NASA Science News
March 15, 2006
March 15, 2006: NASA astronauts are going back to the moon and when
get there they may need quake-proof housing.
That's the surprising conclusion of Clive R. Neal, associate professor
of civil engineering and geological sciences at the University of Notre
Dame after he and a team of 15 other planetary scientists reexamined
Apollo data from the 1970s. "The moon is seismically active," he told a
gathering of scientists at NASA's Lunar Exploration Analysis Group
(LEAG) meeting in League City, Texas, last October.
Between 1969 and 1972, Apollo astronauts
placed seismometers at their landing sites around the moon. The Apollo
12, 14, 15, and 16 instruments faithfully radioed data back to Earth
until they were switched off in 1977.
And what did they reveal?
There are at least four different kinds of moonquakes: (1) deep
moonquakes about 700 km below the surface, probably caused by tides;
vibrations from the impact of meteorites; (3) thermal quakes caused by
the expansion of the frigid crust when first illuminated by the morning
sun after two weeks of deep-freeze lunar night; and (4) shallow
moonquakes only 20 or 30 kilometers below the surface.
The first three were generally mild and harmless. Shallow moonquakes on
the other hand were doozies. Between 1972 and 1977, the Apollo seismic
network saw twenty-eight of them; a few "registered up to 5.5 on the
Richter scale," says Neal. A magnitude 5 quake on Earth is energetic
enough to move heavy furniture and plaster.
Furthermore, shallow moonquakes lasted a remarkably long time. Once
got going, all continued more than 10 minutes. "The moon was ringing
like a bell," Neal says.
On Earth, vibrations from quakes usually die away in only half a
The reason has to do with chemical weathering, Neal explains: "Water
weakens stone, expanding the structure of different minerals. When
energy propagates across such a compressible structure, it acts like a
foam sponge?it deadens the vibrations." Even the biggest earthquakes
stop shaking in less than 2 minutes.
The moon, however, is dry, cool and mostly rigid, like a chunk of stone
or iron. So moonquakes set it vibrating like a tuning fork. Even if a
moonquake isn't intense, "it just keeps going and going," Neal says.
for a lunar habitat, that persistence could be more significant than a
"Any habitat would have to be built of materials that are somewhat
flexible," so no air-leaking cracks would develop. "We'd also need to
know the fatigue threshold of building materials," that is, how much
repeated bending and shaking they could withstand.
What causes the shallow moonquakes? And where do they occur? "We're not
sure," he says. "The Apollo seismometers were all in one relatively
small region on the front side of the moon, so we can't pinpoint [the
exact locations of these quakes]." He and his colleagues do have some
good ideas, among them being the rims of large and relatively young
craters that may occasionally slump.
"We're especially ignorant of the lunar poles," Neal continues. That's
important, because one candidate location for a lunar base is on a
permanently sunlit region on the rim of Shackleton Crater at the Moon's
Neal and his colleagues are developing a proposal to deploy a network
10 to 12 seismometers around the entire moon, to gather data for at
least three to five years. This kind of work is necessary, Neal
believes, to find the safest spots for permanent lunar bases.
And that's just the beginning, he says. Other planets may be shaking,
too: "The moon is a technology test bed for establishing such networks
on Mars and beyond."