Fossil ice found in Earth’s starting material |Curator of Petrology

High-resolution SEM (Scanning Electron Microscope) investigations, along with high-resolution CT imaging 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.  This has allowed Dr Epifanio Vaccaro, Curator of Petrology at the Natural History Museum, along with colleagues in Japan, to create a model of how the asteroids grew and the planets formed, including our own planet Earth.

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Picture of Earth credit NASA

The presence of ice in some asteroids it has been known for a long time, this has been hinted at by the observed alterations caused by the water to the minerals making up the asteroids known as aqueous alterations. However, the direct evidence of the presence of ice was never been observed before. The discovery was made by Dr Epifanio Vaccaro, Curator of Petrology at the Museum, along with a team of Japanese researchers.

Continue reading to find out more about this important discovery.

Where was the fossil ice found exactly?

The fossil asteroidal ice was discovered in a primitive meteorite officially called Acfer 094, that fell in Algeria in 1990. This meteorite was a piece of a larger asteroid that formed at the early stages of formation of our Solar System. In particular, the fossil ice was observed in the matrix of the meteorite, which is the fine-grained material that holds the meteorite together.

By looking at the fine-grained matrix, researchers have been able to see the microscopic pockets that were left behind when the ice they once contained melted. These tiny holes are the direct evidence of the presence of ice in the asteroid and are known as fossil ice.

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SEM image of typical fossil asteroidal ice found in Acfer 094.
SEM images of Acfer 094 polished sections.
Illustration of Acfer 094 polished sections showing that UPLs (coloured in yellow) are widely distributed in the polished sections.Enter a caption Matsumoto et al., 2019

What is the matrix?

The Matrix of a primitive meteorite is the fine-grained material that is found between the larger pieces that make up the meteorite and holds everything together. This is thought to be the remnants of the dust that once made up the protoplanetary disk. The dust within the disk started to stick together, incorporating different elements and materials including the ice, and this is the starting material from which all the planets, including the Earth, came from.

Artist’s impression of the water snowline around the young sta
Artist’s impression of the protoplanetary disk showing the water snowline around a young star By A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO) 
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X-ray computed tomography (XCT) Image of sample Acfer094 matrix showing fossil asteroidal ice Matsumoto et al., 2019

Why is this discovery important?

Based on this finding of asteroidal ice, researchers made a model explaining how asteroids grew and how the planets formed. According to this model fluffy ice and dust particles came together into bigger bodies beyond the snow line. This is the point in the planetary disk beyond which solid water ice can exist because any closer it would be melted by the heat from the developing star. The bodies then migrated inwards, crossing the snow line and as they did so, the ice started melting leaving the fossils in its place. Some of these newly formed bodies of dust and ice compacted into rock and remained small, becoming asteroids whereas others continued to gather dust and evolved into planets.

 

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Schematic illustration of the Acfer 094 parent body formation model. The parent body grew by agglomeration of fluffy source dust with and without ice through its radial migration from the outer to inner regions of the solar nebula across the H2O snow line Matsumoto et al., 2019

These meteorites are made up of very similar material from which our own planet formed, so by studying them, it helps us understand what is the starting material from which the Earth originated.

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