“Big Splat” Update: Harrison Schmitt’s Skeptical Take on the 2-Moon Hypothesis

PM asked Harrison Schmitt, an Apollo 17 astronaut who walked on the moon and a geologist who’s studied its makeup, about a new hypothesis that caught a wave of attention last week: that a second, smaller moon once smacked into the far side of our moon, resulting in the structure we see today. His response? With so little data to back it up, he’s skeptical.

By Skylar Bergl
The Moon before and shortly after the proposed impact.

The Moon before and shortly after the proposed impact.


August 10, 2011 7:00 PM Text Size: A . A . A
Last week, we covered a new study with a provocative explanation for why the two sides of our moon are so different—that a second moon could have grazed one side of it billions of years ago, an event the scientists behind the idea call "the big splat."

We were curious to hear the take of someone who not only knows the geology of the moon, but walked upon our natural satellite. And so we called Apollo 17 moon walker, geologist and advocate of moon mining Harrison Schmitt.

Schmitt's main beef with the the "big splat" idea is that it was created mostly via computer simulation, a fact that makes him uncomfortable. "I'm known as a skeptic," Schmitt said. "Particularly about hypotheses that come from computer models. They have to be tested. Any hypothesis has to be tested and the best way to test it is to try to disprove it."

The news last week was the first that Schmitt had heard of the hypothesis. "It's going to have to be examined," Schmitt said. "I know of no geological evidence that would support it. It's one thing to build a computer model that says it's possible, it's another thing to verify those models against what we know about the moon, and we know an awful lot about the moon."

The lopsidedness of the moon—with the higher mountains on the far side that faces away from Earth—doesn't convince Schmitt that there may have been a collision like the one described in Martin Jutzi and Erik Asphaug's study. To Schmitt, the moon is not all that lopsided anyway; the center figure of the moon is offset from the center of the Moon's mass by only 2 km (1.2 miles).

Furthermore, he points to some of the basins on the Moon's surface that have been subjected to many impacts as well. "The main thing that's absent [is], we don't know the specific ages of these large impacts," Schmitt said. "There is one group, a large group, who claims that most of the basins on the moon formed in a very short period of time around 3.9-3.85 billion years ago. We know that from geological analysis." But those are the youngest basins, he says—there are even older basins whose exact age is uncertain. The upshot is: If there are some on the far side of the moon that are so old they would predate the "big splat," that could cast doubt on the new hypothesis.

"Right now, I would say based on what I understand and have reviewed, there's not much on the moon supporting the idea," he says. "I think the burden of proof is going to be on those who propose the addition of a smaller moon to the current moon's surface."

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