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\DOI{10.5802/crgeos.313}
\datereceived{2024-05-06}
\daterevised{2025-08-01}
\dateaccepted{2025-09-10}
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\dateposted{2025-10-02}
\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\TopicFR{G\'eomorphologie}
\TopicEN{Geomorphology}

\title{Experimental simulation of high mobility  processes and exotic
pseudolevitation}

\alttitle{Simulation exp\'{e}rimentale des ph\'{e}nom\`{e}nes de 
mobilit\'{e} augment\'{e}s par les \'{e}chappements gazeux \`{a} basse 
pression atmosph\'{e}rique}

\author{\firstname{Kelly} \lastname{Pasquon}}
\address{Universit\'{e} Paris-Saclay, Geops, CNRS, Orsay, France}
\address{Cerema, Nantes, France}
\email[K. Pasquon]{kellypasquon@hotmail.com}

\author{\firstname{Julien} \lastname{Gargani}\CDRorcid{0000-0002-3409-4493}\IsCorresp}
\addressSameAs{1}{Universit\'{e} Paris-Saclay, Geops, CNRS, Orsay, France}
\address{Universit\'{e} Paris-Saclay, Centre d'Alembert, France}
\email[J. Gargani]{julien.gargani@universite-paris-saclay.fr}

\author{\firstname{Manish} \lastname{Patel}\CDRorcid{0000-0002-8223-3566}}
\address{Open University, Milton Keynes, UK}
\email[M. Patel]{manish.patel@open.ac.uk}

\author{\firstname{Matthew} \lastname{Sylvest}\CDRorcid{0000-0002-5909-6858}}
\addressSameAs{4}{Open University, Milton Keynes, UK}
\email[M. Sylvest]{matthew.sylvest@gmail.com}

\author{\firstname{Matthew}\nobreakauthor\lastname{Balme}\CDRorcid{0000-0001-5871-7475}}
\addressSameAs{4}{Open University, Milton Keynes, UK}
\email[M. Balme]{matt.balme@open.ac.uk}

\author{\firstname{Susan J.} \lastname{Conway}\CDRorcid{0000-0002-0577-2312}}
\address{Nantes Universit\'{e}, Universit\'{e} d'Angers, Le Mans Universit\'{e}, 
CNRS UMR 6112, Laboratoire de Planetologie et Geosciences, France}
\email[S. J. Conway]{susan.conway@univ-nantes.fr}

\author{\firstname{Marion} \lastname{Masse}\CDRorcid{0000-0003-1743-0667}}
\addressSameAs{5}{Nantes Universit\'{e}, Universit\'{e} d'Angers, Le Mans Universit\'{e}, 
CNRS UMR 6112, Laboratoire de Planetologie et Geosciences, France}
\email[M. Masse]{marion.masse@univ-nantes.fr}

\author{\firstname{Gwenael} \lastname{Jouannic}\CDRorcid{0000-0003-3399-389X}}
\addressSameAs{2}{Cerema, Nantes, France}
\email[G. Jouannic]{gwenael.jouannic@cerema.fr}

\shortrunauthors

\keywords{\kwd{Mars}
\kwd{Low pressure}
\kwd{Friction}
\kwd{Mobility}
\kwd{Runout}
\kwd{Pseudolevitation}
\kwd{Erosion}}

\altkeywords{\kwd{Mars}
\kwd{Basse pression}
\kwd{Friction}
\kwd{Mobilit\'{e}}
\kwd{Transport}
\kwd{Pseudo-l\'{e}vitation}
\kwd{\'{E}rosion}}

\thanks{PNP (CNRS/CNES) and Europlanet}

\begin{abstract} 
Planetary surfaces are transformed by various processes at different
atmospheric pressures. On Mars, fluids are expected to have flowed at
reduced atmospheric pressure during the last 3 billion years.
Nevertheless, the influence of low pressure on surface processes has
been investigated in only a few studies. Our understanding of the
influence of gas in fluids on Martian surface morphologies is poor due
to the low number of experiments performed at low atmospheric pressure
(${\sim}$7 mbar) and the limited potential analogs on Earth.
Here, we present laboratory and numerical experiments to explore the
influence of gas production on fluid-sand mobility at low atmospheric
pressure. Energetic cohesive sand pellets uplifted by escaping gas were
generated, forming narrow channels with levees with lengths of 
80--90 cm, which were approximately 1800 times their widths. The maximum
experimentally determined velocity of the pellets ranged from 60 to 150
cm/s. The channel morphologies were formed by a pseudo-levitation
mechanism corresponding to transport with reduced friction. We propose
that the mechanisms observed in our experiments at low atmospheric
pressure may explain some past Martian surface changes.
\end{abstract}

\begin{altabstract} 
Les surfaces plan\'{e}taires sont transform\'{e}es par diff\'{e}rents
processus de surface g\'{e}n\'{e}rant parfois une mobilit\'{e} accrue.
A basse pression atmosph\'{e}rique, comme sur Mars lors des 3 derniers
milliards d'ann\'{e}es, la propagation des fluides \`{a} la surface en
a \'{e}t\'{e} affect\'{e}e. Cependant, les modes de transport \`{a}
basse pression  (${\sim}$7 mbar) ont \'{e}t\'{e} \'{e}tudi\'{e}s dans un
nombre limit\'{e} d'exp\'{e}rimentation, ce qui limite notre
compr\'{e}hension pr\'{e}cise des \'{e}coulements de surface et des
morphologies produites par ces processus. Notre connaissance de ce type
d'\'{e}coulement est d'autant plus faible qu'il n'existe pas
d'analogues \'{e}vidents actuellement \`{a} la surface de la Terre.
Dans cette \'{e}tude, nous pr\'{e}sentons des r\'{e}sultats
exp\'{e}rimentaux et de mod\'{e}lisation sur le transport de
mat\'{e}riel solide en pr\'{e}sence d'\'{e}chappements gazeux \`{a}
basse pression. La propagation d'un m\'{e}lange coh\'{e}sif de sable
humide, subissant une l\'{e}vitation partielle caus\'{e}e par la
vaporisation de l'eau au contact d'une surface granulaire,
g\'{e}n\`{e}re la formation d'un chenal \'{e}rosif d'une longueur de 
80--90 cm, de lev\'{e}es lat\'{e}ral et frontal \`{a} une vitesse
comprise entre 60 et 150 cm/s. Ce ph\'{e}nom\`{e}ne est assimilable
\`{a} une propagation avec une friction basale r\'{e}duite, mais non
nulle $(\mu_{\mathrm{eff}}\sim 0.2)$. Ce processus de transport et
d'\'{e}rosion pourrait s'\^{e}tre produit ou se produire encore \`{a}
la surface de Mars.
\end{altabstract} 

\maketitle

\vspace*{10pt}

\twocolumngrid

\end{noXML}

\defcitealias{Kiteetal2014}{ibid.}

\section{Introduction}\label{sec1}
The physic of water-sand mixed flow is complex and the effects of low
atmospheric pressure on the mixed flow dynamic are largely unexplored.
Only~a few studies have investigated flows under low atmospheric
pressure  \citep{Jouannicetal2015,Masseetal2016,Hernyetal2018}. These
experiments caused exotic morphologies. The full spectrum of
morphological diversity has not yet been studied. Many parameters and
conditions have not been investigated. Previous studies have focused on
flows under low atmospheric pressure: (i)~caused by pure liquid water
released over a sandy substrate with a low surface temperature, 
${<}$278~K  \citep{Jouannicetal2015}, (ii)~caused by pure liquid water
released over a sandy substrate with surface temperatures reaching
values ${>}$290~K  \citep{Hernyetal2018}, (iii)~caused by pure
ice melting and sublimation on a substrate with a low surface
temperature  \citep{Masseetal2016}. In this study, a mixed flow
(sand--water) is released over a sandy substrate with a temperature
${>}$290~K. The study of mixed flow at low atmospheric
pressure is conducted for basic science purposes, but could also be
used to discuss exotic processes that may have occurred on Mars. 

Mars is thought to have experience of reduced atmospheric pressure for
more than three billion years compared to  Earth \citep{Kiteetal2014},
below the triple point of water. The drop in atmospheric pressure has
occurred during the last three billion years  \citepalias{Kiteetal2014} and
the influence of atmospheric pressure lowering on morphologies is not
well constrained. The variation in atmospheric pressure may have
occurred with the variable presence of liquid water. An ocean is
believed to have existed 3.5 Gyr ago  \citep{DiAchilleHynek2010} and
perhaps until the late Hesperian  \citep{TurbetForget2019}. The
presence of liquid water on Early Mars has also been suggested by
various observations, such as fluvial and coastal (ocean) morphologies 
\citep{MalinEdgett2003,DiAchilleHynek2010}. Geomorphological and
modeling studies have suggested that transient wet conditions may have
occurred during the Hesperian  \citep{Bakeretal1991}, and radar
observations have indicated that subglacial water bodies could still be
present beneath the poles of  Mars 
\citep{Oroseietal2018,Lauroetal2021}. Potentially, liquid water may
have been present during the long transition from high to reduced
atmospheric pressure on Mars. On Mars, low atmospheric pressure and
transient liquid water may have existed simultaneously in the past,
even if this combination of conditions is controversial in explaining
many current processes 
\citep{Thomasetal2010,PilorgetForget2015,Schimdtetal2017,Pasquonetal2019}. 
In any case, the physics of flows under exotic conditions  (i.e.\ 
7~mbar) is not well known and the aim of this study is to describe such
flows. 

The propagation of liquid water and sediments interacting under
low-pressure conditions 
\mbox{representative} of present-day Mars 
(${\sim}$7~mbar) is complex  \citep{Masseetal2016}. Under low
atmospheric pressure  (${\sim}$7~mbar), their transition phase
thresholds of water are significantly different than those at current
terrestrial atmospheric pressure: liquid water boils near its freezing
point, rather than  100~\textdegree C, under terrestrial conditions.
Recent studies have shown that exotic processes may have occurred or
are occurring on Mars, resulting in mechanisms and morphologies that
differ from those on Earth 
\citep{Masseetal2016,Raacketal2017,Hernyetal2018,Sylvestetal2017,Brozetal2020}. 
These surface transformation mechanisms at reduced atmospheric pressure
are important because of the extended period of time Mars has spent in
this atmospheric pressure regime. Mars is thought to have transitioned
to low atmospheric pressure more than 3 Gyr ago  \citep{Kiteetal2014}.
Surface transport, erosion and sedimentation processes may have been
very different and caused exotic morphologies or different erosion and
sedimentation rates over this period. On Earth, reduced atmospheric
pressure less than half that of the present time existed 2.7 Gyrs ago
\citep{Sometal2012}, which may have influenced erosion and deposition
processes as temperatures could more easily reach water's boiling point
(\mbox{${<}80~\text{\textdegree}$C} at this atmospheric pressure).

Many complex morphological changes caused by fluid flow must be
considered under low atmospheric pressures due to the various ways in
which liquids and gases can modify the topography. Terrestrial analogs
are generally used to interpret planetary flow dynamics; however, this
approach is not entirely appropriate for describing low-pressure
processes on Mars. Fluid-sediment interactions at low atmospheric
pressures have been described in a few specific cases using
experimental setups  \citep{Conwayetal2011, Jouannicetal2015,
Masseetal2016, Sylvestetal2017, Raacketal2017}. 
However, considering the recent interest in this research topic, the
high morphological variability of flows at low pressures has not been
described exhaustively, and numerous phenomena and processes remain
unknown. Here we investigate, using an experimental and theoretical
approach, the transport processes associated with the propagation of
wet sand over a sandy surface. The resulting 
dynamic and morphologies
will be described and\unskip\break
\mbox{discussed.}

\section{Method}\label{sec2}

\subsection{Experimental setup}\label{sec2.1}
To understand the influence of low atmospheric pressure on sediment
propagation by a fluid  (liquid or gas), a set of laboratory
experiments was performed in an atmospheric chamber  (Mars chamber at
the Open University) with a controlled atmospheric pressure of  7~mbar.
Experiments were conducted along a  20\textdegree\ inclined metallic
surface 1~m in length and 40~cm in width covered by sand. A reservoir
containing a cold  ($274~\mathrm{K} < T < 278$~K) mixture of water and
sand was released over a hot  ($291~\mathrm{K}<  T < 294$~K) thin 
(1--2~mm) non-cohesive dry sand layer placed on the inclined surface.
The reservoir is 6~cm in width, 6~cm in height and ${>}$10~cm in
length, and was located 4~cm above the surface at a slope of 
50\textdegree. The sand layer on the surface was composed of grain
sizes  200--250~$\upmu$m. The sand in the reservoir was of constant
mass (i.e., 50~g) and a variable mass of water ranging from 0~g to 
100~g was added. Consequently, the ratio between the water and sand
mass $m_{\mathrm{water}}/m_{\mathrm{sand}}$ ranged from 0 to 2. The
variable mass ratio between water and sand was conducted in  17
experiments and allow us the measurement of 135 channels  (length,
width) formed by the propagation of the material. Even if a small
discontinuity could be observed in a channel, it is considered as a
unique channel in the counting. If two different paths with erosive
features, diverging from a unique point, are observed, they are counted
as two different channels. In this study, the movement analysis 
(distance, speed, and time) focused on the pellets that moved over
longer distances using video analysis. Measurements were also performed
manually, but also using image and video analysis to determine the
width, length, and thickness of the resulting morphologies. Two cameras
have been used to make photos and videos of the flows. Measurements
accuracy were 0.5~mm. Video analysis was performed using the
open-source software Kinovea.

\subsection{Numerically modeling transport and gas\newline
escape}\label{sec2.2}
The interpretation of the transport process was also conducted using a
modeling approach. Basically, during the propagation of the material 
(i.e.\ the pellet) along the path, gas escape from the transported
material and interact with the substrate. The contact between the
pellet and the substrate was simulated by a numerical model. The
friction was described by the equation  $\tau =\sigma_n\tan \phi$,
where $\sigma_n$ is the normal stress and $\phi$ is the kinetic
friction angle. The normal stress $\sigma_n$ was calculated by
considering the normal component of the weight of the flow on the
sliding surface as $\sigma_n=\rho_sg\;z\cos \theta$, where $\theta$ is
the local slope,  $z$ is the thickness of the pellet,  $g$ is gravity
and $\rho_s$ is the density of the cohesive sand. The value of  $\phi$
is usually considered to be
${>}$20\textdegree\ \citep[e.g.,][]{Kleinhansetal2011}. 
On Earth and
Mars, the dynamic angle of repose of dry sand dunes ranges from 
30\textdegree\ to 35\textdegree\ \citep{Atwood-StoneMcEwen2013}. We did
not use a more sophisticated formulation of the kinetic friction
coefficient \citep[e.g.,][]{Jopetal2006} because this coefficient has
less influence on the final results than the uncertainty in the
parameters in the more complex descriptions.

The rheological expression was substituted into the Newton's laws of
motion, taking into account the weight of the pellet  $(F_G=mg)$, the
lift force exerted by escaping gas  $(F_L= 0.5 C_{\mathrm{D}} \rho_{\mathrm{gas}}
S\;v_{\mathrm{gas}}^2)$ and the friction force  $(F_R=\sigma_n\tan\phi
x S)$, where  $m$ is the mass of the pellet,  $S$ is the surface at the
base of the pellet,  $C_{\mathrm{D}}$ is the drag coefficient, 
$\rho_{\mathrm{gas}}$ is the gas density, and  $v_{\mathrm{gas}}$ is
the velocity of the gas escaping from the pellet perpendicular to the
slope.

\begin{figure*}
\includegraphics{fig01}
\vspace*{-4pt}
\caption{\label{fig1}Morphology of linear channel, pellet and digitate
flow at low atmospheric pressure for (A)~a reservoir water content of
25~g and 50~g of sand and (B)~a reservoir water content of 50~g and 
50~g of sand. (C)~Schematic representation of the pellet of wet sand
during transport in our experiments. (D)~Measurement of velocity versus
time for different sand--water ratios (50~g of sand with 18~g, 25~g,
37.5~g or 50~g of water) using video analysis. The slope is
20\textdegree, the sand has a grain size of  200--250~$\upmu$m, the
cohesive wet cold sand ranges in temperature from  $275~\mathrm{K} < T
< 278$~K, and the hot substrate temperature ranges from 
$291~\mathrm{K} < T < 294$~K in all experiments. The atmospheric
pressure is  7~mbar.}
\vspace*{-4pt}
\end{figure*}

The resulting motion along the path is described by:
{\begin{equation}\label{eq1}
m\cdot g\cdot \sin \theta-
(\sigma^{\mathrm{eff}}_n\tan
\phi) x
S = m\;\mathrm{d}v_x/\mathrm{d}t 
\end{equation}}\unskip
where  $t$ is the time, $v_x$ is the velocity along the slope, and
{\begin{equation}\label{eq2}
\sigma^{\mathrm{eff}}_n=
(mg\cos\theta- 0.5 C_{\mathrm{D}}
\rho_{\mathrm{gas}} 
S v_{\mathrm{gas}}^2)/S
\end{equation}}\unskip
and $\theta$ is the slope. The 1-D finite difference model for the
dynamic equation of motion~(\ref{eq1}) was solved numerically along the
slope using a finite difference approach leading to the equation:
{\begin{equation}\label{eq3}
v_x
(t+\Delta t)=
v_x(t) + [g\cdot \sin \theta-
(\sigma^{\mathrm{eff}}_n
\tan \phi)x S/m]x\Delta t 
\end{equation}}\unskip

Alternatively, the pellet dynamics along the path can be simulated by a
1-D equation considering that
{\begin{equation}\label{eq4}
\sigma^{\mathrm{eff}}_n
\tan \phi=\sigma_n 
\mu_{\mathrm{eff}}                                                 
\end{equation}}\unskip
or equivalently that  
{\begin{equation}\label{eq5}
\mu_{\mathrm{eff}}= 
[\sigma^{\mathrm{eff}}_n
\tan \phi]/\sigma_n.
\end{equation}}\unskip

In this case, the reduced effective friction can be written as
{\begin{eqnarray}
\mu_{\mathrm{eff}} 
&
= 
&
[1- (0.5 C_{\mathrm{D}} \rho_{\mathrm{gas}} 
v_{\mathrm{gas}}^2)/
(g H \cos\theta\rho_{\mathrm{sand}})]
x^\cdot\tan\phi
\nonumber\\
&
=
&
\tan\phi_{\mathrm{eff}} 
\label{eq6}
\end{eqnarray}}\unskip
where  $H$ is the pellet thickness. Thus, in this case, the equation
that describes the motion of the pellet is
{\begin{equation}\label{eq7}
m\cdot g\cdot\sin\theta-\sigma_n\mu_{\mathrm{eff}} S =
m\;\mathrm{d}v_x/\mathrm{d}t 
\end{equation}}\unskip
where $\phi_{\mathrm{eff}}$ is the effective kinetic friction angle. In
both models, the gas escape velocity $v_{\mathrm{gas}}$ is considered
to be equal to 90~ms$^{-1}$, as estimated by \citet{Masseetal2016}.

\begin{figure*}
\includegraphics{fig02}
\vspace*{4pt}
\caption{\label{fig2}Experiment at low atmospheric pressure.
(A)~Annotated view of the channels produced by the pellets. (B)~Pellet
channels with gaps after splitting. (C)~Influence of the sand--water
ratio  $M_{\mathrm{water}}/M_{\mathrm{sand}}$,  where  $M$ is the mass,
on the number of digitate shapes (i.e.\ pellet-channels).}
\vspace*{4pt}
\end{figure*}

\section{Results}\label{sec3}

\subsection{Experimental results}\label{sec3.1}
We observed an exotic mass transport process that we call digitate flow
because it produces multiple narrow channels with a linear path 
(Figure~\ref{fig1}). Two other morphologies are also observed:  (i)~a
linear channel caused by the mixing flow of sand and liquid water, with
some minor signs of boiling  (bubbles, sand ejection) and  (ii)~a
low-density flow composed of ``individual'' sand grains that were
transported tens of centimeters above the experimental surface, as a
cloud of sand. However, the second process did not influence the
morphology. We focus on the digitate flow morphology, which has never
been accurately described before. Various digitate shapes were observed
(1--25,  Figures~\ref{fig1} and~\ref{fig2}), including long 
(80--90~cm) and narrow (5--10~mm,  Figure~\ref{fig1}) channels. They
were formed by the rapid propagation  (60--150~cm/s) of small pellets
of wet sand with a maximum diameters of  ${\sim}2 \pm0.5$~mm. 
Substrate erosion to a depth of about 1~mm, lateral levees and a
frontal levee at the end of the path are the main 
\mbox{morphological}
features of the channel caused by the pellet propagation. The pellets
formed rapidly after contact with the substrate, and their size
remained constant during the flow, as indicated by the constant channel
width. The pellets rapidly  (1.5--2~s) generated a channel-like
morphology  (${\sim}$1~mm deep) with lateral levees  ${\sim}$1~mm high
on the substrate due to the displacement of the sand substrate by the
direct contact of the pellet and its escaping gas. No liquid flow or
liquid diffusion occurs in these channels, as evidenced by the light
color of the substrate  (contrary to the darker color of the wetted
substrate in the same experiment), which is why this process is called
dry digitate flow.

As the wet sand pellet propagates, water vapor is produced at the
interface between the cold pellet  ($274~\mathrm{K}< T < 278$~K) and
the hot sand substrate  ($291~\mathrm{K}< T< 294$~K). The energetic gas
escape from the pellet causes sand grains to be ejected from the
substrate to the outside of the channel. These channels can have one or
two gaps along their path due to some short  (${\sim}$1~cm)
rebound/levitation events that may be sometimes associated with the
pellet \mbox{splitting.} The maximum velocity of the pellets is not influenced
by the initial amount of sand in our experiments. The dry digitate
flows are always produced during less than 2~s in all experiments. The
velocity of the pellets gradually increases along their path, reaching
a maximum velocity of approximately 60--150~cm/s, before decelerating
rapidly. However, the number of channels produced by the pellet  (i.e.\ 
digitate shapes) depends on the initial mass of liquid water 
(Figure~\ref{fig2}). The higher the water content of the reservoir, the
greater the number of digitate shapes observed. Furthermore, higher
water mass in the reservoir leads to faster pellet motion 
(Figure~\ref{fig1}). The release of additional water vapor during the
splitting may explain why the pellets move over a longer period of time
and faster. The pellets split when the water content is higher. When
the pellets split, it could cause  (i)~two divergent channels, 
(ii)~one or more rebounds associated with disconnection along the
channel,  (iii)~different propagation length of the new formed 
pellets\unskip\break  
(Figure~\ref{fig2}). 

The results of the numerical modeling were then validated by comparison
with the experimental 
results, and next, the pellet motion under
Martian gravity and other kinds of gas escape was
simulated.

\subsection{Reduced effective friction and pseudo-levitation modeling
results}\label{sec3.2} 
Even if one or two short rebound/levitation events occur, the flow has
permanent or semi-permanent contact with the substrate, as shown by the
semi-continuous morphologies of the channel and lateral levees along
the path. The pellet dynamics and digitate shape morphology cannot be
explained by pure levitation processes  $(\mu_{\mathrm{eff}} = 0)$ but
can be explained by reduced effective friction $\mu_{\mathrm{eff}} =
\tan \phi_{\mathrm{eff}}$. The gas escape from the pellet displaces
some isolated dry sand grains, moving the grains away from their
initial position on the substrate. However, the pellet has permanent or
semi-permanent contact with the substrate, as suggested by the
formation of lateral and frontal levees and erosion of the channel. The
semi-permanent contact is characterized by pellet rebounds and small
channel disconnections along the pellet path. The process that causes
the dry digitate shapes is not a pure levitation process due to the
permanent or semi-permanent contact between the pellet and the sand
substrate: it will be called pseudo-levitation. We fit the experimental
observations via simulations, considering a gas escape speed of 
$v_{\mathrm{gas}}$ of  90--100~ms$^{-1}$ and a drag coefficient  $C_{\mathrm{D}}=
1.18$, as in  \citet{Masseetal2016}, at the beginning of the dry
digitate flow, along  ${\sim}$40--70~cm of the path, i.e., from
$t=0.45\ndash0.8$~s (Figure~\ref{fig3}). Then, the rapid decrease in
the lift force produced by the gas escape until $F_L= 0$ increases the
effective friction and abruptly decreases the pellet velocity. These
results were obtained considering a pellet thickness of  $2 \pm0.5$~mm
(Figure~\ref{fig3}A and~\ref{fig2}B). The gas escape allows a sliding
propagation over a longer distance and a higher slip propagation speed
than without gas escape or with a smaller velocity of gas escape. The
presence of a lift force caused by the escaping gas is equivalent to a
reduced effective friction $\mu_{\mathrm{eff}}\sim 0.2\;
(\phi_{\mathrm{eff}} \sim11.5\text{\textdegree})$ along the first  
${\sim}$40--70~cm  ($t=0.45\ndash0.8$~s) of the path before the
friction increases to the friction of the dry sand, i.e.,
$\mu_{\mathrm{eff}} = \mu = 0.58$ ($\phi  \sim30\text{\textdegree}$, 
Figure~\ref{fig3}A).

\begin{figure*}
\vspace*{-2pt}
\includegraphics{fig03}
\vspace*{-4pt}
\caption{\label{fig3}Dynamics of pellet propagation. (A)~Experimental
and numerical model velocity vs.\ distance, with $C_{\mathrm{D}}= 1.18$, a gas
escape speed $v_{\mathrm{gas}}$  of  90~ms$^{-1}$  at  $t=0.45$~s, a
simulated pellet thickness of 1.5~mm, and an initial velocity of
70~cm/s. $\phi=30\text{\textdegree}$ in the blue simulation, and
$\phi_{\mathrm{eff}}= 11.5\text{\textdegree}$  in the yellow
simulation. (B)~Influence of gravity on the pellet velocity and runout.
The blue simulation parameters are identical to those used in the
previous graph, i.e.,  $C_{\mathrm{D}}= 1.18$, the gas speed $v_{\mathrm{gas}}$ is
90~ms$^{-1}$ at  $t=0.45$~s,   $\phi= 30\text{\textdegree}$, and the
initial velocity is 70~cm/s. In the yellow  simulation, 
$g=3.72$~ms$^{-2}$. (C)~Pellet velocity and runout simulation for a
pellet thickness of  $H=2.5$~mm, $C_{\mathrm{D}}= 1.18$ and a gas speed
$v_{\mathrm{gas}}$ of 90~ms$^{-1}$  at  $t=0.8$~s, $\phi=
30\text{\textdegree}$, and an initial velocity of 70~cm/s. (D)~Force
diagram. In all the experiments, as in the numerical models, the  slope
$\theta$  is  20\textdegree.}
\vspace*{-6pt}
\end{figure*}

\subsection{Modeling the high mobility process under Martian
conditions}\label{sec3.3} 
The numerical simulations suggest that under Martian gravitational
conditions  $(g_{\mathrm{Mars}} =3.72~\mathrm{ms}^{-2})$, the mobility
of the sediments could be multiplied by more than 3 for pellet
propagation  (Figure~\ref{fig3}B). For a pellet of 1.5~mm thickness and
a gas escape of 0.45~s, our model suggests that the path length reaches
${\sim}$290~cm under Martian gravitational conditions instead of the 
${\sim}$90~cm path length observed under terrestrial gravitation
conditions. For a 2.5~mm thick pellet and a gas escape of 0.8~s, our
model suggests that the path length reaches ${\sim}$360~cm under
Martian gravity instead of the ${\sim}$90~cm path length observed under
Earth gravity  (Figure~\ref{fig3}C). Moreover, when the Martian gas
density of (CO$_{2}$)$_{\mathrm{gas}}$ is 
\mbox{considered} instead of
(H$_{2}$O)$_{\mathrm{gas}}$, the simulated mobility increases by more
than 4 times (Figure~\ref{fig4}C).

\begin{figure*}
\includegraphics{fig04}
\caption{\label{fig4}(A)~Morphologies on the Kaiser dune in the HiRISE
image ESP\_018608\_1330, Mars, described in \citet{Pasquonetal2018}.
(B)~Linear dune gullies and pits on the Matara dune, Mars, HiRISE image
ESP\_038,255\_1300. (C)~Velocity and runout simulation for CO$_2$
\citep[$\rho_{\mathrm{gas}}= 0.01$~kg/m$^3$ in][]{Diniegaetal2013} and
H$_2$O ($\rho_{\mathrm{gas}} = 0.0042$~kg/m$^3$) gas flux. Here,
$C_{\mathrm{D}}= 1.18$, the gas escape speed is  90~ms$^{-1}$  at 
$t=0.8$~s, $\phi= 30\text{\textdegree}$, and the initial velocity is
70~cm/s. In all the numerical models, the slope $\theta$~is~20\textdegree.}
\end{figure*}

\section{Discussion}\label{sec4}

\subsection{Comparison with previous experimental\newline
results}\label{sec4.1}
From a physical point of view, the experimentally obtained velocity
(i.e.,  60--150~cm/s) is greater than the erosion threshold of sand 
\citep{Gargani2004}. The 
\mbox{experimentally} obtained
velocity is also
faster than the propagation velocity observed in flows in 
\mbox{previous}
low-pressure experiments for gullies obtained with a release of a pure
water flow over a frozen surface  \citep{Jouannicetal2015} or a ``hot''
surface until 297~K \citep{Hernyetal2018}, and for the melting of a
water ice block over a thin dry sand surface  \citep{Masseetal2016}.
The experimental speed obtained in this study is also slightly faster
than the propagation of pellet described in  \citet{Hernyetal2018} of
75~cm/s at the maximum. 

\subsection{Possible Martian landforms analogs}\label{sec4.2}

\subsubsection{Past Martian conditions and potential
analogs}\label{sec4.2.1}
During the last billions of years, low atmospheric pressure 
\citep{Kiteetal2014}, diurnal temperatures ${>}$290~K, and a higher
availability of liquid water than during the late Amazonian are
believed to have existed on Mars. Consequently, pseudo-levitation
processes could have been common in the past on Mars. The degradation
of old Martian surfaces could cause difficulties in identifying the
morphologies formed by these processes. During the last million/billion
years, numerous processes may have degraded the present surface of Mars
and explain the lack of accurate observations: Aeolian processes 
\citep{BalmeGreeley2005}, degradation by sublimation processes 
\citep{Sejourneetal2011,Sejourneetal2012}, crater impacts 
\citep{Lagainetal2021}.

\subsubsection{Debris flows and seasonal activities on
Mars}\label{sec4.2.2} 
Debris flow morphologies on Mars suggest the presence of high mobility
processes due to the 
\mbox{presence} of ice, evaporite, gas or liquid water 
\citep{DeBlasio2011,Lucasetal2014}. The occurrence of past and present
high mobility processes on Mars are compatibles with the potential role
of processes uncommon on Earth caused under different
pressure-temperature conditions. On Mars, high mobility processes are
inferred by morphologies observed on gentle slopes of  6--8\textdegree,
such as the linear gullies of the Russell megadune 
\citep{Jouannicetal2012} or the perennial rill 
\citep{Jouannicetal2018}. Linear gullies are around 1~km in length,
with a  10~m width and lateral levees height of 1~m 
\citep{Pasquonetal2016}. Perennial rills are morphologies with a width
of 1~m that growth seasonally during springtime a few hundreds of
meters along sand dunes, with an increasing number of channels from
upstream to downstream \citep{Jouannicetal2018}. The high mobility of
sand for hundreds of meters along gentle slopes could be due to the
presence of  CO$_{2}$ \citep{Pasquonetal2019} and H$_{2}$O
on and below the surface  \citep{Jouannicetal2012,Jouannicetal2018} or
brines  \citep{Chevrier2025}. On Mars,  CO$_{2}$ ice is seasonally
present on the surface and is known to influence seasonal surface
changes  \citep{PilorgetForget2015,Pasquonetal2019}. The controversial
presence of hydrated salt on the surface after recurring slope lineae
(RSL) formation has been interpreted as a potential proof of the
existence of a transient liquid water phase  \citep{Ojhaetal2015}, but
a dry granular flow is an alternative interpretation 
\citep{Schimdtetal2017}. Deep groundwater has been proposed as the
origin of this water  \citep{AbotalibHeggy2019}. The experimental
morphologies obtained in this study (lateral levees, eroded channel)
are different from the ones of the RSL and do not appear to be a
potential analog. Thick water ice deposits can also be observed 
\citep{Dundasetal2018}, which may influence the transport processes for
granular material and the evolution of surface morphology. The aim of
this study is not to argue for or against the presence of liquid water
or brines on Mars at the present time, but only to test the role of gas
ejection from a solid pellet on the propagation process on a slope and
on morphologies. 

\subsubsection{Martian gullies}\label{sec4.2.3}
The experimental morphologies are characterized by a channel with a
linear morphology  (length ${\gg}$ width), lateral levees, a final pit
at the end of the intermediate channels and several aligned
disconnected channels. From a morphological point of view, some
unexplained Martian morphologies present similar characteristics, such
as long, linear, narrow, partially disconnected channels with lateral
levees and aligned pits diverging from gullies \citep[Figures~\ref{fig2}
and~\ref{fig4};][]{Pasquonetal2018}. In the experiments,
the pellet is deposited at the end of the last channel, whereas Martian
morphologies do not always show a final deposit 
\citep{Pasquonetal2016,Diniegaetal2013}. On Mars, the final pit feature
has been explained by the sublimation of icy blocks 
\citep{Diniegaetal2013}. 

The morphology of the degraded water-rich or ice-rich deposits is
different from the morphology of fresh deposits 
\citep{Jouannicetal2015}, and this could cause difficulties or
misinterpretation of these morphologies. The degradation of pellets
could occur in a few hours or days. The resulting morphologies are
difficult to observe when pellets become dry and the sand becomes
non-cohesive. The resulting morphology could be very small. An ice-rich
block to reach pluri-meter scale is certainly easier to be observed
than a wet sand pellet of   1~m without ice, due to the cohesive
properties of ice. For morphologies with width equal or superior to
1~m, initial cohesion of the pellet must be considered: ice-rich blocks
or pure ice are better explanatory material than wet sand. The
repetition of ice-rich blocks sliding along the same path could
increase the channel width and length. Seasonal water frost thickness
on Mars could reach 1~mm \citep[mean thickness 
180~$\upmu$m;][]{Langeetal2024} and seasonal  CO$_{2}$ frost/snow
thickness could reach 2~m at high latitude  \citep{Smithetal2001}. The
sublimation rate of ice (CO$_{2}$ and H$_{2}$O ice) could reach values
ranging from   0.74~mm/h for water ice \citep{Chittendenetal2008} to
80~cm/s for  CO$_{2}$ ice  \citep{Cedillo-Floresetal2011}. However, in
specific conditions, water ice sublimation is believed to reach 0.3~m/s
\citep{Kimetal2022} and CO$_{2}$ ice 5--15~m/s  \citep{Thomasetal2010}.
A gas escape of 80~cm/s is sufficient to cause pseudo-levitation of
centimeter-thick blocks.

The experimentally obtained velocity is less than the modeling
estimations based on Manning's equation applied to gullies morphologies
along the Russell megadune on Mars 
\citep{Mangoldetal2010,Jouannicetal2012}. Our model suggests that a
velocity similar to those of \citet{Jouannicetal2012} and 
\citet{Mangoldetal2010} could be reached on a gentle slope of 
7--8\textdegree\ by assuming a low effective friction coefficient of
0.2 (i.e., 
$\phi_{\mathrm{eff}}=11\text{\textdegree}\ndash11.5\text{\textdegree}$,
Figure~\ref{fig5}). Theoretically, this low effective friction
coefficient could be obtained from pseudo-levitation of sand mixed with
liquid water, but it is not excluded that sand mixed with water ice, or
with  CO$_{2}$ ice, may generate sufficient sublimation (i.e.\ gas
escape from ice) to cause similar transport processes and morphologies.
Pseudo-levitation of ice--rich material containing CO$_{2}$ ice could
be produced by a minimum gas speed of 80~cm/s for fluidization of 
CO$_{2}$ gas under Martian conditions   \citep{Cedillo-Floresetal2011}.

\begin{figure*}
\vspace*{2pt}
\includegraphics{fig05}
\vspace*{1pt}
\caption{\label{fig5}Numerical modeling of sand displacement along a
slope. (A)~The slope $\theta$ as a function of the distance $x$ in the
area where the perennial rills are located. The green curve is the
slope extracted from the HRSC DTM, and the red curve is the smooth
decrease in the slope, which is used as an input in the numerical
simulation.  (B)~Velocity of the simulated granular flows as a function
of the runout distance for various kinetic effective friction  angles
$\phi_{\mathrm{eff}}$,  including  $\phi_{\mathrm{eff}}=
11\text{\textdegree}$,  12\textdegree, 13\textdegree\ and 
13.8\textdegree, assuming that the initial velocity is zero. Due to
noise in the HRSC DTM and for simplicity, the topography is smoothed by
assuming that the height follows  $H(x) = H_0\exp(-x/L_0)$, where  $x$
is the distance along the sliding path and  $H_0$ and  $L_0$ are two
empirical constants. In the model,  $H_0= 100$~m, $L_0= 400$~m  and 
$g_{\mathrm{Mars}}=3.72~\mathrm{ms}^{-2}$.  (C)~Linear dune gullies
with disconnected pits on Matara dune, Mars described in 
\protect\citet{Pasquonetal2018}, HiRISE image ESP\_020,770\_1300.}
\end{figure*}

\subsection{Implications for terrestrial flows}\label{sec4.3}

\subsubsection{Early earth}\label{sec4.3.1} 
On Earth, lower atmospheric pressure is thought to have occurred 2.7
Gyrs ago  \citep{Sometal2012}, which may have influenced surface
processes. However, weathering conditions and plate tectonics caused
the destruction or non-preservation of old and rare morphologies in the
rock record. The experiments conducted in this study allow us to
discuss the potential role of boiling water on the mobility of
terrestrial surface processes. When the atmospheric pressure is half
the present atmospheric pressure \citep{Sometal2012}, the boiling point
occurs for lower temperatures  (${<}80~\text{\textdegree}$C) and could
be more\break frequent.

\subsubsection{Pyroclastic flows analog}\label{sec4.3.2} 
Several experimental studies investigated the role of fluidization in
granular flows to understand pyroclastic flows 
\citep{Rocheetal2004,Rocheetal2016}. Our experimental flows reached
maximum speeds of 1--1.5~ms$^{-1}$, which is in the range of
experimental studies of pyroclastic flows released by fluidized
granular columns with high interstitial air pore pressure at ambient
temperature and pressure  \citep{Rocheetal2016}. In our experiments,
the Froude number  $\mathrm{Fr}= v_{\mathrm{pellet}}/(g^{\cdot}
H_{\mathrm{pellet}})^{1/2}$ ranged between 3 and 4, which is slightly
larger than the value of  2.4--2.8 observed in pyroclastic flows 
\citep{Rocheetal2004}.

There are too many differences between our experiments and pyroclastic
flow to pretend that it is a perfect analog. However, the influence of
gas escape on mobility in both cases is relevant to discuss the role of
gas escape. Pyroclastic flows are characterized by high mobility 
\citep{ValentineWohletz1989,Gueugneauetal2017} and significant velocity
\citep{Calderetal1999}. During pyroclastic flows, the rock degassing
process or gas escape in the dense current causes increased mobility.
The runout of granular flow is increased by fluidization 
\citep{Rocheetal2004}. Gas is expected to decrease the friction basal
shear stress, which affects the motion of solid materials. Effective
normal stress reduction justifies the use of low effective friction
coefficients for pyroclastic density currents \citep{Breardetal2018}.
The morphology of pyroclastic flows includes central channels, lateral
levees and abrupt termination. In Montserrat, the friction of
pyroclastic flows has been evaluated to range from 0.3 to 0.06 based on
morphological observations of the length and height, whereas
cold-debris avalanches display a wider range of friction values ranging
from 0.5 to 0.03  \citep{Calderetal1999}. During pyroclastic flows,
speeds of 3--25~ms$^{-1}$ have been observed 
\citep{Coleetal1998,Calderetal1999,Yamamotoetal1993,Rocheetal2016}. 

Explosively ejected hot ballistics or hot pyroclastic debris on snow
slopes could also cause water vapor and snow melting and trigger
volcanic ice-slurry flows, volcanic mixed avalanches and lahars.
Volcanic ice-slurry flows and mixed avalanches are characterized by low
friction processes and occur during natural events that involve mixing
hot volcanic material, ice, snow, water and air 
\citep{Kataokaetal2021,PiersonJanda1994}. The melting of glacier ice
and snow by pyroclastic density currents or phreatic explosions is a
common process causing liquefied and lubricated flows 
\citep{Lubeetal2009}. Gas is not necessarily present along the entire
path but may play a role during the beginning of this phenomenon when
hot volcanic rocks  \citep{Croninetal1996} and/or explosive events 
\citep{Waythomasetal2013} are involved. High mobility (i.e.,
0.13--0.16), a significant velocity of  5--14~ms$^{-1}$ and frontal
Froude numbers ${>}$6 have been estimated for volcanic
ice-slurry flows  \citep{Lubeetal2009}. The presence of gas in the
initial phase during this kind of event suggests potential applications
of our experiments and models, to pyroclastic flows.

\subsection{Ice-rich slope on Galilean satellites of
Jupiter?}\label{sec4.4} 
During the last decades, the presence of various icy and water-rich
environments has been described in the solar system. More specifically,
three of the four icy moons of Jupiter (Europe, Ganymede, Callisto) are
known to be ice-rich, but also to have a liquid water ocean in depth,
in the case of Europe and Ganymede  \citep{SpohnSchubert2003}. In the
case of Europe, some geysers of liquid water have been observed 
\citep{Reynoldsetal1983,Steinbruggeetal2020}. Surface processes
associated with the presence of water ice and liquid water under low
atmospheric pressure on these moons are still unknown. Future mission
JUICE may help to describe more accurately the surface evolution of
these extreme environments  \citep{Grassetetal2013} and allow us to
discuss transport mobility on their surfaces.

\section{Conclusion}\label{sec5}
We performed several material transport experiments at a low
atmospheric pressure of 7~mbar along a 20\textdegree\ slope. These
experiments allowed us to observe exotic transport processes involving
multiple (1--25) divergent pellets of wet sand. These generated erosive
channels with lateral and frontal levees. Gas escaped from the cold
pellets that contacted the hot substrate via boiling. The velocity of
this exotic transport process reached 60 to 150~cm/s over distances of
80--90~cm. This high mobility process is caused by the
pseudo-levitation of the wet sand pellets. This exotic transport can be
formalized by a reduced effective friction coefficient. A dry digital
flow produced by the reduced effective friction in relation to CO$_{2}$
or H$_{2}$O gas escaping at the interface between a water-rich cold
sand pellet and a relatively hot substrate is a possible mechanism that
may have transformed the surface of Mars. This kind of process can
influence planetary surfaces and may have caused surface erosion in
prior Martian history.

\section*{Acknowledgments}
This study was funded by the PNP (CNRS/CNES) and Europlanet. The
editorial board, and especially Fran\c{c}ois Chabaux, is acknowledged
for their work. 

\section*{Declaration of interests}
The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

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