Fossil ice found in meteorite is the first direct evidence of ice in asteroids

High-resolution scans of a 4.6 billion- year-old meteorite have revealed ‘fossilised ice’, showing for the first time direct evidence that when early asteroids formed they incorporated frozen water into their matrix.

Researchers from London’s Natural History Museum and several Japanese institutes including Tohoku University have discovered the fossil ice in a meteorite weighing just 82 grams that slammed into Algeria in 1990. Officially called Acfer 094, this meteorite was a piece of a larger asteroid that formed during the early days of the Solar System.

Whilst Acfer 094 has been studied previously, its matrix, which is the material holding the meteorite together, was overlooked. Due to the fine -grained state of the matrix the technology needed to investigate it further simply did not exist. Recent advances, however, now mean that microscopes with more specialised resolution have allowed the team to return to Acfer 094 and study it in more detail than ever before.

The presence of water within asteroids has been hinted at previously by observed changes in thier mineral make up, now known as aqueous alterations. But how this water was distributed throughout the asteroids, and when this ice melted has not previously been understood.

By looking at the fine structure preserved in Acfer 094 the team were able to see the microscopic pockets that were left behind when the ice they once contained melted. These tiny holes that record where the ice once was are known as fossil ice.

Dr Epifanio Vaccaro, Curator of Petrology at the Natural History Museum said, 'The meteorite in question dates to roughly 4.6 billion years ago, when the Sun was born and our solar system formed. The matrix of these meteorites is therefore thought to be the starting material from which all the planets formed.'

When our Sun was born it pulled in the gas and elements that were floating around it creating what is known as a planetary disk. This was made up of many different materials, including iron, silicates, hydrogen and ice and in time this material began to stick together.

Some of the newly formed balls of dust and ice compacted into rock and remained small, becoming asteroids. Other formations continued to gather debris and material, growing larger in size becoming rocky proto-planets which started to heat up, setting in motion a process known as differentiation. As the heat melted the metals in the matrix it caused these heavier elements to sink downwards towards the centre, creating the planet's core. At the same time, the lighter silicates moved towards the outside, forming what we now see as the mantle and the crust.

Dr Vaccaro explains, ‘When this happens, all the starting material that we had in the protoplanetary disk is gone as it went through the process of melting and recrystalisation, which limits what we can learn by studying the rocks of Earth.

‘This means that if we want to understand what the dust was like as the solar system formed, we need to go back and grab some of the material that didn't go through this differentiation process. In some meteorites, such as Acfer 094, we have that starting material preserved.'

The discovery of asteroidial ice has allowed the team to create a model that tells us how the asteroid grew and how the planets formed. From this, researchers concluded that fluffy ice and dust particles came together into bigger bodies beyond the snow line, a point in the planetary disk beyond which solid water ice can exist, and then migrated inwards. As this occurred, the ice started melting leaving the gaps known as fossil ice in its place.

More advanced technology will now allow researchers to look back and re-examine previously discovered specimens. It is hoped that through understanding how these primitive asteroids came into being we will better understand where we ourselves come from.

Dr Vaccaro concludes, 'They are made up of very similar stuff from which our own planet is formed, so by studying these meteorites we can better understand how the Earth evolved.’ 

The study Discovery of fossil asteroidial ice in primitive meteorite Acfer 094 has been published in the journal Science Advances.

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