Workshop on Early Mars (1997) 3017.Pdf the MAGNETIC PROPERTIES EXPERIMENT on MARS PATHFINDER

Workshop on Early Mars (1997) 3017.pdf THE MAGNETIC PROPERTIES EXPERIMENT ON MARS PATHFINDER. Jens Martin Knudsen, Haraldur Pall Gunnlaugsson, Morten Bo Madsen, Stubbe F. Hviid, Walter Goetz, Niels Bohr Institute for Astronomy, Physics and Geo- physics, Universitetsparken 5, DK-2100 Copenhagen , Denmark. Introduction 2 A remarkable result from the Viking missions to Mars in 1976 was the discovery that the Martian soil is highly magnetic, in the sense that the soil is attracted by small permanent 1 magnets [1,2]. 4 The Viking landers carried two types of permanent mag- 3 nets, a weak and a strong magnet. The surface magnetic field and surface magnetic field gradient of the strong type magnet 1 were 250 mT and 100 Tm , respectively. The corresponding 1 numbers for the weak magnet were 70 mT and 30 Tm . Both types were mounted on the backhoe of the soil sampler, where they were exposed to the soil. A strong type magnet 5 was mounted in the Reference Test Chart (RTC) which was exposed to wind born particles exclusively. At both landing sites both backhoemagnets were saturated 3 with magnetic particles after few insertions into the soil. The RTC magnets gradually became saturated with magnetic dust during the mission. ath nder lander. The lab els refer to 1: Based on the returned pictures of the amount of soil Figure 1: The P wo sets of Magnet Arrays, 2: Tip Plate Magnet, 3: clinging to the magnets, it was estimated that the Martian The t dust contain between 1% and 7% of a strongly magnetic Ramp Magnets, 4: Imager IMP and 5: The So journer ver. phase, most probably a ferrimagnetic ferric oxide intimately Ro dispersed throughout the soil. Limits for the saturation 2 1 < magnetization s were advanced: 1 Am (kg (soil)) 1 2 intergrowth of different crystallites in a single dust particle < s 7Am (kg (soil)) . i.e. it may be able to distinguish single phase particles from multiphase particles. Specifically, a color difference between Scientific Goal the material adhering to the five magnets of a Magnet Array will be expected if not all particles in the dust are essentially The Magnetic Properties Experiment (MPE) on Mars Pathfinder identical to one another. consists of two Magnet Arrays located at different sites on Grossly stated two major pathwaysfor the formation of the the Lander (see figure 1), a Tip Plate Magnet (TPM) and two Martian soil may be distinguished, and the resulting magnetic Ramp Magnets. The TPM is mounted on the tripod base (tip phase will be different for the two pathways. plate) of the Imager for Mars Pathfinder (IMP), at distance If the magnetic phase has formed in abounded water via of only 10 cm from the eye of the imager. The two Ramp precipitation, it will be almost pure iron oxide, and will not Magnets are located at the ends of the ramps where the So- contain the element titanium. If, however, the magnetic phase journer Rover will drive off the lander. The Sojourner rover has formed via comminution of the underlying rocks, which, is expected to return to one of the Ramps, and measure the like the Mars meteorites are assumed to contain titanomag- elemental composition of the dust that has been attracted to netite, the resulting magnetic phase will contain the element the Ramp Magnets, using the onboard APX spectrometer. titanium. The scientific goal of the MPE on Mars Pathfinder is to An identification of the magnetic phase in the surface dust identify the strongly magnetic phase on the surface of Mars. on Mars (or identification of its properties) will enhance our The inclusion of weaker magnets on Mars Pathfinder -- as understanding of the alteration processes that have shaped the compared with the Viking missions -- will enable us to give surface of the planet, particularly the role of water. a more precise estimate of the magnetization of the grains in The identification will be based on three results: the dust on Mars. The spectroscopiccapabilities of the IMP are so much bet- The pattern of dust on the two Magnet Arrays and the ter than those of the Viking cameras that it may be possible to TPM. Both the Magnet Array and the TPM have been distinguish a difference in mineralogy between loose soil/dust constructed with a variety of values of the magnetic on the ground and the material that has been magnetically field and magnetic field gradient at the surface of attracted to one of the Magnetic Properties Instruments. This the instrument on which magnetic dust is attracted. By will give informations of crystallite size and the degree of observing on which site on the surface of the instrument THE MAGNETIC PROPERTIES EXPERIMENT ON MARS PATHFINDER: J. M. Knudsen et al. Workshop on Early Mars (1997) 3017.pdf magnetic dust is attracted one can estimate the effective Examples magnetization of the attracted dust grains. Figure 4 shows three samples on the Magnet Array. The The optical properties of the attracted dust. The IMP is capable of acquiring images in 12 distinct spectral bands (The imager is described in detail in an article that will be published in Journal of Geophysical Research [3], special issue on the Pathfinder mission). Using these filters, a rough reflection spectrum can be obtained. The optical properties of Fe(III) compounds are very sensitive to e.g. particle size [4]. Elemental analyses of the dust attracted to the Ramp Magnet performed using the APX instrument on the Sojourner rover. Figure 2 shows a front view of the Magnet Array. The Figure 4: Three di erent samples on the Magnet Array. At the top wehave pure maghemite -Fe O which 2 3 2 1 is strongly magnetic ' 70 Am kg and hence s sticks to all magnets. In the middle wehave basaltic 2 1 b each sand ' 2:0Am kg and at the b ottom we s 2 1 have synthetic hematite -Fe O , ' 0:4Am kg , 3 3 s Figure 2: Front view of the Magnet Array. The p osi- sticking to the strongest magnet only. tions of the magnets b elow the surface are indicated by dashed lines. pattern of particles is significantly influenced by the detailed positions of the magnets below the surface of the instrument magnetic properties of the particles. From the images, essential are indicated (dashed lines) as well as typical values of facts on their magnetic properties can be deduced. B the magnitude of the magnetic field gradient r and the Figure 5 shows three different samples on the TPM. Note magnetic field B at the surface. Each magnet assembly consists of a ring magnet with outer diameter 18 mm and inner diameter 13 mm magnetized opposite to a centrally aligned disc magnet of diameter 6.5 mm. Figure 3 shows the design of the TPM. The magnet itself is essentially a bulls eye pattern magnet, of the same type as the magnets used for the Magnet Array but 4.0 mm thick. The active surface of the instrument is tilted 7 with respect to the Figure 5: Three samples on the Tip Plate Magnet. Left: B underlying magnet to ensure sufficient variation of r and Black basaltic sand from Brdratunga, Iceland. This B . sample contains particles of relatively low saturation 2 1 magnetization ' 1Am kg . Middle: Beach sand s 2 1 from Skerfjafjordur, Iceland ' 10 Am kg , dis- s playing chains formation. Right: Synthetic maghemite 2 1 -Fe O , ' 70 Am kg . 2 3 s the impressive variations of patterns formed on the TPM. Detailed analysis of such patterns will give information on magnetization and crystallite dimensions ront view and top view of the TPM. The Figure 3: F References : [1] Hargraves, R. B. et al., 1976, J. of Geophys ws the \active" surface of the instrument top view sho Res, 82(28) 4547-4558. [2] Hargraves, R. B. et al., 1979, J. bly with indication of the p osition of the magnet assem of Geophys Res, 84(B14), 8379-8384. [3] Morris, R. V. et al., w the surface dashed lines and 5 marks that de ne b elo 1989, J. of Geophys Res, 94(B3), 2760-2778. [4] Smith, P. H. a lo cal co ordinate system on the surface. et al., 1997, submitted to J. of Geophys Res -- Planets..

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