Experimental Tests of the Induction Heating Hypothesis for Planetesimals

Introduction: Induction heating is the release of thermal energy within a body due to the resistance of that material to a current passing through it. This mechanism is also called Joule heating or Ohmic dissipation. Induction heating has been proposed as a process that could have caused metamorphism and melting in planetesimals in the early solar system [1]. Although other theories are more widely studied, this hypothesis is still considered a viable source of asteroid heating [2, 3]. Induction heating requires a magnetic field of sufficient strength to induce a current in the materials present. When Sonnet and co-workers proposed induction heating in the early solar system they assumed that a magnetic field originated in the protostar and was carried outward by the expanding solar wind. Subsequent observations and models resulted in a reduction in the estimated intensity of TTauri stage solar winds, especially in the mid-plane [4, 5]. Some authors have used these observations to argue against induction heating of planetesimals [6]. Recent observations of the Orion cluster made with Chandra have provided new measurements of magnetic field intensities around Young Stellar Objects (YSOs) which are similar to our early Sun. Twenty-eight of these analogs were identified in the COUP field, and found to have an average x-ray flare luminosity of 10ergs/s [7]. Flares occurred an average of 1.5 times for each young solar analog over the 9 day observing period [8]. Characteristics of the brightest flares were used to calculate magnetic field strengths of 10s to 1000s of Gauss [8]. The same authors interpreted the Chandra flare measurements as indicating large magnetic structures are present that would connect the stellar photosphere with the inner rim of the circumstellar disk [8]. While these measurements of magnetic fields around YSOs do not provide a direct measurement of field strengths in the regions we expect planetesimals to form, they do show that the intense field strengths presumed in induction heating models are feasible [i.e. 1]. Meteoritic Constraints: The available meteoritic and asteroidal evidence indicates the presence of an intense, selective, and short-lived heat source in the very early solar system. The chronology of meteoritic heating events, indicators of parent-body size derived from cooling histories, and the spatial distribution of asteroid types provide constraints for the analysis of proposed heating mechanisms. Experimental Methods: We seek an upper limit to the intensity of the magnetic field experienced by chondritic materials in the early solar system by measuring the field strength required to heat chondritic materials in the lab. The materials used in our experiments are pellets of cutting dust from the Fukang pallasite [10], controlled mixtures of olivine and metal, and pure metal reference materials. Grain sizes and composition are measured in control samples in all cases through optical microscopy and electron microprobe analysis. Our procedure is similar to that of previous work [9]. However, samples are placed in vacuum-sealed silica tubes and heated by an Ameritherm HotShot radio frequency (RF) induction heating station. This device is designed to heat small objects using frequencies from 150 to 400 kHz with up to 2 kW of power. The RF radiation is generated using a watercooled copper induction heating coil. The geometry of the coil can be varied to accommodate a variety of sample sizes, currently we are using a 5 cm diameter coil, 2 cm in height, which is wrapped four times. Heating is achieved through Joule heating resulting from Eddy currents in electrically conductive material. Temperatures up to 2000 C can be achieved within seconds in highly conductive materials. The length of the heating cycle can be varied from 10 milliseconds up to 3 hours. Magnetic field strength (B) is calculated using: