Most meteoroids, which end up as meteorites if they land on Earth, are fragments of asteroids (their ‘parent bodies’), and probably most asteroids are fragments of larger bodies that broke up as they collided with each other. Strictly, the term ‘asteroid’ is reserved for objects this side of Jupiter. Some are hundreds of kilometres across, the size of some moons. Those large enough to be detected total more than a million. In order of size the largest are Ceres (945 km), Vesta (525 km), Pallas (512 km) and Hygiea (434 km), accounting for about 44% of the main asteroid belt, but its total mass remains miniscule. The initial mass of the belt is estimated to have been 500–1000 times greater, around that of an Earth-sized planet. Whatever their size, asteroids are traditionally believed to be the more solid remnants of a primeval cloud of dust and gas that, over a few million years, aggregated and condensed into the various bodies of the solar system. Most of the remnants ended up between Mars and Jupiter, where they were prevented from growing further by the disruptive influence of Jupiter’s gravity. There are also more than 27,000 ‘near-Earth objects’.
While this is the text-book explanation of their origin, some researchers are exploring possibilities that to some extent challenge this view. The Galaxy is a violent place. Jets of plasma spurt from newly born stars, massive old stars explode into supernovae, a gravitational sink at the centre of most galaxies gobbles up everything within reach. In its early days the solar system itself was a violent arena, crowded with planetesimals rising through the ranks and erratic ‘oligarchs’ slugging it out for a limited number of permanent positions. Computer simulations re-enact the battle:
In many cases, the smaller planet escapes from the collision highly deformed, spun up, depressurized from equilibrium, stripped of its outer layers, and sometimes pulled apart into a chain of diverse objects. Remnants of these ‘hit-and-run’ collisions are predicted to be common among remnant planet-forming populations, and thus to be relevant to asteroid formation and meteorite petrogenesis.
Erik Asphaug et al., Nature 439:155-60 (2006).
In an effort to explain the genesis of the Moon cosmologists propose that the Earth itself collided with a planet: the latter vaporised, then partially re-constituted itself. A glancing blow by a giant asteroid may have been what flattened Mars’s northern hemisphere; certainly something major must have happened to generate the dichotomy between its highlands and lowlands. Another catastrophe caused Mercury, a tiny planet with an exceptionally large core, to lose most of its mantle, whereupon asteroids pounded its new surface. The cratered moons of the planets beyond Mars shows that all the planets were bombarded.
According to the standard chronology, it was not until hundreds of millions of years after the planets emerged that the solar system quietened down. Impacts gouged out basins up to 3000 km across, in a storm, or series of storms, that rocked the entire solar system. If the pockmarked face of the Moon is any guide, the bombardment did not climax until more than 500 million years after the solar system’s origin. Thereafter colliding asteroids rapidly dwindled, both in size and frequency.
What was going on? Planets supposedly grew in inverse proportion to the amount of material remaining, so why was the solar system not swept clean of leftovers from the formation process long before 4.0 Ga? Why was the cessation of the bombardment so abrupt?