Noise May Reveal Alien World Interiors

[Madame Butterfly 0]

Noise May Reveal Alien World Interiors

By Larry O'Hanlon, Discovery News

 

 

Sept. 26, 2005— Just days after NASA announced its intention to put people back on the moon by 2018, some seismologists have announced a new discovery made with Apollo-era data that could dictate what equipment to take on that trip, and beyond.

 

A re-examination of lunar seismic data brought back by Apollo astronauts in the 1970s has proven that a newly developed method that uses seismic static (as opposed to distinct loud quakes) to see buried geological structures inside the Earth works off-world as well.

 

For decades earthquakes have served like floodlights into the Earth's interior, with different geological structures being revealed by how they predictably reflect, scatter, speed-up or slow down powerful earthquake vibrations passing through the planet, all of it collected by seismic stations worldwide.

 

 

Earlier this year, however, seismologists showed that much shorter frequency seismic "noise" collected by the same seismographs can also be used to illuminate structures inside the planet.

 

Seismic noise is thought to be caused by things like ocean waves and gurgling geysers.

 

Now it looks like seismic noise may be the key to discovering the geological anatomies — and therefore the origins and histories — of worlds thought to have few or no significant earthquakes.

 

Those worlds include the Moon, Mars, Mercury and perhaps some of the moons of Saturn and Jupiter.

 

"It's very exciting," said seismologist Michael Ritzwoller of the University of Colorado.

 

Ritzwoller recently used seismic noise to reveal structures under California, establishing the validity of the technique for studying the Earth's crust.

 

"It's just now being used for the first time on the Earth and now being applied to a celestial object," he said.

 

The big question on the moon, said Ritzwoller, was whether there is even enough ambient seismic noise to echo off internal structures and make a sort of lunar "ultrasound" picture.

 

After all, unlike Earth, there are no oceans, winds or other processes rubbing up against the moon's surface to make seismic noise.

 

That question had a surprising answer, according to the new findings of Eric Larose of Laboratoire de Geophysique Interne et Tectonophysique, in Grenoble, France, and his colleagues.

 

The moon does indeed have seismic noise, but they found it's created by the heating and cooling, expanding and contracting of the moon's surface as it goes from 230° Fahrenheit (110° Celsius) in the lunar day to -274°F (-170°C) over a 29 1/2 day lunar rotation.

 

Their report on the new analysis of Apollo 17 data appears in the current issue of Geophysical Research Letters.

 

What's more, that lunar seismic noise does reflect off buried geological features on the moon.

 

 

"The answer is YES," said Larose. "We can see waves reflected by interfaces and buried scatterers."

 

They also used the rise and fall in seismic noise to reconstruct the cycle of lunar days when the seismic network was in place.

 

In fact, said Ritzwoller, Larose and his team probably could have seen even deeper into the moon had the Apollo seismograph stations been able to record a broader array of vibration frequencies, as do more modern "broadband" seismographs.

 

But those limitations, and the test on the moon, make it clearer how to go about future explorations of the moon's interior, as well as the interior of Mars and perhaps some of the moons of Jupiter and Saturn.

 

"We now think of designing an array that could be transported to Mars and deployed there," said Larose.