地球如何保有磁場？

原文網址：https://carnegiescience.edu/news/how-does-earth-sustain-its-magnetic-field

地球如何保有磁場？

地球核心的化學組成如何塑造地球的歷史並讓地球成為生命可以存在的地方？

太陽風以及來源更遠的宇宙射線裡帶有的游離粒子會對生命造成危害。因此，如果沒有地球磁場使它們偏離地球，我們所知的生命就不可能存在。地球磁場是由外核裡的液態鐵的運動來持續產生，這種現象稱為「地球發電機」。

儘管這一切如此至關重要，對於地球發電機最一開始如何形成，以及億萬年來持續使其運作的能量來源是什麼，卻還有許多問題有待解答。

包括卡內基研究所的科學家Alexander Goncharov、Nicholas Holtgrewe 和Irina Chuvashova在內的國際研究團隊做出的最新成果，探討了在以鐵為主要成份的地核當中，摻有質量較輕的元素會如何影響地球發電機的生成與持續運作。這項發現刊登於《自然通訊》(Nature Communications)。

地球是由太陽年幼時圍繞在其周圍的盤狀塵埃與氣體聚積而成。行星形成過程中密度最高的物質最終會沉入內部，使得地球產生了目前的層狀結構——地核、地函與地殼。雖然地核的主要成分是鐵，但是地震波的資料顯示某些較輕的元素，像是氧、矽、硫、碳、氫也在分化過程中溶到了地核裡面。

隨著時間推移內核開始結晶出來，溫度也從那時起一直漸漸冷卻下來。地球發電機是否光憑這股從地核逸散至地函的熱量就能驅動？或者熱對流只靠熱量並無法產生，還需要輕元素從凝固的內核當中往上浮時所提供的額外推力？

解開地核確切的化學組成有助於解答這項問題。

矽酸鹽是地函的主要組成，同時矽也是繼氧和鐵之後地球含量第三高的元素。因此地核當中可以和鐵形成合金的主要輕元素，一個很有可能的選項就是矽。由任職於中央研究院與國立台灣大學的謝文斌所領導的研究團隊，在實驗室裡仿製出深層地球的環境條件，藉此模擬地球的鐵質核心把熱往外傳輸至地函的過程中，矽的出現會帶來什麼樣的影響。

「地核物質的導熱性越差，要讓地球發電機開始運作的門檻也就越低，」Goncharov解釋。「門檻低到某個程度時，不需要再加上物質的運動，光憑熱對流就足以把地核的熱能傳輸出去。」

團隊發現他們的模擬內核裡矽的濃度佔總重百分之八左右的時候，地球發電機僅透過熱的傳輸就能在整部地球歷史當中一直維持運作著。

放眼未來，他們希望可以拓廣這項成果以了解當地核含有氧、硫、碳的時候，會讓對流作用出現什麼樣的影響。

 

How does Earth sustain its magnetic field?

How did the chemical makeup of our planet’s core shape its geologic history and habitability?

Life as we know it could not exist without Earth’s magnetic field and its ability to deflect dangerous ionizing particles from the solar wind and more far-flung cosmic rays. It is continuously generated by the motion of liquid iron in Earth’s outer core, a phenomenon called the geodynamo.

Despite its fundamental importance, many questions remain unanswered about the geodynamo’s origin and the energy sources that have sustained it over the millennia.

New work from an international team of researchers, including current and former Carnegie scientists Alexander Goncharov, Nicholas Holtgrewe, Sergey Lobanov, and Irina Chuvashova examines how the presence of lighter elements in the predominantly iron core could affect the geodynamo’s genesis and sustainability. Their findings are published by Nature Communications.

Our planet accreted from the disk of dust and gas that surrounded our Sun in its youth. Eventually, the densest material sank inward in the forming planet, creating the layers that exist today—core, mantle, and crust. Although, the core is predominantly iron, seismic data indicates that some lighter elements like oxygen, silicon, sulfur, carbon, and hydrogen, were dissolved into it during the differentiation process.

Over time, the inner core crystallized and has been continuously cooling since then. On its own, could heat flowing out of the core and into the mantle drive the geodynamo? Or does this thermal convection need an extra boost from the buoyancy of light elements, not just heat, moving out of a condensing inner core?

Understanding the specifics of the core’s chemical composition can help answer this question.

Silicates are predominant in the mantle, and after oxygen and iron, silicon is the third-most-abundant element in the Earth, so it is a likely option for one of the main lighter elements that could be alloyed with iron in the core. Led by Wen-Pin Hsieh of Academia Sinica and National Taiwan University, the researchers used lab-based mimicry of deep Earth conditions to simulate how the presence of silicon would affect the transmission of heat from the planet’s iron core out into the mantle.

“The less thermally conductive the core material is, the lower the threshold needed to generate the geodynamo,” Goncharov explained. “With a low enough threshold, the heat flux out of the core could be driven entirely by the thermal convection, with no need for the additional movement of material to make it work.”

The team found that a concentration of about 8 weight percent silicon in their simulated inner core, the geodynamo could have functioned on heat transmission alone for the planet’s entire history.

Looking forward, they want to expand their efforts to understand how the presence of oxygen, sulfur, and carbon in the core would influence this convection process.

原始論文：Wen-Pin Hsieh, Alexander F. Goncharov, Stéphane Labrosse, Nicholas Holtgrewe, Sergey S. Lobanov, Irina Chuvashova, Frédéric Deschamps, Jung-Fu Lin. Low thermal conductivity of iron-silicon alloys at Earth’s core conditions with implications for the geodynamo. Nature Communications, 2020; 11 (1) DOI: 10.1038/s41467-020-17106-7

引用至：Carnegie Institution for Science. "How does Earth sustain its magnetic field?"