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2.1. Remanent Magnetism

Remanent magnetization is familiar through everyday occurrence of permanent magnetism. Ferromagnetic substances may acquire or lose magnetization under some circumstances. Thus ferromagnetic material which cools from a high temperature to a lower one while held in a magnetic field is very efficiently magnetized (called thermo-remanent magnetization). Also, the growth of mineral grains in the presence of a magnetic field produces chemical-remanent magnetization. And the accretion of magnetic particles in the presence of a magnetic field produces depositional-remanent magnetization. The total magnetization of a natural object is often a superposition of magnetization components acquired through several processes over its lifetime. And these different components can be later disentangled, giving some details about the conditions prevailing under each acquired magnetization. Virtually all meteorites carry natural remanent magnetization (e.g., Levy & Sonett 1978).

2.1.1. Meteorites (~ 0.1 to 1 m; 10-17 pc)

How did our solar system form ? The answer lies in part within the asteroidal belt, located about 3 Astronomical Units (~ 4.5 × 108 km) from the Sun, and containing ~ 105 small rocky planetesimals/asteroids. The asteroidal belt is the origin of many meteorites found on Earth. Among the meteorites, the chondrites contain abundant millimeter-sized silicate spherules (chondrules) which were formed ~ 4.5 × 109 years ago within the solar nebula, and have remained relatively unchanged since. Unravelling this record may provide constraints on the type and duration of processes that occurred within the solar nebula.

Chondrites are not entirely pristine, as a few processes may have occurred in the solar nebula, changing the original primary characteristics of chondrites (e.g., Brearly 1997). Examples are the many alterations, either within the original solar nebula before coalescence or accretion into an asteroid, or after accretion within the interior of an asteroid.

The magnetic properties of meteorites are studied to know more about the physical conditions in the early solar system. The meteorites could have been magnetized during the accretion and cooling stages of the formation of the Solar nebula. Origin     Estimates of the primordial magnetizing fields (the fields responsible for the remanence) have been made through various techniques. Careful measurements of the magnetic field properties of meteorites, based on the thermo-remanent magnetization model, have revealed the primeval magnetic field strength required to give the observed remanent magnetization. The evidence seems to show that chondrules (~ 1 mm in size) inside meteorites (~ 10 cm to 1 m in size) were probably magnetized by the interplanetary magnetic field. A predicted theoretical magnetic field in the early solar nebula (~ 30 µT = 0.3 Gauss), which was inherited from an earlier interstellar cloudlet, is about the correct value needed to magnetize the carbonaceous chondrites. In addition to chondrules, small interstellar grains (~ 1 µm in size) have been discovered in meteorites (notably silicon carbide SiC grains, graphite grains, and corundum Al2O3 grains), The distribution of their sizes follows a log-normal equation (e.g., Sandford 1996).

A possible magnetic field for the early solar nebula may have had a dipolar shape with a strength around 1 Gauss. The magnetic field lines could have been perpendicular to the elongated nebular disk, in a dynamo model with a differentially rotating protosolar nebula (gas density 3 × 10-10 cm-3, temperature ~ 200 K, magnetic field ~ 1 Gauss, diameter ~ 7 Astronomical Units, e.g. Levy & Sonett 1978). Such large early interplanetary magnetic fields may have decayed with the dispersal of the early nebular gas, on a time scale of 10 million years (e.g., Umebayashi & Nakano 1984). Evolution     In the case of a well-preserved meteorite, such as the Allende meteorite which fell to Earth on 8 February 1969 in Mexico, paleomagnetism has shown that its chondrules may have acquired their random remanent magnetization before accretion into the meteorite. During or soon after accretion into the meteorite, a sulfidation event occurred which remagnetized most of the meteorite, but a fraction of the pre-accretion remanent magnetism survived. A subsequent shock slightly rotated the chondrules in the meteorite. Chemistry     Magnetic minerals in meteorites are quite often different from those in terrestrial rocks. Kamacite is by far the most abundant and the most common magnetic mineral in meteorites. Others include tretataenite, magnetite, and titanomagnetite. Shu et al. (1997) proposed a model where a magnetosphere in a high magnetic state (inner disk radius located far from star) with low gas temperature (500 K) would allow partial retention of Na and K in rocky chondrules located in the protostellar disk, while a magnetosphere in a low magnetic state (inner disk radius located close to star) with high gas temperature (1500 K) would evaporate Na and K and leave only Ca-Al oxides and silicates in ordinary chondrites in the protostellar disk.

Caveat: a difficulty in meteorite magnetism is that nobody knows what may have happened to the meteorites after their fall to Earth. Generally, atmospheric entry in the Earth affects a meteorite's magnetization only in the outer few centimeters, and it does not interfere with identification of the inner primordial magnetization (e.g., Levy & Sonett 1978). Artificial magnets on Earth may have been used later to identify meteorites - such contacts with artificial magnets could produce a large remanent magnetization in some types of meteorites (e.g., ordinary chondrites), but not in others (e.g., achondrites). Shocks and heat in the absence of a magnetic field may demagnetize the meteorites. A good review on these topics can be found in Sugiura and Strangway (1988).

2.1.2. Comets' Nuclei (~ 1 to 10 km; ~ 10-13 parsec)

Spacecrafts visit comets rarely, for only brief time intervals in their flythroughs, and at different places along the cometary tails. Near the nucleus of a comet, the general ubiquitous interplanetary magnetic field (~ 50 µGauss at 1 AU) gets compressed by the pressure of cometary static ions, to values ~ 50 nT (= 0.5 milliGauss) at 1 AU from the Sun (e.g. Spinrad et al. 1994). There is usually no need for an intrinsic cometary magnetic field attached to the cometary nucleus; all effects are extrinsic. Magnetic disturbances in the interplanetary magnetic field, due to the presence of comet Halley, have been measured by the spacecrafts Giotto (Mazelle et al. 1995), Vega I and Vega II (Mikhajlov and Maslenitsyn 1995).

2.1.3. Asteroids (~ 10 to 100 km; ~ 10-12 pc)

Big asteroids could be viewed as micro-planets. A few of them have been surveyed at a distance by spacecrafts, and deviations of the interplanetary magnetic fields have been measured in their vicinity. The small radius of the asteroid does not permit the setting up of a dynamo magnetic field. The magnetic moment of the asteroid is weak, weak enough that the magnetic field cannot set up a bow shock and cannot carve a recognizable cavity against the solar wind ram pressure, but it may be strong enough to generate a bow wave and dispersive anisotropic MHD waves (e.g. Baumgärtel et al. 1997). An asteroid generates disturbances in the interplanetary plasma flow, launching whistler waves that are swept downstream by the flowing plasma (e.g., Kivelson et al. 1995). The interaction of the solar wind flow with the asteroid may depend on the properties of the asteroid, such as its magnetization and its electrical conductivity. The interplanetary field may become draped around the asteroid (Wang & Kivelson, 1996). Gaspra     The Galileo spacecraft acquired data in 1990 during its passage at 1600 km from Gaspra, consistent with a diversion of the interplanetary flow by the asteroid 951 Gaspra (e.g., Baumgärtel et al. 1994; Kivelson et al. 1995). It is thought that some remanent magnetization, left over from the time of formation of the asteroid, could create a somewhat chaotic magnetic field (perhaps like an imperfect non-ideal line dipole).

Gaspra orbits at a mean distance of 2 AU from the Sun, where the interplanetary magnetic field strength is ~ 2 nT = 20 µGauss. Gaspra's magnetic moment (= Bsurf r3surf) has been estimated around 1.5 × 1011 Gauss m3, predicting a chaotic surface magnetic field around Bsurf ~ 0.5 Gauss at a radius rsurf ~ 7 km (e.g. the dipole model of Baumgärtel et al. 1994). However, the dipole model also predicted a change of magnetic field magnitude which was not observed in the data of the Galileo probe (Wang & Kivelson, 1996). Thus the real magnetic field on Gaspra may be random (not dipolar). Ida     The Galileo spacecraft acquired data in 1993 during its passage at 2400 km from Ida, consistent with a diversion of the interplanetary flow by effects from the asteroid 243 Ida (e.g., Burnham 1994; Kivelson et al. 1995). The asteroid Ida (radius ~ 15 km) may have revealed to the Galileo spacecraft a weak magnetic field, probably remanent. Ida affects the magnetic field of the solar wind sweeping past it. It is not yet known if a model with a conducting Ida or a model with a magnetic moment for Ida could produce the observed signature in the interplanetary flow (Kivelson et al. 1995).

2.1.4. Big Moons Without Dynamos Earth's Moon     The data for Earth's Moon do not show a large scale golbal magnetic field, so the magnetic moment < 1 × 1012 Gauss. m3 (e.g., Lin et al. 1998). The radius of Earth's Moon is ~ 1740 km. The Moon is located at 60 Earth radii from the Earth's center. The magnetic field strength at the equatorial surface is < 2 µGauss (e.g. Kivelson et al. 1996b). The solar wind normally flows virtually unimpeded to the lunar surface, where it is absorbed. Europa     Europa, a large rock at 9 Jupiter radii from Jupiter's center, seems to have an extrinsic magnetic field induced by a current-carrying ionosphere, maintained by Jupiter's background magnetic field of strength ~ 420 nT (= 4.2 milliGauss), as seen by the Galileo probe (Kivelson et al. 1997). The data for Europa are consistent with some kind of passive magnetic dipole, of strength ~ 9 × 1015 Gauss m3. The radius of Europa is ~ 1570 km. The magnetic field strength at the equatorial surface amounts to 240nT = 2.4 mGauss (e.g. Kivelson et al. 1997). Callisto     The Galileo spacecraft detected a small enhancement of the field strength related to small changes in the jovian plasma environment caused by Callisto's presence. Internal magnetic anomalies in the crust of Callisto could also affect the result, being more probable than an internal dynamo.

Callisto, with a radius of 2400-km, is a moon rock at 26 Jupiter radii from Jupiter's center. It shows little or no intrinsic magnetic field (< 30 nT; < 300 microGauss); the magnetic moment is < 4 × 1015 Gauss m3, as measured by the Galileo probe (e.g., Khurana et al. 1997; Gurnett et al. 1997).

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