Gold is one of the heaviest naturally occurring elements found on Earth, but Earth did not create the gold itself. The gold mined from veins, recovered from placer deposits, or extracted from ore bodies consists of atoms that formed before Earth existed. Modern astronomy and nuclear physics indicate that gold is produced through a process called rapid neutron capture, commonly known as the r-process. Observations made during a neutron-star merger in 2017 provided direct evidence that these collisions can create the conditions required for r-process nucleosynthesis. The geological story of gold begins inside Earth, but the atomic story begins in space.
What Scientists Directly Observe
Neutron stars are among the densest objects known in the universe. They form when massive stars exhaust their nuclear fuel and undergo supernova explosions. The outer layers of the star are expelled into space while the core collapses under gravity. If the remaining core mass falls within a specific range, the result is a neutron star composed primarily of neutrons packed together at extremely high densities.[1]
Astronomers have directly observed neutron stars through radio, X-ray, gamma-ray, and optical observations. Some neutron stars exist alone while others occur in binary systems. In binary systems, two neutron stars orbit each other. Albert Einstein’s theory of general relativity predicts that orbiting massive bodies should emit gravitational waves and gradually lose orbital energy. Measurements of binary neutron-star systems have confirmed this prediction.[2]
As orbital energy decreases, the stars move closer together. Eventually they collide. This event is called a neutron-star merger. Prior to 2017, scientists had theoretical reasons to suspect that these mergers could produce heavy elements such as gold, platinum, and uranium. However, direct observational evidence remained limited.[3]
On August 17, 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo detector recorded a gravitational-wave signal designated GW170817. The signal was identified as a neutron-star merger approximately 130 million light-years from Earth. Within hours, telescopes around the world observed the associated explosion in the galaxy NGC 4993. This event became one of the most extensively studied astronomical observations in modern history.[4]
Scientists directly observed gravitational waves from the merger. Scientists directly observed electromagnetic radiation from the explosion. Scientists directly observed the changing brightness and spectrum of the resulting kilonova. These observations form the foundation of the current understanding of heavy-element production during neutron-star mergers.[5]
How Gold Is Thought to Form During a Merger
The creation of gold is not directly observed atom by atom. Instead, scientists use observed spectra, nuclear physics, and computer models to determine what nuclear reactions likely occurred during the merger. This distinction is important because it separates observation from interpretation.[6]
Gold contains 79 protons in its nucleus. Ordinary stellar fusion can create many elements, but it becomes increasingly difficult to build elements heavier than iron through standard fusion reactions. Nuclear physicists therefore looked for environments capable of supplying large numbers of free neutrons in extremely short periods of time.[7]
Neutron-star mergers provide such an environment. During a collision, neutron-rich material is ejected into space at high velocity. According to current nuclear physics models, atomic nuclei within this ejecta can rapidly capture neutrons. This process is known as rapid neutron capture or the r-process.[8]
During the r-process, nuclei absorb neutrons faster than radioactive decay can occur. These unstable nuclei then undergo a sequence of radioactive transformations that produce heavier elements. Gold, platinum, uranium, thorium, and many rare-earth elements can be produced through this mechanism.[9]
The kilonova associated with GW170817 exhibited spectral characteristics predicted for r-process element formation. Scientists therefore concluded that neutron-star mergers can create heavy elements through r-process nucleosynthesis. This conclusion is supported by multiple independent analyses published in peer-reviewed journals.[10]
What remains less certain is the exact amount of gold produced. Different models generate different estimates depending on assumptions regarding ejecta mass, neutron abundance, nuclear reaction pathways, and elemental composition. Scientists therefore generally discuss total r-process material rather than claiming a precisely measured quantity of gold.[11]
Current evidence supports the conclusion that neutron-star mergers can create gold. Current evidence does not permit direct measurement of the exact number of gold atoms produced during GW170817. That distinction is important when discussing scientific certainty.[12]
From a Neutron-Star Collision to a Gold Deposit
The production of gold atoms does not create a gold deposit. This is where astronomy ends and geology begins.
A neutron-star merger produces atoms dispersed into space. These atoms become part of the interstellar medium, which consists of gas and dust distributed throughout galaxies. Over time, interstellar material becomes incorporated into new generations of stars, planets, asteroids, and comets.[13]
The Solar System formed approximately 4.56 billion years ago from such material. Gold atoms present within the solar nebula became incorporated into Earth as the planet formed. Geochemists classify gold as a siderophile element, meaning it tends to associate with metallic iron under certain conditions.[14]
Evidence from geochemistry indicates that much of Earth’s original gold inventory partitioned into the core during planetary differentiation. Partitioning refers to the distribution of an element between different materials under specific temperature and pressure conditions. During Earth’s early molten stages, dense metallic material migrated toward the center of the planet. Gold preferentially entered this metallic phase and was transported downward with it.[15]
The gold accessible to mining today represents gold that remained in the mantle and crust or was later redistributed through geological processes. Magmatic activity, hydrothermal fluids, metamorphism, faulting, uplift, weathering, and erosion concentrated portions of this gold into deposits. Those deposits may later become economic ore bodies if grade, tonnage, accessibility, and market conditions permit profitable extraction.[16]
A distinction must be made between occurrence and economic occurrence. Gold atoms occur throughout Earth’s crust. Gold concentrations occur where geological processes enrich those atoms relative to surrounding rock. Economic gold deposits occur where concentration, tonnage, metallurgy, and economics combine to support mining operations.[17]
The gold recovered by a miner from a placer deposit, quartz vein, epithermal system, or orogenic deposit therefore represents the final stage of a sequence that began long before Earth formed. The atoms were likely produced through r-process nucleosynthesis. Those atoms became incorporated into the material that formed Earth. Geological processes later concentrated part of that gold into deposits. Mining ultimately recovers a portion of those concentrations.
The current scientific evidence supports neutron-star mergers as one important source of the heavy elements required to create gold. Additional research continues regarding the relative contribution of neutron-star mergers and rare supernova events to the total heavy-element inventory of the universe. However, observations from GW170817 established that neutron-star collisions can create the conditions necessary for r-process nucleosynthesis and the formation of heavy elements that include gold. For the first time, scientists were able to observe a neutron-star merger and compare the observations directly with theoretical predictions. That observation transformed a long-standing hypothesis into one supported by direct astronomical evidence.[18]
Related Reading
Why Gold Forms, Moves, and Concentrates
The Complete Guide to Gold Geology and Gold Deposit Types
The Complete Guide to Gold Prospecting Clues: Minerals, Alteration, Veins, and Host Rocks
References
[1] NASA. Neutron Stars. National Aeronautics and Space Administration.
[2] Taylor, J.H., Weisberg, J.M. Further Experimental Tests of Relativistic Gravity Using the Binary Pulsar.
[3] Cowan, J.J., Sneden, C., Lawler, J.E., et al. Origin of the Heaviest Elements: The Rapid Neutron-Capture Process. Reviews of Modern Physics.
[4] Abbott, B.P. et al. GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral. Physical Review Letters.
[5] Abbott, B.P. et al. Multi-Messenger Observations of a Binary Neutron Star Merger. Astrophysical Journal Letters.
[6] National Academies of Sciences. Observational and Theoretical Approaches to Heavy Element Formation.
[7] OpenStax Astronomy. Stellar Nucleosynthesis.
[8] National Science Foundation. Rapid Neutron Capture and Heavy Element Production.
[9] Cowan, J.J. et al. The r-Process and Nucleosynthesis of Heavy Elements.
[10] Kasen, D. et al. Origin of the Heavy Elements in Binary Neutron-Star Mergers from a Gravitational-Wave Event. Nature.
[11] Metzger, B.D. Kilonovae. Living Reviews in Relativity.
[12] LIGO Scientific Collaboration. GW170817 Data Interpretation Papers.
[13] NASA Astrophysics Division. Interstellar Medium and Stellar Evolution.
[14] U.S. Geological Survey. Gold Statistics and Information.
[15] Wood, B.J., Halliday, A.N. The Lead Isotopic Age of the Earth and Core Formation. Na