What Are the Dark Patches on the Moon? Ancient Floods of Lava

The dark regions are flood basalt, meaning lava of low viscosity that flowed far and pooled into level plains, and they are darker than the surrounding highlands because basalt contains more iron and titanium bearing minerals. They fill the floors of enormous impact basins excavated by collisions early in the moon's history, with the lava arriving considerably later than the impacts, having been generated by heating deep in the interior and having risen through fractures the impacts created. The surfaces are smooth compared with the highlands and carry far fewer craters, which is how their relative youth was established before any samples existed, since a surface accumulates craters over time and a lightly cratered one must be younger.

Several features distinguish these plains and are visible with modest equipment:

• •Wrinkle ridges, low sinuous ridges formed as the cooling lava contracted and the crust buckled
• •Rilles, channels that carried flowing lava, some of them collapsed tubes
• •Flow fronts where one lava sheet stopped against another
• •Craters that were flooded, leaving only the rim showing above the plain
• •Domes formed by more viscous eruptions
• •Sharp boundaries against the brighter, rougher, older highland terrain

The distribution is the striking fact, since these plains cover roughly a third of the near side and only a few per cent of the far side, and the asymmetry demanded an explanation as soon as the far side was photographed in 1959. The crust is thicker on the far side, measured directly by gravity mapping, which means magma rising from the interior had further to go and less often reached the surface, so basins there were excavated and never filled. Why the crust is thicker is a separate question with several proposals, including uneven cooling caused by the earth's proximity during the moon's early molten period, and a collision with a second smaller moon that accreted onto the far side. Neither is established, and the asymmetry remains one of the outstanding problems in lunar science.

Returned rock resolved questions that observation alone could not. Radiometric dating of basalt collected from several of these plains gave ages between roughly three and a half and four billion years, confirming that the flooding happened long after the basins formed and establishing that volcanic activity had largely ceased by then. The compositions showed the lava came from deep in the interior and differed between sites, indicating a mantle that was not uniform. The samples contained almost no water, which shaped thinking about the moon's origin for decades until later reanalysis with better instruments found traces after all. Crater counts on the dated surfaces provided the calibration that allows any surface in the solar system to be dated approximately by counting its craters, which is the single most widely used tool in planetary geology.

The names in use reflect a seventeenth century interpretation that was wrong and stuck anyway. Early telescopic observers took the dark smooth regions for bodies of water and named them accordingly, so they carry Latin names meaning sea, ocean, bay, marsh and lake, and those names were fixed by a naming scheme published in 1651 that displaced several competing systems and became standard. The individual names evoke weather and states of mind, giving a Sea of Tranquillity, a Sea of Crises and an Ocean of Storms, which reflects a belief that the moon influenced weather on earth. The water interpretation was abandoned long before the names were, and the terminology now sits in astronomical use as a fossil of an obsolete theory, which is not unusual in the sciences.

The dark plains are flood basalt that pooled in impact basins long after the impacts, darker because of their iron and titanium minerals and smoother and less cratered because they are younger. They cover a third of the near side and a few per cent of the far side, where the crust is thicker and magma rarely reached the surface. The names record a seventeenth century belief that they were seas.