The Bay of Rainbows - by David Teske

"The Lunar Observer" - November 2019

Sinus Iridum, the Bay of Rainbows, is the most splendid of all the lunar bays. Captured near sunset in the image below, I am amazed at the ripples of lava that flow into the bay, much like gentle water waves at the edge of the sea. Sinus Iridum is the striking landmark on the northwest shores of Mare Imbrium. Sinus Iridum has a diameter of about 260 km with a surface area of 237,000 km2, about the size of the state of Kentucky. As such, it may actually be one of the smaller impact basins. It is the remains of a giant impact crater bordered on the northwest by the Jura Mountains, which are a portion of the original crater wall. Named after the Jura Mountains of Earth by Giovanni Riccioli in 1651, the Jura mountains are composed of several lines of parallel, concentric mountains 30 km wide with peaks up to 4,000 m high. These mountains terminate in the southwest with Promontorium Heraclides with a height of 1.7 km. This was named the ‘Moon Maiden’ by Cassini back in 1692 as it has the shape of a lady’s head near lunar sunrise, around 10.5 day past New Moon. The Jura mountains abruptly stop in the southeast at Promontorium Laplace which tower 2.6 km high. The lava plains of Sinus Iridum has several small craterlets and crater pits. Under low sun angles such as the sunset im-age, a series of wrinkle ridges are visible within Sinus Iridum that are parallel to each other and the Jura mountains. The wrinkle ridge that crossed Sinus Iridum to southeast may be related to the inner wall of the Imbrium Basin.

So, what formed this remarkable and beautiful lunar bay? Sinus Iridum was likely formed af-ter the Imbrium impact, but before the various lava flows which flooded the Imbrium Basin. That means that an asteroid some 15 km across slammed into this area 3.2 to 3.8 billion years ago. Perhaps the asteroid slammed into Mare Imbrium at such an angle that the southeast was of Sinus Iridum was totally buried by the basin. Since the southeast wall of Sinus Iridum is missing, the most likely explanation is that the Sinus Iridum projectile impacted on the sloping floor of the Imbrium basin and that the southern rim is buried under mare lavas. Promontorium Heraclides tapers down in height, suggesting this tilting, but Promontorium Laplace ends abruptly, giving evidence for faulting that down-dropped Iridum’s missing rim. If this is the case, could there be a large, Apennine scarp-like feature buried under Imbrium lavas that was the rim of Sinus Iridum? Upon researching this subject, I find another theory that Sinus Iridum was formed before Mare Imbrium and much later filled with lavas from the basin. This theory is supported as the northern edge of the bay lies 600 m below ‘sea level’, as if the lava was unable to fill the bay completely.

As you ponder the origin of the Bay of Rainbows and marvel at its majestic beauty, you are looking at the site of two lunar landings and rovers. The Soviet probe Luna 17 landed 30 km from Promontorium Heraclides on November 17, 1970, which released Lunakhod 1, an eight wheeled rover remotely controlled from Earth. It journeyed around 12 km in the next 11 months. More recently, the first Chinese lunar lander Chang’e 3 landed on December 14, 2013 just southeast of Promontorium Laplace and released the small rover Yutu. The Yutu rover survived and sent back data for 31 months, well beyond the expected three-month lifespan.