几米漫画地下铁:冰冷，死寂的月球如何保持了磁场？

作者：Melissae Fellet

A mystery thrown up by the Apollo moon rocks may finally have been solved. How did the moon remain magnetic tens of millions of years after its molten core stopped sloshing?

一块阿波罗月球陨石所引出的疑团可能最终会被解开。月球炽热的内核停止了翻滚已几千万年了，它是如何保持磁场的？

The moon has no global magnetic field today, but early in its life, it probably had a core hot enough to churn violently, with the movement of this electrically charged fluid creating a magnetic field. But as the core cooled, the convection should have eased enough to kill the field. So it was a puzzle when Apollo moon rocks suggested the moon still had a magnetic field 4.2 billion years ago, millions of years after the powerful mixing is thought to have ended. Now two groups have come up with explanations for what could have kept the core stirred up.

现在，月球的两极已经没有了磁场，可在其早年，它可能拥有过一个炽热得剧烈翻滚的内核，因为这些带电的液体的运动，磁场便产生了。可随着内核的降温，对流的停止足以让磁场消失。因此，当阿波罗月球陨石揭示了42亿年前的月球，在那些威力强大的混合熔岩凝固后，可能仍然存在磁场的时候，这的确令人费解。现在，有两个研究小组对月球保持内核活力的原因提出了解释。

The moon is thought to have formed closer to the Earth than it is now and spun faster, slowing down and moving away over time through tidal interactions with Earth. Christina Dwyer at the University of California, Santa Cruz, and colleagues say previous models did not take into account this faster spin, which would have agitated the molten core like water in a washing machine. This could have enabled the magnetic field to last until 2.7 billion years ago.

月球曾被认为它形成的时候与地球的距离比现在要近，而且旋转比现在更快，它的减速和离去是因为长期和地球发生相互的潮汐的作用导致。加州大学的Christina Dwyer，还有Santa Cruz和同事们说，之前的那些样本并没有把之前更快的旋转速度纳入考虑范围，那急速的旋转搅动着熔岩内核，就像洗衣机搅动水一样。这能令磁场一直持续到27亿年前。

Michael Le Bars at the Non-Equilibrium Phenomena Research Institute in Marseille, France, says large meteorite impacts that occurred until about 3.9 billion years ago also could have set the lunar core sloshing for periods of 10,000 years at a time.

法国马赛的非平衡态研究院的Michael Le Bars说在39亿年前曾经出现过的每一次剧烈的陨石撞击也可能会把月球内核的熔岩运动延续一万年。

Both models offer "a way out of a pretty major conundrum", says Ben Weiss at the Massachusetts Institute of Technology. Some meteorites – thought to be chips off of asteroids – are magnetic, and spacecraft flybys have measured two asteroids with magnetic fields, he says, adding that the models might explain how these space rocks came to be magnetised.