Formation
The impacting body is estimated to have been at least 100km (62 miles) in diameter.
Bodies in the inner solar system experienced a heavy bombardment of large rocky bodies in the first billion years or so of the solar system. The impact which created the Caloris Basin must have occurred after most of the heavy bombardment had finished, because fewer impact craters are seen on its floor than exist on comparably-sized regions outside the crater. Similar impact basins on the Moon such as the Mare Imbrium and Mare Orientale are believed to have formed at about the same time, possibly indicating that there was a 'spike' of large impacts towards the end of the heavy bombardment phase of the early solar system. Based on MESSENGER's photographs, Caloris' age has been determined to be between 3.8 and 3.9 billion years.
Read more about this topic: Caloris Basin
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