A large dark or low area on a planetary surface; pl. maria. The name, which means ‘sea’, is not a geological term; it was originally used in the seventeenth century, when the dark plains of the Moon were thought to be water, and was subsequently applied to the dark patches on Mars in the nineteenth century. The lunar maria are dark, smooth lowland plains of lava that erupted between the end of the late heavy bombardment 3.9 billion years ago and about 2 billion years ago. Samples of the maria have been obtained by the Apollo missions and unmanned Luna landers. The maria consist of solidified basaltic lavas, chemically and mineralogically different from the highlands, being enriched in iron and titanium, and with abundant pyroxene. The lunar mare lavas differ from those on Earth in that they were very fluid, with a viscosity similar to that of motor oil, so that they flowed for great distances. The Martian maria are albedo markings that do not consistently correspond to any kind of topographical feature or geological province, and in most cases appear to consist of dark surface dust. Maria are also now known to exist on Saturn’s moon Titan; these consist of liquid hydrocarbons such as methane and ethane. The largest, Kraken Mare, is nearly 1200 km across.