Ina is a peculiar small depression ("crater" in IAU nomenclature) on the Moon, in Lacus Felicitatis. It is D-shaped, 2.9 km × 1.9 km wide and 64 m deep (from the deepest point of the bottom to the highest point of the rim). Ina is remarkable for several dozens low hills with flat or rounded tops and very sharp rounded boundaries, looking like drops of mercury. Their surface looks like the usual surface of the Moon while the space between them is very different. Ina is the most prominent of several dozen similar features on the Moon. Their origin is unclear.
Discovery, exploration and naming
Ina was discovered on photographs taken in 1971 by the crew of Apollo 15 from lunar orbit. It could have been found 5 years earlier on images taken by Lunar Orbiter 4 had a photographic flaw not prevented this. At the end of 1972 Ina was observed and photographed by the crew of Apollo 17. After Apollo it was reconnoitred by several orbiting spacecraft beginning in 2009 when the Lunar Reconnaissance Orbiter obtained photos of Ina with resolution of about 0.5 m/pixel and with varying illumination angles. On a topophotomap published in 1974 by NASA this feature was given the Latin female name Ina, in accordance with the convention to give small lunar craters human first names. In 1979 this name was adopted by the International Astronomical Union. In Apollo-era publications the feature is called D-caldera due to its shape, and at the time it was believed to be a unique feature on the moon. Two neighbouring features were named in 1976. They are the small craters Osama on Ina's southwestern edge and Dag to its northwest (both 400 m in diameter). The widest hill in the eastern part of Ina (650 m wide) was named Mons Agnes in 1979. In July 2023, NASA announced the selection of a new scientific payload called Dating an Irregular Mare Patch with a Lunar Explorer (DIMPLE) to establish the age and composition of volcanic activity at Ina.
Description Ina is located on top of a rounded upland (dome) 300 m high and 15 km in diameter. It is situated on an elongated plateau (horst) about 30 km wide. This plateau stands in the middle of Lacus Felicitatis ‒ a small lunar lake between Mare Serenitatis, Mare Vaporum and Mare Imbrium. Ina is a D-shaped depression 2.9 km × 1.9 km wide. It has an elevated rim 600–1000 m wide and 30–40 m high. The eastern part of the rim is 10 m higher than the western. Its outer slope is very gently sloping (1°–3°) and lacks a distinct edge while the inner slope is very steep (tens of degrees) and has a very sharp border with the depression. The deepest point of the depression is located somewhat to the northwest of its center. This point is 30 m deeper than the edges of the depression and 64 m deeper than the highest point of the rim. There are 2 clearly distinct types of surface inside Ina: hills and lowlands. The surface of the hills looks like the usual surface of Lacus Felicitatis while the surface of the lowlands is very different. There are several dozen hills within Ina. They are diverse in size and have rounded amoeba-like edges, like drops of mercury. Many of them are interconnected with other hills or with the edge of the depression. They are rather low (5–25 m, usually 10–15 m). Their tops are flat or slightly rounded while their slopes are steep. The edges of the hills are usually very sharp. Often they are surrounded by a little moat. The boundary between the hills and the lowland has the same appearance as the outer boundary of the depression. The surface of the hills is very smooth compared to the lowlands. In addition, although the sample size is not great, the hills do have an impact crater density intermediate between the fresh lowlands and the ancient neighboring plains of Lacus Felicitatis. Ina's lowlands are much rougher than its hills and show lots of small irregular relief features whose height is no more than several meters. Some craters are also discernible there. Some of the lowlands contain small patches with very bright tone. These are outcrops of scattered rocks 1–5 m in size. Such patches are located mainly near the border between the lowlands and the hills, especially in the lowest places. Ina's lowlands are bright bluish-grey. The hills are darker and brown (the usual lunar surface colour). The lowlands resemble freshly exposed basalt with high titanium content like the basalt seen in some young impact craters. Ina is surrounded by a weak dark halo. The surface of its surroundings is slightly more blue than the more distant surface.
Analogs
Ina is the most prominent, large and well-known representative of a class of features called irregular mare patches, which are also known as "meniscus hollows" for the similarity of their edges to a convex meniscus. Several dozen such features are known on the Moon, all located on the maria. They are hypothesized to be volcanic in origin but several alternative hypotheses exist (see Origin below) and their origin is far from certain. Somewhat similar but distinct features, called hollows, are rather common on Mercury. These features differ from lunar meniscus hollows in having bright halos; they are also more widespread, often bigger and usually located in impact craters.
Interpretations
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