Scientists have proposed a new theory of where water came from on Earth

So all of Earth's water is of extraterrestrial origin. Studies of the oldest rocks indicate that liquid water existed already 4.5 billion years ago, almost simultaneously with the formation of the Sun. For a long time astronomers attributed this to comet or asteroid impacts, but a new theory has emerged in the French journal Astronomy & Astrophysics: water could have arrived on Earth by evaporating ice from the original asteroid belt.

Details: Quentin Kral et al, An impact-free mechanism to deliver water to terrestrial planets and exoplanets, Astronomy & Astrophysics(2024). DOI: 10.1051/0004-6361/202451263

Background: comets, asteroids and the "x-factor"

Initially, it was assumed that the Earth itself (through volcanic eruptions and the release of vapour from magma) could provide the water supply. But in the 1990s, scientists analysing the isotopic composition of Earth's water (especially the ratio of deuterium and ordinary hydrogen D/H) concluded that its characteristics are closer to asteroids of the "carbonaceous" type than to comets. So the hypothesis was formed that large portions of water were delivered by asteroids.

The question remained: how could debris from the belt between Mars and Jupiter reach the hot Earth in such quantities as to "create" full-fledged oceans? There have been theories of "gravitational billiards" - a series of perturbations that knock icy objects on a trajectory to the inner planets. A new idea, however, offers a more elegant option.

The essence of the new hypothesis: "ice evaporation" and the formation of a vapour disc

1. Originally "icy" asteroids
   In the proto-evolved protoplanetary disc, where the embryos of planets and the asteroid belt were formed, ice was abundant. As soon as the disc dissipated - which was a few million years after the Sun formed - the icy bodies in the belt began to heat up, losing ice by sublimation.

2. A vapour disc around the Sun
   At near-zero cosmic pressure, water from the asteroids was in the form of vapour and formed a 'secondary' gas disc that spread inwards, closer to the Sun.

3. Contact with the inner planets
   Earth, Mars, as well as Venus and Mercury appeared as if "bathed" by this vapour. The process reached its maximum about 20-30 million years after the birth of the Sun, when its luminosity increased dramatically.

   As a result, the planets captured a noticeable fraction of the vapour gravitationally, creating water reserves, which were sufficient for the formation of oceans and underground reservoirs. Moreover, processing of this vapour into liquid phase and "preservation" of water on the Earth went through the developing water cycle - vapour condensed in clouds and returned by rains.

Confirming factors and perspectives

• D/H (deuterium ratio) measurements
  The author's model accurately reproduces the isotopic composition of Earth's water.
• Recent missions to asteroids (Hayabusa 2, OSIRIS-REx)
  Discovered hydrated minerals on large debris and the asteroids themselves, indicating the presence of water in the past.
• Interest in ALMA observations
  Astronomers can look for water vapour in young stars with massive asteroid belts to confirm in real time the existence of a similar process.

Why it matters

If the hypothesis is correct, we get a new perspective on the history of Earth's water. A system formed from "vapour" could explain not only how significant amounts of H2O appeared on Earth (and Mars), but also why its amount was so consistently maintained for the next 4.5 billion years. The authors suggest that similar mechanisms may operate in other stellar systems, increasing the chances of finding exoplanets with liquid water - a crucial condition for the origin of life.