Geological Chronicle
Sinus Iridum, Latin for the Bay of Rainbows, is a 249-kilometer-wide basaltic lava plain that forms a prominent semicircular embayment along the northwestern rim of Mare Imbrium. The structure originated during the Late Imbrian epoch roughly 3.8 billion years ago as a large complex impact crater. Subsequent eruptions of mare basalt flooded the Imbrium basin and spilled across the crater's broken southeastern rim, submerging its seaward wall and converting the impact structure into an open lunar bay.
The bay is encircled from northeast to southwest by the arcuate peaks of the Montes Jura, which rise up to 3,800 meters above the lava floor. The mountain arc terminates in two prominent headlands that project into Mare Imbrium: Promontorium Laplace in the northeast and Promontorium Heraclides in the southwest. When the lunar terminator reaches the region about ten to eleven days after a new moon, sunlight catches the elevated crests of the Montes Jura while the bay floor remains cast in shadow, producing an observational effect known as the Golden Handle.
The interior floor consists of smooth, iron-rich mare basalts crisscrossed by wrinkle ridges (dorsa) formed by tectonic compression as the cooling lava settled. The bay contains few large post-flooding craters, though several small impact scars pockmark its margins, including Heraclides E in the south, Laplace A in the east, and Bianchini G in the north. The impact crater Bianchini cuts directly into the northwestern arc of the Montes Jura.
Italian astronomer Giovanni Battista Riccioli assigned the feature its name in his 1651 lunar map Almagestum Novum. Modern robotic exploration has focused heavily on the region. In December 2013, China's Chang'e 3 mission landed just south of Sinus Iridum in northern Mare Imbrium, deploying the Yutu rover to analyze the local volcanic stratigraphy and basalt chemistry.