Obliquity dependence of the formation of the martian polar layered deposits

Abstract

Mars' polar layered deposits (PLD) are comprised of layers of varying dust-to-water ice volume mixing ratios (VMR) that are thought to record astronomically-forced climatic variation over Mars’ recent orbital history. Retracing the formation history of these layers by quantifying the sensitivity of deposition rates of polar material to astronomical forcing is critical for the interpretation of this record. Using a Mars global climate model (GCM), we investigate the sensitivity of annual polar water ice and dust surface deposition to a variety of obliquity and surface water ice source configurations at zero eccentricity that provide a reasonable characterization of the evolution of the PLD during recent low-eccentricity epochs. The GCM employs a fully interactive dust lifting/transport scheme and accounts for the coupling effects of the physics of dust and water on the transport and deposition of water ice and dust. Under a range of tested obliquities (15°–35°), the predicted net annual accumulation rates range from −1 ​mm/yr to +14 ​mm/yr for water ice and from +0.003 to +0.3 mm/yr for dust. GCM-derived accumulation rates are ingested into an integration model that simulates polar accumulation of water ice and dust over five consecutive obliquity cycles (∼700 thousand years) during a low eccentricity epoch. A subset of integration model simulations predict combined north polar water and dust accumulation rates that correspond to the observationally-inferred average growth rate of the north PLD (0.5 ​mm/yr). These integration model simulation results are characterized by net water transfer from the south to the north polar region. In the north, a ∼230 ​m-thick deposit is accumulated over ∼700 thousand years. Three types of layers are produced per obliquity cycle: a ∼30 m-thick dust-rich (20–30% dust VMR) layer that forms at high, a ∼0.5 m-thick dust lag deposit (pure dust) that forms at low obliquity, and two ∼10 m-thick dust-poor (∼3%) layers that separate the dust rich layers and form when obliquity is increasing or decreasing. The ∼30 m-thick dust-rich layer is reminiscent of a ∼30 m scale length feature derived from analysis of visible imagery of north PLD vertical structure, while the ∼0.5 m-thick dust lag is only a factor of ∼2 smaller than observed “thin layers”. Overall, this investigation provides further evidence for an obliquity forcing in the PLD climate record, and demonstrates the importance of ice-on-dust nucleation in polar depositional processes.