在地球磁場的維持上月球也許具有關鍵地位

原始網址：www.sciencedaily.com/releases/2016/04/160401075118.htm

The Moon may play a major role in maintaining Earth's magnetic field

在地球磁場的維持上月球也許具有關鍵地位

The Earth's magnetic field permanently protects us from the charged particles and radiation that originate in the Sun. This shield is produced by the geodynamo, the rapid motion of huge quantities of liquid iron alloy in the Earth's outer core. To maintain this magnetic field until the present day, the classical model required the Earth's core to have cooled by around 3,000 °C over the past 4.3 billion years. Now, a team of researchers from CNRS and Université Blaise Pascal[1] suggests that, on the contrary, its temperature has fallen by only 300 °C. The action of the Moon, overlooked until now, is thought to have compensated for this difference and kept the geodynamo active. Their work is published on 30 march 2016 in the journal Earth and Planetary Science Letters.

地球磁場長久以來保護我們免受來自太陽的帶電粒子和輻射所造成的傷害。這道屏障是由「地球發電機」(geodynamo)產生，此效應是因地球外核中的巨量液態鐵合金迅速流動而造成的。傳統模型認為要讓地球磁場維持至今日，地核必須在43億年的歲月中降溫將近3000°C。相反地，法國國家科學研究中心和克萊蒙費朗第二大學的研究人員近日提出的學說認為，地核溫度僅需降溫300°C就能維持地球磁場。他們認為彌補這個溫度差以及維繫地球發電機運作的關鍵因素是直到現在才被注意到的月球運動。這項研究刊登於2016年3月30日的《地球與行星科學通訊》期刊。

The classical model of the formation of Earth's magnetic field raised a major paradox. For the geodynamo to work, the Earth would have had to be totally molten four billion years ago, and its core would have had to slowly cool from around 6800 °C at that time to 3800 °C today. However, recent modeling of the early evolution of the internal temperature of the planet, together with geochemical studies of the composition of the oldest carbonatites and basalts, do not support such cooling. With such high temperatures being ruled out, the researchers propose another source of energy in their study.

傳統模型在解釋地球磁場如何形成時具有一個重大的矛盾。若要讓地球發電機運轉，地球在四十億年前必須要處於完全熔融的狀態，在這之後地核得從將近6800°C的高溫緩緩降溫至現今的 3800 °C。然而，最近模擬早期地球內部溫度如何演變的結果，以及對最古老的碳酸岩(carbonatite)和玄武岩成分進行的地球化學研究，皆不支持這樣的冷卻過程。由於上述結果排除掉如此高溫為可能的能量來源，所以這些研究人員在他們的研究之中提出了另外一種來源。

The Earth has a slightly flattened shape and rotates about an inclined axis that wobbles around the poles. Its mantle deforms elastically due to tidal effects caused by the Moon. The researchers show that this effect could continuously stimulate the motion of the liquid iron alloy making up the outer core, and in return generate Earth's magnetic field. The Earth continuously receives 3,700 billion watts of power through the transfer of the gravitational and rotational energy of the Earth-Moon-Sun system, and over 1,000 billion watts is thought to be available to bring about this type of motion in the outer core. This energy is enough to generate the Earth's magnetic field, which together with the Moon, resolves the major paradox in the classical theory. The effect of gravitational forces on a planet's magnetic field has already been well documented for two of Jupiter's moons, Io and Europa, and for a number of exoplanets.

地球的外型略呈扁平且以一根傾斜的轉軸為軸心自轉，而這根自轉軸本身也會繞著一個圓圈晃動；另外，地函內部也會發生塑性變形。以上的作用或多或少都跟月球造成的潮汐力有關。研究人員顯示這個作用力同樣也能促使組成外核的液態鐵合金不斷運動，進而產生地球磁場。在地球-月球-太陽系統中，重力和彼此繞行時產生的能量會持續轉換至地球身上，功率高達3兆7000億瓦。研究人員相信這些能量當中有超過1兆瓦被用來驅動外核中的這類運動，而如此龐大的能量足以產生地球磁場。因此將月球這項因子考量在內，便能夠解決傳統理論中的重大矛盾。在木星的兩顆衛星，木衛一「埃歐」及木衛二「歐羅巴」，還有一些地外行星上已經詳細記錄到這種重力對星球磁場的影響。

Since neither the Earth's rotation around its axis, nor the direction of its axis, nor the Moon's orbit are perfectly regular, their combined effect on motion in the core is unstable and can cause fluctuations in the geodynamo. This process could account for certain heat pulses in the outer core and at its boundary with the Earth's mantle.

既然地球自轉、地軸指向以及月球軌道這三者都並非是照著完美的規律運作，它們聯合起來對地核流動造成的效應就不會是穩定的，因此會造成地球發電機內部發生擾動。這可以用來解釋外核內部以及核幔邊界處發生的部分熱能高漲現象。

Over the course of time, this may have led to peaks in deep mantle melting and possibly to major volcanic events at the Earth's surface. This new model shows that the Moon's effect on the Earth goes well beyond merely causing tides.

長久下來，這種現象可能使深部地函的頂部熔融，並在地球表面造成重大的火山噴發事件。這個新模型顯示月球對地球造成的影響之大可不僅只有潮汐而已。

[1] At the Laboratoire Magmas et Volcans (CNRS/IRD/Université Blaise Pascal), part of the Observatoire de Physique du Globe de Clermont-Ferrand, the Institut de Recherche sur les Phénomènes Hors-équilibre (CNRS/Aix-Marseille Université/Ecole Centrale Marseille) and the Institut de Recherche en Astrophysique et Planétologie (CNRS/Université Toulouse III -- Paul Sabatier).

來源自：CNRS. "The Moon may play a major role in maintaining Earth's magnetic field." ScienceDaily. ScienceDaily, 1 April 2016.