Magnetic and optical properties of airborne dust and settling rates of dust at the Phoenix landing site

摘要

The Magnetic Properties Experiment (referred to as iSweep or Caltarget) onboard the Phoenix lander was executed in the arctic region of Mars during the mission's 152 sols lifetime. The iSweep experiment involved periodic multispectral imaging of a series of permanent ring magnets. It was designed to attract airborne magnetic dust particles to certain areas on the iSweeps thereby sorting all settling airborne particles at least to some degree according to their magnetic properties. The dust on the area directly above the strong magnets of the iSweep was found to be brighter than that collected on the precursor Sweep Magnet Experiment onboard the Mars Exploration Rovers near Mars' equator, and also this dust is found to be brighter than both surface soil near the lander and soil in the region surrounding the lander. As most other dust and soils on Mars, the Phoenix dust lacks strong spectral signatures of highly crystalline phases. For the first time, based on the complete calibrated data set of images of the iSweeps, spectra were extracted of the putative dust falling on the magnetically protected areas of the iSweeps. These areas are accessible only for particles with a magnetic susceptibility below 10~(-3). Spectra of this nonmagnetic dust are interpreted as signals from nonmagnetic minerals such as tectosilicates or glasses pigmented by poorly crystalline ferric oxides. Rates of dust settling were determined to be 1.08 μm/sol on the magnets and 0.06 μm/sol for the magnetically protected areas.