    Sands at Gusev Crater, Mars

    J. Geophys. Res., Planets

    (2014)
  • Y. Cedillo-Flores et al.

    CO2 gas fluidization in the initiation and formation of Martian polar gullies

    Geophys. Res. Lett.

    (2011)
  • P.R. Christensen

    Formation of recent martian gullies through melting of extensive water-rich snow deposits

    Nature

    (2003)
  • S.J. Conway et al.

    Decametre-thick remnant glacial ice deposits on Mars

    Geophys. Res. Lett.

    (2014)
  • Cited by (0)

    1

    Present address.

    View full text