Abstract. The Mars Pathfinder lander carried two magnet arrays, each containing five small permanent magnets of varying strength. The magnet arrays were passively exposed to the wind borne dust on Mars. By the end of the Mars Pathfinder mission a bull's-eye pattern was visible on the four strongest magnets of the arrays showing the presence of magnetic dust particles. From the images we conclude that the dust suspended in the atmosphere is not solely single phase particles of hematite (a-Fe203) and that single phase particles of the ferrimagnetic minerals maghemite (-y-Fe203) or magnetite (Fe304) are not present as free particles in any appreciable amount. The material on the strongest magnets seems to be indistinguishable from the bright surface material around the lander. From X-ray fluorescence it is known that the soil consists mainly of silicates. The element iron constitutes about 13% of the soil. The particles in the airborne dust seem to be composite, containing a few percent of a strongly magnetic component. We conclude that the magnetic phase present in the airborne dust particles is most likely maghemite. The particles thus appear to consist of silicate aggregates stained or cemented by ferric oxides, some of the stain and cement being maghemite. These results imply that Fe 2+ ions were leached from the bedrock, and after passing through a state as free Fe 2+ ions in liquid water, the Fe 2+ was oxidized to Fe 3+ and then precipitated. It cannot, however, be ruled out that the magnetic particles are titanomagnetite (or titanomaghemite) occurring in palagonite, having been inherited directly from the bedrock. In addition to the backhoe magnets (strong and weak), each Viking lander carried a reference test chart (RTC) magnet, which was passively exposed to the dust suspended in the Martian atmosphere. The RTC magnets were of the strong type only.The backhoe magnets and the RTC magnets on the Viking landers were constructed as disc-shaped magnets of diameter 6.5 mm surrounded by a ring magnet of an outer diameter of

Many of the particles currently suspended in the martian atmosphere are magnetic, with an average saturation magnetization of about 4 A·m 2 /kg (amperes times square meters per kilogram). The particles appear to consist of claylike aggregates stained or cemented with ferric oxide (Fe 2 O 3 ); at least some of the stain and cement is probably maghemite (γ-Fe 2 O 3 ). The presence of the γ phase would imply that Fe 2+ ions leached from the bedrock, passing through a state as free Fe 2+ ions dissolved in liquid water. These particles could be a freeze-dried precipitate from ground water poured out on the surface. An alternative is that the magnetic particles are titanomagnetite occurring in palagonite and inherited directly from a basaltic precursor.

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