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Magnetization of the Martian crust Kathy Whaler ([email protected]), School of GeoSciences, University of Edinburgh Mike Purucker, Planetary Geodynamics Laboratory, NASA/Goddard Space Flight Center, Maryland, USA Introduction The Martian dynamo operated for only the first ~0.5 billion years of the planet’s history. Thus to-day’s magnetic field reflects remanent magnetization proportional to the then dynamo field locked into rocks formed early in its history. Mars Global Surveyor (MGS) satellite vector magnetic measurements show several unexpected features, notably an order of magnitude higher strength than from terrestrial remanent magnetization, and linear features with alternating polarity reminiscent of those associated with sea-floor spreading on Earth (see Fig. 1). Here, we present and interpret models of Martian crustal magnetization deduced from MGS data, using an algorithm developed for satellite measurements of the terrestrial lithospheric field. Figure 2: Declination, D (angle from North) and inclination, I (angle from vertical), where magnetization strength is sufficiently high for the angles to be well determined. T marks Tyhrrhena Patera, shown in more detail in Figure 3. The solid line is the dichotomy. Figure 1: An early (Purucker et al., 2000) compilation of MGS radial component data, reduced to 200km common altitude, superimposed on shaded relief topography. The dark grey stripes were areas without data coverage (subsequently filled). Note the much lower field strength in the relatively flat, low-lying area north of the dichotomy, and the cratered, higher, much more magnetic area to the south. V is a truncated magnetic feature at Valles Marineris, G an offset feature at Ganges Chasm. A and C indicate magnetic features in young terrain west of Olympus Mons (A) and eastern Chryse Planitia (C). Fig. 2 shows a linear ‘channel’ of D 0°, I 90° in the Cimmeria region. Possible causes are a process analogous to terrestrial sea-floor magnetic stripe formation, dyke intrusion over a period during which the magnetic field was steady but different from when the surrounding crust was magnetized, or (since the locus of the boundary is a great circle arc) a transform fault. Figure 3: Radial magnetic field at 200km altitude, and deduced magnetization D and I, over Tyhrrhena Patera. The sudden reversal of inclination from steeply down to steeply up along the ‘arms’ of a triple is the pattern expected over a triple junction formed in a reversing magnetic field. Method Alternating polarity magnetic stripes Our modelling strategy places no restrictions on magnetization direction but involves solving a data-by-data system of linear equations – an intractable problem for large satellite magnetic data sets. However, each datum depends on magnetization only in a small disc of crust directly beneath the satellite (i.e. the satellite footprint is small), so the numerically sparse system was solved using an iterative conjugate gradient technique. The code to calculate the sparse matrix elements parallelizes efficiently, and we use an iterative conjugate gradient technique to find the solution. Results shown here were obtained using 8 processors on Edinburgh Parallel Computing Centre’s sunfire system. Data Cain et al. (2003) assembled a 3-component data set at 111274 positions. Global coverage was achieved with altitudes 102-426km using data from all mission phases. Uncertainties depend on component (horizontal data are more affected by external fields) mission phase, local time, and altitude. Results The magnetization amplitude depends on the misfit to the data, but the pattern is robust. Thus we focus here on the magnetization direction and relative strength. Conclusions MGS data and models show structural and tectonic activity, non-dipole magnetic fields and reversals. Our chronology below aids structural and tectonic interpretation. The strong remanent magnetization is likely due to a combination of more iron in the Martian crust, different mineralogy, and a more powerful dynamo operating during its short lifetime. Table 1: A chronology of events with a magnetic signature Code Event Location Reference Initiation of Martian dynamo Magnetic field creation events 1a Cooling of primordial magma ocean(s) to yield largescale magnetic features Planet-wide Various authors 1b Development of lineated magnetic features associated with crustal recycling Terra Sirenum and Cimmeria Connerney et al., Acuña et al. 1c Development of magnetic features associated with volcanism and plutonism Proto-Apollinarsis Langlais et al. and Patera 1d Development of magnetic features associated with volcanism and tectonism Tyrrhena Patera Whaler and Purucker 1e Impact at eastern end of lineated magnetic feature (1b above) and development of TRM during cooling Terra Sirenum This study Martian dynamo disappears Magnetic field destruction events 2a Internal heating and impact? Elysium Mons Frey et al., this study 2b Internal heating and impact? Ascræus Mons Frey et al., this study 2c Impact Isidis Various authors 2d Impact Hellas Acuña et al. 2e Impact Argyre Acuña et al. Later tectonic events, neither constructive nor destructive Figure 4: Inferred radial magnetization component with features discussed in Table 1 identified. 3a Graben formation Valles Marineris Purucker et al. 3b Tectonism Ganges Chasma Purucker et al. Code Event Location Reference Initiation of Martian dynamo Magnetic field creation events 1a 1b 1c 1d 1e Cooling of primordial magma ocean(s) to yield large- Planet-wide scale magnetic features Development of lineated magnetic features Terra Sirenum and Cimmeria associated with crustal recycling Development of magnetic features associated with Proto-Apollinarsis and Patera volcanism and plutonism Development of magnetic features associated with Tyrrhena Patera volcanism and tectonism Impact at eastern end of lineated magnetic feature Terra Sirenum (1b above) and development of TRM during cooling Martian dynamo disappears Various authors Connerney et al., Acuña et al. Langlais et al. Whaler and Purucker This study Magnetic field destruction events 2a Internal heating and impact? Elysium Mons Frey et al., this study 2b Internal heating and impact? Ascræus Mons Frey et al., this study 2c Impact Isidis Various authors 2d Impact Hellas Acuña et al. 2e Impact Argyre Acuña et al. Later tectonic events, neither constructive nor destructive 3a Graben formation Valles Marineris Purucker et al. 3b Tectonism Ganges Chasma Purucker et al.