Asteroids that traveled from the fringes of the solar system more than 4.5 billion kilometers away may have brought water and organic matter to the primordial Earth, a team of Japanese researchers said Monday.

The hypothesis, published in the scientific journal Nature Astronomy by the Japan Aerospace Exploration Agency and others, was drawn from an analysis of samples from the Ryugu asteroid collected by the Hayabusa2 space probe.

It remains a mystery how the Earth, which consisted almost entirely of molten magma in its initial formation, came to be covered in water.

"In a broad sense, it is possible that small celestial bodies brought things that led to water and life on Earth," said Motoo Ito, a senior researcher of geomaterials science at the Japan Agency for Marine-Earth Science and Technology.

Photo shows a Ryugu asteroid sample analyzed by a team of Japanese researchers. (Photo courtesy of the Japan Agency for Marine-Earth Science and Technology / Phase 2 curation "Team Kochi")(Kyodo)

In December 2020, a capsule that had been carried on a six-year mission by Hayabusa2 delivered more than 5.4 grams of surface material to Earth from the Ryugu asteroid, located over 300 million km away.

While Ryugu particles displayed a close compositional match to that of Earth's water, there were some slight differences, leading researchers to hypothesize that Earth may have also sourced its water from places other than asteroids.

An analysis of eight particles, totaling approximately 59 milligrams, from the Ryugu asteroid found in almost all of them organic matter and water not in liquid form but a hydroxyl group consisting of one oxygen atom bonded to one hydrogen atom, with a composition similar to that of water contained in cosmic dust and comets.

Protected by a "cradle" of phyllosilicates, they were believed to have endured harsh environmental changes after leaving the outer solar system, where water and organic matter are present.

Organic areas comprised of a matter known as aliphatic hydrocarbon were also found within the relatively coarse-grained phyllosilicates. The surrounding coarse grains, which are more resistant to heat, may have prevented water and the organic matter from being altered by the environment.

File photo shows the Ryugu asteroid pictured by Hayabusa2 in November 2019. (Photo courtesy of JAXA)(Kyodo)

Hayabusa2 left Earth in 2014 and reached its stationary position above Ryugu in June 2018 after traveling 3.2 billion km on an elliptical orbit around the Sun for more than three years.

The probe touched down on the asteroid twice the following year, collecting the first-ever subsurface samples from an asteroid.

Researchers have previously found that Ryugu, born from a parent body formed in the outer solar system, traveled to the inner solar system and that its particles contain amino acids -- organic compounds said to be the "source of life."

Related coverage:

NASA spacecraft collected 250-gram sample from asteroid Bennu

Amino acids found in asteroid samples collected by Japan's Hayabusa2 probe