The missing 500 million: Cosmic bombardment melted Earth’s first crust

消失的五亿年：宇宙轰炸熔化了地球的早期地壳

Earth is the only planet we know of with buoyant, silica-rich continents. But, despite decades of research, geologists still don’t agree on how they formed. 地球是我们所知的唯一拥有浮力大、富含硅酸盐大陆的行星。然而，尽管经过了数十年的研究，地质学家们对于这些大陆是如何形成的仍未达成共识。

“The continents started appearing around about four billion years ago—that’s the oldest continental rock we know about,” said Tim Johnson, a geologist at Curtin University in Perth, Australia. “The Earth is four and a half billion years old, so why they started appearing then is unknown, as is the mechanism to make that continental crust.” “大陆大约在四十亿年前开始出现——这是我们所知的最古老的大陆岩石，”澳大利亚珀斯科廷大学的地质学家蒂姆·约翰逊（Tim Johnson）说。“地球已有四十五亿年的历史，所以为什么它们在那时才开始出现，以及形成这种大陆地壳的机制是什么，目前尚不清楚。”

Johnson and his colleagues are now arguing that the formation of continents on Earth was caused largely by an intense, sustained barrage of asteroid impacts that kept the early crust hot and thin enough to make buoyant continents possible. In short, the lands we live on are here because of ancient bombardment from space. 约翰逊和他的同事们现在认为，地球上大陆的形成在很大程度上是由一场强烈且持续的小行星撞击造成的，这些撞击使早期地壳保持在足够热且薄的状态，从而使浮力大陆的形成成为可能。简而言之，我们所居住的陆地之所以存在，是因为来自太空的远古轰炸。

Plates and plumes

板块与地幔柱

The problem with studying the formation of continents is that the geological evidence of this process is almost gone. The oldest known continental-type rocks crystallized around 4.03 billion years ago, right at the end of the Hadean eon (the earliest era in Earth’s history, spanning the first 500 million years of its existence). 研究大陆形成的问题在于，这一过程的地质证据几乎已经消失。已知最古老的大陆型岩石结晶于约四十亿年前，正处于冥古宙（地球历史上最早的时期，涵盖了其存在的前五亿年）的末期。

Rare basaltic rocks date back about 4.2 billion years, and a handful of the oldest zircon crystals push the record back to 4.4 billion years. Beyond that, there’s hardly anything else. So, scientists looking into the origin of continents had to rely largely on educated guesses. 稀有的玄武岩岩石可以追溯到约四十二亿年前，而少数最古老的锆石晶体将记录推回到了四十四亿年前。除此之外，几乎没有其他证据了。因此，研究大陆起源的科学家们不得不主要依赖于科学推测。

“There are huge debates about what was going on in the early Earth, because the data is so scarce,” Johnson said. One dominant idea holds that plate tectonics, much like today’s, was already running in the Hadean, with continental crust forming above subduction zones—areas where tectonic plates collide. “关于早期地球发生了什么，存在着巨大的争论，因为数据太稀缺了，”约翰逊说。一种主流观点认为，冥古宙时期就已经存在类似于今天的板块构造，大陆地壳是在俯冲带（构造板块碰撞的区域）上方形成的。

The other claims that early Earth was too hot for rigid plates, and that crust instead formed above mantle plumes rising from deep within the planet, a phenomenon comparable, Johnson said, to the wax blobs rising inside a lava lamp. 另一种观点则认为，早期地球温度过高，无法形成坚硬的板块，地壳是在从地球深处升起的地幔柱上方形成的。约翰逊说，这种现象类似于熔岩灯中上升的蜡块。

The issue with both these ideas, though, was that Earth, based on most models, appeared too cold for all this to happen. “People have tried to understand Earth’s heat budget through time, and nobody could make it fit,” Johnson said. “Nobody could make it fit because we did not consider the energy coming from outside of Earth.” 然而，这两种观点的问题在于，根据大多数模型，地球似乎太冷了，不足以发生这一切。“人们试图了解地球随时间变化的热量收支，但没有人能得出合理的解释，”约翰逊说。“没有人能解释通，是因为我们没有考虑来自地球外部的能量。”

This energy, he argues, came from asteroid and meteorite impacts that were far more frequent back when the solar system was young. Adding these impacts to the early Earth’s heat budget, though, proved rather challenging because Earth has a peculiar way of healing its scars. 他认为，这种能量来自太阳系年轻时远比现在频繁的小行星和陨石撞击。然而，将这些撞击纳入早期地球的热量收支计算被证明相当具有挑战性，因为地球有一种独特的“愈合伤疤”的方式。

The moon shot

月球视角

The reason we don’t really know what was happening on Earth four billion years ago is that plate tectonics effectively recycles the surface of the planet back into the mantle. “One place where we do know what was going on back then is the Moon,” Johnson said. “We have sent people there. We have collected sample from there. We have immense amounts of high-quality data from the Moon.” 我们之所以不真正了解四十亿年前地球上发生了什么，是因为板块构造有效地将行星表面循环回了地幔。“我们确实知道当时发生了什么的一个地方是月球，”约翰逊说。“我们曾派人去过那里，采集过样本，并拥有大量来自月球的高质量数据。”

Because the Moon does not have plate tectonics, its crust is a single, solid, continuous shell. And this shell, Johnson’s team noted, is peppered with impact craters. Calibrated against dated lunar samples, crater counts on the Moon let Johnson’s team estimate how frequently large bodies were hitting our closest celestial neighbor shortly after the Earth had formed. 由于月球没有板块构造，其地壳是一个单一、坚固且连续的外壳。约翰逊的团队指出，这个外壳上布满了撞击坑。通过对月球样本的测年校准，月球上的陨石坑计数使约翰逊的团队能够估算出在地球形成后不久，大型天体撞击我们最近的邻居的频率。

“Scaling that flux up to Earth’s larger size and stronger gravity makes it clear the planet must have been hit by thousands of impactors that were greater than 10 kilometers in diameter,” Johnson said. When his team determined the most probable frequency of impacts and the size of impactors, they could calculate how much energy this immense bombardment delivered to Earth and, consequently, how much heat it produced. It turned out it was a lot of heat. “将这种通量按比例放大到地球更大的尺寸和更强的引力，很明显，地球一定遭受了数千次直径超过10公里的撞击体撞击，”约翰逊说。当他的团队确定了最可能的撞击频率和撞击体大小时，他们就能计算出这场巨大的轰炸向地球输送了多少能量，进而计算出它产生了多少热量。结果证明，热量非常巨大。

Most prior modeling of early Earth’s heat budget focused on internal sources like heat left over from accretion and core formation plus the ongoing decay of radioactive isotopes—we thought these were absolutely dominant. Johnson’s space bombardment model showed they were not. 此前大多数关于早期地球热量收支的模型都集中在内部来源上，例如吸积和地核形成过程中残留的热量，以及放射性同位素的持续衰变——我们曾认为这些是绝对主导因素。但约翰逊的太空轰炸模型表明并非如此。

Bringing the heat

带来热量

The team focused on modeling how the kinetic energy of each impact would ultimately end up as heat. The physics, Johnson said, is straightforward even if the details are complex. “It really is as simple as converting the size and the velocity of the impactor into energy,” he explains. 该团队专注于模拟每次撞击的动能最终如何转化为热量。约翰逊说，物理原理很简单，尽管细节很复杂。“这实际上就像将撞击体的大小和速度转化为能量一样简单，”他解释道。

When a large body hits, some of the impact energy goes into vaporizing or melting rock right at the impact site. But, especially when an impactor is big, most of it propagates into the mantle below. “This energy basically heats up the entire upper mantle,” Johnson said. 当大型天体撞击时，部分撞击能量会用于汽化或熔化撞击点处的岩石。但是，特别是当撞击体很大时，大部分能量会传播到下方的地幔中。“这种能量基本上加热了整个上地幔，”约翰逊说。

This heat drives more melting and more basaltic volcanism, a process that plays out not just in the minutes-to-hours timescale of the actual collision, but in tens or even hundreds of millions of years afterward. When Johnson and his colleagues added up these contributions, impact heating exceeded radiogenic and core heat for most of the Hadean by roughly an order of magnitude. 这种热量驱动了更多的熔融和玄武岩火山活动，这一过程不仅发生在实际碰撞的几分钟到几小时内，而且在随后的数千万甚至数亿年里持续进行。当约翰逊和他的同事们将这些贡献相加时，发现撞击产生的热量在冥古宙的大部分时间里，比放射性热和地核热高出一个数量级左右。

Feeding this reworked heat budget into geodynamic simulations led the team to the conclusion that the Earth’s crust in the Hadean was thin and largely molten underneath. The models suggest it was less than 5 kilometers thick, with widespread partial melting starting just 2 to 3 kilometers below the surface. 将这一重新计算的热量收支输入地球动力学模拟后，团队得出结论：冥古宙时期的地球地壳很薄，且下方大部分处于熔融状态。模型显示，地壳厚度不到5公里，地表下2到3公里处就开始了广泛的部分熔融。

At around 5 kilometers depth, melt fractions exceeded 30 percent by volume—well past the point where rock can hold together as a coherent slab. The key takeaway was that plate tectonics could not work in such conditions. “Subduction and plate tectonics require that your lithosphere is rigid and it can jostle around and subduct,” Johnson said. “That’s just not possible if our calculations are anywhere close to the mark.” 在约5公里的深度，熔融比例超过了体积的30%——这远远超过了岩石能够作为连贯板块保持在一起的临界点。关键结论是，在这种条件下，板块构造无法运作。“俯冲和板块构造要求岩石圈是坚硬的，并且能够相互挤压和俯冲，”约翰逊说。“如果我们的计算接近事实，那么这种情况根本不可能发生。”