Scientists may have finally unlocked the mystery of this strange Moon feature, nicknamed Ina. NASA/GSFC/ASU

Ina’s secret to keeping her youthful look is a special kind of foamy lava. It’s a surprising discovery for scientists, who had originally thought this geographic feature on the Moon was only about 100 million years old. But a study in Geology suggests the kind of volcanic eruption that created Ina — a depression on the Moon’s surface shaped like the letter “D” — only makes the crater appear that young, while it is really about 3.5 billion years old.

The original age estimate had come about because Ina looked much brighter than the other spots around it, which usually means that not as much dust and rock had built up in this 2.5-square-mile depression, a sign of youth. On top of that, Ina has fewer impact craters — another signal that would seem to indicate an age inconsistent with the land around it.

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One big problem with the previous age theory, however, is that an eruption 100 million years ago would have been strange. According to the study, that is “an extremely young age for volcanism on the Moon,” since the rest of the Moon’s volcanoes stopped pumping about a billion years before that.

“As interesting as it would be for Ina to have formed in the recent geologic past, we just don’t think that’s the case,” study co-author and Brown University professor Jim Head said in a statement from Brown. “I think most people agree that the volcano Ina sits on was formed billions of years ago, which means there would have been a pause in volcanic activity for a billion years or more before the activity that formed Ina.”

The geologists on the case compared Ina, which was first photographed by NASA’s Apollo 15 crew in 1971, to Hawaii’s Kilauea volcano, which created a crater similar to Ina with an eruption of rock known as “magmatic foam” that comes when the stores of lava beneath the volcano get low. It’s “a bubbly mixture of lava and gas,” Brown University explained. “When that foam cools and solidifies, it forms the highly porous surface.”

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The effect of this bubbly lava would have been even more extreme on the Moon, where the decreased gravity and atmosphere would have made it “even fluffier.” The porous surface allows dust and rock to hide in its crevices, making it build up less than in surrounding areas.

According to the study, porous surfaces would also make it seem as though fewer craters have hit, because they would be hard to spot — the airy consistency leads to smaller craters. “Thus extremely young volcanic eruptions are not required to explain the unusual nature of Ina.”

The new information about Ina could potentially be applied to other geographic features of the Moon. According to a previous NASA report, scientists used information from its Lunar Reconnaissance Orbiter to find 70 other spots that are similar to Ina.

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