地球的磁力屏障較過往認為的還要古老

原文網址：www.sciencedaily.com/releases/2015/07/150730162010.htm

Earth's magnetic shield is much older than previously thought

An older geomagnetic field suggests an early start to plate tectonics

地球的磁力屏障較過往認為的還要古老

更古老的磁場代表更早開始的板塊運動

Since 2010, the best estimate of the age of Earth's magnetic field has been 3.45 billion years. But now a researcher responsible for that finding has new data showing the magnetic field is far older.

自2010年起，對於地球磁場的年紀的最佳預估為34.5億年。但提出此發現的科學家現在有新證據顯示磁場其實更加古老。

John Tarduno, a geophysicist at the University of Rochester and a leading expert on Earth's magnetic field, and his team of researchers say they believe the Earth's magnetic field is at least four billion years old.

John Tarduno為羅徹斯特大學(the University of Rochester)的地球物理學家，同時也是地磁領域中首屈一指的專家，他的團隊說他們認為地磁的年紀已有40億年之久。

"A strong magnetic field provides a shield for the atmosphere," said Tarduno, "This is important for the preservation of habitable conditions on Earth.

「強力的磁場可以作為大氣層的護盾，」Tarduno說，「這對地球能夠保持適合生物生存的條件來說相當重要。」

The findings by Tarduno and his team have been published in the latest issue of the journal Science.

Tarduno與他的團隊的發現刊登在期刊《科學》的最新一期。

Earth's magnetic field protects the atmosphere from solar winds -- streams of charged particles shooting from the Sun. The magnetic field helps prevent the solar winds from stripping away the atmosphere and water, which make life on the planet possible.

地磁能夠保護大氣免受太陽風，這種由太陽噴射而出的帶電粒子流的傷害。磁場有助於避免太陽風剝離地球的大氣與水，使得生命有機會出現在地球上。

Earth's magnetic field is generated in its liquid iron core, and this "geodynamo" requires a regular release of heat from the planet to operate. Today, that heat release is aided by plate tectonics, which efficiently transfers heat from the deep interior of the planet to the surface. But, according to Tarduno, the time of origin of plate tectonics is hotly debated, with some scientists arguing that Earth lacked a magnetic field during its youth.

地磁由液態的鐵質地核(外核)產生，這種「地球發電機」(geodynamo)的運作需要地球持續地將熱能發散出去。現今的板塊運動能相當有效率地讓地球深處產生的熱能傳遞至地表，因此可以幫助散熱。然而根據Tarduno所說，板塊運動的開始時間仍有相當激烈的爭議，其中有些科學家的論點為地球在幼年時缺乏磁場，因此也缺乏板塊運動。

Given the importance of the magnetic field, scientists have been trying to determine when it first arose, which could, in turn, provide clues as to when plate tectonics got started and how the planet was able to remain habitable.

由於地磁的重要性，科學家一直試著確認它首度出現的時間，這也能提供板塊運動何時開始，以及地球如何一直保持適居的線索。

Fortunately for scientists, there are minerals -- such as magnetite -- that lock in the magnetic field record at the time the minerals cooled from their molten state. The oldest available minerals can tell scientists the direction and the intensity of the field at the earliest periods of Earth's history. In order to get reliable measurements, it's crucial that the minerals obtained by scientists are pristine and never reached a sufficient heat level that would have allowed the old magnetic information within the minerals to reset to the magnetic field of the later time.

對科學家來說幸運的是，像磁鐵礦這類的礦物從熔化狀態冷卻時，可以將當下的磁場紀錄鎖在其中。取得最古老的這些礦物可以告訴科學家在地球的早期歷史，當時磁場的方向與強度。為了得到可信的測量結果，重要的是科學家取得的這些礦物必須保持在原始的狀態，從未被加熱至會使礦物內舊有的磁場資訊被洗掉，而被往後的磁場資訊取而代之。

The directional information is stored in microscopic grains inside magnetite- a naturally occurring magnetic iron oxide. Within the smallest magnetite grains are regions that have their own individual magnetizations and work like a tape recorder. Just as in magnetic tape, information is recorded at a specific time and remains stored unless it is replaced under specific conditions.

磁鐵礦為自然產生具有磁性的氧化鐵，而磁場的方向資訊即保存於其內的微小顆粒當中。這些極小的磁鐵顆粒中會有區域紀錄它們自身獨有的磁化資訊，作用方式就像磁帶錄音機一樣。如同磁帶一般，特定時間的資訊會被紀錄下來並且保存於其中，除非在特定條件下才會被重置。

Tarduno's new results are based on the record of magnetic field strength fixed within magnetite found within zircon crystals collected from the Jack Hills of Western Australia. The zircons were formed over more than a billion years and have come to rest in an ancient sedimentary deposit. By sampling zircons of different age, the history of the magnetic field can be determined.

Tarduno的新發現建立於從西澳Jack Hills採集的鋯石晶體中的磁鐵礦保存的磁場強度紀錄。這些鋯石形成於超過10億年前，之後停置在這層古老的沉積物。藉由採集不同年代的鋯石樣品，就能確立磁場的歷史。

The ancient zircons are tiny -- about two-tenths of a millimeter -- and measuring their magnetization is a technological challenge. Tarduno and his team used a unique superconducting quantum interference device, or SQUID magnetometer, at the University of Rochester that provides a sensitivity ten times greater than comparable instruments.

這些古老的鋯石相當微小，僅約十分之二厘米左右，因此測量它們的磁化在技術上有相當的難度。Tarduno與他的團隊利用羅徹斯特大學獨具的超導量子干涉儀(superconducting quantum interference device)，或稱作SQUID磁量儀(SQUID magnetometer)來測量，與相似的儀器相比其靈敏度高出了十倍以上。

But in order for today's magnetic intensity readings of the magnetite to reveal the actual conditions of that era, the researchers needed to make sure the magnetite within the zircon remained pristine from the time of formation.

但為了確認現今從磁鐵礦讀出的磁場強度反映出的是當時的真實情況，研究人員必須要先確保這些鋯石中的磁鐵礦仍維持著形成當時的原始狀態。

Of particular concern was a period some 2.6 billion years ago during which temperatures in the rocks of the Jack Hills reached 745℃. Under those conditions, it was possible that the magnetic information recorded in the zircons would have been erased and replaced by a new, younger recording of Earth's magnetic field.

其中特別令人擔憂的是在26億年的某段時期，Jack Hills的岩石的溫度曾到達475℃。在此情況下，鋯石紀錄的磁場資訊可能會被抹去，並被新的、較年輕的地磁資訊取代。

"We know the zircons have not been moved relative to each other from the time they were deposited," said Tarduno. "As a result, if the magnetic information in the zircons had been erased and re-recorded, the magnetic directions would have all been identical."

「我們知道這些鋯石顆粒沉積下來之後，彼此之間就沒有相對移動的發生。」Tarduno說，「因此，如果鋯石中的磁場資訊曾經被消去並重新紀錄，那麼它們的磁場方向應該會變得全部一致。」

Instead, Tarduno found that the minerals revealed varying magnetic directions, convincing him that the intensity measurements recorded in the samples were indeed as old as four billion years.

然而，Tarduno發現這些礦物顯示出的磁場方向相當多變，讓他確信這些樣品所紀錄的磁場強度資訊，年代確實可追溯至40億年前。

The intensity measurements reveal a great deal about the presence of a geodynamo at the Earth's core. Tarduno explains that solar winds could interact with the Earth's atmosphere to create a small magnetic field, even in the absence of a core dynamo. Under those circumstances, he calculates that the maximum strength of a magnetic field would be 0.6 ?T (micro-Teslas). The values measured by Tarduno and his team were much greater than 0.6 ?T, indicating the presence of a geodynamo at the core of the planet, as well as suggesting the existence of the plate tectonics needed to release the built-up heat.

強度測量結果揭露了有關位於地核的地磁發電機出現的大量資訊。Tarduno解釋即使沒有地球發電機的存在，太陽風跟地球大氣層的交互作用也會產生小規模的磁場。在此情況下，他計算出這類磁場的最大強度為0.6mT(micro-Teslas)。但Tarduno及他的團隊測量到的強度卻遠大於0.6mT，顯示地核已經有地球發電機的出現，也代表說板塊運動已經開始運作才能排出不斷累積的熱能。

"There has been no consensus among scientists on when plate tectonics began," said Tarduno. "Our measurements, however, support some previous geochemical measurements on ancient zircons that suggest an age of 4.4 billion years."

「科學家之間對板塊運動的開始時間尚無共識。」Tarduno說，「然而，我們的測量結果支持了先前對古老鋯石的地球化學測量結果，其指出開始於44億年前。」

The magnetic field was of special importance in that eon because solar winds were about 100 times stronger than today. In the absence of a magnetic field, Tarduno says the protons that make up the solar winds would have ionized and stripped light elements from the atmosphere, which, among other things, resulted in the loss of water.

在那個年代磁場特別重要，因為當時的太陽風強度較現今強了約莫100倍。Tarduno說在沒有磁場的狀況下，組成太陽風的質子會把大氣層中的輕元素離子化並颳走，這會造成許多後果，其中包括水氣流失。

Scientists believe that Mars had an active geodynamo when that planet was formed, but that it died off after four billion years. As a result, Tarduno says, the Red Planet had no magnetic field to protect the atmosphere, which may explain why its atmosphere is so thin.

科學家咸信火星在形成時擁有活躍的地球發電機，但在四十億年後已經停止運作了。Tarduno說這導致這顆紅色行星的大氣層缺乏磁場保護，解釋了為何其大氣層如此稀薄。

"It may also be a major reason why Mars was unable to sustain life," said Tarduno.

「這可能也是火星無法支持生命的主要原因。」Tarduno說。

引用自：University of Rochester. "Earth's magnetic shield is much older than previously thought: An older geomagnetic field suggests an early start to plate tectonics." ScienceDaily. ScienceDaily, 30 July 2015.