The mare material within the basin consists of basalt formed in the intermediate to young age group of the Upper Imbrian epoch.

This mare has a slight bluish tint relative to the rest of the Moon and stands out quite well when color is processed and extracted from multiple photographs.

Mascons were identified in the center of other maria (such as Serenitatis or Imbrium) from Doppler tracking of the five Lunar Orbiter spacecraft in 1968.

[4] The gravity field was mapped at higher resolution with later orbiters such as Lunar Prospector and GRAIL, which unveiled an irregular pattern.

Mare Tranquillitatis was named in 1651 by astronomers Francesco Grimaldi and Giovanni Battista Riccioli in their lunar map Almagestum novum.

The "bay" of dark mare (basalt) at left is Sinus Concordiae, with "islands" of older, light highland material.