Gold (Au) is a heavy, dense element known for its distinct yellow luster and resistance to corrosion. While relatively rare in Earth’s accessible layers, fueling a multi-trillion-dollar industry, gold is a common component of planetary bodies throughout the solar system. Determining where the most gold resides requires separating the small amounts humans can find from the massive, unreachable stores locked within planetary interiors.
The Cosmic Origin of Gold
Gold cannot be formed in the standard fusion reactions that power stars. Its existence is a testament to the most violent and energetic processes in the universe. Elements heavier than iron require the rapid neutron-capture process (r-process), where atomic nuclei rapidly absorb a high flux of neutrons before radioactive decay.
The primary cosmic factories for gold are the mergers of two neutron stars, the ultra-dense remnants of massive stars that exploded as supernovae. When these stars spiral and collide, the resulting cataclysm, known as a kilonova, ejects neutron-rich material. The 2017 observation of GW170817 confirmed that these mergers produce prodigious amounts of heavy elements, with a single kilonova manufacturing a mass of gold equivalent to several times that of Earth.
This newly formed gold was dispersed across the galaxy, mixing into the gas and dust that became our solar nebula. As the solar system coalesced approximately 4.6 billion years ago, this distributed gold dust was incorporated into the building blocks of the planets. Every body in the solar system contains a certain amount of this cosmic treasure.
Why Earth’s Gold is Hard to Find
When Earth first formed, it was a molten, homogeneous mass that quickly underwent planetary differentiation. This separation was driven by immense heat, causing the denser materials, primarily iron and nickel, to sink toward the center in an event often called the “Iron Catastrophe.” Gold is a siderophile element, meaning it readily bonds with iron.
As the iron migrated inward to form the core, it scavenged nearly all of the planet’s gold, platinum, and other heavy, iron-loving elements. Scientists estimate that over 99% of Earth’s total gold supply is now sequestered within the metallic core, about 1,800 miles below the surface. If this vast amount of gold were accessible and spread evenly across the surface, it would form a layer up to 50 centimeters thick.
The small amount of gold found in the crust and mantle today is attributed to the “Late Veneer” theory. This suggests that after the core fully formed, Earth was bombarded by a final wave of large, metal-rich meteorites. These late arrivals delivered precious metals to the mantle and crust, which had solidified and were no longer exchanging material with the deep core. This thin, post-differentiation layer accounts for all the gold reserves currently mined by humanity.
Comparing the Solar System’s Gold Reserves
The body holding the most gold mass is likely Earth. Its immense size and highly differentiated structure mean its core contains a staggering, unquantifiable mass of gold. Other rocky inner planets, such as Mars and Venus, also underwent differentiation and likely have the vast majority of their gold locked away in their metallic cores.
Mercury is a unique candidate because it is the most metal-rich body in the solar system, with a core making up about 60% of its mass. Its disproportionately large core suggests it holds an enormous reservoir of gold and other siderophile elements. Like Earth, this gold is trapped beneath a mantle and crust.
The most concentrated and potentially accessible gold reserve is found in the asteroid belt. The metallic asteroid 16 Psyche is believed to be the exposed, partially intact core of a shattered protoplanet, measuring 280 kilometers across. Observations suggest Psyche is composed of a mixture of metal and rock, with metallic content ranging from 30% to over 80%. While its gold concentration is estimated to be around 10 parts per million, the sheer volume of metal means Psyche’s reserves represent a highly concentrated store of material that was never buried beneath a massive rocky mantle.