Geological Chronicle
Perched atop a low volcanic dome in Lacus Felicitatis, Ina is a D-shaped depression measuring 2.98 kilometers in diameter and roughly 64 meters deep. Astronauts aboard Apollo 15 first spotted the distinctive depression from orbit in 1971, initially dubbing it the D-caldera. Its interior splits into two strikingly different terrains: several dozen smooth, rounded mounds that resemble drops of mercury, and a rough, bright bluish-grey floor interspersed with rocky rubble.
The unusual landscape has prompted sharp disagreements over the timing of lunar volcanism. Optical observations and crater counts across the lowlands suggested the floor might be remarkably young, while the edges of the hills could not have maintained their sharpness for 50 million years. Later research led by geologist Head and colleagues examined vesicular magmatic foam that could swallow loose regolith and cushion small impactors, yielding a revised age of about 3.5 billion years old for the mounds.
To resolve whether Ina represents young outgassing or ancient porous lavas, NASA selected the DIMPLE mission in July 2023 as part of its lunar exploration payloads. The spacecraft payload is designed to investigate the composition and absolute age of the depression, addressing whether small irregular mare patches across the Moon formed in the distant past or during unexpected later pulses of activity.