Regional dust deposits on Mars: Physical properties, age, and history

Abstract

Major dust storms on Mars play an important role in the deposition and removal of fine dust material. Thermal, radar, and visual remote sensing observations provide important constraints on the Martian regolith which have been used to determine the location and physical properties of regional dust deposits. These deposits are located in three northern equatorial regions, Tharsis (−20°S to 50°N, 60° to 190°W), Arabia (−5°S to 30°N, 300° to 360°W), and Elysium (10° to 30°N, 210° to 225°W). They are covered by fine (∼2–40 μm), bright (albedo > 0.27) particles, with fewer exposed rocks and coarse deposits than found elsewhere. Dust is currently deposited uniformly throughout the equatorial region at a rate of ∼40 μm/global storm. Over geologic time the rate of accumulation may vary from 0 to 250 μm/yr due to changes in atmospheric conditions produced by orbital variations. Dust deposited during global storms is subsequently removed only from dark regions, resulting in a net accumulation in the low‐inertia, bright regions. The thickness of these current dust deposits is 0.1–2 m. The thermal inertia places a lower limit of ∼0.1 m on the thickness of these deposits, while the sparse but ubiquitous presence of exposed rocks and the degree of visible mantling indicate that the thickness is less than 5 m. Dual‐polarization radar observations of a very rough texture in Tharsis are consistent with this model, with a ∼2‐m‐thick dust layer burying most of the surface rocks but permitting radar sampling of the rough subsurface. Based on their thickness and rate of accumulation, the age of these deposits is 105–106 years, suggesting a cyclic process of deposition and removal. One possible cause may be cyclic variations in the magnitude and location of maximum wind velocities related to variations in Mars' orbit. At present, perihelion and maximum wind velocities occur in the south, whereas regional dust deposits occur in the north, suggesting net transport from south to north. Orbital parameters oscillate with periods ranging from 5×104 to 106 years. The agreement between these periods and the dust deposit age suggests a possible link. At different stages in orbit evolution, maximum wind velocities will occur in the north, with subsequent erosion and redistribution of the accumulated fines. Based on this model, much of the uppermost Martian surface is very young and is being continually reworked.