# Laboratory Investigations of Flow Dynamics and Volume Growth During Erosion of Different Substrates by CO2-Driven Granular Flows in Martian Gullies

Lonneke Roelofs, Susan J. Conway, Bas van Dam, Arjan van Eijk, Jonathan P Merrison, Jens Jacob Iversen, Henk Markies, Marcel van Maarseveen, Tjalling de Haas

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Lonneke Roelofs, Susan J. Conway, Bas van Dam, Arjan van Eijk, Jonathan P Merrison, et al. Laboratory Investigations of Flow Dynamics and Volume Growth During Erosion of Different Substrates by CO2-Driven Granular Flows in Martian Gullies. Journal of Geophysical Research. Planets, 2025, 130 (10), pp.e2025JE008993. 10.1029/2025je008993. hal-05374862

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Distributed under a Creative Commons Attribution 4.0 International License
JGR Planets

RESEARCH ARTICLE

10.1029/2025JE008993

**Key Points:**

* CO₂-driven granular flows are effective erosive agents in Martian gully systems today

* The entrainment of unconsolidated material by CO₂-driven granular flows enhances flow mobility if enough thermal energy is present

* CO₂-driven granular flows have likely played a dominant role in the formation of gully systems on Mars

**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., Conway, S. J., van Dam, B., van Eijk, A., Merrison, J. P., Iversen, J. J., et al. (2025). Laboratory investigations of flow dynamics and volume growth during erosion of different substrates by CO₂-driven granular flows in Martian gullies. *Journal of Geophysical Research: Planets*, 130, e2025JE008993. https://doi.org/10.1029/2025JE008993

Received 17 FEB 2025
Accepted 23 SEP 2025

**Author Contributions:**

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

**Data curation:** Lonneke Roelofs,
Bas van Dam

**Formal analysis:** Lonneke Roelofs,
Jonathan P. Merrison

**Funding acquisition:** Lonneke Roelofs,
Jonathan P. Merrison, Tjalling de Haas

**Investigation:** Lonneke Roelofs,
Bas van Dam, Arjan van Eijk, Jonathan
P. Merrison, Jens Jacob Iversen,
Tjalling de Haas

**Methodology:** Lonneke Roelofs, Susan
J. Conway, Bas van Dam, Arjan van Eijk,
Jonathan P. Merrison, Jens Jacob Iversen,
Henk Markies, Marcel van Maarseveen,
Tjalling de Haas

© 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.

<figure><img src="image_2.png" /></figure>

# Laboratory Investigations of Flow Dynamics and Volume Growth During Erosion of Different Substrates by CO₂-Driven Granular Flows in Martian Gullies

Lonneke Roelofs<fnref n="1" /> ID, Susan J. Conway² ID, Bas van Dam<fnref n="1" /> ID, Arjan van Eijk<fnref n="1" />, Jonathan P. Merrison<fnref n="3" />, Jens Jacob Iversen<fnref n="3" />, Henk Markies<fnref n="1" />, Marcel van Maarseveen<fnref n="1" />, and Tjalling de Haas<fnref n="1" /> ID

**Abstract** Gullies are actively changing landforms on planet Mars. The prevailing hypothesis, supported by a suite of different studies, states that present-day activity in these gullies is caused by fluidized granular flows driven by the sublimation of seasonal CO₂ ice. However, the long-term formation process of gully landscapes is a contentious issue as water-driven debris-flow processes could easily explain erosion. In contrast, we do not know if CO₂-driven granular flows can cause a significant amount of erosion. In this study, we conducted flume experiments investigating the flow dynamics and erosion capacity of CO₂-driven granular flows under different substrate and flow settings. Our experiments show that CO₂-driven granular flows under Martian conditions are efficient erosive agents, which can erode and entrain large volumes of unconsolidated material in various environmental (i.e., substrate and flow) settings. In general, erosion and entrainment enhance the mobility of CO₂-driven flows. However, the frost and thermal conditions of the slopes and the flow composition determine the erosion efficiency of these flows. Finally, based on terrestrial debris-flow erosion theory we estimate that collisional forces at the base of CO₂-driven flows can also cause erosion of more consolidated material such as bedrock, permafrost or Latitude Dependent Mantle.

**Plain Language Summary** Gullies are landforms on hillslopes on planet Mars. These gullies are actively evolving due to the actions of granular flows that are driven by the seasonal sublimation of CO₂ ice. Sublimation is the process in which an ice turns into a gas. CO₂ ice is deposited on the Martian surface during winter when the temperature of the surface becomes –120°C. At the end of the winter, the surface warms up and the CO₂ ice is transformed into CO₂ gas. This phase change is powerful enough to drive granular flows in Martian gullies, with flow dynamics similar to water-driven debris flows on Earth. However, it is unclear if and how CO₂-driven granular flows can erode the underlying soil and rock and thus explain the longer-term formation of the gully systems. In this study, we used experiments in a large environmental chamber to study if and how granular flows driven by CO₂ sublimation can erode material on Mars. The results of these experiments show that CO₂-driven granular flows are very efficient in eroding loose substrates. Erosion of material makes CO₂-driven granular flows even more mobile. Based on calculations we also estimate that CO₂-driven granular flows can erode rocky material in Martian gullies.

## 1. Introduction

Gullies are among the most active and recent landforms on planet Mars today (Diniega et al., 2021). They occur on steep slopes, mostly on crater walls, in the mid-latitudes on both hemispheres (Balme et al., 2006; Bridges & Lackner, 2006; Conway, de Haas, & Harrison, 2019, Conway, Harrison, et al., 2019; Harrison et al., 2015; Heldmann & Mellon, 2004; Kneissl et al., 2010; Malin & Edgett, 2000; Noblet et al., 2024) and are formed during the end of the Late Amazonian period (5 million years ago till present-day) (Conway et al., 2018; Malin & Edgett, 2000; Reiss et al., 2004; Schon et al., 2009). On their upslope ends, gullies have an erosional alcove with a dendritic channel pattern (e.g., Conway et al., 2011; Costard et al., 2002; Malin & Edgett, 2000). These dendritic channels converge and eventually fan out onto a debris apron. The resemblance of Martian gullies to landforms carved by water-driven debris flows on Earth has long led to the belief that liquid water was involved in their formation (e.g., Christensen, 2003; Conway et al., 2011; Costard et al., 2002; de Haas, Hauber, et al., 2015; Dickson et al., 2023; Dickson & Head, 2009; Khuller & Christensen, 2021; Knauth & Burt, 2002; Lanza et al., 2010; Levy et al., 2010; Malin & Edgett, 2000; Williams et al., 2008). This hypothesis fed into the scientific

<footnote n="1">¹Department of Physical Geography, Faculty of Geosciences, Utrecht University, Utrecht, The Netherlands, ²Nantes Université, University Angers, Le Mans Université, CNRS, Laboratoire de Planétologie et Géosciences, Nantes, France,</footnote>

<footnote n="3">³Mars Simulation Laboratory, Aarhus University, Aarhus, Denmark</footnote>
**Project administration:**

Lonneke Roelofs, Tjalling de Haas

**Resources:** Jonathan P. Merrison, Jens

Jacob Iversen

**Supervision:** Susan J. Conway,

Tjalling de Haas

**Visualization:** Lonneke Roelofs

**Writing – original draft:**

Lonneke Roelofs

**Writing – review & editing:** Susan

J. Conway, Jonathan P. Merrison,
Tjalling de Haas

debate about water on Mars, the habitability potential of the planet throughout its geologic history (Cottin et al., 2017; Hoffman, 2002) and thus increased the scientific interest into these landforms.

The discovery of wide-spread present-day activity in Martian gullies over the last decade (Diniega et al., 2010; Dundas et al., 2010, 2015, 2019; Pasquon et al., 2023; Raack et al., 2020; Sinha & Ray, 2023), combined with the lack of stable water on the Martian surface today has slowly shifted the hypothesis on the formation of Martian gullies. The current leading hypothesis, supported by a suite of remote sensing, modeling and experimental studies, states that present-day activity in Martian gullies is caused by a fluidized granular-flow process driven by the sublimation of seasonal CO2 ice at the end of local winter and beginning of local spring (e.g., de Haas, Hauber, et al., 2019; Diniega et al., 2010; C. Dundas et al., 2010, 2012, 2015, 2022; C. M. Dundas et al., 2025; Khuller et al., 2021; Pasquon et al., 2023; Pilorget & Forget, 2016; Raack et al., 2015, 2020; Roelofs, Conway, de Haas, et al., 2024, Roelofs, Conway, van Dam, et al., 2024; Sinha & Ray, 2023).

The observed activity in Martian gullies is widespread and extensive. The channel beds are actively eroded (de Haas, Hauber, et al., 2019; Dundas et al., 2025), new lobate deposits form on debris aprons, meter-scale boulders are transported through the channels and new channels form on debris aprons (Diniega et al., 2010; Dundas et al., 2010, 2015; de Haas, Hauber, et al., 2019; Sinha & Ray, 2023). Recent activity seems to occur in two environmental end-member configurations; a granular flow initiates in a defrosted alcove and flows on top of a frosted debris apron surface (Figure 1c), and in other places, granular flows originate at a location in the alcove that contains CO2 frost while flowing through a mostly defrosted gully (Figure 1b) (Pasquon et al., 2023; Roelofs, Conway, de Haas, et al., 2024). Activity does not occur when both the alcove and the debris apron are fully frosted. These environmental configurations have been observed in various gullies on Mars during local winter to early spring (Figures 1b and 1c). However, observations of frost in gully systems on Mars also illustrate that although these end-members are useful generalizations for modeling purposes, many different configurations between the two end-members exist and that various configurations of frosted and defrosted slopes can exist in a single gully (Figures 1a and 1d). It is even suggested that some gully activity can be explained by granular flows under CO2-slab ice, that is solid CO2 ice (Pilorget & Forget, 2016). Independent of the environmental configuration there are two prerequisites for gully activity; slopes hosting CO2 frost, and slopes with temperatures above the CO2-frost point which form a source of thermal energy (Roelofs, Conway, de Haas, et al., 2024). These prerequisites help explain the trend in gully orientation shifting from pole-facing to equator-facing when going from the lower to the higher latitudes (Conway, de Haas, & Harrison, 2019; Noblet et al., 2024) and can help explain the lack of gully activity and absence of gully landforms on some steep slopes in the mid-latitudes with CO2 frost (a question proposed by Khuller et al., 2021).

To enhance our mechanistic understanding of CO2-driven granular flow processes, experimental and modeling studies have shown that as a result of the low atmospheric pressure on Mars, the sublimation of small amounts of entrained CO2 ice in a granular mixture can create a gas flux powerful enough to both destabilize and fluidize large volumes of unconsolidated granular material (Cedillo-Flores et al., 2011; de Haas, Hauber, et al., 2019; Pilorget & Forget, 2016; Roelofs, Conway, de Haas, et al., 2024, Roelofs, Conway, van Dam, et al., 2024; Sylvest et al., 2016, 2019). The large gas flux decreases intergranular friction in the material (Roelofs, Conway, de Haas, et al., 2024; Sylvest et al., 2016, 2019), and creates a fluidized granular flow with flow dynamics comparable to terrestrial water-driven debris flows and gas-driven pyroclastic density currents (Roelofs, Conway, van Dam, et al., 2024).

Although the CO2-sublimation hypothesis for present-day flow activity in gully systems is well supported by recent scientific evidence, the long-term formation process of gully landscapes is debated. There are two end-member hypotheses for the long-term formation of gully systems on Mars. The first one proposes that although gully systems might be altered by CO2-driven processes today, the long-term formation of these landscapes is explained by water-driven granular flow processes (Costard et al., 2002; de Haas, Hauber, et al., 2015; Dickson et al., 2023; Dickson & Head, 2009; Khuller & Christensen, 2021; Malin & Edgett, 2000). This hypothesis is supported by climate modeling, showing that small quantities of liquid water could have existed in these gully systems at higher obliquity of the planet (Dickson et al., 2023), and the correlation between gully systems and water-ice rich deposits (Christensen, 2003; Conway et al., 2018; Dickson & Head, 2009; Noblet et al., 2024). However, it is unclear how much water could have been produced during these higher obliquity periods at the localities of the gullies, and if the produced volumes of water would have been large enough to drive the evolution of the gullies with water-driven debris flows.
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Images of Martian gullies with frost in different parts of the gully. We assume the frost is mostly comprised of CO₂ ice, but we cannot rule out the presence of H₂O ice. Panel (a) shows three gullies on a crater wall with frost present in the alcove and on the debris fan (Synthetic RGB CaSSIS image using the PAN and BLU channels, where defrosted surfaces appear red and frosted surfaces white, MY37_023207_118_0, Ls 19.7°, Lat. 58.409°, Long. 92.062°). Panel (b) shows gullies on a crater wall, in a crater northeast of Newton crater, with frost present in the alcove (white) and frost-free debris aprons (RGB HiRISE image, ESP_036719_1415, Ls 136.9°, Lat. -38.211°, Long. 206.825°). Panel (c) shows a crater wall gully, in a crater south of Porter crater, with frost (light purple) present on the debris apron and a mostly defrosted alcove. The alcove of this gully is eroded into Latitude Dependent Mantle (Color stretched RGB HiRISE image, ESP_011400_1275, Ls 183.4°, Lat -52.201°, Long. 246.878°). Panel (d), shows a zoom-in of the alcove in panel (c), highlighting that although the alcove is mostly frost-free, there are small patches of frost in the alcove.</figcaption></figure>

The second hypothesis on the long-term formation of gully systems on Mars postulates that the presently observed CO₂-driven granular process altering the gully systems has also formed the gully system on a longer time scale (Pilorget & Forget, 2016; Roelofs, Conway, van Dam, et al., 2024). From observations, it has become clear that significant volumes of material can be moved by CO₂-driven processes (de Haas, Hauber, et al., 2019; Roelofs, Conway, van Dam, et al., 2024; Sinha & Ray, 2023). Extrapolated over longer time scales, and multiple climate cycles, it is hypothesized that it is likely that CO₂-driven mechanisms have played a dominant role in the formation of the gullies (Dundas et al., 2017, 2025; Pilorget & Forget, 2016; Roelofs, Conway, de Haas, et al., 2024, Roelofs, Conway, van Dam, et al., 2024). This hypothesis is supported by the similarity in flow dynamics between the Martian CO₂-driven granular flows and terrestrial water-driven debris flows (Roelofs, Conway, van Dam, et al., 2024), which form gully like landscapes on Earth. In addition, the topographic signature of present-day activity is identical to that of the topographic signature of the entire gully (Dundas et al., 2025), implying that present-day processes are similar to those in the past.
However, to this day it remains elusive how effective CO₂-driven granular processes are in eroding unconsolidated sediments and consolidated bedrock, what typical erosion rates are for specific flow volumes and if these match with gully denudation rates. Note that in our manuscript we use the term *weathering* for the in situ breakdown or alteration of rocks and minerals, *erosion* to describe the process in which material is removed, and *denudation* for the long term lowering of a surface due to erosional processes. Furthermore, it remains unclear under which substrate and flow conditions (e.g., the presence and abundance of CO₂-ice and thermal energy and the availability of unconsolidated material vs. bedrock) the most mobile and erosive CO₂-driven granular flows can exist and how these conditions influence the flow dynamics. These unanswered questions inhibit assessing the possibility that CO₂-driven granular processes dominate gully evolution. Therefore, resolving questions on the gully forming potential of CO₂-driven granular flows is necessary for solving the Martian gully enigma.

We aim to determine the likelihood that CO₂-driven granular flow processes have played an important, if not dominant role, in the formation of the gully systems on Mars during the Late-Amazonian. We do this by conducting flume experiments under Martian atmospheric conditions similar to those conducted by Roelofs, Conway, de Haas, et al. (2024), Roelofs, Conway, van Dam, et al. (2024). These previous experimental studies have shown that, under Martian atmospheric conditions, the sublimation of small quantities of CO₂ ice (only 0.5% of the volume fraction of the flow), can fluidize granular material on gentle slopes (with slope angles as low as 10°) and create deposit morphologies, with levees and lobes, similar to those observed on Mars (Roelofs, Conway, de Haas, et al., 2024, Roelofs, Conway, van Dam, et al., 2024). Furthermore, these experiments have shown that CO₂-driven granular flows are dynamically similar to terrestrial two-phase granular flows, which explains the similarity in deposit morphology.

The novelty of the experiments presented here, is the addition of an erodible bed in the bottom of the flume, which allows us to not only study flow dynamics of CO₂-driven granular flows, but also study their erosion capacity under different thermal (thermal conductivity and temperature of the environment), topographic (slope steepness) and substrate (presence of frost, ice and unconsolidated regolith) settings. Conducting these experiments will help us understand how CO₂-driven granular flow dynamics and erosion in Martian gullies occur and how these gullies could have evolved under climatic changes during the Late-Amazonian period.

## 2. Materials and Methods

### 2.1. Set-Up of the Flume and Environmental Chamber

The experiments conducted for this study were performed under Martian atmospheric conditions in a granular-flow flume installed in the Mars Simulation Wind Tunnel at Aarhus University. The Mars Simulation Wind Tunnel is a cylindrical low-pressure environmental chamber, with a total length of 8 m and an inner diameter of 2.15 m (Holstein-Rathlou et al., 2014) (Figure 2a). The chamber has electrical and mechanical feed-throughs that enable the operation of the experimental set-up from the outside. Furthermore, the chamber has multiple portholes through which the experiments can be observed. The flume and chamber set-up we used for this work is similar to the one presented in Roelofs, Conway, van Dam, et al. (2024) with some minor modifications (Figure 2) that are discussed below. During the experiments, granular mixtures of different compositions were released into the flume while various sensors and cameras captured the flow dynamics.

The flume, constructed out of Lexane, a transparent polycarbonate resin thermoplastic, consisted of a steep and narrow chute, 197 cm in length and 12 cm in width, ending on a wider outflow plain, 210 cm in length and 140 cm in width (Figures 2b and 2c). The slope angle of the chute was adjustable and could be set between 15° and 30°, representing the range in slopes angles in Martian gullies where erosion has been observed (Dundas et al., 2025). On top of the chute an aluminum reservoir, 0.021 m³ in volume, was installed that stored the granular mixture before releasing it into the chute at the start of the experiment via a trap door mechanism. When the trap door opened at the beginning of the experiment, the granular mixture exited the reservoir through an opening of 5 cm in height and 12 cm in width. The bottom of the chute was made interchangeable so that different types of substrates could be inserted and possibly eroded by the flow. The standard substrate consisted of an aluminum plate that was sand blasted to create bed roughness and was kept at a constant 20°C by electrical heating pads. The standard aluminum chute bottom could be lifted out of the chute to reveal a depression that could be filled with a different substrate. The depression is 160 cm long, 10 cm wide and 3 cm deep, and starts 7 cm in front of the reservoir. The outflow plain of the flume had a constant slope of 10°, approximately representing the average slope angle of
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Photos and schematic drawing of chamber (a) and flumes (b–c). The photo in panel (a) depicts the Mars Simulation Wind tunnel at Aarhus University (Denmark). The photo in panel (b) shows the flume set-up with a granular flow deposit on the outflow plain, on the sides of the outflow plain the markers for the digital elevation model creation are visible. Details of the set-up can be found in panel (c). All dimensions are given in cm.</figcaption></figure>

gully fan termini on Mars (Dundas et al., 2025), and was covered with an antislip material (3M Safety-Walk 500 series, equal to 80 grit sandpaper) to create bed roughness.

Along the flume chute, four laser distance sensors (Baumer OADM 20U2480/ S14C) were installed to measure flow depth during the experiment (Figures 2b and 2c). We define flow depth as the distance between the top of the granular flow and the bottom of the initial (uneroded) surface. Based on the flow depth we determined the arrival of the flow front at different places along the chute, which enabled us to calculate frontal flow velocity. In addition to the laser distance sensors, a measurement section was installed at the bottom of the downstream end of the chute. In this measurement section, basal normal force and relative basal pore pressures were recorded with a load cell (HBM PW6D —3 kg) and two differential pressure sensors (Honeywell TruStability HSCDRRD006MGAA5), respectively. The load cell was attached to a force plate of 5 × 5 cm. The two differential pore-pressure sensors were connected to the bottom of the chute via thin tubes and measured the gas pressure at the bottom of the flow relative to the ambient pressure in the chamber.

After the experiments, we used photogrammetry to capture the morphology of the flow deposits, and if applicable, the erosion of material in the flume. For the photogrammetry, we used Agisoft Metashape software to reconstruct digital elevation models (DEMs). For this purpose, multiple markers were installed on the flume chute and outflow plane which can be automatically recognized by Agisoft Metashape (see Figure 2b).

The DEMs were used to study deposit morphology, including the presence of levees and lobes. The DEMs were also used to calculate the maximum outflow length and determine the location of the center of mass of the deposit, both a proxy for flow mobility. The location of the center of mass of the deposit was also used to calculate flow efficiency $1/R$, as defined by Iverson (1997):

$$ \frac{1}{R} = \frac{L_{tot}}{H_{drop}} \qquad (1) $$

with R as a dimensionless net resistance coefficient, $L_{tot}$ the horizontal distance from the source (sediment reservoir) to the deposit (center of mass of the outflow), and $H_{drop}$ the vertical elevation above the center of mass of the deposit. This flow efficiency is derived from equating the total potential energy lost during the motion of the granular material to the total energy degraded to irrecoverable forms by resisting forces. Experimental water-driven terrestrial debris flows in the USGS debris-flow flume yield $L_{tot}/H_{drop} > 2$ for channelized runout (Iverson, 1997).

All experiments were filmed from inside the chamber with four webcam(s) (Logitech Brio). Two webcam(s) filmed the granular flows from the side of the translucent chute, and one from the top of the chute. The fourth webcam filmed the experiment from the top of the chamber and recorded the outflow of the granular material onto the outflow plane. From outside the chamber, a high-speed camera, Edgertronic sc1 monochrome, was used to record detailed flow dynamics at the transition from the chute to the outflow plane by filming at a frame rate of 600 Hz.

## 2.2. Experimental Scenarios

Two sets of experiments were conducted. The first set focused on determining the erosion capacity of CO₂-driven granular flows. For this, we used an idealized case in which a granular flow of sand and CO₂-ice grains flows over an erodible sand bed. The second set of experiments focused on the flow dynamics of different types of CO₂-
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Schematic drawing of the five (a–e) bed and flow conditions tested in this manuscript.</figcaption></figure>

driven granular flows in various substrate and flow settings. In this latter set of experiments, we varied the type and composition of the substrate and the composition of the flow in different combinations that represent possible flow scenarios on Mars (Figure 1 and Roelofs, Conway, de Haas, et al. (2024)).

For the first set of experiments, granular flows comprised of sand and CO₂-ice grains were released into the flume chute in which an erodible sand bed (3 cm thick) was inserted. The sand in the bed was the same as the sand in the granular flow and was approximately at room temperature, ~20°C. This temperature represents the maximum modeled temperature of warm slopes facets in gullies that also host CO₂ ice on Mars (Roelofs, Conway, de Haas, et al., 2024), and ensures surplus of energy available for CO₂ sublimation and thus fluidization of granular material in our flume. This surplus of thermal energy is needed to ensure we actually simulate a stable fluidized granular flow as our experimental flows experience higher flow resisting forces, that is friction and intergranular impacts, than a larger volume granular flow would in a Martian gully. This stems from both scale effects that have been broadly discussed in terrestrial debris flow literature (Iverson, 1997; Iverson & Denlinger, 2001; Iverson et al., 2010) and, in our specific case, the higher gravitational force on Earth compared to that on Mars.

In this first set of experiments, we kept the amount of sand in the granular flow constant at 8 kg and varied the amount of CO₂-ice at the start of the experiment from 0.31 to 1.12 kg. Furthermore, we varied the slope angle of the chute from 15° to 30°. The flow dynamics and outflow morphology of these granular flows were compared to those of experiments with the same granular flow composition and chute slope, but with a non-erodible aluminum bottom plate as the chute bottom. These reference experiments are published in Roelofs, Conway, van Dam, et al. (2024). For a detailed list of experimental settings see Table S1 in Supporting Information S1.

In the second set of experiments, we tested how different types of granular flows move over various substrates in conditions that can be found in Martian gullies (Figure 1). These experiments were performed at a chute angle of 30°. The following five settings were tested (see also the schematics in Figure 3):

1. A granular flow of sand and CO₂-ice grains flowing over a non-erodible bed made of an aluminum plate (Figure 3a). This represents the movement of a CO₂-driven flow in a bedrock-exposed part of the gully.

2. A granular flow of sand and CO₂-ice grains flowing over an erodible bed made of room temperature sand (Figure 3b). This represents the movement of a CO₂-driven flow in a part of the gully where an unconsolidated defrosted substrate is present, that is in the convergent channels or on the apron.

3. A granular flow of sand and CO₂-ice grains flowing over an erodible bed made of sand and CO₂-ice grains in the same ratio as the granular flow (Figure 3c). This represents the movement of a CO₂-driven flow in a part of the gully where an unconsolidated frosted substrate is present, that is in the convergent channels or on the apron.

4. A granular flow of room temperature sand flowing over an erodible bed made of sand and CO₂-ice grains (Figure 3d). This represents the movement of a dry, frost-free, mass movement in a part of the gully where an unconsolidated frosted substrate is present, that is in the convergent channels or on the apron.
5. A granular flow of room temperature sand flowing over a bed made of slabs of solid CO₂ ice (Figure 3e). This represents the movement of a dry, frost-free, mass movement in a part of the gully where slab ice is present.

The experiment testing setting 1 is one of the reference experiments without erosion discussed above and presented in Roelofs, Conway, van Dam, et al. (2024). The experiment that tests setting 2 is one of the standard erosion experiments discussed earlier. For details on the exact composition of the granular mixtures and the experimental settings see Table S1 in Supporting Information S1.

Due to time constraints related to lab access, only the experiments with an erodible bed made of sand and a granular flow of sand and CO₂-ice grains conducted at a flume slope of 30° were repeated. However, the earlier executed repeats of experiments in the same set-up (see Roelofs, Conway, van Dam, et al. (2024)), showed that variation in the experimental results, under similar experimental conditions, is small. We are, therefore, confident that the variation in the results between the experiments with different experimental settings is caused by inherent differences and not by natural variability.

## 2.3. Details on Granular Material, CO₂ Ice, and Experimental Routine

The sand for the experiments was a mixture of fine-grained and coarse-grained sand with a broad grain-size distribution ($D_{50}$ of 310 µm, see Roelofs, Conway, van Dam, et al. (2024) and Figure S2 in Supporting Information S1). The broad grain-size distribution minimizes gas permeability relative to a mono-disperse sand and thus slows down the gas escape rate of the CO₂. To ensure the sand did not contain any water that would affect the experimental results, the sand was dried in the oven and cleared of any excess moisture in the Mars Simulation Wind Tunnel by putting it in a vacuum before the experiments.

The CO₂ ice used for making the granular flows and the erodible bed was ordered in pellet form and crushed to the size of the coarsest sand grains with a KitchenAid 5KGM grain mill. After crushing, the CO₂-ice grains were be mixed with the sand. During the mixing process, part of the CO₂ ice sublimated when coming in contact with the warmer sand. The combined weight of the mixture was therefore monitored during the mixing process and the loss of CO₂ ice was compensated by adding more ice. Once the desired weight of the combination of sediment and CO₂ ice was reached, the mixture was poured into the reservoir on top of the flume chute. Hereafter, the chamber was closed and depressurized until the atmospheric pressure in the chamber reached 8 mbar after around 20–25 min. As soon as the chamber reached this pressure, the granular mixture was released into the flume, while the sensor data was logged and the video cameras recorded the passing of the granular flow. During the depressurization about half of the CO₂ ice would sublimate. The amount of CO₂ ice left at the start of the experiment, that is the initial amount of CO₂ at the start of an experiment, was experimentally quantified by Roelofs, Conway, van Dam, et al. (2024) and is used in this manuscript when referring to the initial amount of CO₂.

For the experiments with the slabs of solid CO₂ ice in the bottom of the chute, slabs of CO₂-ice were cut into pieces of 10 × 20 cm with a bandsaw. These pieces were placed behind each other in the bottom of the chute to fill the entire depression in the chute. All CO₂ ice used in the experiments was ordered from a commercial supplier close to the lab. To limit the contamination of the CO₂ ice with water, the CO₂ ice pellets and slabs were stored in closed polystyrene-foam containers in a sealed freezer until usage.

# 3. Results

The results of the two sets of experiments are discussed in separate sections. The first three paragraphs of the results focus on the experiments performed to determine the erosion capacity of CO₂-driven flows. In these paragraphs, the results of the experiments in which CO₂-driven granular flows flow over an erodible sand bed are presented and compared to the results of experiments in which CO₂-driven granular flows flow over a non-erodible sand bed (presented before in Roelofs, Conway, van Dam, et al. (2024)). The last three paragraphs discuss the flow dynamics and mobility of CO₂-driven flows in different environmental contexts.

## 3.1. General Flow Behavior of Erosive and Non-Erosive Flows

Fluidization of the granular material, sand and CO₂ ice, occurred in both the experiments with and without an erodible sand bed. The granular flows came down the chute as one big surge, with a constant flow depth during 0.5–1.5 s. Occasionally, the main surge was followed by a few smaller surges (Figure 4). The flow depth of the CO₂-driven granular flows was ~1.5 times larger when an erodible sand bed was present in the chute (Figures 4
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Flow depth for two experiments measured at two locations in the flume chute. The blue lines depict data from an experiment without an erodible bed (also published in Roelofs, Conway, van Dam, et al. (2024)), whereas the red lines depict data from an experiment with an erodible sand bed. Sensor 1 is located 47 cm downstream of the reservoir, sensor 3 is located 157 cm downstream of the reservoir. Both experiments were conducted with a flume slope angle of 30° and an initial amount of 1.12 kg of CO₂ ice and 8 kg of sand in the granular flow.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 5.** Flow characteristics of the granular flows in experiments with an erodible sand bed against flume slopes for different amounts of CO₂ in the granular flow. The results of two experiments without an erodible bed in the chute are also plotted as a reference. Panel (a) shows the maximum flow depth at 157 cm from the reservoir, panel (b) shows the frontal velocity of the granular flows, panel (c) depicts the density of the flow calculated from the load cell (weight) data and the flow depth, and panel (d) displays the maximum differential basal pressure observed by the two pore-pressure sensors.</figcaption></figure>
and 5a). For a chute slope of 30°, the flow depth over a non-erodible aluminum bed at the first laser distance sensor reached 19 mm, whereas the flow depth over an erodible sand bed reached 25 mm at the same location (Figure 4). The flow velocity of the granular flows over an erodible sand bed was similar to those of granular flows over a non-erodible bed, with the same arrival time of the flow front at the different laser distance scanners (Figures 4 and 5b).

Flow depth was generally larger for the flows over erodible beds with larger amounts of CO₂ ice in the granular flow mixture (Figure 5a). Furthermore, for the granular flows with the largest amount of CO₂ (1.12 kg), a smaller chute slope angle resulted in slightly higher flow depths, with the largest flow depths recorded at a chute slope of 20° (Figure 5a).

The frontal flow velocity of the granular flows over erodible sand beds increased with increasing slope angle of the chute (Figure 5b). The frontal flow velocity was similar for the granular flows with an initial amount of 1.12 and 0.59 kg of CO₂ in the granular mixture but was much lower for the granular mixture with an initial amount of CO₂ of 0.31 kg.

The CO₂-driven granular flows over an erodible sand bed had a density that was two times smaller than the density of CO₂-driven granular flows over a non-erodible bed with the same amount of CO₂ in the granular mixture (Figure 5c). The densities of the flows, with an initial amount of 1.12 kg of CO₂, flowing over an erodible sand bed were around 500 kg/m³, whereas the same flows had a density of 1,100 kg/m³ when flowing over a non-erodible bed. The density of the granular flows flowing over an erodible bed increased by a factor >2 when the initial CO₂ content decreased from 0.59 to 0.31 kg (Figure 5c).

For all flows, the basal pore pressures were between 0.2 and 1.3 mbar higher than the atmospheric pressure of ~8 mbar in the chamber (Figure 5d). In general, differential basal pore pressures were higher on lower slope angles. The only exception for this are the granular flows with the lowest amount of initial CO₂ (0.31 kg). Furthermore, the differential basal pore pressures were higher when the bed of the chute was non-erodible compared to when the bed of the chute was made of sand (see flows with an initial 1.12 kg of CO₂ in Figure 5d).

## 3.2. Erosion of Unconsolidated Substrate by CO₂-Driven Granular Flows

The CO₂-driven granular flows flowing over an erodible sand bed erode significant amounts of material when enough CO₂ ice is present in the mixture (Figure 6a–6e). When the amount of CO₂ in the mixture becomes too small, at 0.31 kg, and the slope angle of the chute <20° no net erosion of material in the chute occurs. For the other granular flows, with higher amounts of CO₂ in the mixture, a higher slope angle of the chute linearly relates to more erosion (Figure 6e). At a chute angle of 30°, the amount of eroded sand was equal to the initial amount of sand in the granular flow, assuming a dry bulk density of 1,600 kg/m³ for the sand. When the chute angle was decreased to 15°, the amount of material being eroded dropped to 10% of the initial amount of sand in the granular flow. The erosion of the erodible bed in the chute was relatively spatially homogenous, that is no extreme upstream scour was observed (Figures 6a and 6b).

## 3.3. Outflow Morphology and Mobility of Erosive CO₂-Driven Granular Flows

Compared to the CO₂-driven granular flows flowing over a non-erodible bed, the flows flowing over an erodible sand bed had longer runout length (Figures 7h, 7i, 7k, and 7l and Figure 8a). For the granular flows flowing over an erodible sand bed, an increase in chute slope angle or an increase in CO₂ content resulted in an increase in the runout length of the deposit onto the outflow plain (Figure 7) and resulted in the center of mass, a proxy for flow mobility to be located further onto the outflow plain (Figure 8a). At the highest CO₂ content tested, 1.12 kg, the chute angle mattered less for the total outflow length and the location of the center of mass, than for the granular flows with lower CO₂ fractions (Figures 7 and 8a).

However, when we determine the flow efficiency of the different CO₂-driven granular flows, and thus correct the outflow length for the potential energy the flows had at the time of release, the story changes. In general, flows with higher fractions of CO₂ are more flow-efficient, and granular flows moving over an erodible bed are still more efficient than flows moving over a non-erodible bed (Figure 8b). However, these differences in flow efficiency are smaller than for the location of the center of mass on the outflow plane (Figures 8a and 8b). More importantly, when studying the flow efficiency it becomes clear that CO₂-driven granular flows released in chutes
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Hillshaded digital elevation models (DEMs) of the erodible bed in the chute of the flume after three experiments on different flume angles (a–c), width averaged change of these three experiments (d), and net change of the erodible bed in the chute against chute angle, for all experiments with an erodible sand bed (e). The three experiments depicted in panels (a–d) are performed at three different flume angles; 30°, 20° and 15°, but with the same amount of initial CO₂ in the granular flow (1.12 kg). In panel (a–c), elevations below zero imply erosion of material, and elevations above zero imply deposition of material. Note that the erodible bed is present between 7 and 167 cm. For experiments with an initial amount of CO₂ of 0.31 kg and a chute angle below 30°, deposition occurred in the entire chute and DEMs could not be reconstructed. Therefore, we only know that the net change, depicted in panel (e), should be above zero.</figcaption></figure>

with a smaller slope angle are more efficient, and thus lose less energy to friction, than those in chutes with a steeper slope (Figure 8b).

## 3.4. Flow Dynamics of Granular Flows in Different Environmental Contexts

The granular flows in the five different flow and substrate conditions tested have varying flow characteristics (Figures 9 and 10 and Table S2 in Supporting Information S1). When a granular mixture of CO₂ ice and sand flowed over a solid aluminum bed or a unconsolidated sand bed, the flows mostly come down in one big surge, with a relatively stable flow depth for at least a second (Figures 9a and 9b). When the granular mixture encountered a bed containing CO₂-ice grains, or solid CO₂ ice, the flow was characterized by multiple big or small surges or flow peaks (Figures 9c and 9d).

The highest flow depths were reached when sand and CO₂-ice grains were present in the bed, independent of the composition of the initial granular flow, followed by the flow containing sand and CO₂-ice grains flowing over a sand bed (Figures 9b and 9d). The highest flow velocities at the end of the chute were recorded for experiments with a granular flow containing sand and CO₂-ice grains flowing over a sandy bed, followed by a granular flow of sand and CO₂-ice grains flowing over a solid aluminum floor (Figure 10). The lowest flow velocities were recorded for a granular flow of sand and CO₂-ice flowing over a bed of sand and CO₂ ice.
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Digital elevation models (DEMs) for the outflow deposits of 12 experiments under Martian atmospheric pressures conducted under different slope angles of the chute (from 15° to 30°) and different amounts of CO₂ in the granular flow. Nine experiments are conducted with an erodible sand bed in the chute (a–i), 3 are conducted with a non-erodible aluminum bed in the chute (j–l). Note that the DEMs depicted in panel (k) and (l) have been published before in Roelofs, Conway, van Dam, et al. (2024).</figcaption></figure>

Under three specific bed and flow conditions, the flow velocity keeps increasing in the downslope direction; the flow of CO₂ ice and sand flowing over a solid bed, the flow of CO₂ ice and sand flowing over a sandy bed, and the flow of sand flowing over a bed of CO₂ ice and sand (Figure 10). In the other two conditions, with CO₂ ice present in both flow and bed and with solid CO₂-slab ice in the bed, the flow front slows down at the end of the chute.

Differential basal pore pressures were largest for the flow with sand and CO₂-ice grains in the flow mixture and the bed, followed by the basal pressures of the granular flow of sand and CO₂-ice grains flowing over a solid aluminum bed (Figure 9 and Table S2 in Supporting Information S1). Negative differential pore pressures were observed under the granular flow made of sand flowing over solid CO₂-slab ice. This is likely a measurement error instead of a valid observation as it is clear from the high flow velocities during this experiment (Figure 10) that the granular material is behaving as a fluid.

Flow densities vary under the five conditions tested but all stay below 1,500 kg/m³ (Figure 9), which is approximately the dry bulk density of the sand. For the flows with CO₂ ice in the initial granular flow mixture, the flow front, or main body of the flow, has a smaller density than the tail of the flow (Figures 9a–9c). When a
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Location of the center of mass on the outflow plain in the down-slope direction against the slope angle of the chute (a), and the drop height divided by the location of the center of mass along the flume against the slope angle of the chute for all experiments with an erodible sand bed in the chute (b). As a reference, the results of two experiments without an erodible bed in the chute are also plotted. The drop height is calculated as the vertical distance between the outflow point of the reservoir and the center of mass. The location of the center of mass is determined as the horizontal distance between the outflow point of the reservoir and the center of mass. The dotted lines between the individual data points are meant to help the reader observe the trends in the data for flows with the same CO₂ composition more easily.</figcaption></figure>

mixture of CO₂ ice and sand flow over a bed of CO₂ ice and sand, the flow comes down in multiple surges (Figures 9a–9c). The front of every surge is relatively low in density (around 1,000 kg/m³), whereas the tail of every surge is relatively high in density (around 1,500 kg/m³).

### 3.5. Flow States and Flow Velocity Profiles in Different Environmental Contexts

In addition to the flow dynamics recorded by the sensors in the flume, the high-speed footage reveals the existence of two flow states that occurred under different flow and bed conditions. The first flow state can be described as a dynamic homogeneous fluidized bed, referring to and building on static fluidized-bed theory (Kunii & Levenspiel, 1991). This state is observed in the chute and on the outflow plane in the experiments with a granular flow composed of sand and CO₂-ice grains and on the outflow plane when a granular flow of sand flows over a slab of CO₂-ice (Figures 11a and 11e). In this flow state, the granular flow seems to be close to a steady state, with the exception of small surface undulations and possibly small-scale turbulence that is difficult to observe from the high-speed footage.

The second flow state can be best described as a dynamic bubbling bed, or even a turbulent bed. This state is observed in all experiments in which an unconsolidated erodible bed is present in the chute of the flume (Figures 11b–11d). In this flow state, gas pockets, or bubbles, rise to the surface of the flowing granular mixture (Figures 11b–11d). The bubbling bed flow state presents itself slightly differently depending on the flow and/or bed conditions. The biggest bubbles, ~6 cm, were observed in the chute when a granular flow of sand flowed over a bed of sand and CO₂-ice grains (Figure 11d). However, after ~35 cm onto the outflow plane, the gas bubbles suddenly ceased to exist and the flow halted. Smaller gas pockets, ~4 cm, were observed in the chute when a granular flow of sand and CO₂-ice grains flowed over a bed of sand (Figure 11b). However, under these conditions, the gas pockets were focused in the center of the flow and continued to exist further onto the outflow plane. For the granular flows composed of sand and CO₂-ice grains flowing over a bed of sand and CO₂-ice grains bubbles were not observed in the flume chute, but were observed on the outflow plane, where the flow also split in two lobes because of the existence of two flow fronts (Figure 11c). In addition, very small gas bubbles, <1 cm, were observed in the flow chute when sand flowed over a solid CO₂-ice bed.

### 3.6. Mobility, Morphology and Erosion by Granular Flows in Different Environmental Contexts

The outflow deposits of the CO₂-driven granular flows flowing over a warm, either non-erodible or erodible beds, were similar in morphology despite differing in length (Figures 12a, 12b, 12f, and 12g). These outflow deposits
<figure><img src="image_11.png" /><figcaption>**Figure 9.** Flow depth, differential pore pressure (sensor 1), and flow density during the five experiments performed under different substrate and flow conditions. All experiments were conducted on a chute slope of 30°. Flow density is only calculated for the main surge and is missing for the experiment in panel (d) due to measurement errors. For the experiment in panel (e) differential pore pressure was negative during flow passing and only reached above zero at the end of the experiment.</figcaption></figure>

consisted of one big lobate deposit with a few smaller lobate deposits superposed. The granular flow consisting of sand and CO₂-ice grains, flowing over a bed of sand and CO₂-ice grains deposited itself into two distinct larger lobes, that originated simultaneously (Figures 12c and 12h). These two larger lobes were also built up from multiple smaller lobes.

The deposits of the flows that consisted of sand only, flowing over a sand and CO₂ bed, or a bed made of solid CO₂ ice, differ in morphology from the ones previously discussed. They are smaller in extent, both in length and width, and are not comprised of multiple individual smaller lobes (Figures 12d, 12i, 12e, and 12j). The deposit of
<figure><img src="image_12.png" /><figcaption>**Figure 10.** Frontal flow velocity at different locations along the flume for the experiments performed under different substrate and flow conditions. The velocity was calculated by the difference in arrival time between the laser distance sensors.</figcaption></figure>

the granular flow consisting of sand, flowing over solid CO₂ ice is most limited in size and has the shape of a rectangular prism instead of a lobe (Figures 12e and 12j).

Of the granular flows flowing over an erodible unconsolidated bed, the flow made of sand and CO₂-ice grains flowing over an erodible sand bed has the longest outflow and highest flow mobility (Table S2 in Supporting Information S1). Furthermore, this flow and bed configuration results in a doubling of the initial sand volume by erosion and entrainment of material. The flow made of sand and CO₂-ice grains flowing over an erodible sand and CO₂ bed is far less mobile than the former, but is still capable of eroding and entraining the same volume of material (see the final deposit volume in Table S2 in Supporting Information S1). The granular flow consisting of sand and CO₂-ice grains flowing over a warm sand bed is the least mobile and erosive of all flows interacting with an erodible unconsolidated bed, with only a 50% gain in volume due to erosion and entrainment of material in the chute.

# 4. Discussion

## 4.1. Flow Dynamics of (Erosive) CO₂-Driven Granular Flows in Different Environmental Contexts

Recently, the flow dynamics of CO₂-driven flows under Martian atmospheric conditions were experimentally determined for cases when these granular flows move over a non-erodible bed (Roelofs, Conway, de Haas, et al., 2024). These experiments showed that the sublimation of small quantities of CO₂ ice within the granular mixture, caused by direct contact with warmer surfaces, creates a gas flux large enough to decrease intergranular friction and fluidize the material, even on small slope angles. Furthermore, the experiments showed that Martian CO₂-driven flows are dynamically similar to terrestrial water-driven debris flows and pyroclastic density currents (Roelofs, Conway, de Haas, et al., 2024).

However, the previous experimental conditions simplified the complex interplay of topography, substrate, thermal and frost conditions in Martian gullies. On Mars, CO₂-driven granular flows initiate high up in gully alcoves, that are eroded into bedrock or Latitude Dependent Mantle (LDM), made of cemented dust and ice (de Haas, Conway, et al., 2019; Sinha & Ray, 2023). In the tributary channels of the alcoves, loose sediment is present that has either been transported there by CO₂-driven granular flows or has fallen into the channels by other geomorphic processes, like rockfall (Tesson et al., 2020) and small-scale landslides (Pasquon et al., 2019). Further down-slope in the gully, more loose sediment is present as it is transported and deposited by granular flow processes downslope (Sinha & Ray, 2023). Eventually, the material ends up on the debris apron, composed of multiple lobate deposits and channels (Dundas et al., 2019; Sinha et al., 2020). At the end of local winter and the beginning of spring, slope facets in the gully system with different aspects will either be covered by CO₂ frost and
<figure><img src="image_13.png" /><figcaption>**Figure 11.** Stills of high-speed footage of the five experiments conducted under different substrate and flow conditions (a–e). The high-speed camera filmed the granular flows on the transition from the flume chute to the outflow plain. On the stills, the presence of gas bubbles and surface ripples in the flows are annotated in white. A schematic representation of the flow velocity profile is given in blue. The white grid on the outflow plane can be used for scale, every square is 5 × 5 cm. See Movies S1–S5, https://geo public.data.uu.nl/vault-erosion-co2-driven-flows-on-mars/research-erosion-co2-driven-flows-on-mars%5B1737457221%5D/ original/SupVideos/ for the full length high-speed footage of the five experiments.</figcaption></figure>

ice or will be heated up by direct sunlight (Pasquon et al., 2023; Roelofs, Conway, de Haas, et al., 2024) depending on the time of day and the exact time during the season. Therefore, understanding the flow dynamics and erodibility of CO₂-driven granular flows in different environmental contexts is crucial in understanding the role these flows potentially play in landscape evolution in Martian gullies.

Our experiments show that granular material can be fluidized under Martian atmospheric conditions in various flow and substrate settings. Granular material can be fluidized by the sublimation of CO₂ ice when the flow is composed of sand and CO₂-ice grains and the bed is made of non-erodable material or sand at room temperature (e.g., Figure 7, Figures 11a and 11b). Fluidization of granular material also occurs when room-temperature sand flows over an erodable bed of sand and CO₂-ice grains or a bed made of solid CO₂ ice (Figures 11d–11e). As long as CO₂-ice is present in the bed or in the granular material and there is a source of thermal energy, fluidization will occur. Even in conditions when both the granular flow and the erodable bed contain CO₂ ice, fluidization by sublimation can occur due to thermal energy coming from heated-up surfaces on the sidewalls of the gully system,
<figure><img src="image_14.png" /><figcaption>**Figure 12.** Hillshaded digital elevation models (DEMs) of the outflow deposits of 5 experiments (f–j) conducted under different substrate and flow conditions (schematized in a and b). All experiments were conducted on a chute slope of 30°. The center of mass of the deposits is marked by an x on top of the DEMs.</figcaption></figure>

with a minor contribution of frictional heating within the flow (as also suggested by Dundas et al. (2017); Roelofs, Conway, van Dam, et al. (2024)) (Figure 11c).

When CO₂-driven granular flows move over an erodible, unconsolidated, surface, the behavior of the flows slightly changes compared to flows moving over a non-erodible surface. Under these erosive conditions, the fluidization regime of the granular material changes from fluidized (Figure 11a), to bubbling (Figure 11e), or even to turbulent fluidization (Figures 11d and 11d) (see Kunii and Levenspiel (1991) for fluidized bed theory).

Transitioning from one fluidization regime to the next, while keeping the material constant, is caused by an increase in the upward velocity of the gas (Kunii & Levenspiel, 1991). In our experiments, the transition from normal fluidization to bubbling and turbulent fluidization is caused by a higher sublimation rate of the CO₂ ice in the granular mixture. When a granular flow made of CO₂ and sand erodes and entrains warm sand particles, it brings thermal energy into the flow. Direct contact between the warmer sand grains and the CO₂-ice grains throughout the flow column increases the conduction of heat, and thereby the amount of CO₂ sublimating and thus increases the CO₂ gas flux within the granular mixture. The same occurs for granular flows of sand flowing over and entraining granular material consisting of sand and CO₂ grains.

The experiment with a granular flow of sand and CO₂ ice, flowing over a bed of sand and CO₂ again highlights the importance of thermal energy for the fluidization of these types of flows. In the flume chute, sublimation of CO₂-ice grains mostly occurred along the warm chute walls resulting in the formation of the two flow fronts. When reaching the relatively warm outflow plain, the flow regime of the granular flow changed; the flow dispersed over a larger area, creating a larger surface area for the newly entrained CO₂-ice grains to come into contact with a warm surface, increasing the sublimation rate and thus shifting the fluidization regime to bubbling on the outflow plane (Figure 11c).

The experimental observation of a bubbling or turbulent fluidization regime in Martian CO₂-driven flows is interesting from a fluid dynamical point of view, as it has never been described before in this environmental context. In theory, the gas velocity at which the transition between fluidization regimes occurs is dependent on the density and diameter of the grains, the fraction of smaller particles in the flow, the permeability of the granular material, the dynamic viscosity of the gas, the inclination of the slope and gravity (Boonprasop et al., 2019; Kunii & Levenspiel, 1991; O'Dea et al., 1990; Wytrwat et al., 2020). Numerical models could solve the exact fluidization regime in specific environmental settings on Mars but this is part of future research. Validating hypotheses on specific fluidization regimes in gullies on Mars will, however, be difficult, as our experiments show that the direct morphologic effects of the fluidization regime are small. Fluidized, bubbling

and turbulent granular flows result in similar outflow deposits (Figures 7 and 12), and no distinct identifiable differences in small-scale morphological features were observed (Figures 7 and 12).

## 4.2. Erosion Capacity and Mobility of CO₂-Driven Granular Flows

Our experiments show that CO₂-driven granular flows can erode unconsolidated material under Martian atmospheric conditions and different substrate and flow settings. This explains the wide range of environmental settings in which fresh granular flow deposits are observed and confirms earlier hypotheses on the range of boundary conditions needed for these flows to occur (Pasquon et al., 2023; Roelofs, Conway, de Haas,
et al., 2024). Erosion occurs when CO₂-ice grains are present in the flow, when they are only present in the substrate, or when they are present in both. Furthermore, there is a positive feedback between the erosion of material and the degree of fluidization if enough CO₂ and thermal energy are present. This might help explain the lack of clear source points for granular flow activity in Martian gullies (Dundas et al., 2017); what starts as a small initial mass movement, can quickly grow into a large volume granular flow by erosion-induced enhanced fluidization. However, limits to the high erosion capacity of CO₂-driven flows exist. The amount of erosion caused by a flow depends on the abundance of CO₂ ice, thermal energy for sublimation and how well these two mix. In our experiments, erosion is most effective when CO₂-ice grains are present in the flow and the bed consists of relatively warm sand.

Depending on the slope angle, the volume growth of a granular flow composed of sand and CO₂-ice particles, with an initial mass of 10 kg, is between 10% and 100%, for respectively slope angles of 15° and 30°. This means that the erosion of loose, unconsolidated material by CO₂-driven flows in our lab set-up is more efficient than erosion by experimental terrestrial water-driven debris flows in a comparable set-up. For example, terrestrial water-driven debris flows of ~80 kg, simulated in a flume with an erodible bed of 2.5 m in length, and a flume slope angle of 34°, erode and entrain only 0%–15% of their initial volume, depending on flow and bed composition (Roelofs et al., 2022, 2023). The entrainment magnitude by our experimental Martian granular flows confirms the model conclusion by de Haas, Hauber, et al. (2019) that stated that entrainment of material was necessary to explain the final deposit volume of gullies in Hale crater. They showed that entrainment led to final flow volumes that were ~2.5–5.5 times larger than initially released.

The high effectivity of CO₂-driven granular flows to erode unconsolidated materials implies that unconsolidated material in active gully alcoves, produced by for example rockfall and landslides, can be easily entrained by these granular flows and transported downslope when CO₂ ice and thermal energy are present in abundance. This leaves the bedrock in the alcoves exposed, which increases thermal weathering, and thus increases the production of unconsolidated material that is then transported downslope by CO₂-driven granular flow processes.

Further down-slope in the gully system, in the convergent channels and on the debris apron, the CO₂-driven flows remain highly effective in eroding material because on low slope angles the flows increase in flow efficiency (Figure 8b). Under high flow efficiency less potential energy is lost to friction within the flow, which helps the flow maintain momentum and erosive power. Due to the high erosion capacity on low slopes, deep channels are cut in earlier deposited loose material in the convergent tributary alcove channels and on the debris aprons of the gullies, as observed by Diniega et al. (2010); Dundas et al. (2010); de Haas, Hauber, et al. (2019); Sinha and Ray (2023).

### 4.3. Favourability of Different Environmental Settings for CO₂-Driven Gully Evolution

Although CO₂-driven flows are highly efficient at eroding material there are limits of the boundary conditions under which these erosive flows occur. Our experiments show that erosion is most effective when CO₂-ice grains are present in the flow and the bed is made of relatively warm sand. This would mean that on Mars, CO₂-driven flows are most efficient in transporting sediments to the debris apron in gully systems where CO₂ ice is present in the alcove at the end of winter while the rest of the system is partly or fully defrosted (see Figure 13a). Another efficient environmental setting for transporting material is when there is plenty of warm and loose regolith in the alcove that falls into frosted channels and flows further downstream on a frosted fan (see Figure 13b). In both cases, enough thermal energy and CO₂ ice are available for efficient sediment transport over long distances. The amount of sediment that can be easily transported in these ideal cases is not only set by the thermal and frost conditions in the gully but also by the amount of loose unconsolidated regolith. The latter was identified by Sinha and Ray (2023), who observed frequent and extensive activity gullies rich in unconsolidated sediments.

In gullies where limited CO₂ ice is present in the alcove, or where most of the gully surface is frosted, transport of granular material is less efficient (see Figure 14). In these cases, the production of sufficient amounts of CO₂ gas is hindered by limited availability of either CO₂ ice or thermal energy. This limits fluidization of the flow, increasing granular friction, and limits erosion and runout (Figure 10). However, to fully understand under which thermal and substrate conditions full-scale CO₂-driven granular flows can efficiently occur and cause erosion in Martian gullies detailed numerical geophysical flow models need to be developed.
<figure><img src="image_15.png" /><figcaption>**Figure 13.** Illustrations of two environmental settings in gullies on Mars that produce efficient fluidized granular flows driven by CO₂ sublimation. These efficient flows will flow out far onto the debris apron and will be capable of eroding and entraining significant amounts of material. Panel (a) illustrates an environmental setting in which CO₂ frost is present in the alcove and the rest of the system is defrosted and heated up, providing the necessary thermal energy for sublimation. Panel (b) illustrates an environmental setting in which the material in the alcove is defrosted and heated up, providing the necessary thermal energy for sublimation, and the debris apron is covered by CO₂ frost.</figcaption></figure>

In gully systems where loose material is efficiently transported downslope by CO₂-driven processes, and the loose regolith and debris are regularly removed from the alcove tributaries, the pace of landscape evolution will be set by bedrock weathering and bedrock erosion processes. Weathering of the bedrock or the water-ice rich LDM in the gully alcove can happen due to thermal weathering (Viles et al., 2010) and frost weathering (by the actions of melting and refreezing of water ice) (Conway & Balme, 2014; Conway, de Haas, & Harrison, 2019; Eppes et al., 2015; Tesson et al., 2020), enhanced by mechanical weathering caused by geomorphic processes like rockfall (Bickel et al., 2020; Tesson et al., 2020) and sublimation induced slumping (Sylvest et al., 2016, 2019)

<figure><img src="image_16.png" /><figcaption>**Figure 14.** Illustrations of two environmental settings in gullies on Mars that do not produce efficient fluidized granular flows driven by CO₂ sublimation. Panel (a) illustrates an environmental setting in which very limited amounts of CO₂ frost are present in the alcove while the rest of the gully system is completely defrosted and heated up. Panel (b) illustrates an environmental setting in which the alcove and debris apron contains a lot of CO₂ frost and a very limited amount of hot surfaces that can provide the necessary thermal energy for sublimation.</figcaption></figure>
that decrease the strength of the bedrock (Beer et al., 2024) or LDM. Once the bedrock or LDM material is weak enough it could slump, slide or fall into one of the alcove tributaries to be picked up and transported downslope by CO₂-driven granular flows. Gullies with alcoves that produce larger amounts of finer-grained unconsolidated material will likely see more frequent activity and change at a quicker pace than those that produce less or coarser material (Sinha et al., 2018).

Although the discussion above, describing the efficiency of CO₂-driven gully evolution in different environmental settings is useful for interpreting general gully activity trends, in time and space with varying climatic conditions, it does not provide precise limits on the occurrence and efficiency of CO₂-driven granular flow erosion processes under the influence of Martian gravity in specific environmental conditions, related to thermal conditions, the amount of available CO₂ ice, different type of sediments and slope angles. To determine these precise limits a combination of more experiments and detailed numerical modeling is needed. However, this is outside of the scope of this research.

### 4.4. Implications for the Formation and Evolution of Martian Gullies by CO₂-Driven Processes

The general high erosion capacity of CO₂-driven flows makes it likely that they have played a dominant role in the formation and evolution of Martian gullies during periods in which the climate was, and is, favorable for CO₂-driven granular flows to occur. However, the difference between erosion efficiency in various frost, thermal and substrate settings implies that certain gullies might be evolving quicker than others at present-day.

Our experiments show that the gullies where an excess of CO₂ frost co-occurs with warm slopes during an extended period during the year are likely changing at a higher pace than gullies where either CO₂ frost, warm surfaces or the time they co-occur is limited. This would also imply that under the influence of changing obliquity, which drives climatic change and the cycling of volatiles on Mars (Forget et al., 2017; Laskar & Robutel, 1993), the window of opportunity for rapid gully evolution might shift from lower latitudes to higher latitudes and back. The close link between climate and the evolution and formation of gullies was also recognized by Noblet et al. (2025), who showed a link between global patterns in gully distribution and the local climates ability to emplace and remove volatiles from the surface to explain our results. If true, gullies would go through a cycle of rapid change (when sufficient CO₂ and thermal energy are present) to slow/no change (when there is only a limited amount of CO₂ or thermal energy available). This climate-driven activity cycle could help explain inactivity in the low-latitude gullies in the northern hemisphere (Dundas et al., 2022) and the variation in activity frequency and magnitude in currently active gullies.

### 4.5. Possibility for Bedrock Erosion by CO₂-Driven Granular Flows

From the sediment cascade in gully alcoves described in Section 4.3, with bedrock weathering and CO₂-driven granular flows transporting the loose debris, it becomes evident that CO₂-driven granular flows can be the dominant formative process behind Martian gullies without causing bedrock erosion (as earlier proposed by Dundas et al. (2022)). However, understanding the potential of CO₂-driven granular flows to cause bedrock erosion and erosion of the consolidated icy LDM material can help us understand differences in denudation rates in gullies on Mars. Note that in this discussion we see LDM as another type of “rock” as it is a consolidated material.

From terrestrial water-driven debris flows we know that the balance between rock strength on the one hand, and the two erosive forces, shear and impact, on the other hand, determines bedrock erosion rate (Hsu et al., 2008; Stock & Dietrich, 2003, 2006). On Mars, the low gravity combined with the low density of the fluidized CO₂-driven granular flows, between 350 and 1,600 kg/m², will make the shear forces working on the bed relatively small compared to water-driven debris flows on Earth, that have bulk densities around 2,000 kg/m² (Costa, 1984). The magnitude of the basal shear force is estimated by the shear stress under a steady uniform flow:

$$ \tau = \rho g H \sin S \qquad (2) $$

where ρ is the density of the flow (kg/m³), g is the gravitational acceleration (m/s²), H is the flow depth (m) and S is the channel-bed slope. Compared to a terrestrial water-driven debris flow of a certain depth on a specific slope
angle, the shear force at the bottom of a CO₂-driven granular flow on Mars of the same depth and at the same slope angle will be 3–15 times smaller (Table S3 in Supporting Information S1).

Collisional stress within a granular flow is given by (cf. Hsu et al. (2008); Stock and Dietrich (2006)):

$$ \sigma = v_s \rho_s D^2 \gamma^2 \quad (3) $$

where $v_s$ is the volumetric solids fraction (–), $\rho_s$ is the density of the solid particles (kg/m³) and D is the characteristic particle diameter (m), and $\gamma$ is the shear rate (the rate at which the fluid passes over an adjacent layer) given by:

$$ \gamma = \frac{u}{H} \quad (4) $$

in which u is the flow velocity. Based on our experimental results, showcasing the state of fluidization of the granular material by the sublimation CO₂ ice, we can assume that the $v_s$ of Martian CO₂-driven flows is similar to that of terrestrial water-driven debris flows. In addition, the flow velocities and flow depths of our experimental CO₂-driven debris flows are similar to experimental terrestrial debris flows in similar size set-ups (de Haas & Woerkom, 2016; Roelofs et al., 2022), resulting in similar shear rates. As the Martian surface mostly consists of basaltic rock, with a density of ~3,000 kg/m³, $\rho_s$ of the sediment in Martian CO₂-driven flows will not be dissimilar from that of terrestrial water-driven debris flows. Furthermore, sedimentological and surface studies of Martian gully fan deposits, show the presence of large boulders (>1 m in size) (de Haas, Ventra, et al., 2015; Dundas et al., 2022), which highlights the similarity in effective grain size of sediment in Martian granular flows and terrestrial water-driven debris flows. Therefore, in contrast to shear forces, the magnitude of impact forces, or collisional stresses, under CO₂-driven granular forces on Mars can be expected to be similar to those of terrestrial water-driven debris flows.

We further assume that the strength of the bedrock in Martian gullies, that is the erosion resistance, is in the same order of magnitude, to that of rock in debris-flow catchments on Earth. Depending on the rock type, the tensile strength of rock on Earth varies between 0.1 MPa for weathered sandstone, 1 MPa for weathered granite, 100 MPa for basalt, and 200 MPa for quartzite (Sklar & Dietrich, 2001). For the LDM material in the gullies it is safe to assume that the tensile strength is equal or lower than that of the underlying bedrock, as (impure) water ice has a tensile strength in the range of 0.1–2 MPa (Litwin et al., 2012). In addition, bedrock in debris-flow catchments on Earth is often highly fractured and as fracture density in the rock increases, the normal rock tensile strength decreases and general erosion rates increase (DiBiase et al., 2018; Molnar, 2004; Neely et al., 2019; Scott & Wohl, 2019). Likewise, the bedrock in Martian gullies is often highly fractured as 65% of gullies occur on crater walls (Harrison et al., 2015) that formed under extreme stresses during meteorite impacts. The highly fractured and low-strength nature of bedrock on crater rims is reflected in the enhanced back-weathering rates on fresh crater rims (de Haas, Conway, & Krautblatter, 2015).

As the balance between impact stresses and bedrock or LDM strength can directly be related to erosion rates (Hsu et al., 2008), we hypothesize that the impact forces underneath CO₂-driven granular flows on Mars will be large enough to cause erosion of LDM material and bedrock in the gully alcoves, despite the relatively small shear forces. We deem it likely that impact forces by large sediment particles will cause bedrock erosion in gullies on Mars regularly as boulder transport in gullies is not a rare event (Dundas et al., 2022); in 21% of all documented flows boulders are being transported. Impact forces will be higher, and thus erosion more effective, when the effective grain size of the material in the granular flow is larger. This implies that in gully alcoves where many boulders are produced, bedrock erosion rates will be higher.

## 5. Conclusions

We conducted flume experiments under Martian atmospheric conditions to determine the likelihood of CO₂-driven granular flows playing an important, if not dominant, role in the formation of gully systems on Mars. We investigated the flow dynamics of CO₂-driven granular flows under different substrate and flow settings and we investigated the erosion capacity of these flows.
Our experiments show that CO₂-driven granular flows under Martian atmospheric conditions are efficient erosive agents, that can erode and entrain large volumes of unconsolidated material, given the right thermal and CO₂ ice conditions are met. In our experimental set-up (atmospheric pressure of 8 mbar and thermal contrast of 140°C), the flows grew in volume by a factor of two over a short distance of 2 m due to erosion and entrainment of material. This is likely caused by the high pore pressures in CO₂-driven granular flows. We, therefore, conclude that CO₂-driven granular flows can be the dominant process behind the formation and evolution of gully systems on Mars. We attribute the high erosion capacity of CO₂-driven granular flows to the effectiveness at which the production of CO₂ gas from sublimation under Martian atmospheric pressure fluidizes granular material.

Furthermore, our results show that CO₂-driven granular flows are mobile and erosive under various substrate and flow conditions. As long as CO₂ ice and thermal energy are present in the flow or in the environment, the flows remain mobile and erosive. Overall, the entrainment of unconsolidated material has a positive effect on the mobility of CO₂-driven granular flows. This is the case for when the flow contains CO₂ ice, or if the erodible bed contains CO₂ ice. In both cases, the sublimation of CO₂ ice is enhanced by the mixing of thermal energy (in the form of warm sand grains) and CO₂-ice particles throughout the flow column. The resulting enhanced production of CO₂ gas shifts the fluidization regime of the granular material from fluidized, to bubbling fluidization or even turbulent fluidization. In addition, flows become more efficient, that is loose less energy to friction, on slopes with lower slope angles, due to prolonged sublimation.

The effectiveness of CO₂-driven flows to erode unconsolidated material in certain substrate and flow conditions implies that gullies evolve quicker when sufficient CO₂ frost and thermal energy co-exist in the gully. This would mean that under the influence of changing obliquity and thus changing thermal and frost conditions, gullies go through a cycle of rapid change–slow change–limited change. Rapid change happens when sufficient CO₂ and thermal energy are present and slow/no change occurs when a limited amount of CO₂ or thermal energy is available.

Furthermore, the effectiveness at which CO₂-driven flows clean out the gully alcoves until the bedrock implies that bedrock weathering and erosion rate are setting the upper limit for the pace of landscape evolution in these systems on the long term. If CO₂-driven flows play a role in bedrock erosion themselves remains speculative. However, based on the estimated collisional forces of observed coarse-grained CO₂-driven granular flows in Martian gullies we expect that CO₂-driven granular flows can effectively erode bedrock, permafrost and LDM.

## Conflict of Interest

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

## Data Availability Statement

For the experiments presented in this manuscript the data collected by the sensors in the flume, the DEMs of Difference, and the Movies S1–S5 are available via Yoda (online repository of Utrecht University; https://geo/public.data.uu.nl/vault-erosion-co2-driven-flows-on-mars/research-erosion-co2-driven-flows-on-mars%5B1737457221%5D/original/SupVideos/). The data and instructions on how we processed the raw data can be found in Roelofs et al. (2025).

## Acknowledgments

LR was supported by the Dutch Research Council (NWO)—Grant OCENW. KLEIN.495 to TdH. The visit to the lab was funded by Europlanet-project number 20-EPN-023 to LR. We also acknowledge funding from the CNRS INSU Programme Nationale de Planétologie. SJC is grateful for financial support of the French Space Agency CNES for her HiRISE and CaSSIS work. The Mars Simulation Wind Tunnel laboratory is a member of Europlanet (2024) RI and has received funding from the European Union's Horizon 2020 research and innovation programme under Grant agreement 871149.

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