Asteroids with low albedo (the amount of light reflected by the surface) preserve a record of planetesimals, their formation and evolution, and the conditions of the solar nebula at some of the earliest stages of solar system history. These “dark” asteroids are located in the middle portion of the outer main belt of asteroids, at roughly 2.5 to 4 astronomical units (AU, with one AU being the distance from the Sun to Earth). Dark asteroids are believed to be very primitive, preserving remnants of the first planetesimals that accreted in the solar system and possibly the building blocks of planets. They are linked to carbonaceous chondrite meteorites based on spectroscopy. However, the origins and evolutionary pathways of these primitive asteroids are not well known.
New work by Driss Takir (Jacobs, NASA Johnson Space Center) and colleagues used astronomical observations of these primitive asteroids from various types of spectroscopy, combined with thermal and dynamical models, to provide interpretations for their origin and evolution. Their work identified many of these primitive asteroids as belonging to a Ceres-like group located between 3.0 and 3.4 AU. Numerical modeling showed that a highly porous interior best characterizes the observations for the size of the asteroids in the Ceres-like group. These models also suggest a relatively late accretion age of 1.5 to 3.5 million years after solar system formation and a maximum interior temperature of 900 K (~262°C or 1160°F). This contradicts the classical view that accretion time was correlated with heliocentric distance and points toward large-scale migrations in the early solar system.
The dynamical constraints placed on the Ceres-like group of dark asteroids indicate that they were scattered into the main asteroid belt from their original formation locations at ~20-40 AU due to the migration of the giant outer solar system planets. READ MORE