Ina, the puzzling patch

Ina is a small, strange hollow on the Moon’s near side, in a little dark plain called Lacus Felicitatis, the ‘Lake of Happiness’. It is shaped like the letter D, about 2.9 by 1.9 km, and roughly 64 m deep. Its floor is covered with smooth, rounded mounds that look like blobs of frosting, surrounded by lower ground that is rough, blocky and brighter.

What makes Ina puzzling is how few craters it has. On the Moon, more craters usually means an older surface, because meteorites keep hitting it over time. Ina has so few that it looks almost brand new. If it really is young, it would mean the Moon was still having small volcanic eruptions long after scientists thought its volcanoes had gone quiet. Not everyone agrees, and that debate is still going on.

How it was found

The astronauts of Apollo 15 photographed Ina from lunar orbit in 1971. An earlier robotic orbiter had taken a picture of the area, but a flaw in the photo hid the feature. Early reports called it the ‘D-caldera’ because of its shape. A NASA map from 1974 gave it the name Ina, a Latin woman’s name, and the International Astronomical Union made the name official in 1979.

The mystery

In 2014, a team using NASA’s Lunar Reconnaissance Orbiter found 70 similar ‘irregular mare patches’ across the near side, most of them smaller than 500 m. By counting the few craters on three of them, the team estimated they were less than 100 million years old, and Ina perhaps less than 50 million. That is young for the Moon, whose big lava plains formed billions of years ago. It would mean the Moon’s inside stayed hot enough to make magma for much longer than expected.

What scientists think now

In 2017, another team suggested a different answer. They argued that Ina’s mounds are made of very porous, foamy lava. A foamy surface would make new impact craters smaller and harder to see, so the patch would only look young. By their estimate, Ina could be about 3.5 billion years old, like the lava plain around it. Other ideas include gas escaping from below or soil sinking into hidden holes. A sample brought back from Ina could settle the argument.

On the 3D Moon

On the 3D Moon you can find Ina north of the Moon’s center, in Lacus Felicitatis. Open its card to read why this D-shaped patch has scientists arguing about its age.