Source-to-Sink: An Earth/Mars Comparison of Boundary Conditions for Eolian Dune Systems

摘要

Eolian dune fields on Earth and Mars evolve as complex systems within a set of boundary conditions. A source-to-sink comparison indicates that although differences exist in sediment production and transport, the systems largely converge at the dune-flow and pattern-development levels, but again differ in modes of accumulation and preservation. On Earth, where winds frequently exceed threshold speeds, dune fields are sourced primarily through deflation of subaqueous deposits as these sediments become available for transport. Limited weathering, widespread permafrost, and the low-density atmosphere on Mars imply that sediment production, sediment availability, and sand-transporting winds are all episodic. Possible sediment sources include relict sediments from the wetter Noachian; slow physical weathering in a cold, water-limited environment; and episodic sediment production associated with climatic cycles, outflow events, and impacts. Similarities in dune morphology, secondary airflow patterns over the dunes, and pattern evolution through dune interactions imply that dune stratification and bounding surfaces on Mars are comparable to those on Earth, an observation supported by outcrops of the Burns formation. The accumulation of eolian deposits occurs on Earth through the dynamics of dry, wet, and stabilizing eolian systems. Dry-system accumulation by flow deceleration into topographic basins has occurred throughout Martian history, whereas wet-system accumulation with a rising capillary fringe is restricted to Noachian times. The greatest difference in accumulation occurs with stabilizing systems, as manifested by the north polar Planum Boreum cavi unit, where accumulation has occurred through stabilization by permafrost development. Preservation of eolian accumulations on Earth typically occurs by sediment burial within subsiding basins or a relative rise of the water table or sea level. Preservation on Mars, measured as the generation of a stratigraphic record and not time, has an Earth analog with infill of impact-created and other basins, but differs with the cavi unit, where preservation is by burial beneath layered ice with a climatic driver.