About 3.8 to 4.1 billion years ago, when our Solar System was only 600-900 million years old, an event occurred which has been named “The Late Heavy Bombardment”.
Wikipedia tells us that during this interval, a disproportionately large number of asteroids apparently collided with the early terrestrial planets in the inner solar system, including Mercury, Venus, Earth and Mars. The LHB happened “late” in the Solar System’s accretion period when the Earth and other rocky planets formed and accreted most of their mass; it is a period still early in the history of the solar system as a whole.
But now the likely origin of the LHB has emerged unexpectedly through an analysis of how solar systems form, called the “Cosmic Smog Model”. Australian researcher David Noel had assembled evidence on the orbits of planets, planetoids, comets, and dwarf planets in our solar system.
He noticed that all the true planets, out as far as Neptune, some 30 AU from the Sun, orbited in a rough plane — the plane of the ecliptic. Maximum inclination of planetary orbits is only about 7 degrees. But beyond Neptune, orbits of objects became more and more inclined to the ecliptic.
Immediately beyond Neptune and out to about 50 AU lies the inner Kuiper Belt, home of dwarf planets Pluto (orbit inclined at 17 degrees to the ecliptic) and Makemake (29 degrees). Out further, from 50 to 100 AU from the Sun, lies the Outer Kuiper Belt or Scattered Disc, including the most massive known dwarf planet, Eris, with an orbit inclined at 44 degrees to the ecliptic.
Beyond about 100 AU lies the Oort Cloud, source of long-period comets. Orbits of these comets are random with respect to the ecliptic. The conclusion drawn from the analysis is that the Sun’s gravitational influence essentially ends at the edge of a sphere 100 AU in radius.
Inside this sphere, the Sun has flattened the orbits of objects to conform with the ecliptic, closer objects very much so, more distant ones to a lesser extent. The analysis concludes that our solar system formed by reworking existing matter within this 100 AU radius sphere.
As a consequence of this gradual regularizing of solar system object orbits down towards the plane of the ecliptic, these objects would be more likely to come into contact. Formerly random orbits were squeezed down so that they all tended to lie in the same plane. A consequence of the Sun’s gravitation, the small but exceedingly long-lasting effects show up first in the inner and then the outer planets, moving out then to the Kuiper Belt.
The same phenomenon is apparently the reason for formation of the Rings of Saturn, which form a disc some 280,000 km across and about 10 metres thick, easily the flattest object in the solar system.
Here then is an obvious reason for the Late Heavy Bombardment and for its timing in the solar system’s history.
The source article is at: The Cosmic Smog model for solar system formation, and the nature of ‘Dark Matter’.