Lunar Tunes: Do Impacts Ring a Bell?

By looking at current dust and craters, cosmologists think they can hear the echoes of an impact that created the moon.  Is that lunar, or looney?

The news media were abuzz recently with claims, published by Science Magazine, that dust from the primordial impact that formed the moon has been discovered.  The theoretical impacting body was even given its mythical name, “Theia,” in the title: “Identification of the giant impactor Theia in lunar rocks” (Herwartz, Pack, Friedrichs and Bischoff).  Reporters took off like a rocket:

• Lunar rock chemistry supports big-smash theory (Nature News)
• Traces of another world found on the Moon (Pallab Ghosh in the BBC News)
• Part of infant Earth survived moon’s shocking birth (New Scientist)
• Lunar Rocks Are First Direct Evidence of Collision That Formed Moon: Lunar samples from Apollo landings confirm a long-held theory (National Geographic)

The original paper, though, allowed for other interpretations from the data, which consists only of isotope ratios for oxygen-17.  “An alternative explanation for the isotope difference between Earth and the Moon is that the Δ¹⁷O value of Earth was modified by late-accreting material (late veneer) after the formation of the Moon,” the authors say at the end of the paper, without discounting that possibility.  Philosophers might add an infinite number of alternatives, due to the principle of underdetermination of theory by data.  The best that can be said is that the ratios are “consistent” with the authors’ favored interpretation.  Nature News did give voice to some critics of the claim; Dan Clery in a companion piece on Science Magazine also took note of the alternative scenario.

Other Lunar News

Where have all the craters gone?  The Geological Society of America (GSA) asked that question in relation to the noticeable difference in crater abundances between the earth and moon.  The short answer is that earth erodes its craters through weather and active geological processes, but the moon does not.  “If Earth weren’t so dynamic, its surface would be heavily cratered like the Moon or Mercury,” the GSA news story states.  A corollary is that “craters on Earth cannot be used to understand Earth’s bombardment history.”  Because the craters have mostly been eroded away (except for a few spectacular examples), determining the earth’s impact history will require indirect methods, such as looking for molten layers of rock blasted out by the impacts.  The short article did not address the shortage of impact data and meteorites in the lower strata of the earth.

How dry am I?  Research from the University of Hawaii, reported by Science Daily, shows that the amount of water in lunar rocks varies from region to region.  (The moon is still much more arid than earth, Science Magazine notes.)  The origin of water on the moon was an Apollo surprise, the first article says: “When water was first discovered in lunar samples in 2008, it was very surprising because from the time Apollo astronauts brought lunar samples, scientists thought that the Moon contained virtually no water.”  Its discovery sent astronomers scrambling to account for it.  One of the benefits of the Giant Impact Hypothesis has been incorporating a source of water to the moon.  Looking for it in Apollo samples is still quite new, though, and surprises are still forthcoming:  “Our work is surprising because it shows that lunar formation and accretion were more complex than previously thought.”  This is often code for “poorly understood.”

Put another 60 million candles on the birthday cake:  Whoops, moon’s age was wrong.  Earth and moon just got back-dated by 60 million years, PhysOrg says.  “From today, the Earth is around 60 million years older—and so is the Moon.”  Reason?  The crystal ball gets cloudy: “Looking back into ‘deep time’ it becomes more difficult to put a date on early Earth events.”  The crystal from South Africa yielded a new answer that is now the truth until the next revision.  So the earth-moon system aged 60 million years in one day.  Most things age by one year per year.

Unrequited love:  Why is the moon leaving us?  On Universe Today, in video and text, Fraser Cain weaves a sad tale about how the moon keeps moving farther from the earth. “We had a good run, us and the Moon,” he begins.  “Grab your special edition NASA space tissues because today we’re embarking on a tale of orbital companionship, childhood sweethearts and heartache.”  Because of tidal effects, the moon gets farther from us 1-2 cm per year, he says.  Good thing he didn’t extrapolate that back in time, or he would have had a real problem explaining how emerging life and continents might have survived ravaging tides from a moon just 24,000 miles away (currently ten times that far, on average).

How do you solve a problem like maria?  A 55-year-old puzzle has been “solved,” Penn State News says: why does the moon have a light side and a dark side?  The near side we see from earth has large, dark lava lakes called maria, whereas the farside is brighter and heavily cratered.  It wasn’t till Soviet orbiter Luna 3 photographed the lunar farside in 1959 that the “Lunar Farside Highlands Problem” came to light.  Now, “Penn State astrophysicists think they know why.”  It’s related to the consensus theory of the moon’s formation by a Mars-sized impactor, the “Giant Impact Hypothesis.”  Because the initial fragments were hot, and tidal locking was rapid due to the original proximity of the bodies, the near side of the primordial moon was kept hotter than the farside, due to the earth’s radiative heat.  As a result, low-melting-point elements like calcium and aluminum preferentially accumulated on the farside.  Combining with silicates, the crust thickened more on the farside compared to the near side.  Later large impacts were able to punch though the thinner crust on the near side more than on the farside, forming the lava lakes we call maria.  “When meteoroids struck the farside of the moon, in most cases the crust was too thick and no magmatic basalt welled up, creating the dark side of the moon with valleys, craters and highlands, but almost no maria.”  It’s an interesting story, but the near side still seems to show more large impact basins than the farside.  The press release does not say if the theorists considered the gravitational anomalies measured by lunar orbiters in their hypothesis.  And if the consensus “Giant Impact Hypothesis” falls into disrepute, as some feel it has, this theory of the dichotomy will fall with it.

How do you solve a problem like maria?

How do you catch a stone and rain it down?

How do you find a theory panacea?

A flibbertijibbet! A will-o’-the wisp! A clown!

Many a thing we know we’d like to tell them

Many a thing they ought to understand

But how do you make them stay

And listen to all you say

How do you keep their critics from being banned

Oh, how do you solve a problem like maria?

How to sing Looney Tunes in your rock band?