Magnetic fields on asteroids and planetesimals

Introduction Extraterrestrial planetary magnetism was first confirmed when Burke and Franklin (1955) detected non-thermal radio emissions attributable to charged particles accelerating in Jupiter’s rotating magnetosphere. During the second half of the twentieth century, exploratory missions revealed that planetary magnetic fields were quite common and that the characteristics of each planet’s magnetic field were remarkably diverse (Ness, 2010). Active dynamos are observationally confirmed within Mercury, Earth, Ganymede, and all four giant planets. Remote sensing and paleomagnetic studies indicate the past existence of internally generated magnetic fields on Mars and the Moon despite the lack of present-day dynamos (Acuña et al., 2008; Weiss and Tikoo, 2014). Magnetization of meteorites indicates that planetesimals also likely generated transient dynamos (Weiss et al., 2010), and the nebula itself contained a large-scale magnetic field (Fu et al., 2014b). A better understanding of the evolution of dynamo fields in planetesimals provides a context with which to interpret future paleomagnetic, geochemical, and geophysical observations. Increased understanding of these ancient terrestrial dynamos can in turn aid our understanding of dynamos presently active in our solar system and inform modeling of potential exoplanetary dynamos. In this chapter, we summarize the present state of observation and theory of magnetism in these small bodies. First, we summarize the methods and results of paleomagnetism studies of meteorite samples and in situ observation of asteroids. Next, we examine the conditions necessary for core formation, a prerequisite of dynamo action. We then summarize dynamo theory and its implementation in numerical modeling, focusing particularly on planetesimal dynamos. Following this, we consider the effect of core crystallization on core dynamos. We conclude with a discussion of alternative proposed explanations for meteorite magnetization. Meteoritic Paleomagnetism Meteorites with a wide diversity of compositions and geologic histories have been recovered on Earth. These samples include both unmelted accretionary rocks known as chondrites and partially and fully melted igneous rocks known as achondrites (Weisberg et al., 2006). As such, meteorites contain records of the formation and evolution of solid bodies in the early solar system.