The Big Whack : Pieces of the Moon that never rise. Where did the moon come from?

The lost popular theory is that about 4.5 billion years ago, a Mars size protoplanet slammed into Earth. Some of the resulting debris, tossed into orbit, coalesced to form the moon.

The idea, known as the ''big whack'' would explain much about the moon. But scientists have lacked smoking-gun evidence like a crater of pieces of the protoplanet, which they have named Theia.

In a study published in the journal Nature, researchers assert that pieces of Theia did survive the impact - but they are way down, on the boundary between Earth's mantle and more, 1,800 miles [2,900 kilometers] below the surface.

'' We looked into the deeper Earth,'' said Qian Yuan, a postdoctoral researcher at the California Institute of Technology, who led the research.

'' We found big chunks of the impactor Theia.''

Dr. Yuan's interest grew out of a planetary geochemistry class while he was a graduate student at Arizona State University. He recalled that the professor asked a straightforward question :

''Where is the impactor Theia right now?'''

Some of Theia now makes up the moon. But if Theia was the size of Mars, then about 90 percent of its mass must have ended up back on Earth. Some of that certainly melted and mixed in with Earth's minerals, but perhaps some pieces of the protoplanet persisted almost intact.

Dr. Yuan wondered if these pieces might be what makes up two mysterious structures deep inside the Earth, at the boundary between the core and the mantle.  

The blobs - one under West Africa, the other under the Pacific Ocean - span an area as large as a continent and stretch upward hundreds of miles into the mantle.

They were first spotted a half-century ago, when researchers realized that seismic waves - the shaking generated by earthquakes - slowed down when passing through these regions. 

It is difficult to discern much about the structures, other than that they exist. The seismic data offer fuzzy impressionistic views that do not tell what the structures are made of. It is impossible to drill that far into the planet to pull up the samples.

The blog under West Africa is known as Tuzo, after J. Tuzo Wilson, the one under the Pacific Ocean, is called Jason, after W Jason Morgan. Both did groundbreaking work in the study of plate tectonics.

Dr. Yuan noted the volumes of Tuzo and Jason were roughly comparable to that of the moon, leading him and his colleagues to wonder if they might be additional pieces of Theia.

With that as a starting point, he arrived at his theory on the basis of computer simulations of such a collision.

The World Students Society thanks Kenneth Chang.


Post a Comment

Grace A Comment!