Geological Chronicle
Mare Tranquillitatis is an irregular basaltic plain spanning 873 kilometers across the eastern near side of the Moon. Unlike circular lunar maria such as Serenitatis or Crisium, Tranquillitatis lacks a multi-ring mountain rim. Its irregular boundary formed from overlapping impacts during the Pre-Nectarian period, more than 3.9 billion years ago, where the structural rings of older basins intersected. Successive volcanic eruptions during the Upper Imbrian epoch flooded the floor with fluid basaltic lavas, which also spilled into adjacent depressions like Palus Somni.
The basalts of Mare Tranquillitatis contain the highest titanium concentrations on the Moon, reaching up to 11 weight percent titanium dioxide (TiO2). Most of this titanium is locked within the opaque oxide mineral ilmenite (FeTiO3). In multispectral orbital imagery from the Clementine and Lunar Reconnaissance Orbiter spacecraft, these ilmenite-rich basalts produce a distinct bluish spectral hue that contrasts sharply with adjacent, low-titanium maria.
Orbital gravity data from NASA's Lunar Prospector and the GRAIL mission show that Mare Tranquillitatis lacks a centralized mass concentration (mascon). Circular lunar basins exhibit strong positive gravity anomalies caused by mantle uplift and thick volcanic plugs. In contrast, the gravity field across Tranquillitatis is diffuse and asymmetric, reflecting an uneven basin floor filled with relatively shallow lava sheets.
On July 20, 1969, the Apollo 11 Lunar Module Eagle touched down in the southwestern plain at 0.674° N, 23.473° E, establishing Statio Tranquillitatis (Tranquility Base). Astronauts Neil Armstrong and Buzz Aldrin collected 21.55 kilograms of rock and soil samples, confirming the high ilmenite content of the mare floor. Robotic missions had surveyed the region earlier, including Ranger 8, which transmitted 7,137 photographs before impacting in February 1965, and Surveyor 5, which performed alpha-scattering chemical analyses in September 1967.