Asteroid Density, Porosity, and Structure

Abstract

New data from observations of asteroid mutual perturbation events, observations of aster-oid satellites, and spacecraft encounters have revolutionized our understanding of asteroid bulk density. Most asteroids appear to have bulk densities that are well below the grain density of their likely meteorite analogs. This indicates that many asteroids have significant porosity. High porosity attenuates shock propagation, strongly affecting the nature of cratering and greatly lengthening the collisional lifetimes of porous asteroids. Analysis of density trends suggests that asteroids are divided into three general groups: (1) asteroids that are essentially solid objects, (2) asteroids with macroporosities ~20% that are probably heavily fractured, and (3) as-teroids with macroporosities >30% that are loosely consolidated " rubble pile " structures. 1. OVERVIEW: THE DENSITY AND POROSITY OF ASTEROIDS Data from a number of sources indicate that many aster-oids have significant porosity. In some cases, porosities are large enough to affect asteroid internal structure, gravity field, impact dynamics, and collisional lifetimes. Porosity can also affect a range of asteroid physical properties including thermal diffusivity, seismic velocity, cosmic-ray exposure, and dielectric permeability. The thermal and seismic effects can in turn affect asteroid internal evolution, metamorphism, shock dissipation, and elastic properties, which can deter-mine whether colliding asteroids accrete or disrupt. The study of asteroid bulk density, along with supporting studies of meteorite porosity and physical properties, is just begin-ning. These data allow for new views of asteroid belt structure and evolution. This chapter will review some of the basic data accumulated on asteroid bulk density and meteorite grain densities. Using these data we will estimate the porosi-ties of asteroids, outline the implications of high porosity on internal structure, examine how porous asteroids respond to impacts and shock, and interpret these data to character-ize the distribution of porous objects in the asteroid belt.