Editing Oceanus Procellarum (section)

==Origin==
{{multiple image|total_width=600|align=center
|title=Moon – Oceanus Procellarum ("Ocean of Storms") |width1=614 |height1=228 |image1=14-236-LunarGrailMission-OceanusProcellarum-Rifts-Overall-20141001.jpg|caption1=Ancient [[rift valley]]s – rectangular structure (visible – topography – [[Gravity Recovery and Interior Laboratory|GRAIL gravity gradients]]) (October 1, 2014)|width2=1500  |height2=1500 |image2=PIA18822-LunarGrailMission-OceanusProcellarum-Rifts-Overall-20141001.jpg|caption2=Ancient [[rift valley]]s – context|width3=1546 |height3=905 |image3=PIA18821-LunarGrailMission-OceanusProcellarum-Rifts-Closeup-20141001.jpg|caption3=Ancient [[rift valley]]s – closeup (artist's concept)}}
[[File:Procellarum Rifts.tif|thumb|Gravity anomalies (red) bordering the Procellarum region overlaid on a global elevation map]]

There are several hypotheses about the origin of Oceanus Procellarum and a related asymmetry between the near and far sides of the Moon. One of the most likely is that Procellarum was a result of an ancient giant impact on the near side of the Moon. The size of the impact basin has been estimated to be more than 3,000 kilometers, which would make it [[List of largest craters in the Solar System|one of the three largest craters in the Solar System]].<ref name=":0">{{Cite web|last=October 2012|first=Charles Q. Choi 29|title=Moon's Mysterious 'Ocean of Storms' Explained|url=https://www.space.com/18276-moon-dark-spot-ocean-storms.html|access-date=2021-05-07|website=Space.com|date=29 October 2012 |language=en}}</ref> 

The impact likely happened very early in the Moon's history: at the time when [[Lunar magma ocean|magma ocean]] still existed or just ceased to exist. It deposited 5–30&nbsp;km of crustal material on the far side forming highlands. If this is the case, all impact related structures such as crater [[Impact crater|rim]], central peak etc. have been obliterated by later impacts and [[volcanism]]. One piece of evidence in support of this hypothesis is concentration of incompatible elements ([[KREEP]]) and low calcium [[pyroxene]] around Oceanus Procellarum.<ref name=Nakamura2012>{{Cite journal | last1 = Nakamura | first1 = R. | last2 = Yamamoto | first2 = S. | last3 = Matsunaga | first3 = T. | last4 = Ishihara | first4 = Y. | last5 = Morota | first5 = T. | last6 = Hiroi | first6 = T. | last7 = Takeda | first7 = H. | last8 = Ogawa | first8 = Y. | last9 = Yokota | first9 = Y. | last10 = Hirata | doi = 10.1038/NGEO1614 | first10 = N. | last11 = Ohtake | first11 = M. | last12 = Saiki | first12 = K. | title = Compositional evidence for an impact origin of the Moon's Procellarum basin | journal = Nature Geoscience | volume = 5 | issue = 11 | pages = 775 | year = 2012 | bibcode = 2012NatGe...5..775N }}</ref><ref name=Byrne2007>{{Cite journal | last1 = Byrne | first1 = C. J. | title = A Large Basin on the Near Side of the Moon | doi = 10.1007/s11038-007-9225-8 | journal = Earth, Moon, and Planets | volume = 101 | issue = 3–4 | pages = 153–188 | year = 2008 |bibcode = 2007EM&P..101..153B | s2cid = 121092521 }}</ref>

Procellarum may have also been formed by spatially inhomogeneous heating during the Moon's formation.<ref name=Nakamura2012/> The GRAIL mission, which mapped the gravity gradients of the Moon, found square formations resembling rift valleys surrounding the region beneath the lava plains, suggesting the basin was formed by heating and cooling of the lunar surface by internal processes rather than by an impact, which would have left a round crater.<ref name=NASA2014>{{cite press release |author=<!--Staff writer(s); no by-line.--> |title=NASA Mission Points to Origin of "Ocean of Storms" on Earth's Moon |url=http://www.nasa.gov/press/2014/october/nasa-mission-points-to-origin-of-ocean-of-storms-on-earth-s-moon |agency=NASA |date=2014-10-01 |access-date=2014-10-02}}</ref>

Other hypotheses include a late accretion of a companion Moon on the far side. The latter postulates that in addition to the present Moon, another smaller (about 1,200&nbsp;km in diameter) moon was formed from debris of the [[giant-impact hypothesis|giant impact]]. After a few tens of millions of years it collided with the Moon and due to a small collisional velocity simply piled up on one side of the Moon forming what is now known as far side [[Lunar highlands|highlands]].<ref name=Jutzi2011>{{Cite journal | last1 = Jutzi | first1 = M. | last2 = Asphaug | first2 = E. | doi = 10.1038/nature10289 | title = Forming the lunar farside highlands by accretion of a companion moon | journal = Nature | volume = 476 | issue = 7358 | pages = 69–72 | year = 2011 | pmid =  21814278| bibcode = 2011Natur.476...69J | s2cid = 84558 }}</ref>