The internal structures and densities of asteroids

Abstract

Abstract-Four asteroidal bodies (the Martian satellites Phobos and Deimos and the main-belt asteroids 243Ida and 253 Mathilde) have now been the subjects of sufficiently close encounters by spacecraft that the masses and sizes and, hence, the densities of these bodies can be estimated to -10%. All of these asteroids are significantly less dense than most members of the classes of meteorites identified as being compositionally most nearly similar to them on the basis of spectral characteristics. We show that two processes can act, independently or in concert, during the evolutionary histories of asteroids to produce a low bulk density. One of these processes is the result of one or more impact events and can affect any asteroid type, whereas the other can occur only for certain types of small asteroids that have undergone aqueous alteration.

An Estimate of Eros's Porosity and Implications for Internal Structure

“…First, the range of estimates for Eros's macroporosity, while not conclusive, are lower than rubble pile models would suggest. Modeling of the collisional breakup and gravitational assembly process predicts porosities of ∼40% for asteroids that have undergone breakup and reassembly (Wilson et al 1999), and rubble pile analogs such as the lunar regolith and unconsolidated terrestrial sediments have porosities greater than 40% (Table I). Second, the apparent homogeneity in mass distribution within Eros (Thomas et al 2001 suggests an implausible continuity of density for a rubble pile.…”

“…For the purposes of this paper we adopt the definition that a rubble pile is an asteroid that was reaccreted from the remnants of a disrupted parent body into a gravitationally bound granular body (i.e., consistent with descriptions in Hartmann (1979), Asphaug et al (1998), and Wilson et al (1999)). This definition implies little internal strength and a relatively high porosity as a result of the voids created by the reassembly of the dispersed fragments.…”

“…An estimate of lunar regolith porosity in situ (for the top 0-60 cm) is 46 ± 2% (Carrier et al 1991). Finally, modeling of the collisional breakup and gravitational assembly process predict porosities of ∼40% for asteroids that have undergone breakup and reassembly (Wilson et al 1999). However, subsequent settling and relithification may lower this value to the range of ∼30%.…”

“…Modeling of the collisional breakup and gravitational assembly process predict porosities of ∼40% for asteroids that have undergone breakup and reassembly (Wilson et al 1999). Variations in porosity within an object might be detected by a difference in the object's gravity field from that of a completely homogeneous object.…”

Comment on "Determining Chondritic Impactor Size from the Marine Osmium Isotope Record"

“…Projectile size is also not a particularly useful measure of an impacting body (mass is better), as assumptions have to be made about density and shape. Densities of meteorites that are measured on Earth (9) are typically higher than the densities of bodies with the same composition measured in space (10). Orbiting bodies are in tension, are often more porous, and show highly variable densities.…”