The Latin nouns “planitia” and “planum” both mean “plain”—in astronomy “low plain” and “high plain,” respectively

The Latin nouns planitia and planum both mean flat surface, plain or level ground [1,2]. Their plural forms are planitiae and plana, respectively. In astronomy, the meaning of these words have a significance of distinguishing topographical areas: planitia stands for low plain and planum for high plain [1-3].

Triggered by new data on Martian topography delivered by the Mariner 9 mission, the International Astronomical Union (IAU) decided in 1973 on a nomenclature in which the term planitia refers to a plain below the zero-elevation level and the term planum refers to an elevated plain (plateau) [3].

This terminology is now applied to various celestial objects. For example, planitiae of different size are known on planets Mars and Venus as well as on moons in the solar system [4]. Similarly, a large number of plana have been described on Mars, Venus, Neptun moon Triton and Jupiter moon Io [5]. Of course, their are planitiae and plana on Earth (with the standard sea level as zero elevation), but typically their names are derived without incorporating these two Latin nouns.

Keywords: astronomy, planetary science, areography, geological features, Latin, terminology.

References and more to explore
[1] Wiktionary > planitia [en.wiktionary.org/wiki/planitia] and planum [en.wiktionary.org/wiki/planum].
[2] MyEtymology > Etymology of the Latin word planitia [www.myetymology.com/latin/planitia.html] and Etymology of the Latin word planum [www.myetymology.com/latin/planum.html].
[3] Ulf von Rauchhaupt: Der Neunte Kontinent - Die wissenschaftliche Eroberung des Mars. Fischer Taschenbuch Verlag, Frankfurt am Main, November 2010; page 107. (Tiefebene und Hochebene are the German words for planitia  and planum).
[4] FindTheData > Astrogeology > Planitia, planitiae: astrogeology.findthedata.org/d/d/Europe/Planitia,-planitiae.
[5] FindTheData > Astrogeology > Planum, plana: astrogeology.findthedata.org/d/d/Europe/Planum,-plana.