An Introduction to aeolian processes and deposition, comparing the conditions on Earth and Mars.
Mars current climate is basically a frigid desert at all latitudes, including the ice-bound poles. Martian weather, like Earth, is seasonal by virtue of Mars’ obliquity and an elliptical orbit around the sun. And like Earth, global atmospheric circulation on Mars involves Hadley Cell-like overturning at equatorial latitudes, and cyclone-anticyclone systems at mid- and polar latitudes in each hemisphere. Hadley cells can extend almost 90o poleward during Martian solstices. Vortices also exist at the poles and the associated jet streams move air masses from west to east.
However, there are also important differences borne of:
- Mars’ rotational axis is currently 25o but varies from 15o to 45o on Milankovitch obliquity time scales. In comparison, Earth’s axis is currently about 23.4o, and varies by <2o.
- Mars rotation period is 24 hours and 39 minutes (1 sol) compared with an Earth solar day of 24 hours (the sidereal day is 23 hours, 56 minutes, and 4 seconds). However, the Martian year is 668.6 sols or 687 Earth days.
- Mars orbital ellipse is significantly greater than Earth’s, with perihelion about 1.4 AU, and aphelion about 1.7AU (Earth values are 1.01 and 0.98 AU respectively).
- Mars is about 0.5 AU farther from the sun than Earth.
- Solar radiance on Mars is about 50% that on Earth.
- The density of Mars atmosphere (~0.02 kg/m3) is less than 2% of that on Earth. Carbon dioxide constitutes 95% of its atmosphere. Surface pressure on Mars is about three orders of magnitude less than the pressure at sea level on Earth.
- Mars gravity constant is about 38% of Earth’s (3.73 m/s2 compared with 9.8 m/s2 respectively).
- Mars has had no oceans, lakes, or river systems for the past 3 billion years.
Hadley cells
Air pressures and temperature gradients between tropical and subtropical latitudes on Earth (ie. 0o to 30o) cause global-scale, meridional (longitudinal) circulation gyres, referred to as Hadley cells or Hadley circulation (after George Hadley, 1735). Air heated at the tropics rises to the top of the troposphere and flows poleward where it cools, sinks, and flows back to the topics. Separate cells usually develop in each hemisphere during spring and autumn. These two cells merge during the summer and winter months to form a single cross-hemisphere cell where flow is from the warm summer to cold winter hemisphere, with return flow to the warm side. Analogous circulation cells form in the mid latitudes (approximately 30o to 60o N and S) and over the poles.
Atmospheric circulation models for Mars are based on multi-year measurements obtained from multiple orbiting satellites and landed rovers. Martian circulation appears to be best described by equatorial Hadley cells that expand and contract longitudinally with the seasons. Hadley circulation is also hypothesized for Venus and Titan.
The meridional component of circulation in these plots reflects seasonal changes in (contoured) atmospheric mass transport and overturning of warm and cold air (shown here as clockwise and anticlockwise flow). Seasonal changes in circulation will also manifest as changing wind patterns and temperatures across the Martian surface. Winter temperatures at the poles are cold enough to freeze CO2 as frost and snow, and any water that remains. Frosty conditions extend to the mid latitudes. Seasonal warming results in sublimation with return of these two phases to the atmosphere. It is expected that ground freezing will reduce the availability of granular sediment that would otherwise be incorporated into dunes and sand sheets.
Dust storms are also an important part of atmospheric cycling; the storms are more prominent in the northern hemisphere during autumn and winter and can completely encircle the planet. Dust also increases the radiative capacity of the atmosphere.
Sand dune distribution
Active and recently active sand dunes and wind streaks are widespread across Martian landscapes, on basin plains, crater floors, ancient river valleys and canyons, structural depressions, and volcanic paterae (caldera-like depressions). Dune fields range in size from localized accumulations confined by valley or crater walls to fields extensive enough to be called ergs (sand seas). Dune crest heights range from about 120 m to <2 m.
Active dunes require a supply of sand and wind velocities at or greater than the fluid and impact thresholds for grain saltation and grain creep. The average global dune migration rate for barchan dune is about 0.5 m/year, based on multi-year images of dunes and dune fields, but can be as rapid as 12 m/year (Bridges et al., 2017, OA).
The amount of sand supplied to individual dunes can be approximated by measurements from multi-year satellite imagery. Sand flux measures are based on the product of dune crest height and crest migration distance over a specified time – expressed as m3.m-1.yr-1. Chojnacki et al., (2019, OA) identified some of the conditions required to maintain sand flux and dune migration (using images obtained by the HiRISE camera on Mars Reconnaissance Orbiter). For regions of high sand flux, these conditions include:
- Significant transitions in topography and elevation, from lowland basins like those overlying massive impact craters to higher relief uplands. Elevation gains of several kilometres are possible from basin to range.
- Seasonal and diurnal thermal effects resulting in part from differences in albedo between basin and uplands are superimposed on meridional, mid-, and polar atmospheric circulation cells. Extensive dune fields adjacent the north polar ice cap experience katabatic winds that are strong enough to maintain some of the most extensive, active dune fields on Mars (cold air over the ice cap flows downslope). Contrasting conditions exist in large fields like Syrtis Major (about 30o N) and Hellespontus Montes (about 30o S). where thermally-driven anabatic winds result from significant differences in albedo between impact basins and volcanic uplands (anabatic winds are driven upslope by thermal gradients). The anabatic winds in these regions are probably superimposed on meridional circulation.
- The winter months on Mars see an expansion of the polar ice caps (mostly frozen CO2) and frost-cemented ground at lower latitudes. Freezing inhibits sand entrainment. The ice caps retreat during spring and summer thaw so that exposed sand becomes available for redistribution dune bedforms reactivated by strong katabatic winds. Chojnacki et al., (op.cit.) note that dune lee slopes are prone to failure because of ice loading. Collapse of frozen sand from dune crests and lee-slope grain flows may be more common in segments of a dune that thaw more rapidly than other segments.
- Dust storms: Dust storms are a regular feature on Mars; major storm systems that can encircle the entire planet surface occur every 4-5 Earth years. The storms have a significant impact on solar insolation, atmospheric temperatures, surface albedo, wind patterns and ferocity, and the CO2 and water vapour flux to the polar caps. They also move massive amounts of particulate matter at local and global scales. Airborne dust presents a real hazard for mobile rovers and for their future human counterparts; NASA’s Opportunity rover mission was terminated by a massive storm in June, 2018 (after 5111 sols of mission activity). Dust storms are most common during perihelion when Mars receives almost 40% more sunlight. Surface heated air rises rapidly in plumes that carry massive amounts of very fine solid particles. Localised dust storms can be as brief as a single sol, but major events can keep dust lofted for months. (Guzewich et al., 2019, OA).
