Wind-blown sand on Mars, Venus, and Titan

An Introduction to wind-blown, aeolian deposition, comparing the conditions on Earth, Mars, Venus, and Titan.

Modern sand dunes cover extensive tracts on the surface of Mars. We have at our disposal a wonderful collection of images captured by orbiting satellites and landed rovers that show dune fields on crater floors and broad plains. There is a remarkable similarity of dune types between Earth and Mars – barchan, parabolic, longitudinal, and star dunes. Aeolian processes including dunes have also been observed on Venus and Saturn’s moon Titan.  One could be forgiven for thinking that the conditions of dune formation on these extraterrestrial bodies are the same as those on Earth, but there are important differences.

Solid phase – fluid phase density ratios

R.A. Bagnold’s iconic book The Physics of Blown Sand and Sand Dunes was first published in 1941. It is a foundational text that ever since has set the scene for studies of aeolian processes and deposits on Earth, but the basic principles also apply to those planets and moons that possess atmospheres and granular sediment. In the Preface to the 1954 reprinted edition, Bagnold writes “In the case of wind-blown sand the grains are some 2000 times as heavy as the fluid. In that of water-driven natural sand the immersed weight of the grains is only one and a half times the weight of the fluid. Since no really intermediate pair of substances exist a big difference in the behaviour of the grains in the two fluids is only to be expected.” In other words, the huge difference between the density ratios of the solid granular phase to the fluid phase in water and air (ρ_solid/ ρ_fluid) inevitably leads to fundamental differences in sediment entrainment and deposition between water and air.

Bagnold’s conclusion was based on terrestrial conditions where the density of water and air, and the gravitational potential at Earth’s surface are reasonably constant. The density of water (1000-1025 kg.m^-3) is about 800 times the density of air at sea level. Their density ratios are even further apart – ρ_s/ρ_water ≈ 2.6; ρ_s/ρ_air ≈ 2200. The plot below shows the relative density ratios for those planetary bodies where we are fairly certain that aeolian processes are persistent and of regional extent; the solid phase density is that of quartz (except for Titan) – it is assumed that rock and mineral densities are the same as those on Earth.

[Keep in mind the difference between mass and weight. Mass is a (scalar) measure of the amount of material in a body; it is directly proportional to inertia. Weight is a (vector) measure of the gravitational response to that body. Thus, the mass of a sand grain will be the same on Earth and Mars, but the weights will be different]

Why density ratios are important

In classical mechanics, the inertia of a body is directly proportional to its mass; this is encapsulated in Newton’s Laws. As the body size increases, so too does its inertia which means greater forces are necessary to accelerate or decelerate a body. The density of a substance is a statement about the amount of mass per unit volume. Thus, density ratios like ρ_solid/ρ_water and ρ_solid/ρ_air are a measure of the relative inertia of a solid particle in a fluid – density ratios are dimensionless numbers.

Sand grains in water have relatively small density ratios and low relative inertia. Thus, sand will move under relatively low flow velocities where the force of fluid flow is greater than resisting forces like friction, gravity, or cohesion. This is the main reason why the characteristic mode of movement is traction, where grains roll, tumble, and slide across the sediment-water interface.  Three-dimensional bedforms are the spontaneous, self-organized result of traction-dominated bedload transport of sediment in water. Saltation is also an important transport mode in high-velocity flows, but it is not the dominant mode. Furthermore, saltation trajectories in water are significantly shorter than those in air.

Density ratios with very high numerical values like ρ_solid/ρ_air, indicate that the particle has high inertia relative to drag forces – in other words, the greater the ratio (relative inertia), the greater the velocity required by air flow to overcome the drag forces and set grains in motion. This also means that sand grains moved by air flow have greater momentum (momentum is directly proportional to velocity). The general outcome is that the reaction of sand grains to air flow is fundamentally different to sand movement in water. The dominant, or characteristic transport mode in terrestrial aeolian systems is saltation. The elevated density ratios (and grain momentum) also mean that air turbulence will have little effect on saltation pathways (Southard, 2021).

Saltation and grain impact

If sufficient energy is imparted to a sand grain by the moving air, the grain will leave the sediment bed and move through the fluid in an arcuate trajectory. The beginning of movement is called the fluid threshold (or aerodynamic threshold) that is approximated by the Shield parameter.

Typically, the trajectory is asymmetric – experimental data indicates that takeoff angles average about 50^o and landing angles usually less than 20^o. The trajectory length is much greater than its height. Trajectory lengths of several metres are possible in air.

Particles are launched when:

1. Airflow velocity produces lift forces sufficient to overcome surface resisting forces.
2. Grains are launched during impact from incoming particles; also called splashing. Splashing also promotes grain movement along the bed – this is called surface creep or impact creep. Grain abrasion results from saltation impacts.

Aeolian processes on other planets and moons

Aeolian processes and bedforms have been observed on Venus, Titan, and Mars (Greeley et al., 1992 ; Greeley et al., 2001;  Craddock, 2011;  ; McSween et al., 2019). All observations are based on satellite imagery and remote sensing (e.g., radar). We also have exceptional images and data from Mars rovers Curiosity, Opportunity, Perseverance, Spirit, and Sojourner that have crawled through dune fields and across dune surfaces (Greely et al., 1992). Structures of putative aeolian origin have also been hypothesized on Pluto and the comet 67P/Churyumov-Gerasimenko that may involve sublimation, outgassing, and very thin, transient atmospheres (e.g., Moore et al., 2016).

Mars

The atmospheric conditions and gravitational potentials in each of these distant worlds are fundamentally different to conditions on Earth. Mars atmosphere is significantly less dense (0.02 kg.m^-3) than on Earth (1.22 kg.m^-3). Mars density ratio ρ_s/ρ_air and therefore relative inertia is much greater than Earth values, which means that higher velocity winds are required to move particles of similar size. The expected outcome for Mars is that saltation path lengths will generally be greater, and that impact splash will be more effective at launching other grains and establishing impact creep. Measured wind speeds to 100 km/hr (~28 m/s) are not uncommon (e.g., Curiosity Rover) which are more than enough to keep Martian sand moving. It also means that, theoretically, air turbulence will have little influence on saltating grain trajectories.

Venus

At the other extreme, Venus’ atmospheric pressure of ~90 atmospheres, and density of 65 kg.m^-3 means that the density ratio is roughly equivalent to ultra-heavy minerals (like gold) in water on Earth (the surface temperature is a balmy 460^oC). Thus, relatively low velocity winds should be capable of moving significantly larger grains than equivalent velocities on Earth. For example, the fluid threshold velocities for fine grained sand on Earth (100-250 μm) are about 0.2 – 0.3 m.s^-1, compared with about 0.02-0.03 m.s^-1 on Venus – almost an order of magnitude lower. In this case, air turbulence will have much greater influence on grain trajectories than on Earth or Mars.

Titan

Titan’s atmosphere contains about 95% N and 5% Methane. Surface temperatures average -179^o C which is close to the freezing point of methane. Broad-spectrum radar imaging and spectroscopic data from Cassini orbiter indicate aeolian-like structures that appear to be composed of solid, granular methane. Titan’s atmosphere is about 5 times as dense as Earth’s (~5.2 kg.m^-3); solid particle density is estimated at 950 ±450 kg.m^-3 (slightly less than liquid water). Fluid velocity threshold values will also be influenced by cohesive forces at grain-to-grain contacts to a greater degree than on Earth or Mars. Modeling, experimental, and theoretical considerations by Comola et al., (2022, OA) suggest a grain size of 2 mm for the minimum fluid threshold that leads to aerodynamic lift of grains, and 0.1 mm for impact thresholds. This means that saltation is possible at relatively low flow velocities but grain launching by impact splash may be more important than forces directly involved with aerodynamic lift.  

Aeolian features on Venus

Of all the rocky planets, Venus’ surface is the youngest. Resurfacing by geologically young impacts, volcanism, plus mechanical and chemical weathering has replaced or covered rocks older than about 500 million years (age estimate based on crater densities). Our first close views of Venus’ surface were delivered by the Magellan orbiter (1990-1994) that conducted extensive radar surveys of Venus, mapping about 98% of its surface. The Magellan data provided evidence for winds strong enough to erode and mould geomorphic structures like yardangs and disperse sediment as dune fields and wind streaks. The dunes have the appearance of self-organized bedforms like those on Earth. More recent views of Venus in the visible part of the light spectrum were obtained by the Parker Solar Probe’s flyby in 2021.

Wind streaks: Wind streaks are one of the most common aeolian structures on all the planetary bodies. They form from the linear distribution of granular material downwind of geomorphic obstacles such as hills, ridges, volcanic cones, large sand dunes, or large impact ejecta fragments. Streak sediment dispersal becomes increasingly diffuse downwind. Almost 6000 streaks have been identified on Venus. They have been used to decipher wind directions in the lower atmosphere.

Dune fields: Two extensive dune fields have been identified from the Magellan radar imagery: Menat Undae (Aglaonice dune field), and Al-Uzza Undae (Fortuna–Meskhent dune field). They are inferred to be transverse dunes based on their consistently parallel crests, that also tend to be normal to wind streaks – these geometric trends are clearly visible in the Fortuna–Meskhent field imaged above. Dune crest lengths range from 0.5 km to 5 km. Each dune ridge also shows a consistent radar bright side (facing top left), inferred to be dune lee faces (down-wind face) (Greeley et al. 1992; op. cit.; Carter et al., 2023, OA).

Yardangs: Yardangs on Earth are subparallel ridges and furrows formed by wind erosion and deflation of sediment or bedrock. They tend to be streamlined teardrop shaped with the broader, steeper face upwind. The consistency of these geometrical attributes means they can be used to decipher wind directions. Yardangs tend to have well defined boundaries and do not originate from obstacles, characteristics that distinguish them from wind streaks. Greeley et al., (op. cit.) identified an area of possible yardangs near Mead crater that are about 25 km long and 0.5 km wide.

Aeolian features on Titan

The most recent data on Titan is from the highly successful Cassini orbiter that made more than 100 flybys of the moon from 2004 to 2017 (NASA-ESA) . Instrumentation included radar, infrared and visible range imaging, as well as the Huygens Lander that survived 72 hours on Titan’s surface. Mapping at 10 km and in some cases 1 km surface-feature resolution was possible.

Three sand seas consisting of extremely large longitudinal dunes have been mapped in the equatorial regions of Titan; Belet and Fensal (imaged below), and Shangri-La sand seas. They cover about 13% of Titan’s surface. Radar imaging shows dune characteristics that are analogous to terrestrial dunes, including (Jaumann et al., 2009; Radebaugh et al., 2022):

• Straight to slightly arcuate crest lines with relatively consistent orientations over large areas. They range to over 400 km in length, averaging 40 km. Morphologically they are like longitudinal dunes in the Namib Sand Sea. Orientation is predominantly east-west.
• Dune size and shape appears to vary according to latitude and altitude, perhaps a function of sediment availability and wind strength.
• Local bifurcation of crest lines
• Deflection around topographic obstacles (the bright radar patches in Belet dunes shown below).
• Streaking and sediment dispersal downwind of topographic obstacles.

The existence of dunes presents a dilemma because the source, composition, and possible cohesiveness of granular sand-sized grains is poorly constrained, although it is highly likely they consist of organic compounds like solid methane and ethane. One generally accepted explanation posits sand solidifying from rain (see references in Comola et al., 2022, op. cit.). The other problem is wind strength – is it sufficient to maintain fluid threshold velocities for extended periods?

It is anticipated that the primary mode of aeolian sand transport on Titan is saltation. The mechanics of sand grain entrainment needs to account for an atmosphere about 5 times as dense as that on Earth, grains that are less dense than water and probably more cohesive than grains like quartz and feldspar. The gravity field is about 14% of that on Earth.

The threshold for launching grains depends on the difference between aerodynamic lift forces and forces that resist movement (gravity, friction, cohesion). Comola et al., (2022, op. cit.) note that the prevailing winds may not be strong enough to mobilize grain entrainment, but that storm winds may be sufficient to provide aerodynamic lift. Their modeling and wind-tunnel experiments indicate that the threshold for grain launching by impact splash is lower than that for aerodynamic lift. They suggest impact splash may be a more important process and that dune migration may be more intermittent than their terrestrial counterparts.

Aeolian features on Mars

Aeolian processes involving erosion and deposition have moulded the Martian landscape over the last 3 billion years. Images obtained by multiple satellite orbiters and landers show a veneer of sand in dune bedforms of shapes and sizes comparable with those on Earth, in broad sand seas (ergs), and more localized accumulations on crater floors and ancient river valleys. Erosional features include yardangs and deflation surfaces.

Aeolian processes were also important contributors to the rock record during the first billion years of Martian history (Noachian and Hesperian periods). Rock units like the Stimson Formation are stratigraphically associated with deposits of fluvial and lacustrine environments, and Martian seas.

Images and discussion of modern and ancient Martian sand dunes are presented in separate articles.

Reference cited but not sighted

Radebaugh, J., Rose, D., Wright, M., Lake, B., Tass, S., Christiansen, E., Rodriguez, S., and Turtle, E.: Dune length, width and orientation in the sand seas of Titan reveal regional properties, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8961 (paywalled).