지구가 훔(hum) 소리를 낸다

[완순선생]

	Source of Earth's Hum Revealed, Space Symphony Possible By Robert Roy Britt Senior Science Writer posted: 10:18 am ET 26 March 2000

Competing with the natural emissions from stars and other celestial objects, our Earth sings like a canary -- it drones on in a constant hum of a gazillion notes. If it were several octaves higher, and hence, audible to the human ear, it could probably drown out the noise from a hundred TV talk shows.

In recent years scientists have used seismographs to sort out these subsurface sound waves from earthquakes (all seismic waves are, essentially, the in-ground equivalent of sound waves). But what causes the hum, which researchers call the background-free oscillation, has been a mystery.

The apparent answer, revealed in the March 24 issue of the journal Science, is as surprising as the hum itself.

Kiwamu Nishida of the University of Tokyo's Earthquake Research Institute has, along with colleagues, analyzed 10 years of seismic data and tied the seismic waves to similar oscillations in the atmosphere.

Inaudible sound waves in the lower atmosphere push and pull on the ground, the researchers say, creating coupled "sound" waves, or seismic waves, inside Earth. The initial source, as yet not determined, could be changes in atmospheric pressure. The researchers also did not rule out possible oceanic sources, such as pounding waves, as the cause of Earth's hum.

The strange-but-true solution was first proposed in 1997 by Naoki Kobayashi, a theorist at the Tokyo Institute of Technology and co-author of the new paper.

A space symphony?

Because Mars and Venus are both solid bodies with atmospheres, Nishida told SPACE.com that our two nearest planetary neighbors are probably humming too, creating a miniature symphony in space.

"Because the density of these atmospheres is different, amplitudes of the 'sound' might be different," Nishida said. "On the other hand, the amplitudes [within the planets] might be similar to that of the solid Earth."

The sounds are below 10 millihertz, whereas 10,000 millihertz is about the lowest audible to the human ear. Which means you can't hear the hum. Good thing, because the discordant sound has been described by one geophysicist as "a very messy noise."

The sound of Mars

Philippe Lognonne of the University of Paris said a proposed future mission to Mars, known as NetLander and expected to launch between 2005 and 2007, would explore the seismic waves of the Red Planet and their possible connection to the atmosphere.

"We expect this signal to exist also on Mars, and to detect it will therefore give us the possibility to see the free oscillations of Mars," Lognonne told SPACE.com. "It is clearly a technical challenge, but it might be possible."

The NetLander mission would use a network of seismic detectors to measure Mars-quakes -- theorists expect the gradual cooling of the planet generates 50 or more temblors a year with magnitude 3.5 or greater. Studying how the seismic waves move through subsurface layers of different composition would help scientists determine the diameter of the planet's core and whether it is solid or liquid.

A set of four meteorology stations would monitor the martian atmosphere and how it interacts with the surface.

출처:http://www.space.com/scienceastronomy/planetearth/space_symphony_000323.html