The Bagnold dune field, Gale crater

A series introducing aeolian processes and deposition, comparing the conditions on Earth and Mars.

Mars’ landscaping over the past three billion years has been dominated by aeolian processes, interspersed with the occasional impact and volcanic eruption (this is the Amazonian Period). Bedrock erosion and deflation, and wind-blown transport of granular sediment and dust have contributed to the current distribution of aeolian bedforms – sand dunes and ripples of all sizes and geometries, sand sheets, and wind streaks. Orbiting satellites from the earliest Mariner and Viking missions, to the current Mars Express, Mars Reconnaissance Orbiter (that has the extraordinary HiRISE imaging system), and Tianwen-1 missions have provided a wealth of data and imagery. However, it is Curiosity rover’s exploits in Gale crater (2012 to present) that have provided a much closer, almost hands-on examination of aeolian bedforms in the Bagnold dune field. Curiosity’s field work has shown that many aspects of sand dune formation and structure are similar to those seen on Earth, but there are also bedforms that prove the exception.

Gale crater is an impact structure. Its age has been determined from two dating methods. A potassium-argon age date performed by Curiosity’s mobile lab gave an age range of 4.21 ±0.35 billion years (the first radiometric dating conducted on another planet; Farley et al., 2013). Age estimates using the crater density method give a range of 3.6 to 4.1 billion years. In a time-stratigraphic context this puts the impact in the Noachian to early Hesperian.

Most of the information for this post is gleaned from Ewing et al., (2017, OA); Bridges et al., (2017, OA); and Chojnacki and Fenton (2017, OA) – see references for links.

The Bagnold dune field

The field, named after Ralph A. Bagnold (1896–1990), is a narrow band of active dunes bordering the north rim of Aeolis Mons (Mt. Sharp) in the Noachian – Early Hesperian Gale crater. Longitudinal dunes and sand sheets occupy much of the inner field; barchan dunes appear to be strung along the outer, northern margin of the dune field. Larger dune fields are located west, south and east of Aeolis Mons.

Curiosity’s 68 sols of field work focused on two barchan dunes informally named High dune and Namib dune (Sol is the abbreviation for solar day). The dunes have characteristic crescentic lee faces, up to 4 m high on Namib dune, with active slopes of 29^o-33^o (approximating the angle of repose). Unlike Earth, the dunes have two active lee faces; a dominant primary face, and a secondary face. Stoss slopes up to 130 m wide (measured normal to the lee face) dip 7^o-10^o. HiRISE image analysis of large dune lee faces and crest line orientations of smaller scale ripples indicate winds predominantly from the west, with diurnal rotation to ENE.

Rover excursions included the lower stoss slope of High dune, and a frontal attack along the base of the active lee faces of Namib dune. Bedform analysis in these domains is based on HiRISE images and derived digital elevation models, and Curiosity MastCam images; Mars Hand Lens Imager (MAHLI) provided data for grain size analysis. Primary sedimentary structures include:

The main dunes.
Small stoss slope ripple trains referred to as impact ripples, superposed on
Metre-high stoss-slope ripples (large ripples).
Lee slope grain flows.
Slope failure and translational gravity slides.

Grain size and composition

Estimates of Martian grain size distributions utilize handlens images like the one shown here. Statistical measures that describe these sands are based on grid counting, assuming grain orientations that exposed their long and intermediate axes (Ewing et al., op. Cit). Note that this differs from standard sieve methods that assume minimum grain diameters that will pass through a specified mesh. The modal long axis length is 0.11 mm; the average is 0.135 mm. Both qualify as fine to very fine sand. The grain size spread includes sand as coarse as 0.3 – 0.35 mm.

Mineral compositions are determined from Curiosity’s elemental analytical capabilities that include surface sampling tools, spectrometers, X-ray diffraction, and the chemistry camera (ChemCam). Bulk sand compositions are basaltic. Enrichment in Si-Mg-Fe-Ni are correlated with minerals like olivine and pyroxene, calcic feldspar, and subordinate quartz, magnetite, ilmenite, hematite, and anhydrite. There is also amorphous silica that may represent volcanic glass or meteorite impact melt.

Metre-scale (large)ripples

These bedforms are actively forming on the stoss and lee slopes of both dunes. They are visible in HiRISE imagery. They were first reported by Lapotre et al., (2016). They have no counterparts on Earth’s aeolian dunes.

Large ripple amplitudes are on the order of 10-15 cm and mean wavelengths of 2.1 m. Crest lines are sinuous or arcuate. Dominant crest orientations of NW and NE give the appearance of cross-hatching when viewed with orbital images. Ripple profiles show some variability between the High dune and Namib dune. Large ripples were also observed on the lee slope of Namib dune.

On Namib dune ripple profiles are strongly asymmetrical with prominent lee faces; on High dune, the profiles are more symmetrical. Lee face dips on Namib dune are angle of repose, but commonly are oversteepened resulting from local collapse of ripple crests. Grain flows are also common on angle of repose slopes (these structures are described below). Lee slopes on High dune tend to be shallower. Large ripples on High dune differ further from those on Namib dune where grain size partitioning results in slightly coarser sand accumulating along the crests.

Formation of the large ripples remains a puzzle. They appear to have formed in concert with the small-scale ripples, by virtue of the parallelism of their crest lines. However, there is no terrestrial analogue for two scales of ripples forming simultaneously by saltation impact – as such the large ripples are deemed not to be impact structures.

Lapotre et al., (op. cit) and Lapotre et al., (2021, OA) suggested the large ripples resemble fluid drag bedforms that on Earth abound in subaqueous environments. On Earth, subaqueous fluid drag moves sediment along a traction carpet and, if flow velocities are high enough, with an added saltation load. They suggest that similar processes may exist on Mars because of the higher kinematic viscosity in the low-density atmosphere – in other words the bedforms are larger because of the lower density atmosphere.

Small-scale ripples (impact ripples)

Small-scale ripples are everywhere superimposed on the metre-scale structures. They also occur on interdune flats and across the lee slopes of High and Namib dunes. Most have straight crest lines that occasionally bifurcate. Amplitudes are on the order of 10 mm; ripple wavelengths range from about 50 mm to 200 mm. Profile symmetry seems to be variable where some bedforms have distinct lee slopes and others are more symmetrical. They are composed on fine to very fine sand and tend not to show grain size partitioning over crests or the base of lee slopes, unlike some larger ripples.

Sets of small ripples tend to parallel the average crest orientations of large ripple, but there is some variation where they wrap around large ripples. Small ripples also form on the lee slope of High and Namib dunes where they are parallel or slightly oblique to lee slope dips. These bedforms are like the rippled surfaces of dunes on Earth. Ripple formation and migration is usually attributed to saltation impact and impact creep.

Grain flows

Grain flows are a common feature of terrestrial dune lee slopes, and this appears to be the case for Martian dunes and large ripples. Subaqueous and subaerial grain flows are well documented on Earth. They are dense mixtures of a fluid, and granular sediment that tends to be well sorted. In aeolian settings, flows, or sand avalanches are maintained by dispersive pressures generated by inter-grain collisions.

Grain flows can form anywhere across sand dune lee slopes. The flow head-wall is amphitheatre-like, bound by a shallow scarp. Flows typically taper and broaden downslope. Toes tend to be thickened. Toes may also show a kind of stacking where successive flow pulses terminate progressively upslope. Flows can occur as single events, or as a series of overlapping lobes (e.g., Cornwall et al., 2018, OA). Grain flows are important processes in the overall migration pathways of active dunes. Triggering mechanisms include:

Lee slope steepening during periods of strong winds and increased sand movement over the dune brink (grain fall) or steepening by gravitational collapse.
Loss of cohesion from wet to dry conditions (more likely on Earth).
Loss of cohesion during spring thaw and sublimation of Martian frost and ice cements.

Grain flows and translational slides commonly occur together on Martian dunes and large-scale ripples.

Translational slides

Failure of relatively steep lee slopes is common on terrestrial aeolian dunes and on Martian dunes and large ripples. Slides move as relatively coherent blocks of sand several laminae thick, or as disaggregated blocks that rotate as they move down slope on a common detachment surface. Slide head-walls are usually steep, amphitheatre-like but extend deeper into the dune deposits than grain flow headwalls. Structural stacking of dislocated blocks can also occur at slide toes. Grain flows may evolve from the toes of slides.

Translational slides have been imaged on both Namib and High dune lee slopes, and on large ripple lee slopes, particularly where profiles are highly asymmetric.

Reference Links

Bridges et al., 2017, OA; Martian aeolian activity at the Bagnold Dunes, Gale crater: The view from the surface and orbit.

Chojnacki and Fenton 2017, OA; The Geologic exploration of the Bagnold dune field at Gale crater by Curiosity Rover.

Ewing et al., 2017, OA; Sedimentary processes of the Bagnold Dunes: Implications for the aeolian rock-record of Mars.

Farley, K.A., et al., 2013. In Situ Radiometric and Exposure Age Dating of the Martian Surface. Science, Vol 343, Issue 6169.