送信者: "Kumao KANEKO" <kkaneko@eeecom.jp>
件名 : EEE会議（地球の核心を核爆発で探る！）
日時 : 2003年5月15日 16:56

各位殿

人類は宇宙については太陽系の外まで探査ロケットを送って色々調査をしています
が、肝心の地球の核心部については、高々地表から１０キロメートル程度しか探査し
ておらず、地球の起源や地質構造については分からないことだらけです。

そこで、米国のカリフォルニア工科大学のDavid Stevenson博士は、グレープフルー
ツ大の地殻探査機を地中に打ち込む計画を発表したが（"Nature" 誌最新号）、それ
によると探査機は１週間かかって地球の核心に到達し、温度、地圧、化学的組成等に
関するデータを高周波地震波で地表に送ってくることになっているが、その前に、こ
の探査機をどうして地球の核心まで打ち込むかが大問題で、計算では、広島型原爆の
100〜1,000倍の核爆発を起こし、その結果出来た地球の亀裂に1億〜100億トン（コン
サートホール13個分）の鉄を溶かして流し込む必要があるので、実行不可能に近い、
つまり構想倒れということのようです。いかにもアメリカ人らしい奇抜で、人を食っ
た発想ですが、ちょっと夢があって面白いとも思いますが・・・。
詳しくは、次の記事("New Scientist"、5月14日号）をどうぞ。
金子熊夫

*****************************************

Journey to centre of Earth proposed


19:00 14 May 03

NewScientist.com news service

A proposal to send a probe to the centre of the Earth has been
published in a leading scientific journal. The wacky scheme would need the
world's largest nuclear bomb and enough iron to fill 13 large concert halls.

David Stevenson at Caltech in Pasadena, California, admits that his
suggestion is "slightly tongue in cheek" and faces some massive engineering
and funding challenges. But, he says, the proposal has a serious side -
highlighting the complete lack of direct knowledge of the Earth's interior.

Space probes have sent back data from beyond the Solar System, but
researchers have never drilled more than 10 kilometres into the crust, he
point out. So geologists know very little about the Earth's composition and
hence its origins and evolution.

The thought of having data direct from the Earth's core makes
geologists' eyes light up. "Oh boy," says Michael Wysession at Washington
University in St. Louis. "There's a lot we would like to know."

The most important question is which element or elements other than
iron make up the core. That would give clues to what drives the movement of
tectonic places on the surface, as well as how the Earth formed in the first
place.


Grapefruit-sized probe


Stevenson's plan, set out in Nature, is to send the probe down via a
self-propagating crack in the rock. The energy the carry it down comes from
gravity acting on the higher density of the molten iron compared to the
surrounding rock.

The grapefruit-sized probe would be carried in the iron and would
reach the Earth's core after about a week. It would then start sending back
data on temperature, pressure and chemical composition via high frequency
seismic waves.

The scheme should work in theory, but there are numerous practical
problems that could spell disaster. "I can see the probe getting stranded,"
says Kathy Whaler at Edinburgh University in Scotland. She believes that the
iron might drain away from the probe, leaving it behind.

Even if the probe makes it, Wysession thinks getting a signal back
will be a big stumbling block. He points out that oil companies have spent a
lot of money developing remote sensing technology that can only give data
about the upper few kilometres. The problem is that high frequency seismic
waves are attenuated very quickly in rock.

For Stevenson, it is getting the crack going in the first place that
is the big issue. One option would be to use a nuclear device a hundred to a
thousand times the size of the Hiroshima bomb. But immediately after the
explosion, you would have to inject between 100 million and 10 billion
tonnes of molten iron into the resulting crack. "I'm not going to suggest a
possible location," he laughs.

Journal reference: Nature (vol 423, p 239)


James Randerson