Formation of the Moon

According to the widely accepted giant impact hypothesis, the moon formed about 4.5 billion years ago when a Mars-sized body called Theia collided with Earth. Simulations and analysis of moon rocks suggest that the moon is primarily composed of material from Theia’s mantle, which is similar in composition to Earth’s mantle.

Chemical Composition of the Moon

However, planets typically have distinct chemical compositions. If Theia formed far from Earth, its composition should have been different, and the moon’s composition should not resemble Earth’s mantle.

The Tungsten Puzzle

One element that complicates the moon’s origin story is tungsten. Tungsten is an iron-loving element that tends to sink towards the cores of planets. The moon and Earth should therefore have very different amounts of tungsten, as Theia’s tungsten-rich mantle would have been incorporated into the moon during the impact.

Isotopic Similarities

Two independent studies examined the ratio of two tungsten isotopes in moon rocks and Earth samples. They found that moon rocks have slightly more tungsten-182 than Earth, an intriguing finding because tungsten-182 is produced by the radioactive decay of hafnium-182, which has a short half-life.

The Late Veneer Hypothesis

The simplest solution to the tungsten puzzle is the late veneer hypothesis. This hypothesis suggests that Earth and the proto-moon initially had similar tungsten isotope ratios. However, Earth, being larger and more massive, continued to attract planetesimals after the impact, adding new material to its mantle. This late veneer would have had more tungsten-184 relative to tungsten-182, while the moon would have retained the ratio from the impact.

Evidence for a Late Veneer

The late veneer hypothesis is supported by the fact that Earth has more siderophile elements (elements that love iron) in its mantle than expected. These elements should have sunk into the core but must have been brought to Earth after core formation by meteorite impacts.

Similarity of Tungsten Isotope Ratios

For the proto-moon to match Earth’s tungsten ratio, Theia and Earth must have started with very similar tungsten abundances. Solving this puzzle will require further planetary studies, but the lunar origin story is becoming clearer.

Role of Planetesimals in Lunar Formation

Simulations have shown that it is more likely for large impacts to occur between bodies that formed close together and therefore have similar compositions. This supports the idea that Theia formed relatively near Earth.

Planetesimals and the Late Veneer

Planetesimals continued to bombard the young solar system after the moon’s formation. Earth picked up more of this late veneer material than the moon, further contributing to the differences in their compositions.

The Solar Nebula: A New Source of Earth’s Water

For decades, scientists have believed that Earth’s water came from ice-filled comets and asteroids. However, new research suggests that the solar nebula, clouds of gas and dust that formed after the sun’s誕生, may have also played a role.

The chemical composition of water is simple: two parts hydrogen and one part oxygen. Hydrogen is abundant in the universe, so any source of hydrogen could have contributed to Earth’s water.

Hydrogen from the Solar Nebula

Hydrogen gas within the solar nebula was incorporated into planets during their formation. Most of this hydrogen remains trapped in Earth’s core, but some escaped and contributed to the building blocks of water molecules. This hydrogen has a lower ratio of deuterium, a heavy hydrogen isotope, to normal hydrogen than water from asteroids or comets.

Water-Logged Asteroids and Solar Nebula Interactions

Early in Earth’s history, water-logged asteroids crashed into each other, forming planetary embryos with an outer layer of magma. Hydrogen-heavy solar nebula gas encountered this magma, creating an atmosphere and sending dissolved hydrogen into the embryos’ interiors.

Isotopic Fractionation and Earth’s Water Distribution

Isotopic fractionation caused normal hydrogen to move deeper into the core, while deuterium isotopes remained in the mantle. As Earth merged with other celestial bodies, it gained enough water and mass to reach its final size.

The Importance of Solar Nebula Hydrogen

Asteroid impacts generated most of Earth’s water, but a small portion near the core appears to originate from the solar nebula. This finding suggests that even planets far from water-rich asteroids may have water.

Implications for Exoplanet Habitability

The team’s findings could help scientists better understand the habitability of other planets. They indicate that planets may have a “floor” of water regardless of their distance from water sources. This supports the idea of rapid planetary growth and the potential for life on other worlds.

Additional Insights

• Water found deep within Earth’s interior has a different ratio of heavy hydrogen isotopes and normal hydrogen, indicating a separate point of origin from asteroids and comets.
• The solar nebula gas contributed to the formation of one out of every 100 water molecules on Earth.
• Earth’s water is likely a combination of sources, including asteroids, comets, and the solar nebula.
• The presence of solar nebula hydrogen in Earth’s water has implications for understanding the habitability of other planets.