PhD defense by Line Drube – Niels Bohr Institute - University of Copenhagen

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PhD defense by Line Drube

Martian Airborne Dust - Magnetic Properties Experiment on Phoenix and dust on the MSL, Calibration Target

The Magnetic Properties Experiment (referred to as iSweep or Caltarget) onboard the Phoenix lander was carried out in the arctic region of Mars throughout the mission's 152 sols lifetime. The iSweep experiment involved periodic multispectral imaging of a target containing a series of ring-shaped permanent magnets. It was designed to attract airborne magnetic dust particles to certain areas on the iSweeps and prevent their accumulation in other areas. The dust settling rates on the different areas of the iSweep were investigated using data from terrestrial laboratory experiments with Martian dust analogues (JSC Mars-1 and HWMK101). Spectroscopic analysis of Phoenix data found the dust on the area directly above the magnets of the iSweep to be brighter in all spectral regions (420-1005 nm, though more in the infrared) than dust collected on the precursor Sweep Magnet Experiment onboard the two Mars Exploration Rovers, and brighter than surface soil near the lander. Like most other dust and soils on Mars, the Phoenix dust lacks the strong spectral signatures characteristic of highly crystalline phases. The areas directly above the open centers of the ring-magnets are accessible only to particles with a magnetic susceptibility below 10-3, and very little dust settled there. The ring-magnets had two purposes on the mission; one was to act as a magnetic properties experiment on airborne dust and the other was to keep certain colored areas as free of dust as possible, for use in radiometric calibration of images acquired by the Surface Stereo Imager.

The iSweep magnet was tested using the Mars (magnetic) dust analogue Salten Skov dust in a wind tunnel at Martian pressure and wind speeds between 1.0 m/s and 10 m/s, and it was found that the ring center accumulated less dust than a reference area with no magnetic field influence and that a less dusty region also extended downwind from the lee-side of the magnet.

Permanent ring-magnets have also been built into the calibration target of the Mars Science Laboratory (MSL), the same type of ring-magnet used in the Sweep magnet experiment on the Mars Exploration Rovers (MERs). Unfortunately, on MSL the ring-magnets were included at a very late stage in the development of the target (actually the target was a flight spare unit from the MER mission). This resulted in the ring-magnets being positioned at a depth of 0.8-1.0 mm below the surface instead of the 0.4 mm used on the MERs and Phoenix. From preliminary computer simulations this didn't appear to make a significant difference, other than in the size of the magnetically protected area. However, wind tunnel experiments using Salten Skov dust have now demonstrated that this relatively small difference in depth causes the "protected" area to disappear, so that with this new configuration the ring center will accumulate more dust than the reference areas free of influence from any magnetic field. With no clean area at all, magnets in this configuration will have the opposite effect to what they were intended to provide, attracting significant amounts of dust and retaining it on areas that are meant to be used as "dust-free" calibration standards.

Supervisor: Morten Bo Madsen, Niels Bohr Institute, University of  Copenhagen