A Gravitational Battle Within the Earth Is Changing the Length of Days

地球内部的一场引力之战正在改变一天的时长

Earth’s days are not exactly the same length. Scientists have been recording small variations in for decades. These changes are minuscule—sometimes just a few milliseconds—and mostly imperceptible, but scientists are keen to understand them as they point to a much more monumental change: Our planet’s rotation speed is shifting.

地球上一天的时长并非完全相同。几十年来，科学家们一直在记录这些微小的变化。这些变化极其细微——有时仅有几毫秒——且大多无法察觉，但科学家们非常渴望了解它们，因为它们指向了一个更为重大的变化：我们行星的自转速度正在发生偏移。

Scientists have already pinpointed a handful of compounding causes. Fluctuations in the atmosphere and oceans can slightly speed up or slow down the Earth’s rotation; major earthquakes can redistribute part of the planet’s mass; and by depositing enormous amounts of water into the oceans, melting continental ice is also a factor.

科学家们已经确定了几个复合原因。大气和海洋的波动可以轻微地加速或减慢地球的自转；大地震可以重新分配地球的部分质量；此外，大陆冰川融化将大量水注入海洋，也是一个影响因素。

These influences account for some of the more sudden and recent changes, but on a several decades-long timescale, some of the variation appears to originate from a source much deeper within the Earth. Beneath our planet’s rocky mantle lies the outer core—a 1,400 mile thick layer of ultra-hot liquid metal that is in constant motion. As these flows change, they can slightly alter the speed at which the rest of the planet rotates.

这些影响解释了近期一些较为突发的变化，但在长达数十年的时间尺度上，部分变化似乎源自地球更深处。在我们星球的岩石地幔之下是外核——一层厚达 1400 英里的超高温液态金属，它处于持续运动状态。随着这些流动的变化，它们会轻微改变地球其余部分的自转速度。

However, how these forces are transmitted from one region to another has remained a mystery. Friction between the core and the mantle is too weak, so geologists have been searching for other mechanisms operating in the Earth’s interior.

然而，这些力是如何从一个区域传递到另一个区域的，一直是个谜。地核与地幔之间的摩擦力太弱，因此地质学家一直在寻找地球内部运作的其他机制。

In a new study published this week in Nature, researchers believe they have identified one of these hidden gears. They propose that the gravity of the inner core pushes rotation in one direction, while other interactions between the core and the mantle oppose it. The result of this competition, accumulated over decades, would be small accelerations and decelerations in the planet’s spin.

在本周发表于《自然》杂志的一项新研究中，研究人员认为他们已经找到了其中一个隐藏的“齿轮”。他们提出，内核的引力推动自转朝一个方向运动，而地核与地幔之间的其他相互作用则产生阻力。这种竞争的结果在数十年间积累，便形成了地球自转的微小加速和减速。

The idea is based on the fact that the inner core does not rotate at the same rate as the rest of the planet, causing it to become slightly misaligned with certain mass irregularities in the mantle. But the core “wants to be aligned,” Mathieu Dumberry, a geophysicist at the University of Alberta involved in the study, said in a press release. What this means is that gravity tries to pull the layers of the inner Earth back into their more harmonic position, which influences—albeit minutely—the rotation of the planet’s surface.

这一观点基于这样一个事实：内核的自转速度与地球其他部分并不一致，导致它与地幔中某些质量不规则区域产生轻微错位。参与该研究的阿尔伯塔大学地球物理学家马修·邓伯里（Mathieu Dumberry）在新闻稿中表示，内核“想要保持对齐”。这意味着引力试图将地球内部的各层拉回到更和谐的位置，从而影响了地球表面的自转——尽管这种影响极其微小。

The authors reconstructed how this interaction influenced Earth’s spin between 1964 and 2019 using seismic estimates of the inner core’s rotation and models of liquid metal flows in the outer core. Their gravitational model closely reproduced both the timing and the magnitude of real gradual changes that had been observed over the decades.

研究人员利用内核自转的地震估算数据以及外核液态金属流动的模型，重建了这种相互作用在 1964 年至 2019 年间如何影响地球自转的过程。他们的引力模型精确地再现了过去几十年中所观测到的真实渐进变化的发生时间和幅度。

Other yet-unknown factors are likely involved, but this proposal unravels a major component of our variable days. The model also didn’t account for all the forces competing involved, but it didn’t need to to produce a compelling result. Because it might be a tug-of-war, Dumberry said, but “gravity still wins.”

虽然可能还涉及其他未知的因素，但这一提议揭示了我们一天时长变化的一个主要组成部分。该模型虽然没有考虑到所有参与竞争的力，但要得出令人信服的结果并不需要面面俱到。邓伯里说，这可能是一场拔河比赛，但“引力依然是赢家”。