October 8, 2014 | David F. Coppedge

Lunar Impact: Major Moon Basin Was Not a Big Hit

The theory of how the largest impact basin on the moon was formed has been turned upside down.

Oceanus Procellarum, the large dark feature often called the “Man in the moon,” has a new story to tell lunar geologists: “I’m a volcano.”  In a surprise reversal, scientists are saying that the huge basin is “not an impact crater” (Nature News). “Gravity data suggest flats of volcanic basalt formed from tectonic stretching,” the subtitle reads: in other words, a large volcanic plume created most of the maria on the near side of the moon.  Oceanus Procellarum is 17% of the lunar surface, constituting most of the near side visible to Earth.

Scientists working data from NASA’s GRAIL orbiter (Gravity Recovery and Interior Laboratory), which measures gravity anomalies on the moon, knew they had found something odd when the outlines of the basin looked a bit rectangular rather than circular.  The results, published in Nature, “revealed anomalies buried beneath the plains’ basalt surface, which the authors interpret as valleys where the crust of the Moon has been stretched and thinned, a process that on Earth happens as tectonic plates move apart.”

This was a big surprise:

The findings were unexpected, says Jeffrey Andrews-Hanna, a planetary scientist at the Colorado School of Mines in Golden who led the study and was a member of the GRAIL science team. “Rift zones aren’t something known on the Moon, although we see them on Earth, Venus and Mars,” he says.

Andrews-Hanna, sees similarities to Enceladus, the geysering moon of Saturn.  “Despite the many differences between the moons, similar physical processes may have occurred on them, he suggests.”  The paper makes this comparison, too, and also points out a region on Mercury with similar characteristics.

The Nature paper estimates the rift basin formed 3.5 billion years ago.  The findings were also reported by Science Daily, Science Magazine and the BBC News.  The BBC quoted Andrews-Hanna in consternation:

“When we first saw it in the Grail data, we were struck by how big it was, how clear it was, but also by how unexpected it was.

“No-one ever thought you’d see a square or a rectangle on this scale on any planet.”

None of the articles or papers mentioned how this turnaround in thinking might affect notions about the “Late Heavy Bombardment” (LHB) that presumably formed many of the impact craters after the moon’s formation (see 4/26/12, 1/09/12, 9/17/10).  A recent article on PhysOrg discusses it, but recognizes that the idea is controversial: “While the LHB is the most popular explanation for the lunar rocks, there are some who disagree and have proposed alternate models,” the article ends.  “We can’t be certain yet that such a heavy bombardment period actually happened. But it certainly seems that Earth’s early period was quite tumultuous.”  As the new finding about lunar maria shows, though, things are not always what they seem.

This paradigm shift may take time to fully sort out, but it’s huge.  What will be the “impact” on other beliefs about the moon’s age and formation?  This could really shake up the sequence.  It also raises many new questions; if it’s a volcanic feature, why on one side of the moon and not elsewhere?  How long was magma that close to the surface?  How could the moon have rift zones?  Why the resemblance to Enceladus?  What other features on the moon might need re-interpretation?  One thing is clear: all the textbooks and TV documentaries just went crashing down.

Remember that lunar history is tied into Earth history.  Evolutionary theories are all connected.  A major rethink about the moon could affect thinking about the early Earth.  Let this be another reminder not to lean too hard on what scientists say about origins.  Today’s consensus is tomorrow’s rethink.  Maybe scientists need to take down the “Think” signs above their desks and replace them with “Rethink” signs.


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