Gold, a heavy element known for its luster and resistance to corrosion, is abundantly distributed throughout the cosmos. This precious metal is a product of some of the most violent and energetic phenomena in the universe. Its presence in our solar system and on our planet is a direct result of these extreme cosmic events that occurred long before the sun ignited.
The Cosmic Forge: How Gold is Created
The creation of gold requires an environment far more energetic than the nuclear fusion processes occurring within a typical star, which can only fuse elements up to iron. Elements heavier than iron, including gold, platinum, and uranium, are forged almost entirely by the rapid neutron capture process, known as the r-process. This process involves an atomic nucleus quickly absorbing a burst of neutrons before the newly formed, unstable nucleus has time to radioactively decay.
The primary site for this rapid process is the collision and merger of two neutron stars. When these dense stellar remnants spiral into one another, they eject matter at high speed, creating a neutron-rich environment perfect for the r-process. Observations of a neutron star merger in 2017 provided the first direct evidence that these events, called kilonovae, are the universe’s most prolific gold factories.
A smaller fraction of gold originates from massive supernovae, the explosive deaths of large stars. These rare events can briefly achieve the necessary density and neutron flux to drive the r-process, scattering newly synthesized gold into the interstellar medium. This gold is then incorporated into subsequent generations of stars, planets, and asteroids.
Where Gold Resides in the Solar System
Within our solar system, gold is distributed according to its classification as a highly siderophile, or “iron-loving,” element. During the formation of Earth and other rocky planets, most of the initial supply of gold chemically bonded with molten iron and sank to form the core. This process, known as planetary differentiation, effectively sequestered the vast majority of the element deep within the planet’s interior.
The most concentrated accessible reserves of extraterrestrial gold are found within a specific class of asteroids. M-type asteroids are metallic and believed to be the exposed cores of shattered planetesimals, containing high concentrations of gold and other precious metals. The asteroid 16 Psyche, for instance, is thought to be a remnant core composed of nickel and iron.
Gold also exists in trace amounts on the surfaces of the Moon and Mars, delivered by meteorite impacts. While not concentrated enough to form terrestrial-style ore bodies, these traces confirm the widespread distribution of gold-bearing material.
Delivery to Earth
The presence of gold and other highly siderophile elements in the Earth’s mantle and crust, far from the core where they should have sunk, is explained by the “Late Veneer” theory. This hypothesis suggests that after Earth’s initial molten phase ended and the core had largely solidified, a final, intense bombardment of large planetesimals occurred.
The material from these late impacts, which included gold-rich meteorites, was deposited onto the already-formed silicate mantle and crust. Because the core was no longer fully molten and actively collecting metal, this late-arriving gold was prevented from sinking. Instead, it remained dispersed throughout the mantle and crust, becoming the source for all of Earth’s current economically viable gold deposits.
Resource Potential of Extraterrestrial Gold
The high concentrations of gold and platinum-group metals found in M-type asteroids have spurred interest in space mining. While Earth’s average crustal concentration of gold is extremely low, some metallic asteroids could contain significantly higher concentrations, potentially making them rich targets.
However, the logistical and technological challenges associated with extracting this gold are immense. The energy required to change an object’s velocity (delta-V) to reach a metallic asteroid is enormous, making transportation of equipment and personnel costly. Furthermore, developing autonomous, zero-gravity processing and refining technology capable of separating metals in the harsh environment of space is a significant engineering hurdle. It is likely that any gold mined in space would initially be used to support in-space construction and manufacturing rather than being shipped back to Earth.