The late heavy bombardment between 3.8 and 4.1 billion years ago, is a period towards the end of the formation of the Solar System, The inner planets are believed to have been bombarded (supported by analysis of moon rocks from the NASA landings, indicating that most impact melt rocks formed between those times)) by large numbers of asteroids, with Mercury and the Moon forming the cratered appearance which persists today.
The reason for the late heavy bombardment is not known. Possibilitities include:
Planet migration – the gas giant planets changed orbits, and perturbed objects in the Kuiperbelts on eccentric orbits so that they entered the inner Solar Sytem, sometimes striking planets.
The existence of a planet between Mars and the asteroid belt, whose orbit became gradually eccentric, perterbing the asteroids on orbits that entered the inner solar system, before itself entering the Sun.
A fifth gas giant in an orbit between Saturn and Uranus, theorised to have been flung out of the Solar System after an encounter with Jupiter, causing Jupiter to loseangular momentumand recede from the Sun, perturbing the orbits of asteroids and causing some of them to enter the inner solar system.
A series of simulations start with aSolar Systemwhere the gas giant planets are in a tight but stable orbital configuration. Assuming a richtrans-Neptunian belt, stray transneptunians interacted with these planets, causing them to migrate slowly during a time of several hundred million years. Jupiter is predicted to migrate inward, whereas the other planets go outwards. The Solar System became catastrophically unstable when Jupiter and Saturn reached a 2:1orbital resonance, causing the outer Solar System to reconfigure rapidly to a wide jovian system. As these planets migrated, resonances would be "swept" through the asteroid belt and Kuiper belt. These resonances would increase theorbital eccentricityof the objects, allowing them to enter the inner Solar System and impact with the terrestrial planets.Recent work suggests that the impactors which caused the late heavy bombardment were sourced from a now almost entirely depleted inner band of theasteroid belt, close toMars.
Extrapolating lunar cratering ratesto Earth at this time suggest that the following number of craters would have formed:
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22,000 or moreimpact craterswith diameters >20km (12mi),
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about 40impact basinswith diameters about 1,000km (620mi),
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several impact basins with diameter about 5,000km (3,100mi),
A major impact would have occurred every 100 years or so.
Prior to the introduction of the Late Heavy Bombardment theory, it was generally assumed that the Earth had remained molten. Older rocks could be found, however, in the form of chips offasteroidsthat fall to Earth asmeteorites. Like the rocks on Earth, asteroids also show a strong cutoff point, at about 4.6billion years ago, which is assumed to be the time when the first solids formed in theprotoplanetary diskaround the young Sun.
Later calculations showed that the rate of collapse and cooling was dependent on the size of the body, and applying this to an Earth-sized mass suggested this should have happened quite quickly, as quickly as 100 million years, a difference now expalained possibly, by the late heavy bombardment.