maria, lunar(mah -ree-ă, mair -ee-ă) (ring plains) Expanses of volcanic lavas of low albedo (i.e. comparatively dark) that were erupted onto the lunar surface mainly between 3900 and 3000 million years ago. The maria are largely confined to the nearside of the Moon where the crust is thinnest. The youngest basins were flooded to produce the circular maria with their associated mascons. The irregular maria are shallower and fill the older less well-defined basins. The mare rocks are basalts. Chemical variations between the maria are revealed in reflectance spectra and by color-difference photography. Age differences are apparent from their crater densities, which are invariably lower than in the older highlands. Maria are mainly named after mental or meteorological states (see table); the terms lacus, sinus, and palus describe smaller lunar maria while oceanus describes a large feature. See also Moon rocks.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006