A feature on the Moon thought to be evidence of recent volcanic activity is likely much older than thought, according to a new study, casting doubt on an intriguing theory.

The feature, known as an Irregular Mare Patch, is called Ina. It was first spotted from orbit by astronauts aboard the Apollo 15 mission in 1971. It’s about 3.2 kilometers (2 miles) long and 1.6 kilometers (1 mile) wide, but it appears brighter than other regions owing to having less regolith – loose rock and dust – which should have accumulated over time.

In addition, it has 80 or so large mounds of rock that tower 30 meters (100 feet) up. These have far fewer impact craters than regions nearby, suggesting the area was reshaped by lava in just the last 50 to 100 million years.

This would have been a billion years after volcanic activity on the Moon was thought to have finished. Thus, it posed a bit of a mystery.

But a new study from Brown University in Rhode Island, published in the journal Geology, provides an answer. They suggest that the region is actually highly porous, allowing loose rock and dust on the surface to filter into voids. This makes it appear that less regolith has built up.

"It's like banging on the side of a sieve to make the flour go through," said co-author of the study Jim Head in a statement. "Regolith is jostled into holes rather than sitting on the surface, which makes Ina look a lot younger."

Meteorites impacting this area would also produce smaller craters, or perhaps ones not visible at all. An impact that would create a 30-meter (100-foot) crater somewhere else on the Moon would only create a 9-meter (30-foot) dent here.

The researchers came to their conclusion after studying volcanoes on Earth, notably the Kilauea volcano in Hawaii. This has a pit crater similar to one on Ina, which erupted in 1959 and created a highly porous rock layer. This was the result of a “magmatic foam”, a bubbly mixture of lava and gas.

Thus, the researchers suggest volcanic activity at Ina took place 3.5 billion years ago, much older than previous estimates but in line with other theories. That’s sad news for anyone who’d hoped the Moon was active relatively recently.

"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," said Head. "The model we've developed for Ina's formation puts it firmly within the period of peak volcanic activity on the Moon several billion years ago."

And speaking to IFLScience, Head said this may apply to other similar regions on the Moon. "Indeed, although not discussed in detail in our work reported in the Geology paper, we think that this mechanism is applicable to most other Irregular Mare Patches," he said.