Observations and results from the magnetic properties experiments on MER – Københavns Universitet

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Results from the magnetic properties experiments on MER

Below is a summary of observations and results from the magnetic properties experiments (MPE) on the Mars Exploration Rovers and previous missions. 

Magnetite or titanomagnetite makes the Martian dust magnetic

The magnetism of the airborne dust is due to its content of magnetite (Fe3O4). This was revealed from Mössbauer spectroscopy data of dust accumulated on the Spirit capture magnet after several integrations by the Mössbauer spectrometer, both before and after significant wind gust events that helped concentrate the most magnetic particles on the magnet.

It is possible that the mineral responsible for the magnetic properties of the Martian dust is titanomagnetite (Fe3-xTixO4) rather than pure magnetite. This idea comes from the APXS data from the Opportunity capture magnet from sol 53 through sol 335, where it is seen that the increase of iron and titanium over time is well correlated.

The ratio Fe/Ti is almost constant over time, perhaps increasing a little. If titanium was in a different phase than the iron, the titanium signal would decrease after a strong wind gust, because this phase would not be magnetic. But since the iron and titanium signals are correlated, the iron and titanium are most likely in the same phase.

The small increase in the iron signal could be due to strong wind effects removing a larger fraction of more strongly Ti-substituted magnetite than pure magnetite, since the latter is more magnetic. The suggestion of the presence of titanomagnetite rather than pure magnetite is supported by the Mössbauer spectrum of the same sample, which is not exactly consistent with pure stoichiometric magnetite.

Martian dust is not magnetically homogeneous

Several observations indicate that the Martian dust is not magnetically homogeneous, but can be separated into more and less magnetic grains. One indication of this is that the saturation magnetization of airborne dust accumulated on the MPE magnets on Mars has been shown to be higher than the saturation magnetization of a population of dust particles observed in dusty areas on the ground.

Another indication is observations of color variations on the MER capture magnet, where darker dust was observed to accumulate on the edge, and brighter dust in the center of the magnet.
Numerical modeling of dust grain capture, taking into account wind speed, grain size and dust magnetic properties, shows that highly magnetic dust grains will prefer to stick on the edge and less magnetic grains in the center of the magnet. This can be explained by the fact that the strong magnetic field provided by the permanent magnets will deflect the paths of the most magnetic particles in the vicinity of the magnet and these attracted particles will most probably stick to the magnet at the edge, which is where they will first encounter a very strong field.
Since magnetite, which is supposedly the most magnetic component of the dust, is very dark, this explanation is consistent with the observations of color variations. Similar color variations were observed on the MER sweep magnet.

Image Credit: NASA/JPL/Cornell/NBI-UCPH.

The image shows the capture (top) and filter (bottom) magnet on MER-A (Spirit) on sol 240 (left) and sol 425 (right). A strong wind gust passed the rover on approximately sol 418. Apparently the wind has blown away the less magnetic particles leaving behind the more magnetic ones, which are darker.

Also the ability of strong wind effects to remove some of the material, leaving behind the darker material, supports the statement that the dust is magnetically separable with magnetite as the most important magnetic component.

The MER capture magnet captures any magnetic particle from a small volume above the magnet, while the MER filter magnet captures only the most strongly magnetic particles, although from a somewhat larger volume. Early in the mission the dust on the filter magnet on both rovers was observed to be significantly darker than the dust on the capture magnet. Again this is consistent with magnetite being the dominant magnetic component of Martian airborne dust.

All Martian atmospheric dust particles are magnetic

Observations of the MER sweep magnet implies that there is no, or at least only a small amount of, completely non-magnetic dust in the Martian atmosphere. The sweep magnet was designed to keep even slightly magnetic dust away from its center, allowing only non-magnetic dust to settle, and no dust has been observed to settle in the center. It is of course possible that a small amount of non-magnetic dust has settled in the center without detection.

This ability of the sweep magnet to keep the central area clean has later been put to use in an improved sensitivity sweep magnet experiment (iSweep) on the Phoenix mission, which was again used to examine whether non-magnetic dust had settled on the center of any of its six sweep magnets.

Image credits:
Left: NASA/JPL/Cornell/NBI-UCPH.
Right: LPL-University of Arizona/NASA-JPL/NBI-UCPH/Texas A&M University.

The image on the left shows the Sweep magnet on MER-A (Spirit), sol 206. The image on the right shows the iSweep after exposure to magnetic Salten Skov dust in our Mars Simulation Dust Sedimentation Chamber (Salten Skov is an area in Denmark, near the city of Aarhus).

Strongly magnetic particles form chains

Chain formation has been observed after strong wind events, where the wind has blown away the less magnetic particles and the remaining strong magnetic particles form chains. Chain formation is characteristic of strongly magnetic material in grains which do not contain a large amount of non-magnetic material.

Image Credit: NASA/JPL/Cornell/NBI-UCPH/USGS.

The image shows magnetic chain formation as seen on the MER-B (Opportunity) capture magnet. On the left is an image acquired by the Microscopy Imager on sol 337 after a strong wind gust on sol 331 had removed some of the lesser magnetic material. On the right is a zoom of that image.

Saturation magnetization of Martian dust

The dust investigated so far has been shown to have a saturation magnetization of less than 2 Am2/kg. However it is possible to separate the airborne dust into populations of more and less magnetic grains. Magnetizations above 7.2 Am2/kg have been observed in the most magnetic of these grains.