Origin And Evolution Of The Asteroids

Available evidence indicates that the asteroids are the remnants of a "stillborn" planet. It is thought that at the time the planets were forming from the low-velocity collisions among asteroid-size planetesimals, one of them, Jupiter, grew at a high rate and to a size larger than the others. In the final stages of its formation, Jupiter gravitationally scattered large planetesimals, some of which may have been as massive as Earth is today. These planetesimals were eventually either captured by Jupiter or another of the giant planets or ejected from the solar system. While they were passing through the inner solar system, however, such large plane-tesimals strongly perturbed the orbits of the planetesimals in the region of the asteroid belt, raising their mutual velocities to the average 5 km (3 miles) per second they exhibit today. The increased velocities ended the accretionary collisions in this region by transforming them into catastrophic disruptions. Only objects larger than about 500 km (310 miles) in diameter could have survived collisions with objects of comparable size at collision velocities of 5 km (3 miles) per second.

Since that time, the asteroids have been collisionally evolving so that, with the exception of the very largest, most present-day asteroids are either remnants or fragments of past collisions.

As collisions break down larger asteroids into smaller ones, they expose deeper layers of asteroidal material. If asteroids were compositionally homogeneous, this would have no noticeable result. Some of them, however, have become differentiated since their formation. This means that some asteroids, originally formed from so-called primitive material (i.e., material of solar composition with the volatile components removed), were heated, perhaps by short-lived radionuclides or solar magnetic induction, to the point where their interiors melted and geochemical processes occurred. In certain cases, temperatures became high enough for metallic iron to separate out. Being denser than other materials, the iron then sank to the centre, forming an iron core and forcing the less-dense basaltic lavas onto the surface. At least two asteroids with basaltic surfaces, Vesta and Magnya, survive to this day. Other differentiated asteroids, found today among the M-class asteroids, were disrupted by collisions that stripped away their crusts and mantles and exposed their iron cores. Still others may have had only their crusts partially stripped away, which exposed surfaces such as those visible today on the A-, E-, and R-class asteroids. Collisions were responsible for the formation of the

One artist's conception of an asteroid strike, highlighted by a tremendous impact cloud rising from a Caribbean island. Collisions between near-Earth objects and Earth have caught the imagination of people throughout the ages. NASA; illustration by Don Davis

Hirayama families and at least some of the planet-crossing asteroids.

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