The Shape and Surface Variation of 2 Pallas from the Hubble Space Telescope

Protoplanet 2 Pallas With a diameter of 265 kilometers, 2 Pallas is one of the largest bodies in the main asteroid belt. Now Schmidt et al. (p. 275) have characterized its surface and shape using images from the Hubble Space Telescope. Color variations and topography were revealed that are possibly linked to the asteroid's thermal evolution and to the formation of its orbital family—the population of asteroids that share the same properties as 2 Pallas and are thought to be the fragments of a collision. In particular, a large-impact crater was observed that could represent the source of the Pallas family. 2 Pallas represents the third intact protoplanet in the main asteroid belt, joining asteroids 1 Ceres and 4 Vesta. Like the asteroids Ceres and Vesta, 2 Pallas is a protoplanet that has remained intact since its formation. We obtained Hubble Space Telescope images of 2 Pallas in September 2007 that reveal distinct color and albedo variations across the surface of this large asteroid. Pallas’s shape is an ellipsoid with radii of 291 (±9), 278 (±9), and 250 (±9) kilometers, implying a density of 2400 (±250) kilograms per cubic meter—a value consistent with a body that formed from water-rich material. Our observations are consistent with the presence of an impact feature, 240 (±25) kilometers in diameter, within Pallas’s ultraviolet-dark terrain. Our observations imply that Pallas is an intact protoplanet that has undergone impact excavation and probable internal alteration.

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