地球深处的中微子揭示地幔新图景
内容总结：
地球内部中微子揭示地幔新图景
在加拿大萨德伯里地下两公里深处的SNO+中微子实验设施中，探测器技术专家马特·德帕蒂乘橡皮筏进入一个装满7000吨超纯水的 cavern，检查一个用于捕捉最难以捉摸的粒子——中微子的探测器。这个实验位于地下，以屏蔽宇宙辐射的干扰，捕捉那些极其罕见的中微子闪烁信号。
中微子是宇宙中数量最多但质量极小的粒子，几乎不与物质相互作用。每秒有数万亿个中微子穿过人体，但探测器多年只捕捉到几十万个信号。更为罕见的是“地球中微子”（geoneutrinos），它们产生于地球内部加热过程中，这种热量驱动地幔岩石流动，塑造板块构造和地球磁场。地球热量主要来自形成时的余热和铀、钍、钾等放射性元素的衰变，后者对维持地球地质活动至关重要。
通过计数地球中微子，物理学家可以直接测量地球内部产热元素的含量。2005年，日本Kamland探测器首次探测到地球中微子；2009年，意大利Borexino探测器也捕获了数十个。2025年11月，SNO+报告了其首次探测结果，使已知地球中微子总数增加约50个。SNO+的特殊之处在于其地理位置——这是西半球首次测得的地球中微子，为研究地球放射性内部提供了新视角。
各实验点的测量结果可能存在差异，这或许暗示地幔并非均匀分布。传统地球化学假设认为地幔中放射性元素均匀分布，因为流动的岩石应会混合一切。但不同位置的测量显示，产生最多地球中微子的区域可能位于地震学上发现的巨大低速异常体（LLSVP）上方，这些异常体分别位于非洲和太平洋下方的地核两侧。研究人员推测，可能存在尚未理解的地球深部结构，它们会富集特定元素。
中微子有望为这些深部结构的起源提供新线索，甚至绘制出“地球内部的化学地图”。然而，目前各探测器的结果仍有巨大不确定性，难以确定测量差异是否真实存在。科学家需要排除来自核反应堆、地壳等其他来源的干扰信号，其中地壳贡献最大的是探测器周围数百公里范围内的信号。
SNO+团队正努力理解探测器周围地质环境的干扰，包括一个18亿年前巨大撞击形成的盆地。地幔中放射性物质总量的估算也存在不确定性，这些元素对地热贡献从几个百分点到一半不等，差异相当于数万座核电站的发电量。
中微子研究正迎来激动人心的时刻：位于中国广州附近山下、拥有超过2万吨闪烁体的JUNO实验预计将于今年晚些时候报告其首批地球中微子数据，其探测能力有望在首年就超过其他三个探测器数十年的总和。此外，长期倡导在海底建造中微子探测器的专家威廉·麦克唐纳认为，远离大陆岩石的海洋地壳更薄更均匀，能将不确定性降到最低，使探测目标仅限于地幔。尽管这一构想耗资巨大，但他希望中国能支持这一项目。在那之前，物理学家仍将继续在地下深处探寻答案。
中文翻译：
地球深处的中微子揭示地幔新图景
引言
在一座地下2公里的实验室里，一台起重机将探测器技术员马特·德帕蒂通过舱口缓缓放入一间白色墙壁的洞室中，洞室内装满了约7000吨超纯水，在灯光照射下呈现出像玻璃水一样湛蓝的荧光。“溅落，”德帕蒂通过对讲机说道，随即踏上一艘在下方等候的充气筏。
通常情况下，这个洞室是地球上最黑暗的地方之一，但今天为了维护工作，室内灯火通明，让我们难得有机会一窥其内部。我透过舱口注视着德帕蒂，他划着筏子前去检查水下的实验装置。他正在检查一个房屋大小的探测器，这个探测器是为捕捉物理学中最难以捉摸的粒子——中微子——而建造的。
这就是SNO+中微子实验，深埋于加拿大萨德伯里地下物理实验室斯诺实验室的克赖顿矿中。SNO+由一个丙烯酸球体构成，球体内衬近10000个灵敏的光探测器，并充满780吨油性液体闪烁体，当高能粒子撞击时会发出闪光。装置周围的水和上方的岩石将探测器与宇宙辐射的强光隔绝开来，使得较为罕见的粒子相互作用所产生的微弱闪光得以显现。
我们一路深入的过程本身就是追求绝对黑暗的一部分，甚至超越了可见光谱的范畴。当我们乘坐主井罐笼下降，穿过岩石隧道走向实验室时，我们的身体和衣物沾染了微量的放射性氡尘埃。在进入主实验室区域之前，我们脱掉矿工服，淋浴，换上电蓝色连体工作服并戴上发网，以尽量减少带入的污染。“淋浴不是为了你们，”德帕蒂在我们换衣服时说，“是为了科学。”
当你试图捕捉幽灵时，这样的极端措施是必要的——在这里，这些幽灵可能有助于揭示地球深处不可抵达区域的秘密。
放射性行星
中微子是所有有质量粒子中数量最丰富的一种。但它的质量极小：仅为电子质量的百万分之一。由于质量如此之小且电磁性质中性，这些粒子几乎从不与其他物质发生相互作用。每秒有数万亿个中微子——主要来自太阳——穿过我们的身体，然而经过多年用SNO+等探测器追踪，研究人员仅捕获了几十万次珍贵的闪光信号。
更加难以捉摸的是地球中微子——经过数十年的搜寻，科学家仅探测到几百个。
地球中微子产生于加热行星内部的物理过程中。这些热量在驱动地幔岩石流动中发挥着重要作用，影响着从板块构造到地球磁场的一切。热量有两个主要来源：行星形成时残留的热量，以及地幔和地壳岩石中铀、钍和钾衰变产生的热量。如果没有第二个来源，地球早已冷却，成为一个地质构造上死寂的行星。
通过计数地球中微子，物理学家可以直接测量地球产生生命所必需的热量元素。“这是我们工作中唯一聚焦地球本身的部分，”安大略省女王大学的粒子天体物理学家瑞安·贝斯说，他参与SNO+项目工作。“我们做的其他所有事都更侧重于接收来自宇宙其他地方的信息。”
2005年，日本一个名为神冈的探测器首次报告探测到地球中微子。2009年，意大利的博瑞克西诺探测器报告又捕获了几十个。2025年11月，SNO+报告了其首次探测结果，将观测到的地球中微子数量增加了约50个。
SNO+探测的特殊之处在于该实验的地理位置：这是在西半球首次测得的地球中微子，为地球放射性内部提供了新的视角。
在解读这些实验结果时仍存在重大不确定性，但研究人员的最佳估计表明，每个站点测量到的通量各不相同。“这可能是地幔并非均匀的第一个线索，”女王大学的粒子天体物理学家、SNO+合作组主任陈马克说。
地球化学家传统上假设放射性元素在地幔中均匀分布，因为流动的岩石应该会将所有物质混合均匀。但不同地点测得的地球中微子通量可能暗示事实并非如此。
产生最多地球中微子的区域大致位于异常高温、高密度物质的巨大块体上方，这些块体被称为大型低剪切波速省，地震学家已在核芯两侧绘制出它们的地图。一个位于非洲下方，另一个位于太平洋下方。“地幔中可能存在尚未被理解的深层地球结构，”陈说，“也许这些结构集中了某些元素。”
中微子有朝一日可能为这些深层结构的仍属神秘的起源提供新的见解，更广泛地说，为构成地球系统诸多方面基础的地幔模式提供新的认知。“这确实将是一种绘制地球内部化学地图的方法，”中国科学院的地球化学家威廉·麦克唐纳说，他长期以来一直是地球中微子搜寻领域的领军人物。
捕捉地球中微子
悬而未决的问题是：地球中微子测量结果究竟是揭示了各实验下方地幔区域的差异，还是差异源于不同实验计数地球中微子的方式不同。所有研究人员都表示，目前不确定性太大，无法下定论。
“如果仅从表面数据来看，我们可以说西半球的放射性物质可能比东半球多得多，”麦克唐纳说。但有一些理由需要对这些估计保持谨慎。“意大利人的结果对吗？日本人的结果对吗？两者都对吗？还是有什么地方出了问题？”他说。
贝斯表示，每个探测器结果所伴随的不确定性来自物理学家识别地球中微子所需的筛选过程。“它们不会突然出现并说，‘你好，我是地球中微子。’”
科学家必须排除能量过高的粒子信号、螺旋度属性测量不正确的粒子信号，以及预期来自核反应堆的粒子信号。他们还必须排除来自地壳的地球中微子，其中最大贡献来自探测器周围数百公里范围内。当这些其他探测信号从总数中扣除后，剩下的应该就全部是来自地幔的地球中微子信号。
总体而言，来自地幔的地球中微子通量在博瑞克西诺似乎非常高，在神冈似乎非常低，尽管不确定性很大。对SNO+结果的详细地质学解释仍在进行中，陈说，但到目前为止，科学家们在加拿大下方看到的是“介于两者之间”的地幔。
SNO+科学家面临的一个特别挑战是理解来自探测器周围环境的中微子，包括一个18亿年前由巨大天体撞击形成的盆地。“这个地质区域存在大量未知因素，”意大利费拉拉大学的研究员弗吉尼亚·斯特拉蒂说，她帮助开发了斯诺实验室的当地放射性模型。
不确定性还来自对加热地幔的放射性物质总量的估计。地球中微子通量表明，这些元素可能只贡献了其热量的很小百分比，也可能贡献了一半——这一差异相当于数万座核电站的功率输出。
两种不确定性来源都使得检测特定地幔区段化学成分之间的差异更加困难。不同放射性元素分布所预期的地球中微子通量差异“非常小，被隐藏在这些不确定性之中，”斯特拉蒂说。
未来的通量
SNO+的探测正值地球中微子研究一个激动人心的时刻：江门中微子实验（JUNO）——中国目前正在收集数据的另一个大型中微子实验——预计将在今年晚些时候报告其首个地球中微子通量，增加第四个且明显更为丰富的观测视角。该实验拥有超过20000吨闪烁体，埋藏在广州市郊外的一座山下，规模如此之大，预计其第一年探测到的地球中微子数量将超过神冈、博瑞克西诺和SNO+数十年的总和。
每个实验的地球中微子通量更清晰的估算可以来自更详细的地质数据，以及各站点进一步的地球中微子计数。然而，麦克唐纳表示，最好的做法是在海底建造一个中微子探测器。这是麦克唐纳数十年来一直倡导的想法。
这样的探测器将远离富含放射性元素的大陆岩石；海洋地壳也更薄、更均匀。与地壳相关的不确定性将大幅降低，以至于“你就处于纯粹地幔区域了，”他说。
海底探测器的构想估计耗资数亿美元，但迄今为止很少获得政府资助方的支持。但麦克唐纳希望他能在中国促成此事，中国已经为其他大型地球科学项目开了绿灯。“这完全有可能，”他说。在那之前，物理学家们将继续在地下深处划桨寻找答案。
英文来源：
Neutrinos From Deep Inside Earth Provide a New Picture of the Mantle
Introduction
In a laboratory 2 kilometers underground, a crane lowers Matt Depatie, a detector technologist, through a hatch into a white-walled cavern filled with about 7,000 tons of ultrapure water that glows as blue as wiper fluid in the light. “Splashdown,” Depatie says over a radio as he steps into an inflatable raft waiting below.
Normally, the cavern is one of the darkest places on Earth, but today, it is lit up for maintenance, offering us a rare chance to see inside. I peer through the hatch at Depatie as he paddles over to examine the submerged experiment. He’s inspecting a house-size detector built to catch some of the most elusive particles known to physics: neutrinos.
This is the SNO+ neutrino experiment, buried deep within the Creighton mine at Snolab, an underground physics laboratory in Sudbury, Canada. SNO+ consists of an acrylic sphere lined with nearly 10,000 sensitive light detectors and filled with 780 tons of oily liquid scintillator, which flashes when lit up by energetic particles. The water around the device and the rock above it shield the detector from the glare of cosmic radiation, allowing the flickers of less common particle interactions to shine through.
Our whole journey down has been part of the crusade to maintain absolute darkness, even beyond the visible spectrum of light. As we descended the main shaft and walked through a rocky tunnel to the lab, our bodies and clothes collected minute amounts of radioactive radon dust. Before entering the main laboratory space, we tossed our mine clothes, showered, and changed into electric blue jumpsuits and hairnets to minimize the contamination we carried in with us. “The showers aren’t for you,” Depatie said as we changed, “they’re for the science.”
Such extremes are necessary when you’re trying to catch ghosts — in this case, ghosts that may help reveal the secrets of inaccessible regions deep within the Earth.
Radioactive Planet
Neutrinos are the most abundant of all the particles that have mass. But that mass is tiny: just a millionth the mass of an electron. With such little heft and a neutral electromagnetic charge, the particles hardly ever interact with other matter. Trillions of neutrinos — mostly those produced in the sun — pass through our bodies every second, yet after years of hunting them with detectors such as SNO+, researchers have captured only a few hundred thousand of their precious flashes.
Even more elusive — so much so that after decades of searching, scientists have detected only a few hundred of them — are geoneutrinos.
Geoneutrinos are produced in processes that heat the interior of the planet. This heat plays a major role in powering the flow of rocks in the mantle, which shapes everything from plate tectonics to Earth’s magnetic field. It comes from two main sources: heat left over from the planet’s formation, and heat produced by the decay of uranium, thorium, and potassium in the rocks of the mantle and crust. Without this second source, Earth would have long since cooled off and become a tectonically dead planet.
In counting geoneutrinos, physicists can get a direct measure of Earth’s vital heat-producing elements. “It’s the one thing we do that focuses on the Earth,” said Ryan Bayes, a particle astrophysicist at Queen’s University in Ontario who works on SNO+. “Everything else we do is more focused on what we receive from other places in the universe.”
The first detection of geoneutrinos, by an instrument in Japan called Kamland, was reported in 2005. In 2009, the Borexino detector in Italy reported catching several dozen more. In November 2025, SNO+ reported its first detection, bumping up the number of observed geoneutrinos by about 50.
What makes the detections at SNO+ special is the experiment’s location: These are the first geoneutrinos measured in the western hemisphere, offering a new perspective on Earth’s radioactive interior.
Major uncertainties remain in interpreting the results from these experiments, but researchers’ best estimates suggest that each site is measuring a different flux. “It could be that that’s the first hint that the mantle is not uniform,” said Mark Chen, a particle astrophysicist at Queen’s University and director of the SNO+ collaboration.
Geochemists have conventionally assumed that radioactive elements are distributed evenly throughout the mantle, because the flowing rock should mix everything together. But the measurements of geoneutrinos in different locations could hint that this is not the case.
The regions that seem to be producing the most geoneutrinos sit roughly above continent-size blobs of anomalously hot, dense material, known as large low-shear-velocity provinces, or LLSVPs, which seismologists have mapped on either side of the core. One is under Africa, the other under the Pacific Ocean. “There may be deep Earth structures in the mantle that are not understood,” Chen said. “It could be that [they] concentrate some kinds of elements.”
Neutrinos could one day offer new insight into the still mysterious origin of these deep structures and, more broadly, the patterns in the mantle that underlie many aspects of the Earth system. “It really would be a way to make a chemical map of the Earth’s interior,” said William McDonough, a geochemist at the Chinese Academy of Sciences who has long been a leading voice in the search for geoneutrinos.
Catching Geoneutrinos
The outstanding question is whether the geoneutrino measurements reveal differences between the areas of mantle below each experiment, or if the imbalance originates in the way the different experiments count their geoneutrinos. Researchers across the board say there’s still so much uncertainty that it’s impossible to say.
“If we take it at face value, we could say maybe the western hemisphere has a lot more radioactive material in it than the eastern hemisphere,” McDonough said. But there are reasons to be wary of these estimates. “Are the Italians right? Are the Japanese right? Are they both right? Or is something wrong?” he said.
The uncertainties associated with each detector’s results come from the sorting process that physicists go through to identify geoneutrinos, Bayes said. “They don’t just show up and say, ‘Hi, I’m a geoneutrino.’”
Scientists must eliminate signals from particles with too much energy, particles with the wrong measure of a property called helicity, and particles that they expect to see flowing from nuclear reactors. They must also eliminate geoneutrinos coming from Earth’s crust, with the largest contribution coming from the area within a few hundred kilometers of the detector. When those other detections are subtracted from the total count, the geoneutrino signal from the mantle should be all that’s left.
In general terms, the geoneutrino flux from the mantle seems to be very high at Borexino and very low at Kamland, though the uncertainties are great. A detailed geological interpretation of the SNO+ results is still in the works, Chen said, but so far scientists see a “pretty in-between” mantle below Canada.
A particular challenge for SNO+ scientists is understanding the neutrinos coming from the detector’s surroundings, including a basin formed 1.8 billion years ago by a giant impactor. “There are lots of unknowns in this geological area,” said Virginia Strati, a researcher at the University of Ferrara in Italy who helped develop a local model of radioactivity for Snolab.
Uncertainty also comes from estimates of the total amount of radioactive material heating the mantle. The flux of geoneutrinos suggests that these elements could contribute anywhere from just a small percentage of its heat to half of it — a discrepancy equivalent to the output of tens of thousands of nuclear power plants.
Both sources of uncertainty make it even more difficult to detect any differences between the chemical makeup of particular sections of the mantle. The difference in the geoneutrino flux expected from various distributions of radioactive elements “is very small, and is hidden in these uncertainties,” Strati said.
Future Flux
The detection at SNO+ comes at an exciting moment for geoneutrino research: JUNO, another huge neutrino experiment currently collecting data in China, is expected to report its first geoneutrino flux later this year, adding a fourth and notably richer view. With more than 20,000 tons of scintillator, the experiment — buried under a mountain outside the city of Guangzhou — is so large that it is expected to detect more geoneutrinos in its first year than the combined output of Kamland, Borexino, and SNO+ over decades.
Clearer estimates of the geoneutrino flux at each experiment could come from more detailed geological data, as well as further geoneutrino counts at each site. However, McDonough says the best thing would be to build a neutrino detector at the bottom of the ocean. It’s an idea McDonough has championed for decades.
Such a detector would be far from continental rocks, which are rich in radioactive elements; oceanic crust is also thinner and more uniform. Crust-related uncertainties go down so much that “you are in mantle-only territory,” he said.
The idea of an ocean-bottom detector, estimated to cost hundreds of millions of dollars, has seen little take-up from government funders to date. But McDonough is hoping he can make something happen in China, which has given the green light to other big geoscience projects. “It’s very possible,” he said. Until then, physicists will keep paddling around for answers deep underground.