New data from observations of asteroid mutural perturbation events, observations of asteroid satellites, and spacecraft encounters have revolutionized our understanding of asteroid bulk density.

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Asteroid Density, Porosity, and Structure

A refined “standard” thermal model for asteroids based on observations of 1 Ceres and 2 Pallas

Systematic biases in radiometric diameter determinations

Claims that Kuiper belt body 2003 UB313 is the ‘tenth planet’, and the ongoing dispute about the ‘planet-ness’ of Pluto, mean that the question of whether there is a firm line between planets and asteroids could not be more topical. A new study of Hubble Space Telescope images of the largest known asteroid encourages the view that the dividing line is at best blurred. Ceres has many of the characteristics of the much larger planets, and a similar history. It is a lot smaller than our Moon, yet HST images show it is a smooth, ellipsoid, probably shaped by separation of a rocky core from an icy mantle. Unlike the icy satellites of Saturn and Jupiter, Ceres is dark, suggesting that its mantle may underlie a thin crust of rocky material. The accretion of bodies in the asteroid belt was halted nearly 4.6 billion years ago by the gravitational influence of the newly formed giant planet Jupiter. The asteroid belt therefore preserves a record of both this earliest epoch of Solar System formation and variation of conditions within the solar nebula. Spectral features in reflected sunlight indicate that some asteroids have experienced sufficient thermal evolution to differentiate into layered structures1. The second most massive asteroid—4 Vesta—has differentiated to a crust, mantle and core2,3. 1 Ceres, the largest and most massive asteroid, has in contrast been presumed to be homogeneous, in part because of its low density, low albedo and relatively featureless visible reflectance spectrum, similar to carbonaceous meteorites that have suffered minimal thermal processing4. Here we show that Ceres has a shape and smoothness indicative of a gravitationally relaxed object. Its shape is significantly less flattened than that expected for a homogeneous object, but is consistent with a central mass concentration indicative of differentiation. Possible interior configurations include water-ice-rich mantles over a rocky core.

Differentiation of the asteroid Ceres as revealed by its shape

[1] We modeled several thermal evolution scenarios for Ceres to explore the nature of large, wet protoplanets and to predict current-day evidence that might be found by close inspection, such as by the Dawn mission. The density for Ceres is near 2.1, suggesting a water content between 17% and 27% by mass. Short- and long-lived radioactive nuclide heating is considered. Even if only long-lived radionuclide heating is assumed, the water ice in Ceres melts quickly and a water mantle forms, but an approximately 10-km crust does not melt. The circulating warm water would alter the silicates. As heat is lost by conduction through the frozen crust, water begins to freeze out at the base of the crust. Solid-state convection begins and transports more heat as well as perhaps material dissolved or entrained in the water to or near the surface. Ceres' water layer eventually (but perhaps not entirely) freezes, forming a layered density structure with perhaps some liquid water remaining today. Our differentiated models are in agreement with the recently measured difference between the equatorial and polar radii. We find that Ceres' existence and evolution depend critically on it containing water at formation, and this depends strongly on the combination of when it accreted and the amount of 26Al present in the pre-Ceres ∼1-km-sized objects; slightly more 26Al or earlier accretion produces a dry Vesta-like object. Melting and freezing plus mineralization would lead to several dimensional changes over time, creating topographic features, zones of weakness, and perhaps disruptions in the crust.

R. C. Oliver

Papers

The size, shape, density, and Albedo of Ceres from its occultation of BD+8°471

Observations of the Occultation of BD+8DEG471 by Ceres