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Apollo 11 astronaut Buzz Aldrin deploys a lunar seismometer.  (Image courtesy NASA)

Apollo 11 astronaut Buzz Aldrin deploys a lunar seismometer. (Image courtesy NASA)

10/11/2006

Strange Quarks Attack!

By Kate Fink

Something strange, from outside our solar system, may be crashing into the moon and triggering "moonquakes."  A new analysis of seismic data from the moon suggests that exotic, theoretical particles called "strange quark matter" might sometimes be to blame.

Our seemingly quiet, crater-pocked satellite is actually rattled by four types of seismic activity: deep, shallow, and thermal moonquakes, and meteorite impacts. These quakes can shake the moon's surface for an hour or more at magnitudes up to five on the Richter scale.  On Earth such a quake would be strong enough to move heavy furniture and crack walls.

Each type of moonquake has a distinct seismic fingerprint, and scientists have linked each to specific causes—except shallow moonquakes. Gravitational and tidal forces cause deep moonquakes, temperature changes induce thermal quakes as the moon moves between sunlight and darkness, and meteorite impacts, well, the name says it all. But the cause of shallow moonquakes remains a mystery.

Cliff Frohlich and Yosio Nakamura, both of the Institute for Geophysics at the University of Texas at Austin, may have discovered a clue. While analyzing data from seismometers placed on the moon by Apollo astronauts, they noticed something intriguing: Of the 28 shallow moonquakes recorded in the data, 23 occurred while that area of the moon faced the constellations Leo and Cancer. “You might well ask, if the moonquakes occur when the moon is pointed in a certain direction, what’s this all about?” Frohlich said.

Their answer: something coming from outside our solar system could be striking the moon and causing the quakes. In a paper published recently in the journal Icarus, they suggest that strange quark matter, an extremely dense particle with extra-solar system origins, could perhaps solve the mystery.

But strange quark matter is rather mysterious itself. Physicist Edward Witten first proposed the existence of these extremely small, dense, and fast particles in 1984. In theory, they would zip through space at hundreds of kilometers per second. And because they would pack three types of quarks into their structure—called "up," "down," and "strange"—their density would be more than a trillion times greater than the familiar matter around us, which incorporates only two quark varieties. So despite being as tiny as an atom's nucleus, a particle of strange quark matter could weigh several kilograms. “A ton of it would be the size of a blood cell,” said Vigdor Teplitz of the Astrophysics Science Division at Goddard Space Flight Center via email.

Amazing stuff—if it exists. Strange quark matter has never been observed.

“This might be the first actual evidence that strange quark matter exists,” Frohlich said.  But, he poetically cautions, “there’s many a slip between theory and the lip.”

At the theoretical density and speed of strange quark matter, it could fly right through the moon, like a BB shot through butter. This might leave no seismic signature at all, or it might leave seismic data on both sides of the moon. Neither of these scenarios matches what the scientists saw.

They speculate that, instead, these little nuggets of matter may have a slightly lower density and larger girth, thus penetrating into the moon but not all the way through, more like shooting an M&M, broadside, at a stick of butter. This revised picture better explains why their seismic data show shallow moonquake impacts only on one side of the moon. But with these adjustments, "it's not exactly the same particle," Nakamura said. "It may be something different, something we have never thought about."

For now, the idea that strange quark matter might cause shallow moonquakes remains merely an intriguing possibility. "It's a fascinating idea," said Peter Shearer of the Institute of Geophysics and Planetary Physics at the University of California at San Diego. "[But] when you have really important new claims, you have to have a fairly high standard of proof," Shearer said.
 
Validating or refuting strange quark matter's contentious relationship with the moon calls for accumulating more data—a tricky proposition since the four seismometers on the moon shut down in 1977. Researchers are limited to this pool of old data when fishing for answers. Putting more seismometers on the moon "would be a great idea, but it's going to be a lot of money," Shearer said. 

But shouldn't these particles, if they exist, sometimes pass through the earth as well? Some scientists think that nuggets of strange quark matter might leave a wisp of seismic data as they pierce the earth. But finding such a signal on the seismically noisy earth would be hard; so far, combing records of earthquakes hasn't turned up any evidence. "The moon is a better place to look for unusual phenomenon," Frohlich said. "It's quiet. Real quiet."

Until the next moonquake strikes.

--Additional Reporting By Molly Wetterschneider


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