While orbiting the Moon in 1971, the crew of Apollo 15 photographed a strange geological feature—a bumpy, D-shaped depression about two miles long and a mile wide—that has fascinated planetary scientists ever since.
Some have suggested that the phenomenon, known as Ina, is evidence of a volcanic eruption on the moon within the past 100 million years—a billion years or so after most volcanic activity is thought to have stopped.
But Ina may not be so young after all. A new analysis concludes that an eruption actually formed it around 3.5 billion years ago, making it about the same age as the dark volcanic deposits we see on the nearside of the moon. It’s the peculiar type of lava that erupted from Ina that helps hide its age, researchers say.
“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,” says Jim Head, professor of earth, environmental, and planetary sciences at Brown University, and coauthor of the new paper in the journal Geology.
“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.”
Ina’s puzzling features
Ina sits near the summit of a gently sloped mound of basaltic rock, which led many scientists to conclude that it was likely the caldera of an ancient lunar volcano. But just how ancient wasn’t clear. While the flanks of the volcano look billions of years old, the Ina caldera itself looks much younger.
One sign of youth is its bright appearance relative to its surroundings. The brightness suggests Ina hasn’t had time to accumulate as much regolith—the layer of loose rock and dust that builds up on the surface over time.
Then there are Ina’s distinctive mounds—80 or so smooth hills of rock, some standing as tall as 100 feet, which dominate the landscape within the caldera. The mounds appear to have far fewer impact craters on them compared to the surrounding area, another sign of relative youth. Over time, it’s expected that a surface should accumulate craters of various sizes at fairly constant rates. So scientists use the number and size of craters to estimate the relative age of a surface. In 2014, a team of researchers did a careful crater-count on Ina’s mounds and concluded that they must have been formed by lava that erupted to the surface within the last 50 to 100 million years.
“That was a really puzzling finding,” Head says. “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. We wanted to see if there might be something about geologic structure within Ina that throws off our estimation of its age.”
The researchers looked at well-studied volcanoes on Earth that might be similar to Ina. Ina appears to be a pit crater on a shield volcano, a gently sloping mountain similar to the Kilauea volcano in Hawaii. Kilauea has a pit crater similar to Ina known as the Kilauea Iki crater, which erupted in 1959.
As lava from that eruption solidified, it created a highly porous rock layer inside the pit, with underground vesicles as large as three feet in diameter and surface void space as deep as two feet. That porous surface is created by the nature of the lava erupted in the late stages of events like this one. As the subsurface lava supply starts to diminish, it erupts as “magmatic foam”—a bubbly mixture of lava and gas. When that foam cools and solidifies, it forms the highly porous surface.
The researchers suggest that an Ina eruption would have also produced magmatic foam. And because of the moon’s decreased gravity and nearly absent atmosphere, the lunar foam would have been even fluffier than on Earth, so it’s expected that the structures within Ina are even more porous than on Earth.
It’s the high porosity of those surfaces that throws off date estimates for Ina, both by hiding the buildup of regolith and by throwing off crater counts.
A highly porous surface would allow loose rock and dust to filter into surface void space, making it appear as though less regolith has built up. That process would be perpetuated by seismic shaking in the region, much of which is caused by ongoing meteor impacts.
“It’s like banging on the side of a sieve to make the flour go through,” Head says. “Regolith is jostled into holes rather than sitting on the surface, which makes Ina look a lot younger.”
Porosity could also skew crater counts. Laboratory experiments using a high-speed projectile cannon have shown that impacts into porous targets make much smaller craters. Because of Ina’s extreme porosity its craters are much smaller than they would normally be, and many craters might not be visible at all. That could drastically alter the age estimate derived from crater counts.
The porous surface would reduce by a factor of three the size of craters on Ina’s mounds, researchers say. In other words, an impactor that would make a 100-foot-diameter crater in lunar basalt bedrock would make a crater of a little over 30 feet in a foam deposit. Taking that scaling relationship into account, the team gets a revised age for the Ina mounds of about 3.5 billion years old. That’s similar to the surface age of the volcanic shield that surrounds Ina, and places the Ina activity within the timeframe of common volcanism on the moon.
The new findings offer a plausible explanation for Ina’s formation without having to invoke the puzzling billion-year pause in volcanic activity.
“We think the young-looking features in Ina are the natural consequence of magmatic foam eruptions on the moon,” Head says. “These landforms created by these foams simply look a lot younger than they are.”
Le Qiao, a visiting graduate student from the Chinese University of Geosciences, is the lead author of the study. NASA’s Lunar Orbiter Laser Altimeter (LOLA) experiment, the National Natural Science Foundation of China, and the State Scholarship Fund supported the work.
Source: Brown University