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Glacial and Gully Erosion on Mars: a Terrestrial Perspective

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Glacial and Gully Erosion on Mars: a Terrestrial Perspective Title Glacial and gully erosion on Mars: A terrestrial perspective Authors Conway, SJ; Butcher, FE; de Haas, T; Deijns, AA; M. Grindrod, P; Davis, JM Description publisher: Elsevier articletitle: Glacial and gully erosion on Mars: A terrestrial perspective journaltitle: Geomorphology articlelink: https://doi.org/10.1016/j.geomorph.2018.05.019 content_type: article copyright: © 2018 Elsevier B.V. All rights reserved. Date Submitted 2018-08-17 1Glacial and gully erosion on Mars: a terrestrial perspective 2Susan J. Conway1* 3Frances Butcher2 4Tjalling de Haas3,4 5Axel J. Deijns4 6Peter M. Grindrod5,6 7Joel Davis6 81. CNRS, UMR 6112 Laboratoire de Planétologie et Géodynamique, Université de Nantes, 9France. 102. School of Physical Sciences, Open University, Milton Keynes, MK7 6AA, UK. 113. Department of Geography, Durham University, South Road, Durham DH1 3LE, UK. 124. Faculty of Geoscience, Universiteit Utrecht, Heidelberglaan 2, 3584 CS Utrecht, The 13Netherlands. 145. Work done at: Centre for Planetary Sciences at UCL/Birkbeck, University of London, 15Malet Street, Bloomsbury, London WC1E 7HX, UK. 166. Now at: Department of Earth Sciences, The Natural History Museum, Cromwell Road, 17London SW7 5BD, UK. 18*corresponding author: [email protected] 1 19Abstract 20The mid-to-high latitudes of Mars host assemblages of landforms consistent with a receding 21glacial landscape on Earth. It is hypothesised that these landforms are a result of dramatic 22changes in climate brought about by swings in Mars’ orbital obliquity, which can vary 23between 15° and 35° on timescales of ~100,000 years. At the highest obliquities it is thought 24that water ice is driven off the two permanent polar caps and redistributed to lower latitudes, 25and as the obliquity swings to lower values water ice is transported in the opposite sense. 26Here, we report on the relationship in time and space of two suites of landforms: gullies and 27glacial landforms. Gullies are kilometre-scale erosion-deposition systems comprising a 28source alcove, transportation channel and deposition apron or fan. The glacial landforms we 29describe here fall into two categories – extant viscous flow features where ice could still be 30present and relicts of glaciation including arcuate ridges commonly interpreted as moraines. 31Both gullies and glacial landforms are particularly common at the mid-latitudes – hinting at a 32common climatological origin. Here, we measure headwall retreat associated with the glacial 33landforms and date the host-craters to constrain the retreat rates. Our analyses show that 34the phase of glaciation recorded by the youngest suite of glacial landforms had headwall 35retreat rates up to ~103 m Myr-1, equivalent to erosion rates of wet-based glaciers on Earth 36and to headwall retreat rates associated with extensive martian bedrock gully systems. In 37addition, we do not find evidence for repeated cycles of erosion, but rather a single episode 38possibly 5-10 Ma. Gullies seem to postdate this episode and have caused cyclical reworking 39of the terrain on the steepest slopes. The majority of the glacial ice preserved on Mars as 40Viscous Flow Features (VFF) pre-dates this erosional episode. Additional evidence that this 41accelerated rate of glacial erosion was facilitated by small quantities of basal pore water is 42provided by the presence of arcuate ridges consistent with terrestrial glaciotectonic features 43which require liquid water to form; textural alteration of the eroded bedrock surface 44consistent with ice-segregation and frost-shattering; and finally the presence of downslope 45pasted-on terrain, which could represent glacial deposits (till). The pasted-on terrain is 2 46usually considered as a thicker latitude dependant mantle located on sloping terrain formed 47from airfall of ice nucleated on dust, but our reinterpretation suggests the inclusion of more 48debris than previously assumed. Although our results cannot substantiate that gullies are 49produced by meltwater, the discovery of this “wet” glacial event does provide evidence for 50widespread meltwater generation in Mars’ recent history. 51 52Keywords: Mars; martian gullies; Viscous Flow Features; Glacier-like forms; Liquid water 3 531. Introduction 54The martian mid-latitudes are host to a suite of landforms that indicate significant 55geologically recent (10s – 100s Ma) surface-atmosphere exchanges of water ice. This study 56focuses on two of the most common landforms: martian gullies and glacier-like forms. We 57examine the role they have played in landscape evolution over the last ten to hundreds of 58millions of years by using statistical analysis of topographic data. In this introduction, we first 59present a brief overview of the state of knowledge concerning the present and past 60distribution of ice and related landforms on Mars, then specify how martian gullies fit into this 61context and finally present the scope of the present study. 621.1 The distribution of water ice on Mars 63Water ice is stable and exposed at the surface at the two polar caps of Mars, which each 64contain a volume of ice similar to the Greenland ice sheet on Earth – approximately 106 km3 65. Water vapour is contributed to the atmosphere by seasonal sublimation of the north polar 66cap, which has higher summer temperatures than the southern cap, because of its lower 67altitude and higher atmospheric pressure . It also has a larger part of the water ice cap 68exposed at the surface compared to the south, where the rest of the surface is partially 69hidden by a perennial, thin, CO2 ice layer and by surface debris . The water vapour 70contributed to the atmosphere is redistributed across the planet and can be deposited as 71surface frosts down to the mid-latitudes . Theoretical modelling predicts that ground ice on 72Mars should exist in diffusive equilibrium with the atmospheric water vapour – it should be 73cold trapped into the pores of the regolith . This idea is supported by observations from the 74Neutron and Gamma Ray Spectrometers on Mars Global Survey that found abundant 75hydrogen in the top metre of the regolith down to ~50° latitude in both hemispheres, which 76can be explained by an ice content of 4% to >64% in the regolith . This geophysical 77evidence is further supported by the observation of ubiquitous polygonally patterned ground 78in the same latitudinal band consistent with thermal contraction cracks formed in ice- 4 79cemented soil over annual timescales . A trench dug by the Phoenix lander at 68°N found 80both excess and pore ice centimetres below the surface , newly formed impact craters have 81exposed water ice in their ejecta at latitudes down to 39° N and exposures of almost pure 82ice in eight ~hundred metre scarps have been found at ~55° latitude . 83The latitude zone hosting ground ice is also an area that is smooth at scales of a kilometre , 84as measured from elevation data from the Mars Orbiter Laser Altimeter (MOLA). This 85smoothing is partly attributed to the presence of the latitude dependant mantle (LDM) - a 86deposit thought to consist of the remnants of an airfall deposit of ice nucleated on dust, 87where the dust forms a sublimation lag protecting the remaining deposits from sublimation. 88Mustard et al. and Milliken et al. found that the LDM exhibited degraded or pitted textures at 89latitudes between 30 and 50° N and S; this change in texture coinciding with the change in 90surface roughness found from MOLA data . The surface age of the LDM has been estimated 91at ~0.1-5 Ma, and decreases in age with latitude . The LDM generally consists of alternating 92relatively ice- and dust-rich layers, indicating multiple generations of deposits formed under 93varying climatic conditions . There are various surface textures/draping deposits that have 94been grouped under the term “LDM”, but we will argue below that not all of these 95necessarily represent airfall dust-ice deposits. In total, LDM deposits cover 23% of the 96surface of Mars and are thought to contain 103-104 km3 of ice . 5 97 98Figure 1: Viscous flow features on Mars, north is up in all panels. (a) Lobate debris apron in 99Deuteronilus Mensae, CTX image F22_044466_2278. (b) Concentric crater fill in Utopia 100Planitia, CTX image P15_007067_2124. (c) Lineated valley fill in the Arabia Terra region, 101CTX image F06_038017_2202 (d) Glacier-like form in Protonilus Mensae, CTX image 102G03_019358_2225. 103Another significant reservoir of ice on Mars are the viscous flow features (VFF) found in the 104martian mid-latitudes . They have a total estimated volume of 4.2 x 105 km3, which is ~20% 105of the total volume of the polar caps . We will use VFF is an umbrella term for a range of 106landforms thought to be similar to debris covered glaciers on Earth. They include the 107following (illustrated in Figure 1): 108 LDA, lobate debris aprons – these are smooth aprons that extend from and 109 encompass mesas. They can extend several to tens of kilometres from their mesa 110 and orbital radar observations have confirmed that they can be up to 1 km thick and 6 111 contain >90% ice . Their ages range from ~40-500 Ma to ~1 Ga . LDA are the 112 largest reservoir of ice among the VFFs . 113 Crater interior ice-deposits, including concentric crater fill (CCF) – these deposits fill 114 and smooth-over the floors of impact craters and are ubiquitous in the mid-latitudes . 115 Levy et al. estimate that ice thicknesses can range up to 1.7 kilometres. The surface 116 of these fills often expresses lineations either concentric to the crater walls (in this 117 case they are called CCF), or can instead be oriented in one direction , and these 118 orientations vary with latitude (pole-facing 30-45° and concentric >45°). They have 119 been dated to be as young as 10 Ma and as old as 700 Ma . Crater interior ice- 120 deposits can cover part or all of the crater floor. Crater interior deposits are found in 121 craters 2-72 km in diameter , and represent the second largest reservoir of ice of the 122 VFFs.
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