Imperial researchers have found that, 4.5 billion years ago, ice modified asteroids in a way that may have led to the beginning of life on Earth.

The study, led by Dr Matt Genge from the Department of Earth Science and Engineering at Imperial with collaborators at the Natural History Museum, the University of Kent and the Japanese Space Agency (JAXA) suggests that rocks from asteroid Ryugu, which were brought back to Earth by the JAXA’s Hayabusa2 mission, show signs of having been fractured by ice.

The fractures identified by the team were filled with clay and sulfide minerals formed by water and seem to have been shaped by a freeze-thaw process, where ice expands and contracts as it melts and refreezes, fracturing rock as a result.

In this way, water seems to have played a key role in modifying the asteroid's composition in the early Solar System. Researchers believe this change may have helped deliver essential organic matter together with clay and sulfide minerals, as well as water, to Earth upon impact billions of years ago – potentially leading to the emergence of life on Earth.

How freeze-thaw cycles led to life on Earth

Dr Genge said: “Our findings suggest that the repeated melting and freezing of ice on asteroids may have helped life form on Earth.

“Our calculations indicate that the pressure exerted by ice as it grows is sufficient to fracture asteroids to their core, allowing water to spread throughout them. Water then interacts with the minerals inside the asteroid to create essential organic matter – which would have been delivered to early Earth generating the oceans and the organic building blocks of life.” 

While collisions with other asteroids can also fracture asteroids, the team concluded that the specific shapes of the fractures on the Ryugu asteroid could only have been formed by the freeze-thaw process.

These freeze-thaw cycles not only fractured Ryugu, but also allowed water to escape through these fractures creating geysers on the asteroid's surface.

The paper is published in Nature Astronomy.

Deciphering Ryugu’s tiny fractures

The team carried out their research on a millimetre-sized piece of Ryugu brought back to Earth by JAXA. To analyse the sample, they used X-ray Computed Tomography (XCT), which is similar to a medical CT scan but for rocks – allowing them to see the 3D shapes of the fractures on the asteroid.

As well as the thin fractures which pointed to the role of the ice creating the fractures on the asteroid, researchers also found ‘veins’ containing framboidal magnetite—tiny, spherical crystals of magnetic iron oxide – which represent further evidence of the presence of water.

Researchers noticed that the fractures and veins had strange curved shapes and appeared as a series of cusps. It was the distinctive shape of the veins that helped them understand the role of ice: the team performed experiments on ice-grains embedded within clay and found similar cusp-like fractures forming around the ice-grains.

“It is the fracturing of asteroids by freeze-thaw that ensured asteroids were thoroughly altered by water. Without it, these life-giving materials may have been much rarer. The cosmic game of ‘Rock, Scissors, Ice’ may well be an essential part of how life came to be,” concluded Dr Genge.

'Evidence from 162173 Ryugu for the influence of freeze–thaw on the hydration of asteroids', by Genge et al., published 26 September 2024 in Nature Astronomy.