Editing Caloris Planitia (section)

==Formation==
[[File:Caloris Basin comparison.jpg|thumb|right|200px|Comparison of the original estimated size of the Caloris basin (yellow) with that based on images from the ''MESSENGER'' probe (blue).]]
The impacting body is estimated to have been at least 100&nbsp;km (62 miles) in diameter.<ref>{{cite web |url=http://www.universetoday.com/34568/caloris-basin/ |author=Coffey, Jerry |title=Caloris Basin |publisher=[[Universe Today]] |date=July 9, 2009 |access-date=July 1, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120831000433/http://www.universetoday.com/34568/caloris-basin/ |archive-date=August 31, 2012 }}</ref>

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 that created Caloris must have occurred after most of the heavy bombardment had finished, because fewer [[impact crater]]s 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.<ref>{{cite journal |last=Gault |first=D. E. |author2=Cassen, P. |author3=Burns, J. A. |author4= Strom, R. G.  |year=1977 |title=Mercury |journal=Annual Review of Astronomy and Astrophysics |volume=15 |pages=97–126 |doi=10.1146/annurev.aa.15.090177.000525 |bibcode=1977ARA&A..15...97G}}</ref> Based on ''MESSENGER''{{'}}s photographs, Caloris' age has been determined to be between 3.8 and 3.9 billion years.<ref name="newscientist30012008"/>

A gravitational high, also known as a [[Mass concentration (astronomy)|mascon]], is centered on Caloris Planitia.<ref>[https://photojournal.jpl.nasa.gov/catalog/PIA19285 PIA19285: Gravity Anomalies], Jet Propulsion Laboratory Photojournal</ref>  Most large impact basins on the moon, such as [[Mare Imbrium]] and [[Mare Crisium]], are also the site of mascons.