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Late heavy bombardment (LHB) is a theorized era of many impacts within the inner solar system within the span of time from 4.1 to 3.8 billion years ago, with asteroids hitting the Earth, Moon, Mars, Venus, and Mercury. This span begins nearly half a billion years after the formation of the solar system. Earth's acquisition of water and formation of life was soon after or possibly was during the LHB or before.
Evidence for the LHB exists in lunar samples, from dating their most recent heating through radioactive dating. There are multiple theories regarding the cause, one theory (the Nice model) being that planetary migration of the outer planets (Jupiter and those beyond) entered and exited various orbital resonances with each other, increasing their orbits' eccentricity, which led to encounters with the asteroids of the asteroid belt, which was much more populous at the time, often throwing asteroids into eccentric orbits, leading to many impacts with the inner-solar-system rocky planets, and reducing the asteroid belt to its current population. This model fits scenarios in which Neptune falls into its current resonance with Pluto.