Experimental determination of remanent magnetism of dusty ice deposits

Abstract

Accumulations of ice and dust mixtures may acquire magnetization during deposition in a manner analogous to sedimentary rocks. Here, we consider the process of particles descending through an atmosphere and depositing in a preferential orientation that serves to record the ambient magnetic field during emplacement. We use a simple model for the settling and reorientation of ice particles with magnetic inclusions that includes magnetic torque, aerodynamic forces and gravity, to investigate the parameter space governing the process. For fields in the range of 10's – 100's μT we find that ice particles of sizes up to ∼100 μm which contain smaller magnetic grains as nuclei will produce a deposit indeed magnetized in the direction aligned with the applied field, but with a moment that is independent of the field strength. For particles in the 100's μm range, the magnetic moment increases with the field strength. To demonstrate the effect experimentally, we performed a suite of laboratory deposition simulations followed by measurements of the magnetic moment of the samples. We show that in the idealized laboratory conditions dusty ice magnetizes in the direction of the applied field, with the alignment increasing with its intensity. For the chosen conditions, the magnetization increases rapidly with field intensity in the range 10 – 200 μT, and approaches a maximal value above that. For a mixture with dust/ice ratio of 5×10−3 we obtained maximal magnetization values in the range 1.6×10−5 – 3×10−3 Am2/kg, depending on the distribution of particle sizes. We show that magnetic particle concentration in the ice determines the level of magnetic remanence, and conclude that the remanent magnetization of natural ice deposit in various settings may be measurable (if unobscured by post-depositional, wind, or other effects) and thus could provide a new paleomagnetic record on Earth and other planetary objects.