Magma ocean

Magma oceans are huge areas of melted rock that cover the surface of a planet or moon during its early stages of formation. These oceans of magma appear when a celestial body is just beginning to come together, either by gathering smaller pieces called planetesimals or by being struck by other large objects in space.

In the early days of our Solar System, magma oceans formed when small pieces of rock melted due to heat from radioactive decay, especially from a type of aluminium called aluminium-26. As planets grew bigger, the energy from huge impacts provided the heat needed to melt the surface. These magma oceans play a key role in shaping the planet, helping to separate heavier metals to form a core and releasing gases that create an atmosphere.

There is evidence that both the Earth and the Moon had magma oceans in their early history. These oceans of molten rock could last for millions of years before cooling down and allowing the planet to develop a more stable surface.

Magma ocean heat sources

The early solar system had several sources of energy that helped form magma oceans. One important source was the radioactive decay of aluminium-26, which produced heat and melted small rocky bodies called planetesimals. This melting started inside the planetesimals and moved heat outward through convection.

Another source of heat came from impacts as planets grew, known as accretionary impacts, and from the energy released when a planet’s core formed. When heavy metals sank to form a core, they released a lot of heat, melting the planet from the inside. A famous example is the Moon-forming impact on Earth, which may have created a very deep magma ocean.

Lunar magma ocean

The Apollo missions helped scientists discover evidence that the Moon once had a huge ocean of magma. They found rocks made of a mineral called anorthite, which is less dense than magma. This suggested that these rocks floated to the surface from a deep layer of melted rock when the Moon was very young.

The lunar magma ocean was thought to be 200-300 kilometers thick and very hot, about 2000 Kelvin. As the Moon cooled, this ocean of magma slowly hardened, leaving behind clues like special rocks and elements in the Moon’s surface.

Earth's magma ocean

During its formation, Earth likely had large fields of molten rock, called magma oceans, caused by huge impacts, including the one that formed the Moon. Scientists believe these magma oceans were very deep, about 1,000 kilometers, based on certain elements found in Earth's mantle.

Today, Earth has a liquid outer core made mostly of molten iron and nickel, which can be thought of as an ocean of molten metal inside our planet. This layer surrounds Earth's solid inner core and lies below the mantle.