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May 22, 2014 Vasudevan Mukunth
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The meteorite that crashed near the Russian city of Chelyabinsk in early 2013 collided with an asteroid around or after 290 million years ago. Where does this leave our ability to predict future crashes?

The Chelyabinsk meteorite streaking across the sky on February 15, 2013. The Chelyabinsk meteorite streaking across the sky on February 15, 2013.

With the second largest air burst recorded in history, a meteorite exploded over the southern Ural region of Russia in February 2013 and crashed near the city of Chelyabinsk. During its journey through Earth's atmosphere, it underwent intense heating, eventually glowing brighter than the Sun, and blew up with a bright flash.

The accompanying shockwave damaged over 7,000 buildings and injured 1,500. The crash disintegrated the rock into fragments.

When analyzing some of these fragments, scientists from the Tohoku University, Japan, detected the presence of a mineral called jadeite. Jadeite is a major constituent of jade, the hard rock that has been used since prehistoric times for fashioning ornaments. The mineral forms only under extreme pressure and temperature.

"Generally, jadeite is not included in meteorites as a primary mineral," said Shin Ozawa, a graduate school student at Tohoku University and lead author of his team's paper published in Scientific Reports on May 22.

The implication is that the Chelyabinsk meteorite, originally an asteroid, could have had a violent past leading to its undergoing immense heating and compression.

Piecing evidence together

"The jadeite reported in our paper is considered to have crystallized from a melt of sodium-rich plagioclase under high-pressure and high-temperature conditions caused by an impact," Ozawa explained. Plagioclase (NaAlSi3O8) is a silicate mineral found in meteorites as well as terrestrial rocks.

The impact would have been in the form of the Chelyabinsk asteroid - or its parent body - colliding with another rock in space.

To arrive at distinct estimates of how this collision could have occurred, Ozawa and his colleagues connected two bits of evidence and solved it like an algebraic equation. In this case, the equations are called the Rankine-Hugoniot relations.

First, they observed the jadeite was found embedded in black seams in the rock called shock-melt veins. "They are formed by localized melting of rocks probably due to frictional heat, accompanied with shear movements of material within the rocks during an impact," Ozawa explained.

The molten rock then solidifies due to high pressure. The amount of time for which this pressure is maintained - i.e. duration of the impact - was calculated based on how long it would have taken a shock-melt vein of its composition to solidify.

Second, they knew the conditions under which jadeite forms, which require a certain minimum impact pressure which, in turn, is related to the speed at which the two bodies smashed into one another.

Based on this information, Ozawa reasons the Chelyabinsk meteorite - or its parent body - could have collided with another space-rock "at least 150 metres in diameter" at 0.4 to 1.5 km/s.

The impact itself could have occurred around or after 290 million years ago, according to a study published in Geochemistry International in 2013, titled 'Analytical results for the material of the Chelyabinsk meteorite'. It also reports that the meteorite is 4.4-4.6 billion years old.

Collision course

Ozawa's results aren't the end of the road, however, in understanding the meteorite's past, a 4-billion-year journey that ended on the only planet known to harbor life. In fact, nobody noticed it hurtling toward our planet until it entered the atmosphere and started glowing.

Earth has been subjected to many asteroid-crashes because of its proximity to the asteroid belt between Mars and Jupiter. In this region, according to Ozawa, asteroids exist in a stable state. So violent collisions with other asteroids could be one of the triggers that could set these rocks on a path toward Earth.

Ozawa speculated that such events wouldn't be uncommon. A report released by the B612 Foundation in April this year attests to that. It states that asteroids caused 26 nuclear-scale explosions in Earth's atmosphere between 2000 and 2013. As The Guardian wrote, "the evidence was a sobering reminder of how vulnerable the Earth was to the threat from space".

The difficulty in detecting the Chelyabinsk asteroid was also compounded by the fact that it came from the direction of the Sun. "If it had approached the Earth from a different direction," Ozawa added, "its detection might have been easier."

Thus, such collisions cause essentially random upheavals in our ability to predict when one of these rocks might threaten to get too close. By studying their past, scientists can piece together when and how these collisions occur, and get a grip on the threat-levels.

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