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Saturday, October 23, 2021

Explained: How an Indian meteorite helped study Earth’s formation

By studying the composition of these meteorite fragments, researchers have unravelled the composition expected to be present in the Earth’s lower mantle which is at about 660 km deep.

By: Kochi Written by Aswathi Pacha , Edited by Explained Desk | Kochi |
Updated: October 12, 2021 2:52:40 pm
The international team of scientists examined a section of the highly-shocked meteorite from Katol (Source: meteorites.asu.edu)

On May 22, 2012, a large meteor shower occurred near the town of Katol in Nagpur. As it happened at noon, the villagers missed the light show but the shower caused sonic booms or thunder-like noises, initially spreading rumours that an aircraft had crashed.

The next day, researchers from the Geological Survey of India collected about 30 meteorite fragments with the largest weighing around a kilogram.

Initial studies revealed that the host rock was mainly composed of olivine, an olive-green mineral. Olivine is the most abundant phase in our Earth’s upper mantle. Our Earth is composed of different layers including the outer crust, followed by the mantle and then the inner core. You can reach the upper mantle if you drill for about 410 kilometers.

Now, by studying the composition of these meteorite fragments, researchers have unravelled the composition expected to be present in the Earth’s lower mantle which is at about 660 km deep.

Studying the meteorite could also tell us more about how our Earth evolved from being a magma ocean to a rocky planet.

How to study a meteorite?

The researchers took a small sample of the meteorite and examined it using special microscopy techniques. The mineralogy was determined using a laser micro-Raman spectrometer.

These techniques helped the team identify, characterise the crystal structure of the meteorite and determine its chemical composition and texture.

What does the new study show?

The international team of scientists examined a section of the highly-shocked meteorite from Katol.

The paper published this month in PNAS reports the first natural occurrence of a mineral called bridgmanite. The mineral was named in 2014 after Prof. Percy W. Bridgman, recipient of the 1946 Nobel Prize in Physics.

Various computational and experimental studies have shown that about 80% of the Earth’s lower mantle is made up of bridgmanite. By studying this meteorite sample, scientists can decode how bridgmanite crystallized during the final stages of our Earth’s formation.

Bridgmanite on Earth VS on meteorite

The bridgmanite in the meteorite was found to be formed at pressures of about 23 to 25 gigapascals generated by the shock event. The high temperature and pressure in our Earth’s interior have changed over billions of years causing crystallisation, melting, remelting of the different minerals before they reached their current state. It is important to study these individual minerals to get a thorough idea of how and when the Earth’s layers formed.

Dr Sujoy Ghosh, Assistant Professor from the Department of Geology and Geophysics, Indian Institute of Technology Kharagpur explains: “Katol meteorite is a unique sample and it is a significant discovery. Though previous studies on other meteorite samples (Tenham and Suizhou samples) have shown the presence of much more magnesium and iron components, they were different from bridgmanite present in the Earth’s lower mantle. The composition of Katol bridgmanite closely matches those synthesized in different laboratories around the globe over the last three decades.” He is the corresponding author of the paper.

Evolution of Earth

“The inner planets or terrestrial planets or rocky planets Mercury, Venus, Earth, and Mars are formed by accretion or by rocky pieces coming together and forming a planet by increased pressure and high temperature caused by radioactive elements and gravitational forces,” explains Kishan Tiwari, research scholar from the Department of Geology and Geophysics, Indian Institute of Technology Kharagpur. “Our Earth was an ocean of magma before the elements crystallised and stabilised and the different layers such as core, mantle were formed. The heavier elements like iron went to the core while the lighter silicates stayed in the mantle. By using the meteorite as an analog for Earth, we can unearth more details about the formation.” He is one of the authors of the paper.

Dr. Ghosh added: “Our findings led to numerous other advances to understand how the Earth’s core formed about 4.5 billion years ago. Our discovery could also help investigations of high-pressure phase transformation mechanisms in the deep Earth”.

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