# Sliding and Burrowing Blocks of CO 2 Create Sinuous “Linear Dune Gullies” on Martian Dunes by Explosive Sublimation-Induced Particle Transport

Lonneke Roelofs, Simone R Visschers, Susan J. Conway, Tjalling de Haas, Maarten G Kleinhans, Sylvain Douté, Kelly Pasquon, Matthew E Sylvest, Manish R Patel

► To cite this version:

Lonneke Roelofs, Simone R Visschers, Susan J. Conway, Tjalling de Haas, Maarten G Kleinhans, et al. Sliding and Burrowing Blocks of CO 2 Create Sinuous “Linear Dune Gullies” on Martian Dunes by Explosive Sublimation-Induced Particle Transport. Geophysical Research Letters, 2025, 52 (19), 10.1029/2024gl112860. [https://doi.org/10.1029/2024gl112860](https://doi.org/10.1029/2024gl112860). [hal-05374866](https://HAL.inhal-05374866)

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Distributed under a Creative Commons Attribution 4.0 International License
# Geophysical Research Letters®

## RESEARCH LETTER

10.1029/2024GL112860

**Key Points:**

* Linear dune gullies on Mars are created by sliding and burrowing blocks of sublimating CO₂ ice

* Slope angle and grain size of the bed determine whether a CO₂-ice block slides or burrows down slope on a Martian dune

* Burrowing CO₂-ice blocks create deep gullies with high levees, sharp bends, and pit-shaped channel endings

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

L. Roelofs,
l.roelofs@uu.nl

**Citation:**

Roelofs, L., Visschers, S. R., Conway, S. J., de Haas, T., Kleinhans, M. G., Douté, S., et al. (2025). Sliding and burrowing blocks of CO₂ create sinuous "linear dune gullies" on Martian dunes by explosive sublimation-induced particle transport. *Geophysical Research Letters*, *52*, e2024GL112860. https://doi.org/10.1029/2024GL112860

Received 8 OCT 2024
Accepted 12 SEP 2025

**Author Contributions:**

**Conceptualization:** Lonneke Roelofs, Susan J. Conway, Tjalling de Haas, Maarten G. Kleinhans

**Data curation:** Lonneke Roelofs, Simone R. Visschers

**Formal analysis:** Lonneke Roelofs, Tjalling de Haas

**Funding acquisition:** Lonneke Roelofs, Tjalling de Haas, Kelly Pasquon, Manish R. Patel

**Investigation:** Lonneke Roelofs, Simone R. Visschers

**Methodology:** Lonneke Roelofs, Susan J. Conway, Tjalling de Haas

**Project administration:** Lonneke Roelofs

© 2025. The Author(s).
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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## Sliding and Burrowing Blocks of CO₂ Create Sinuous “Linear Dune Gullies” on Martian Dunes by Explosive Sublimation-Induced Particle Transport

Lonneke Roelofs¹ Ⓓ, Simone R. Visschers¹, Susan J. Conway² Ⓓ, Tjalling de Haas¹ Ⓓ, Maarten G. Kleinhans¹ Ⓓ, Sylvain Douté³, Kelly Pasquon², Matthew E. Sylvest⁴ Ⓓ, and Manish R. Patel⁴ Ⓓ

¹Department of Physical Geography, Faculty of Geosciences, Utrecht University, Utrecht, The Netherlands, ²Laboratoire de Planétologie et Géosciences, Nantes Université, Univ Angers, Le Mans Université, CNRS, Nantes, France, ³Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), CNRS-Université Grenoble Alpes, Grenoble, France, ⁴School of Physical Sciences, The Open University, Milton Keynes, UK

**Abstract** Martian linear dune gullies are landforms consisting of parallel and often sinuous channels with distinct levees and pit-shaped endings that occur in the mid-latitudes on Mars. Recent observations link their formation to sublimating CO₂-ice blocks that slide downslope in early spring. Here we combine laboratory experiments in which we release CO₂-ice blocks on sandy slopes under Martian atmospheric pressure with observations of linear dune gullies on Russell crater mega dune to test this hypothesis. We show that, on steep slopes (>22.5° in our experiments) sublimating CO₂-ice blocks slide down-slope, creating straight, shallow channels with indistinct levees. On more gentle slopes (<22.5° in our experiments), sublimating CO₂-ice blocks burrow themself into the sand and slowly migrate downslope by excavating their surroundings by explosive gas-driven sediment transport. The burrowing movement helps explain the formation of unique characteristics of linear dune gullies; high levees, deep channels, channel sinuosity, and pit-shaped channel endings.

**Plain Language Summary** On some of the dune fields on Mars, parallel channels, called linear dune gullies, are found. Contrary to their name these gullies are often highly sinuous. In the past, it was believed that these linear dune gullies were formed by debris flow processes driven by liquid water. However, new evidence from satellite images shows that these channels are active in local spring by a process driven by CO₂ ice. The CO₂ ice forms on the Martian dunes in local winter and breaks off at the top of the dune at the beginning of spring. With laboratory experiments in a Mars chamber, we show that linear dune gullies are being formed by blocks of CO₂-ice that slide or burrow themselves into the sandy slope of the dune and move themselves downslope by violently ejecting the surrounding sand. This excavation process is caused by powerful gas flows, created by the sublimation of the CO₂-ice, when CO₂ ice is turned into CO₂ gas. While sliding blocks of CO₂ ice explain the formation of shallow channels, burrowing CO₂-ice explains the creation of the deep, and sinuous channels on the dunes of Mars.

## 1. Introduction

Linear dune gullies are unique and enigmatic landforms that are found on dune fields in the mid-latitudes on planet Mars (Dundas et al., 2012; Mangold et al., 2003; Pasquon et al., 2016; Reiss et al., 2010; Reiss & Jaumann, 2003). These parallel, and contrary to their name, often sinuous channels, with sharp bends, restricted source areas, distinct levees, and pit-shaped channel endings (Figure 1), do not have an Earth analog. Linear dune gullies are distinct from the classical gully landforms that are observed on steep slopes on Mars and Earth, which are composed of an erosional alcove, channel, and depositional apron (Conway et al., 2011; Costard et al., 2002; de Haas et al., 2015; Johnsson et al., 2014; Malin & Edgett, 2000), and are often larger than linear dune gullies (Dundas et al., 2012; Mangold et al., 2003; Pasquon et al., 2016; Reiss et al., 2010; Reiss & Jaumann, 2003). Mapping studies have so far identified 33 intra-crater dune fields on Mars with linear gullies, primarily between 36.3°S and 54.3°S and occasionally between 64.6°S and 70.4°S (Dundas et al., 2012; Mangold et al., 2003; Pasquon et al., 2016; Reiss et al., 2007; Reiss & Jaumann, 2003).

Two hypotheses have been proposed to explain the formation of linear dune gullies on Mars. Initially, studies suggested that linear dune gullies were formed by water-driven, debris-flow-like processes during periods of
**Resources:** Lonneke Roelofs, Sylvain Douté, Matthew E. Sylvest, Manish R. Patel

**Supervision:** Lonneke Roelofs

**Validation:** Lonneke Roelofs

**Visualization:** Lonneke Roelofs

**Writing – original draft:**

Lonneke Roelofs

**Writing – review & editing:**

Lonneke Roelofs, Simone R. Visschers, Susan J. Conway, Tjalling de Haas, Maarten G. Kleinhans, Sylvain Douté, Kelly Pasquon, Matthew E. Sylvest, Manish R. Patel

higher planetary obliquity when liquid water could hypothetically have existed on the surface of Mars (Costard et al., 2002; Jouannic et al., 2012; Mangold et al., 2003; Reiss & Jaumann, 2003). This hypothesis links the formation of linear gullies to the presence of water on Mars and therefore planetary habitability. More recent studies propose that linear gullies are actively being formed by CO₂-driven processes (Diniega et al., 2013; Dinwiddie & Titus, 2021; Dundas et al., 2012; Pasquon et al., 2016, 2019). This hypothesis is based on (a) the limited geographical and thus climatological occurrence of the landforms (Pasquon et al., 2016) and (b) present-day observations of activity in more than half of the linear dune gully sites in local late winter and early spring. The latter coincides with the present-day seasonal deposition and sublimation of CO₂ ice on the surface of Mars (Dundas et al., 2012; Pasquon et al., 2016, 2019). In addition, there have been direct observations of CO₂-ice blocks in the downslope pits at the end of linear dune gullies on Russell crater megadune (Dundas et al., 2012) as well as the movement of fine sediment out of the channels, in the form of dust clouds, in early spring (Dinwiddie & Titus, 2021).

The observed activity is currently explained as follows: in winter CO₂ is deposited from the atmosphere onto the surface as frost and ice, in spring this CO₂ ice sublimates back into the atmosphere. The linear gullies are formed on pole-facing slopes at the transition of local winter to spring (Dinwiddie & Titus, 2021; Dundas et al., 2012; Pasquon et al., 2016), by sliding blocks of CO₂-ice remnants that break off from the dune crest (Diniega et al., 2013; Dinwiddie & Titus, 2021). While sliding down, the CO₂-ice blocks are thought to carve the distinct channels by sublimation-driven gas fluxes that are strong enough to transport sediment away from the sublimating ice (Diniega et al., 2013; Dinwiddie & Titus, 2021). At the dune foot, continued sublimation of stationary CO₂-ice blocks would explain the formation of the observed pits (Diniega et al., 2013).

Despite the extensive evidence for the formation of linear dune gullies by sliding blocks of CO₂ ice, there are key morphological elements in the linear dune gullies that the sliding CO₂-ice block hypothesis cannot explain. Field experiments with sliding blocks of CO₂ ice on terrestrial hot desert dunes have shown that the sublimation of CO₂ ice, induced by the temperature difference between the ice and the sand, creates a gas flux large enough for the CO₂-ice block to slightly hover above the sand and slide downslope (Diniega et al., 2013). Moving downslope, the sliding CO₂ blocks carved shallow grooves (<block thickness) with shallow levees that formed due to sand being pushed to the edge of the block (Diniega et al., 2013). The movement of the CO₂-ice blocks was unaffected by small-scale topographic features; they plowed through ripples and sandy obstacles (Diniega et al., 2013). Experiments with static CO₂ blocks on the hot desert dune, showed the formation of shallow pits (~1 cm deep) (Diniega et al., 2013). Although delivering promising results on the ability of CO₂ blocks to move downslope on hot terrestrial dessert dunes, the terrestrial experiments did not produce the distinct morphology of Martian linear dune gullies. The sliding process did not produce deep channels with distinct levees, and the ease at which sliding blocks plow through obstacles likely rejects the hypothesis that repeated sliding of CO₂ blocks through the same channel could potentially create higher levees. Furthermore, the sliding blocks did not create sharp channel bends or the distinct deep terminal pits that we observe in linear dune gullies on Mars (Figure 1). While the observational evidence shows that it is likely that blocks of CO₂ ice carve linear dune gullies, we do not understand the formation process of these landforms and their unique morphological elements; deep channels with distinct levees, sharp bends, and pit-shaped channel endings. This leaves open the possibility for water-driven formation process and thwarts the further exploration of these landforms as a microclimate proxy on present-day and potentially ancient Mars.

We hypothesize that the thin Martian atmosphere is a critical element in explaining the difference between the terrestrial experiments (Diniega et al., 2013) and the process that carves the linear dune gullies on Mars. The key process for forming linear gullies is creating a gas flux, due to CO₂ sublimation, strong enough to transport large amounts of sediment away from the sublimating ice, that can build up high lateral levees and form deep channels. The gas flux depends on the difference between the density of CO₂ ice and gas in the atmosphere (de Haas et al., 2019; Roelofs, Conway, de Haas et al., 2024; Roelofs, Conway, van Dam et al., 2024). In the thin Martian atmosphere (~700 Pa), the gas flux created by CO₂ sublimation is >100 larger than under Earth's atmosphere and sufficient to transport sediments (Cedillo-Flores et al., 2011; de Haas et al., 2019; Mc Keown et al., 2021; Roelofs, Conway, de Haas et al., 2024; Roelofs, Conway, van Dam et al., 2024).

To test if CO₂-ice blocks are capable of creating linear dune gullies under the thin Martian atmosphere we combine novel small-scale experiments under Martian atmospheric pressure (700 Pa) with detailed topographic and morphometric observations of linear dune gullies on Russell crater megadune. With our experiments, we test
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Two examples of Martian dunes with linear dune gullies. North is up in both images. (a) Linear dune gullies on a dune field in Galle crater (HiRISE image ESP_077477_1280_MIRB). (b) Linear dune gullies on a dune field in an unnamed crater in the centre of Hellas Planitia (HiRISE image ESP_051770_1345).</figcaption></figure>

how blocks of CO₂ ice of different sizes travel downslope on sand beds of varying slope angles and grain sizes. Furthermore, we compare the morphology of the levees and channels created in our experiments with those on Russell crater megadune.

## 2. Materials and Methods

### 2.1. Experiments Under Martian Atmospheric Conditions

To simulate the breaking off and sliding down of CO₂-ice blocks on Martian dunes in laboratory experiments we designed a set-up consisting of a tilted tray, 1.2 m in length and 0.4 m in width, with an adjustable slope and a CO₂-ice block release mechanism at the top (Figure S1. a in Supporting Information S1). The inside of the tilted tray was covered with antislip material (3M Safety-Walk 500 series, equal to 80 grit sandpaper with 0.2 mm median sand diameter) and filled with a 5 cm layer of sand. Three different sediment types were tested of which the results of one type are presented in the main text (a well-rounded, highly spherical, fine aeolian sand with a medium grain size, D₅₀, of 210 µm). The results of the experiments with the two other, medium sands (D₅₀ of 310 and 320 µm), are presented in Figures S3 and S4 in Supporting Information S1. The rotating release mechanism held blocks of CO₂ ice with maximum dimensions of 4.5 × 4.5 × 2 cm during the experiments. We obtained digital elevation models (DEMs) of the experimentally created gully morphologies using photogrammetry with Agisoft Metashape software (Roelofs, Conway, de Haas et al., 2024; Roelofs, Conway, van Dam et al., 2024). DEMs were created of the bed before and after the experiments. The before and after DEMs were subtracted form eachother to create DEMs of Difference (DoDs) that highlight places where material is eroded or deposited.

The experimental set-up was placed in “George,” the Large Dirty Mars chamber of the Hypervelocity Impact and Space and Planetary Environments laboratory at the Open University in Milton Keynes (UK) to conduct the experiments under Martian atmospheric pressure (700 Pa) (Figure S1. b in Supporting Information S1). “George” is a cylindrical low-pressure chamber with a length of 2 m and an inner diameter of 0.9 m (Conway et al., 2011; Roelofs, Conway, de Haas et al., 2024; Sylvest et al., 2016). Three video cameras were installed around the chamber to film the experiments and track the movement and velocity of the CO₂-ice blocks downslope. The average downslope velocity of a CO₂ block was calculated by dividing the length of the tray by the time it took the block to travel downslope. Every experiment was recorded with a Go-Pro camera from the side and two camcorders from the front and the top. Seven experiments were filmed with a high-speed camera at 1,200 fps (Phantom Miro C110 with a RICOH FL-CC6Z1218-VG lens, for set-up see, Figure S1. b in Supporting Information S1) to make detailed observations of the dynamics of the sublimating CO₂-ice blocks and the movement of the sand around the sublimating blocks.

Several experimental parameters were explored, that is, slope angle, initial CO₂ block size, atmospheric pressure, and grain size of the sandy slope. The slope of the tray was adjusted between 20° and 28°, the initial block size was varied between 2.5 × 2.5 × 2 cm and 4.5 × 4.5 × 2 cm, and atmospheric pressure was set at 700 Pa for all Martian experiments and kept at ~100 kPa or for terrestrial reference experiments. We did not vary the temperature
<figure><img src="image_4.png" /><figcaption>**Figure 2.** DEMs of Difference (DoDs) for five key experiments with a decreasing slope (a–e), and elevation profiles over the main feature (f–j). Experiments were conducted with CO₂ ice blocks of 3.5 by 3.5 cm by 2 cm, and a sand with a D₅₀ grain size of 210 µm. The location of the elevation profile is indicated on the DoDs by a dashed line. The cross-section (f–j) corresponds to the respective DoD above (a–e) and the vertical exaggeration is the same as in the profiles of the cross-sections presented in Figure 4. For the DoDs, the top of the image is the upper part of the slope. In panels (a) and (f), a reference experiment conducted under Earth's atmosphere is presented. In the other panels, experiments are presented that were conducted under Martian atmospheric pressure.</figcaption></figure>

conditions of the sand in our tray, all experiments were conducted at room temperature. The slope angles were chosen to reflect steep natural dune slopes (with 30° being the angle of repose). Furthermore, we choose the maximum CO₂ block size in such a way that the ballistically induced sediment transport did not go beyond the edges of the tray when the block was initially dropped along the center axis. The thickness of the CO₂ blocks was dictated by the maximum thickness of the commercially purchased CO₂ slabs.

## 2.2. Morphometric Analysis of Martian and Experimental Linear Dune Gullies

To compare the experimental gully morphologies with real Martian linear dune gully morphology we (a) visually compared channel-levee morphology, focusing on visual distinctiveness of the levees, the concavity/convexity of levee slopes and presence of sharp bends, and we measured (b) channel depth (defined as the elevation difference between the top of the highest levee and the deepest point in the channel) and (c) channel width (defined as the distance between the two levee tops) of both our experimental gullies and of linear dune gullies on Russell crater megadune. We studied the linear gullies of Russell crater megadune because it is the only place with linear dune gullies on Mars of which a refined Digital Elevation Model (DEM) of high enough resolution exist. The refinement of the original HiRISE DEM, by means of photoclinometry performed by Douté and Jiang (2020), is needed to enable detailed study of the morphology of the linear dune gully channels and levees. Furthermore, we (c) calculated the slope angle of the dune along the linear gullies and classified different types of channel morphology; indistinct channels with indistinct levees, and distinct channels with distinct levees.

# 3. Results

## 3.1. Experimental Linear Dune Gullies Formed by Sliding and Burrowing CO₂-Ice Blocks

Our experiments show that under Martian atmospheric conditions, CO₂-ice blocks can move on a sandy slope by two different modes that create different channel-levee morphologies (Figures 2 and 3). The first transport mode
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Series of stills of high-speed videography of two experiments showing the sliding CO₂-block mechanism (a–d) and the burrowing CO₂-block mechanism (e–h). On the images the time stamp of the still in the corresponding video (see Movies S6 and S7) is indicated, note that the image time is 40 times the real time. This means that going from panel (a) to panel (d), 4 s in video time is passed and 0.1 s in real time. Going from panel (e) to panel (h), 56 s in video time is passed and 1.4 s in real time. The mechanisms of sand transport are indicated by orange lines and arrows for downslope rolling, creep or fluidized sheet flow and blue arrows for ballistic, airborne particle transport. The bottom two panels have been intentionally left without markings to allow the reader an unobstructed view of the mechanisms. In Movies S1–S5 the video footage from the frontal camera is presented.</figcaption></figure>

is the earlier hypothesized and studied sliding mechanism (Diniega et al., 2013; Dinwiddie & Titus, 2021) (Figure 3a–3d). This mode of transport is fast, with CO₂-ice block velocities around 0.8 m/s in our experiments, and creates relatively shallow and narrow channels, with small and indistinct levees (Figures 2b, 2c, 2g and 2h and Figures S2a–S2c in Supporting Information S1). During the sliding process, sand is mostly transported away from the CO₂-ice block close to the bed by rolling and fluidized sheet flow (Figures 3a–3d). The second transport mode is a newly discovered mechanism of downslope burrowing (Figure 3e–3h), in which the CO₂-ice block burrows
itself partly into the sand by fluidizing the sediment and excavating its surroundings. This burrowing process is slow, with average downslope velocities of 0.003 m/s, and creates deep and relatively wide channels with high and distinct levees and a frontal moon-shaped bar (Figures 2d, 2e, 2i and 2j and Supplementary Figure S2a–S2c in Supporting Information S1). During burrowing, sand is transported far away from the block by vigorous ballistic ejection, as well as rolling and sheet flow (Figures 3e–3g). When CO₂-ice blocks burrow, sharp bends form in the channels, likely as a result of small heterogeneities in the bed, causing the block to alter its course (Figure 2d). Under Martian atmospheric conditions, both transport mechanisms, sliding and burrowing, are driven by large gas fluxes produced by CO₂-ice sublimation under the thin Martian atmosphere induced by the contact of the CO₂ ice and the warmer sand.

The transport mode of the CO₂-ice blocks, either sliding or burrowing, depends on the slope of the sand bed, which influences the initial block velocity, and the grain size of the sand. In our experiments with fine aeolian sand (D₅₀ = 210 µm), CO₂-ice blocks slide downslope at flume angles of >25° and burrow at flume angles of <22.5° (Figures 2d–2e and Figures S2a–S2c in Supporting Information S1). For the tested ranges in slope angle and CO₂-block size, the mode of transport is independent of the width and length of the CO₂-ice blocks themselves (see Figures S2a–S2c and Figure S4c in Supporting Information S1). On coarser sand that is still relatively well sorted, some blocks still burrow at flume angles <22.5° but get stuck on their way down behind the frontal dam they create (Figure S4b in Supporting Information S1). On equally coarse but more poorly sorted sand, D₅₀ = 310 µm, the blocks slide on all slope angles. On the steepest slopes (25–28°), the blocks start to behave in a stick-slip type manner, like skipping pebbles on water, creating a washboard pattern on the bed (see Figure S4a in Supporting Information S1). The wavelength of this quasi-periodic washboard pattern decreases with decreasing slope (Figure 2 and Figure S3 in Supporting Information S1).

## 3.2. Russell Crater Linear Dune Gullies

At the top of Russell crater megadune, where the slope is the steepest (12.5°–30°), the planform morphology of the gully channels is tributary in nature; multiple shallow channels, without distinct levees, that emerge from the dune crest, converge and merge lower down on the dune slope where the slope is less steep (Figure 4a-b).

On slopes lower than 12.5°, the gully channels are deeper, and distinct levees can be observed on both sides of the channel (Figure 4c-e). Further downslope, the selected channel becomes deeper and the levees higher (Figure 4c-e). The channel-levee morphology on these slopes is comparable to the morphology of the experimental gullies formed by burrowing CO₂-ice blocks. In addition, going downslope, the whole channel-levee system lies more perched on top of the dune slope (as also observed by Jouannic et al., 2012), gets slightly sinuous in some places and eventually ends in a pit-shaped depression (Figure S5 in Supporting Information S1). The depth-to-width ratios of different linear gullies on Russell crater megadune is related to the local dune slope around the channels with relatively deeper channels with higher levees existing on more gentle slopes (Figure 2d). Based on the experimental results, we hypothesize that this is related to the velocity of the CO₂-ice blocks that create these gullies. On smaller slope angles, the gas fluxes from the sublimation of the CO₂ ice have more time to excavate the channel and build up higher levees. The parts of the channels with indistinct morphologies occur on the steepest slopes and have the lowest depth-to-width ratio of the individual channel.

## 4. Upscaling the Burrowing Process to Mars

There are clear differences in the size of the linear dune gullies with deep channels and distinct levees created by CO₂ ice in our experiments (~7–15 cm in width, and ~0.5–2 cm in depth) and linear dune gullies with distinct levees on Russell crater megadune (~10 m in width and ~1 m in depth). Diniega et al. (2013) showed, with a simple model that the gas flux created by CO₂ ice sublimation on a Martian dune is large enough to levitate and move small and large blocks of CO₂ ice downslope by means of sliding. However, this model neglects the actual formation of the key morphological characteristics of linear dune gullies, for example, deep channels with distinct levees, for which gas-driven sediment transport is essential. As burrowing blocks of CO₂ ice created the largest amount of sediment transport resulting in the formation of deep channels, with distinct levees, we use a numerical model, inspired by the ballistic transport of ejected sediment particles, as previously described for Martian scoria cone development (Brož et al., 2015), to test the scalability of the burrowing process observed in our experiments.
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Hillshade of the refined Digital Elevation Model (DEM) (Douté & Jiang, 2020) of a part of Russell crater megadune gullies with slope classes superimposed (a). Original DEM based on PSP_007018_1255 and PSP_0077229_1255. In the map, a distinction has been made between regions containing gully channels without clear and distinct levees and regions with gully channels with clear and distinct levees (indicated by the dashed white line). Four elevation profiles have been drawn (b–e) for four cross-sections of one of the linear gully channels. This channel was chosen as its morphology is not visibly influenced by neighboring channels. The locations of the cross-sections are indicated in the map in panel (a). The x- and y-axis of the elevation profiles (b–e) differ, but the vertical exaggeration is kept constant.</figcaption></figure>

The numerical model (see in Supporting Information S1 for description, assumptions and derivations) describes the particle motion of ejected sand grains during the burrowing process as a ballistic trajectory. With this simplification, we assume that the particle is brought into motion from the fluidized bed surrounding the CO₂-ice block by a gas force created by CO₂ sublimation, and that this force only works on the particle for a certain distance. We also assume that this distance is related to the thickness of the CO₂-ice block, as particles get accelerated by the gas pressure along the edges of the block (Figure S6 in Supporting Information S1). We have estimated this distance to be 0.5 times the thickness of the CO₂-ice block.

Our ballistic model shows that in our experiments under Martian atmosphere on Earth, sand grains can travel between 0 and 0.2 m along the horizontal plane and up to 0.1 m vertically, depending on the angle at which they are ejected (see Figure S7a in Supporting Information S1). This corresponds to the observed maximum distance that sand grains traveled in our experiments with burrowing CO₂ ice (Figures 2d and 2e and cross-section levee in Figure S7a in Supporting Information S1). When we correct the experimental ballistic trajectories for the lower gravity on Mars, we predict that sand grains travel between 0 and 0.55 m in the horizontal direction and more than 0.25 m in the vertical (Figure S7a in Supporting Information S1). This implies that the maximum dimensions of levees and channels would be larger in our experimental set-up under Martian gravity. However, as ballistically ejected sand particles would also be distributed over a larger area, it is likely that if these experiments could be repeated using Martian gravity, multiple CO₂-ice blocks would be needed to travel along the same path to create levees of the same height as in our experiments under terrestrial gravity.
When we increase the path length over which ejected particles accelerate, which under our assumptions means an increase in CO₂-ice block thickness, the ballistic transport becomes even more effective. Increasing CO₂ thickness to 1 m (values corresponding to the size of the blocks found in the Russell gullies (Dundas et al., 2012)) results in a maximum horizontal displacement >13 m (Figure S7b in Supporting Information S1), which corresponds approximately to the width of the levees of the linear dune gullies on Russell (Figures 4c-4e). Slightly smaller CO₂ thickness values, ranging between 0.25 and 0.55 m, result in smaller initial velocities that result in smaller horizontal and vertical displacements (Figure S7b in Supporting Information S1).

Despite the simplifications in our model, our model results imply that the burrowing process that creates linear dune gullies in our experiments is scalable to Mars. Ballistic sediment transport, driven by CO₂ gas fluxes, is able to transport sediment far away from the sublimating CO₂-ice block, and is thus able to create the deep linear gully channels with distinct levees we observe, especially when we consider repeat activity of burrowing CO₂-ice blocks in the same gully. Repeat activity by burrowing blocks in the same channel is likely as the trajectory of burrowing CO₂-ice blocks in downslope direction seems to be highly influenced by even small heterogeneities on the dune surface (Figure 2). Over time, repeat burrowing action in the same channel will build up high levees with a maximum width and height determined by the ballistic trajectories. As levees grow in height, and the channels deepen, the maximum horizontal distance the grains will travel by ballistic transport will become smaller and deposition of grains will mainly occur in areas closer to the channel, giving rise to the distinct levees we observe.

## 5. Do Sliding or Burrowing CO₂-Ice Blocks Create Martian Linear Dune Gullies?

Based on the morphology of our experimental Martian linear dune gullies, the scalability of the burrowing process, and the varying morphology of the linear gully channels on Russell and on other dunes (see Figure 1), we hypothesize that a combination of sliding and burrowing CO₂-ice blocks that break off from the crest of Martian dunes create linear dune gullies. On the steepest parts of the dune, the CO₂-ice blocks slide downslope. On these steep slopes, sediment is moved by gas-induced sheet flow from underneath the sliding block to its edge. This creates the tributary planform shape of the gullies at the dune crest with shallow channels and indistinct levees, similar to the experimental gullies create by sliding blocks of CO₂ ice (this study, and Diniega et al., 2013). As the dune slope gets less steep, the blocks slow down and start to burrow, ballistically ejecting sand grains over large distance of up to 10–20 m. This burrowing process then excavates deeper channels and builds distinct levees; the slower the CO₂-ice block travels downslope the more time the CO₂-gas flux has to excavate its surroundings, and thus the deeper the channel and larger the levees become. Once the CO₂-ice block reaches the dune foot and halts, warm dune sand will continue to drive CO₂-ice sublimation and vigorous ballistic transport of sand grains, which creates the pit-shaped channel endings with distinct rims.

In our experiments the transition of sliding to burrowing seems to occur around slopes of 22.5°, whereas on Russell crater mega dune the morphological transition towards deep channels with distinct levees occurs around 12.5°. The reason for the difference in transitional slope between our experiments and Mars is not completely clear. It could be that the surface area to thickness ratio of the CO₂ blocks is much larger on Mars; thinner blocks of CO₂ ice will submerge less deep into the fluidized sand, which could potentially promote sliding instead of burrowing on more gentle slopes. However, as Dundas et al. (2012) observed meter scale blocks of CO₂ ice in the linear dune gullies on Russell, we deem the above hypothesis unlikely. We, therefore, hypothesize that the initially high downward momentum of our experimental CO₂ blocks, caused by the rotating dropping mechanisms, causes CO₂ blocks to burrow at a steeper dune slopes than they would when they break off more gently from the Martian dune crest.

Based on our experimental results, we further hypothesize that the nature of small heterogeneities in the sand bed, can cause a burrowing CO₂-ice block to change course, in contrast to sliding CO₂-ice blocks that plow through these small heterogeneities (Diniega et al., 2013). Therefore, we hypothesize that smaller morphological features, like ripples, and/or small differences in sand temperature, can cause a burrowing CO₂-ice block on a Martian dune to change course, giving rise to sharp bends and possibly the sinuosity we observe in many linear dune gully channels (Figure 1). However, where the regularity in the sinuosity of some linear dune gullies comes from, and why some but not all linear dune gullies have sinuous reaches needs to be investigated further.

At the transition between sliding and burrowing on Russell crater mega dune, individual indistinct channels that originated from slightly different parts of the dune crest merge (Figure 4). This shows that multiple CO₂-ice blocks travel through, excavate and extend one linear dune gully channel. Over multiple years, repeat break-offs
of CO₂-ice blocks can continuously excavate and extend the existing channel (Pasquon et al., 2016), and thus also build up the extensive levees we observe today by continuous ballistic ejection of material from the channel onto the levee, with a maximum width and height determined by the ballistic trajectories. As levees grow in height, and the channels deepen, the maximum horizontal distance the grains will travel by ballistic transport will become smaller and deposition of grains will mainly occur in areas closer to the channel, giving rise to the distinct peaky levees we observe. In addition to building up large levees, the continuous excavation by burrowing blocks of CO₂ ice will slowly move sand from the top of the dune to the toe of the dune, making the dune less steep, and the channel on the downslope end more perched on top of the dune slope (Jouannic et al., 2012).

**6. Conclusions and Implications**

Our results show that linear dune gullies on Mars are carved by both sliding and burrowing blocks of CO₂ ice. Where sliding blocks of CO₂ ice can explain the shallow channels with indistinct levees on the steepest parts of the dune crest, burrowing blocks of CO₂ ice can explain the unique morphological elements of linear dune gullies lower down on the dune slope; deep channels with distinct levees, sharp channel bends, and pit-shaped channel endings. This implies that water has no role in the formation of linear dune gullies on Mars and that morphological features like sharp bends/sinuous channels are not definitive proof of the action of liquids on planetary surfaces. Hence, if similar landforms are found in high-resolution images of other planetary surfaces during future exploration of the Solar System (e.g., JUICE, Dragonfly, Europa Clipper, Comet Interceptor, and BepiColombo), we can now better constrain surface conditions.

Our process-based findings further highlight the specific local climatological and topographical conditions under which linear dune gullies originate on Mars. First, CO₂ ice needs to accumulate into a significant thickness to form blocks that create gullies of the size we observe (Dundas et al., 2012). Second, the dune needs to be able to keep this CO₂ ice on its crest while the rest of the dune slope defrosts. The defrosting and heating up of a part of the dune slope is necessary for creating enough thermal energy for the sublimation of the CO₂ ice driving the formation of the gully. Combined with the existing observations of the geographically limited occurrence of linear dune gullies and their season-sensitive activity, this sheds light on how specific (micro) climatic conditions, topography, surface characteristics and the atmosphere interact to form unique planetary landforms. By constraining the size of the CO₂-ice blocks forming the gullies based on gully morphometry at specific localities and using the timing and location of linear dune gully activity we could better verify CO₂ deposition and sublimation processes predicted by existing Martian climate models.

Furthermore, our experiments show that linear dune gullies can only be formed on sandy slopes of very specific grain size distributions. Fine-grained, well-rounded and well-sorted sediments are a prerequisite for the burrowing process to occur on more gentle slopes. Consequently, linear dune gullies can only form on a limited type of substrates, even though blocks of CO₂ ice might be more widely present in the Martian environment. This would explain the lack of observations of linear gully morphology on classical gully fans and talus slopes, both composed of more coarser-grained, angular and poorly sorted material.

Lastly, linear dune gullies actively alter the dune morphology and stratigraphy. It is therefore important to keep this process in mind when studying Martian aeolian deposits, for example, the Stimson formation visited by Curiosity (Banham et al., 2018). Stratigraphic structures in ancient aeolian deposits or on lithified dunes on the surface that show signs of slumping, small-scale channel formation and/or undulating erosional and depositional surfaces might be evidence of a linear dune gully-rich past of the dune field. If such features were identified they would be a unique local climate marker within the aeolian deposit.

**Conflict of Interest**

The authors declare no conflicts of interest relevant to this study.

**Data Availability Statement**

A list of experiments (with accompanying experimental parameters and resulting block transport regime), DEMs of experiments, videos of the five key experiments shown in Figure 2, and high-speed footage of the experiments depicted in Figure 3, are given in the data supplement Roelofs, Visschers, et al. (2024). Via the data supplement
Roelofs, Visschers, et al. (2024) the original HiRISE DEM of the studied area on Russell crater mega dune and the refined DEM created by Douté and Jiang (2020) can also be downloaded.

Acknowledgments

LR was supported by the Dutch Research Council (NWO) - Grant OCENW. KLEIN.495 to TdH. The visits to the HVI-SPE-lab at the Open University were supported by the British Society of Geomorphology - Postgraduate research Grant BSG-2023-16 to LR, Europlanet - project number 20-EPN2-093 to KP, and partly in-kind by the School of Physical Sciences of the Open University under the leadership of MRP through UK Space Agency Grant ST/X006549/1. SJC is grateful for financial support of the French Space Agency CNES for her HiRISE work. The Large Dirty Mars chamber of the HVI-SPE-lab at the Open University is a member of Europlanet (2024) RI and has received funding from the European Union's Horizon 2020 research and innovation programme under Grant 871149.

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