The Spirit rover -- and its companion Opportunity -- both landed on opposite sides of Mars earlier this year, each carrying permanent magnets to collect airborne magnetic dust particles and magnetic particles in the surfaces of rocks. One set of magnets was carried by a tool that grinds into rock surfaces. Another two magnets, meanwhile, were mounted on the front of the rover at an angle so that all non-magnetic airborne particles slipped off, while retaining any magnetic airborne dust.

Both of these two magnets are positioned near a panoramic camera that measures how the dust absorbs light as a function of wavelength. The scientists found that the dust collected by the stronger, "filter" magnet had a lower reflectance. It is therefore intrinsically darker in colour and contains either magnetite or maghemite (γ-Fe2O3). However, the dust collected by the weaker, "capture" magnet, is mostly haematite (α-Fe2O3), which is red. The results were obtained from dust in the Gusev crater landing site, which lies 15 degrees south of the equator and is thought to have once been a water-filled lake.

The Danish-German-US group now hopes to obtain chemical and mineralogical information on the dust collected by the two magnets when alpha-particle X-ray and Mössbauer spectra are recorded and analysed. "We believe that after this mission, we will know why Martian dust is magnetic," says team member Preben Bertelsen from the University of Copenhagen. "Furthermore, we will compare the magnetic properties results from the two rovers to see if the atmospheric dust has the same composition on both sides of the planet."

Since all airborne dust particles are magnetic, the magnets might also be used as dust shields to keep nearby surfaces clean during a long mission to Mars. "They may even help to protect future astronauts from dust," adds Bertelsen. With additional collection and analysis of magnetic dust, scientists now hope to identify how water, volcanic activity and erosion have altered the surface of Mars. The results are presented in a special issue of Science, which contains a total of 10 papers by researchers based on the first 90 Martian days of the Spirit rover's mission to the Gusev crater.