Small-scale lobate hillslope features on Mars: A comparative 3D morphological study with terrestrial solifluction lobes and zebra stripe lobes

Highlights

• Small scale lobes (SSL) on Mars are more like terrestrial solifluction lobes than lobes from seismic shaking in the Atacama

• Martian SSL only occur on slopes >10°, terrestrial lobes >0° perhaps due to differences in material or thermal properties

• Martian SSL differ from terrestrial lobes in that they can have larger dimensions and occur near the top of the slope

Abstract

Small-scale lobes (SSL) observed at the surface of Mars are thought to be the result of solifluction. Hence, their existence has important implications for our understanding of recent climate history, the distribution of thaw liquids and its geomorphic effects, as water is believed not to have been recently stable at the martian surface. These small-scale lobes are limited to sloping terrains and often occur alongside polygonal patterned ground attesting to the presence of ice in the environment. In this study, we perform a comparative 3D morphometric analysis of the lobes on Mars with terrestrial analogues. We use data from High Resolution Imaging Science Experiment (HiRISE) camera in order to create Digital Terrain Models (DTMs) on Mars at 1 m/pix. For the terrestrial analogues, we use DTMs and orthophotos from five periglacial environments with solifluction lobes and from one site with lobate features found in the Atacama Desert. The lobate features in the Atacama Desert are found within features called “zebra stripes”, which are apparently unique to the Atacama and are now generally accepted to be the result of seismic shaking. We show that SSL on Mars overlap morphometrically with terrestrial solifluction lobes, and are less similar to zebra stripe lobes. The majority of martian SSL occur below the angle of repose which demonstrates that a dry fall mechanism cannot be responsible for their formation and that lubrication of the subsurface is required. We found that martian SSL only occur on slopes greater than ~10°, a constraint not observed for terrestrial solifluction lobes nor zebra stripe lobes in Atacama. We hypothesise that this apparent constraint does not result from the lowered gravity on Mars, but instead from differing soil properties and/or thermal regime between Mars and Earth which affects the slope at which creep can initiate. We cannot rule out a role of CO₂ sublimation in lobe-formation based on our terrestrial measurements.

Introduction

Solifluction landforms, in the form of downslope oriented lobes, are common in cold-climate environments on Earth. Mars is currently a cold hyper-arid desert planet where contemporary liquid water may be extremely localized and rare (e.g., Ojha et al., 2015). Lobate hillslope features, similar in morphology to terrestrial solifluction lobes have been observed on Mars (Mangold, 2005; Gallagher et al., 2011; Hauber et al., 2011; Johnsson et al., 2012; Balme et al., 2013; Johnsson et al., 2018). These landforms, termed “Small-Scale Lobes” (SSL), are interpreted to have been formed by solifluction processes due to their morphological similarity to solifluction lobes on Earth (Fig. 1; Gallagher et al., 2011; Johnsson et al., 2012).

We identified a new terrestrial environment where lobate forms are observed on sloping terrain and could therefore provide an alternative mechanism to solifluction as an explanation for martian SSL. In the Atacama Desert, lobate forms are visible in aerial images on small hills, have been attributed to the action of salts (Beaty, 1983), overland flow (Owen et al., 2013), and seismic shaking (May et al., 2019). This desert is the driest on Earth, and is one of the closest analogues for the current martian climate. One of the main problems facing the solifluction interpretation of SSL on Mars is the fact that extensive thaw is not thought to have been possible in the last 1 Ga (e.g., Kreslavsky et al., 2008). Hence, if a landform similar to SSL can be found in a dry terrestrial environment, it could provide a possible solution to this problem. The lobes are found within zebra stripes in Atacama, whose origin is debated, but is generally accepted to be the result of seismic shaking (May et al., 2019). As seismic shaking from impacts or marsquakes is becoming recognised as a potential element in martian landscape evolution (Roberts et al., 2012; Brown and Roberts, 2019; Kumar et al., 2019), this forms a potentially interesting alternative formation mechanism for martian SSL.

In this study, we performed a comparative morphometric analysis of SSL on Mars, terrestrial periglacial solifluction lobes and the Atacama lobate forms. Previous works relied on plan-view measurements and observations of martian SSL, such as the study of the distribution of small-scale lobes using HiRISE images, or their relationship to other possible ground ice–related landforms such as gullies, polygonally patterned ground, and sorted stripes (Gallagher et al., 2011; Johnsson et al., 2012; Nyström and Johnsson, 2014; Johnsson et al., 2018). Here, we add topographic context and quantitative 3D analysis of these landforms. These are the first systematic 3D measurements of these features. For this we used data from HiRISE to create DTMs (Digital Terrain Models). For the terrestrial analogues, we used DTMs and orthophotos from Svalbard (Adventdalen), Greenland (Carlsberg Fjord), Sweden (Hödj), the French Alps (Termignon), Iceland (Tindastóll), and the Atacama Desert (Antofagasta). The aim of the study is to use both the new topographic information and comparison with an alternate terrestrial lobate form to assess whether the solifluction interpretation for the martian SSL should be maintained. If the small-scale lobes observed on Mars are indeed the result of solifluction, their existence has important implications for our understanding of recent climate history, the distribution of thaw liquids near the surface, its geomorphic effects and implications for Special Regions for planetary protection (Rummel et al., 2014).

Mars has abundant ground ice from the mid- to high latitudes as shown by GRS (Gamma Ray Spectrometer on Mars Odyssey) (Boynton et al., 2002; Pathare et al., 2018), radar (Mouginot et al., 2010; Bramson et al., 2015; Stuurman et al., 2016) and imaging datasets of fresh craters exposing ice (Byrne et al., 2009; Dundas et al., 2014). In particular the ubiquitous presence of polygonally patterned ground at latitudes higher than ~55° and discontinuously between 30° and 55° is thought to represent the thermal contraction of these ground ice deposits (Mangold et al., 2004; Levy et al., 2009). 98% of martian SSL are found adjacent to, and sometimes superposed on, polygonally patterned ground (Nyström and Johnsson, 2014). If martian SSL are formed by solifluction then they could indicate thaw of this ground ice in the recent past. In support of this hypothesis is the rare observation of ploughing boulders associated with martian SSL (Fig. 2; Johnsson et al., 2018), which on Earth, are intimately associated with solifluction (French, 1976; Ballantyne and Harris, 1994; Ballantyne, 2001). The latitude dependence of SSL orientation also suggests a climatic driver in their formation. SSL are limited to sloping terrain and mainly observed in the northern hemisphere (e.g., Johnsson et al., 2018; Barrett et al., 2018). The southern hemisphere has more steep slopes (Kreslavsky and Head, 2003), so the relative paucity of SSL is not clearly understood, but could be linked to hemispherical differences in surface properties (Johnsson et al., 2018). Mid-latitude (55° to 65°) SSL have a pole-facing preference whereas high-latitude (65° to 80°) ones tend to have an equator-facing preference (Nyström and Johnsson, 2014).

In the arid climate of present-day Mars, the production of liquid water is still a point of debate, and is considered to be slightly more probable in the recent past (e.g., Richardson and Mischna, 2005). This is because Mars has undergone recent climatic shifts due to the variations in its orbital parameters. Contrary to the Earth which is stabilized by the Moon, the obliquity of Mars, currently equal to 25.2°, changes by approximately 20° on a 100 ka cycle (Laskar et al., 2004). According to modelling by Kreslavsky et al. (2008), conditions favouring the formation of an active layer (a thawed layer atop the permafrost) could have last been reached between 10 and 5 Ma.

SSL have been studied by several authors using HiRISE images (Gallagher et al., 2011; Johnsson et al., 2012; Nyström and Johnsson, 2014; Johnsson et al., 2018). As reported by Nyström and Johnsson (2014), 48% of martian SSL occur in craters that also host gullies. SSL superpose gullies (Gallagher and Balme, 2011) suggesting their age to be equivalent to those of gullies (i.e. the last few Ma; Reiss et al., 2004; Schon et al., 2009; De Haas et al., 2015a). Whether gullies are caused by flowing liquid water/brine or CO₂ sublimation triggered flows is a current point of debate (e.g., Conway et al., 2018). Their intimate association with martian SSL suggests that the two landforms are likely to be linked as a landform assemblage, i.e. formed by related processes (Gallagher et al., 2011). Liquid water could be favoured by the presence of perchlorates in the regolith, which were found at the Phoenix site. Perchlorates are molecules which have a eutectic point at 240 K, that maintain water in a liquid state at temperatures below its freezing point (Pestova et al., 2005; Marion et al., 2010). Sublimation of CO₂ frost (Sylvest et al., 2016) is not currently favoured to explain martian SSL, because lobes at the foot of the slope are hard to explain as they are lower than the dynamic angle of friction which limits mass wasting processes (Johnsson et al., 2012, Johnsson et al., 2018). However, the distribution of seasonal CO₂ matches with the observed latitudinal distribution of martian SSL and a recent study has revealed active rock motion associated with SSL and the seasonal CO₂ ice deposits (Dundas et al., 2019).

Finally martian SSL are often associated with landforms that resemble terrestrial sorted patterned ground (Johnsson et al., 2012, Johnsson et al., 2018), which are found in environments with active freeze-thaw cycling. The patterns are outlined by increased concentrations of clasts and can take the form of piles, nets, polygons, circles or stripes depending on the clast concentration and surface slope (Kessler and Werner, 2003). Such patterns have been reported on Mars (Soare et al., 2016; Barrett et al., 2017) and are found in proximity to SSL (Gallagher et al., 2011; Johnsson et al., 2018).

Mass wasting is defined as the downslope movement of soil/rock induced by gravity. Mass wasting encompasses a number of processes, which can operate together or separately (e.g., Selby, 1982) (Fig. 3a). However, not many of these processes produce lobes. Solifluction processes are known to produce lobes whereas soil creep on its own does not, as shown by field observations (Fig. 3a). Commonly, soil creep is described as the process of downslope soil movement driven by a range of processes (Heimsath and Jungers, 2013) such as moisture and/or temperature fluctuations, or dissolution and recrystallization of salts (Pawlik and Šamonil, 2018). Solifluction, on the other hand, refers to slow downslope movement of water-saturated debris or soils (Gallagher et al., 2011) by cyclic freezing and thawing of water (e.g., Matsuoka, 2001).

Solifluction can be divided into four processes (Matsuoka, 2001). 1) Frost creep is a ratchet-like downslope movement due to a normal frost heave followed by thaw and near vertical settling (Fig. 3b). Frost creep is divided into diurnal and annual frost creep depending on the timescale over which the soil has undergone the freeze-thaw cycle. The timescale of the freezing affects the depth of the movement, for example diurnal frost creep mainly affects the first centimetres of the soil (Matsuoka, 1998) whereas annual frost creep affects a few decimetres (Smith, 1988). 2) Needle ice creep is special kind of diurnal frost creep, occurring when superficial surface debris is lifted perpendicular to the surface by ice needles and then falls again vertically on thawing (Li et al., 2018). 3) Gelifluction occurs on thaw when self-compaction leads to high pore-water pressures resulting in plastic deformation of the soil (Fig. 3b; Harris et al., 2008). 4) When conditions of two-sided freezing occur, there is an additional process called “plug-like” flow in cold permafrost conditions. In this case, the whole active layer is set in motion (Matsuoka, 2001). Morphologically, solifluction lobes can be divided into stone- and turf-banked lobes. However, this morphometric classification does not imply a difference in process forming the lobe. It is likely that vegetation (and animals) strongly influence creep processes and thereby also landform development and morphometry (Eichel et al., 2017). Turf-banked lobes have, for example, a higher sediment thickness and a lower surface velocity than vegetation-free lobes (stone-banked lobes) (Matsuoka, 2001; Harris et al., 2008).

Debris flows are another lobe producing process (Draebing and Eichel, 2018). Debris flows are rapid mass movements of mixtures of sediment (40–50%) and water moving under gravity (Iverson, 1997) and deposit lateral levees as well as terminal lobes of unsorted materials. Draebing and Eichel (2018) compared former debris flow lobes reworked by solifluction with solifluction lobes and only found slight geomorphic differences, with reworked debris flow lobes possessing a more half-moon-like shape while solifluction lobes were more tongue-shaped. However, lobes developed from debris flow lobes have steep catchment areas and a debris flow origin can be deduced from their context (e.g. foot of proximal moraines or alluvial fan; De Haas et al., 2015b; Draebing and Eichel, 2018). We do not examine this particular lobe-forming process in any further detail as we consider similar environmental conditions are required as those that form solifluction lobes sensu stricto.