"Today's asteroids have very little water — that's clear," Altwegg added. "But that was probably not always the case. During the Late Heavy Bombardment 3.8 billion years ago, at that time, asteroids could have had much more water than they could now." [
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The asteroids seen now "have stayed in the vicinity of the sun for 4.6 billion years," Altwegg said. "They've lost water due to the sun, due to heat. But to start with, they might have had much more water than they have now." Future analysis of ice-rich bodies in the asteroid belt could shed light on whether Earth's water really did come from there, Altwegg said.