Sinus Iridum - The Bay of Rainbows

 

Images captured from the Jodrell Plank Observatory using the 127 mm. Meade Apo Refractor and the Seestar S30. Data and image credit: Pip Stakkert.

"The other evening, our imaging technician Pip was using the  Seestar S30 to photograph the waxing gibbous lunar disc. He noticed that the 'Terminator' or 'daybreak on the Moon' was about to cross the prominent feature Sinus Iridum - The Bay of Rainbows. Sunlight had just touched the peaks of the crater walls creating the effect known as the 'golden handle'. This can just be seen top left in the bottom image". - Joel Cairo CEO of the Jodrell Plank Observatory.

Key features of Sinus Iridum - Lunar notes - from Professor G.P.T Chat visiting astrophysicist at the JPO.

Sinus Iridum (Latin for "Bay of Rainbows") is one of the most striking basalt-flooded impact structures on the near side of the Moon. It forms a broad semicircular embayment on the northwestern margin of Mare Imbrium and is enclosed by the rugged arc of the Montes Jura mountain range. To lunar observers it appears as a near-perfect luminous crescent when illuminated at low solar angles, making it one of the most recognizable features on the lunar disc.

Position on the Lunar Disc

Sinus Iridum lies at approximately 44°N latitude and 31°W longitude on the Moon's near side. Because it occupies the northwestern sector of Mare Imbrium, it appears in the Moon's upper-left quadrant as viewed through most astronomical telescopes that present an upright image. The feature is roughly 240–260 km in diameter and opens southeastward into Mare Imbrium. The enclosing Jura Mountains are remnants of the original crater rim, rising locally several kilometres above the mare floor.

Origin as a Large Impact Basin

Sinus Iridum began as a major impact crater formed during the late stages of the heavy bombardment that shaped much of the lunar crust. The impact excavated a large bowl-shaped basin and produced an elevated rim composed largely of anorthositic highland material. The southeastern portion of the rim was later breached and largely buried when extensive volcanic flooding associated with Mare Imbrium spread into the crater.

The result is not a true bay in the terrestrial sense, but rather the flooded remains of a large impact structure whose interior became connected to the surrounding mare plains. The surviving rim forms the dramatic semicircular wall visible today.

Geological Composition

The floor of Sinus Iridum consists predominantly of mare basalts emplaced during multiple volcanic episodes. Remote-sensing studies using data from the Clementine mission, Chandrayaan-1, the Lunar Reconnaissance Orbiter, and China's Chang'E program show that these lavas vary in composition and age across the basin.

Key geological characteristics include:

• Basaltic mare plains rich in pyroxene-bearing volcanic rocks.
• Progression from low-titanium basalts in older lava units to medium-titanium basalts in younger units.
• Evidence for increasing olivine abundance in some younger volcanic materials.
• Wrinkle ridges, tectonic deformation features caused by contraction of cooling lava plains.
• Small impact craters, crater chains, and rilles recording later geological modification.

The surrounding Montes Jura remain compositionally distinct from the mare floor, consisting mainly of feldspathic highland crust excavated during the original impact event.

Age and Volcanic History

Modern crater-count dating reveals that Sinus Iridum experienced a prolonged history of volcanic resurfacing rather than a single flooding event.

The oldest exposed mare units have model ages of approximately 3.37 billion years, corresponding to the Imbrian period. Younger lava flows continued entering the basin from Mare Imbrium for more than two billion years afterward. Some of the youngest recognized basaltic units have ages near 1.24 billion years, making them among the youngest extensive mare volcanics on the Moon.

The sequence is interpreted as repeated episodes of lava entering the partially enclosed basin from the larger Imbrium volcanic province. Rather than being filled from a single central vent, Sinus Iridum appears to have been resurfaced multiple times by flows arriving from adjacent mare regions.

Tectonic Evolution

Following emplacement of the mare basalts, the region underwent tectonic deformation associated with cooling and subsidence of the volcanic plains.

Researchers using data from the Japanese SELENE (Kaguya) mission and NASA's Lunar Reconnaissance Orbiter identified wrinkle ridges and compressional structures whose formation may have continued into relatively recent lunar history. These structures reflect crustal shortening caused by the weight and contraction of the basaltic fill.

Spacecraft Investigations

Several lunar missions have studied Sinus Iridum in detail.

NASA Missions

• The Lunar Reconnaissance Orbiter has provided high-resolution imagery, topographic measurements from LOLA, and compositional information used in modern geological mapping.
• Earlier missions including Clementine supplied multispectral data that helped determine iron and titanium abundances.

Japanese Investigations

• SELENE obtained detailed terrain and imaging data used to investigate tectonic structures and wrinkle ridges throughout northwestern Mare Imbrium and Sinus Iridum.

Chinese Investigations

• Chang'e 2 produced high-resolution imagery used in detailed geological mapping and age determinations.
• Sinus Iridum was seriously evaluated as a candidate landing area for later Chinese robotic and sample-return missions because of its smooth terrain and geological diversity.

Although no spacecraft has yet landed within Sinus Iridum itself, it remains scientifically attractive because it exposes the interaction between impact-basin formation, mare volcanism, and tectonic deformation in a single locality. 

Captured from the JPO
and previously published on the blog